Diene Cyclohexene Bicyclo[3.1.0]Hexane 4-Methyl-2-Pentyne (Two Double Bonds) (One Ring, One (Two Rings) (One Triple Bond) Double Bond)

Organic Chemistry I

Mohammad Jafarzadeh Faculty of Chemistry, Razi University

Organic Chemistry, (9th edition) By John McMurry, Cengage Learning, 2016 1 4. Alkenes and Alkynes

An overview of Organic Reactions

Organic chemical reactions can be organized in two ways: what kinds of reactions occur? and how those reactions occur?

!"# !"#$%&' ( #) *+&'+,&-Four types *. *'/#),!of organic '&#!%,reactions*)0 : additions, eliminations, substitutions, and rearrangements.

Addition reactions occur when two reactants add together to form a single • Addition reactionsproduct withoccur no atomswhen “lefttwo over.”reactants An exampleadd together that we’llto form be studyinga single soonproduct with no is the reaction of an alkene, such as ethylene, with HBr to yield an alkyl atoms “left over.” An example is the reaction of an alkene, such as ethylene, with HBr to yield bromide. an alkyl bromide.

H H H Br These two . . . add to give reactants . . . C C + H Br H C C H this product. HH H H Ethylene Bromoethane (an alkene) (an alkyl halide) 2

Elimination reactions are, in a sense, the opposite of addition reactions. They occur when a single reactant splits into two products, often with the formation of a small molecule such as water or HBr. An example is the acid-catalyzed reaction of an alcohol to yield water and an alkene.

H OH H H This one Acid catalyst . . . gives these reactant . . . H C C H CC + H2O two products. H H HH

Ethanol Ethylene (an alcohol) (an alkene)

Substitution reactions occur when two reactants exchange parts to give two new products. An example is the reaction of an ester such as methyl acetate with water to yield a carboxylic acid plus an alcohol. Similar reactions occur in many biological pathways, including the metabolism of dietary fats.

O O These two …give these Acid reactants… C CH3 + H H C H + H CH3 two products. H3C O O catalyst H3C O O

Methyl acetate Acetic acid Methanol (an ester) (a carboxylic acid) (an alcohol)

Rearrangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product. An example is the conversion of dihydroxyacetone phosphate into its constitutional isomer glyceraldehyde 3-phosphate, a step in the glycolysis pathway by which carbohydrates are metabolized.

O H OH 2– 2– This reactant… O3PO C OH O3PO C O …gives this C C C C isomeric product. HHHH HH H

Dihydroxyacetone Glyceraldehyde phosphate 3-phosphate

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Addition reactionsAddition reactions occur when occur two when reactants two reactants add together add together to form to a formsingle a single product withproduct no atoms with no“left atoms over.” “left An over.” example An example that we’ll that be we’ll studying be studying soon soon is the reactionis the of reaction an alkene, of an such alkene, as ethylene,such as ethylene, with HBr with to HBryield to an yield alkyl an alkyl bromide. bromide.

H Br H H H Br TheseH two H . . . add to give These two . . . add to give reactants . . . C C + H Br H C C H this product. reactants . . . C C + H Br H C C H this product. HH H H HH H H Ethylene Bromoethane Ethylene (an alkene) Bromoethane(an alkyl halide) (an alkene) (an alkyl halide) Elimination reactions are, in a sense, the opposite of addition reactions. EliminationThey reactions occur when are, ain single a sense, reactant the oppositesplits into of two addition products, reactions. often with the • Elimination Theyreactions occurformation whenare the a singleofopposite a small reactant moleculeof splitsaddition such into reactionsastwo water products, or. HBr.They often Anoccur withexample thewhen is thea single reactant splits formationinto two products,acid-catalyzed of a smalloften molecule reactionwith such theof an formation asalcohol water to or yieldof HBr.a watersmall An example andmolecule an alkene. is thesuch as water or HBr. An exampleacid-catalyzedis the acid reaction-catalyzed of an reactionalcohol toof yieldan alcohol water andto yieldan alkene.water and an alkene. H OH H H This one Acid catalyst . . . gives these H OH reactant . . . H C C H H H CC + H2O two products. This one Acid catalyst . . . gives these reactant . . . H C C H H H CC HH+ H2O two products. HH H H Ethanol Ethylene (an alcohol) (an alkene) Ethanol Ethylene (an alcohol) (an alkene) Substitution reactions occur when two reactants exchange parts to give two new products. An example is the reaction of an ester such as methyl • Substitution Substitutionreactionsacetate reactionsoccur with waterwhen occur to whenyieldtwo areactants two carboxylic reactants acidexchange exchange plus an alcohol.parts parts to Similar givegive reactwo- new two new products. An example is the reaction of an ester such as methyl products. An example istionsthe reactionoccur in manyof an biologicalester such pathways,as methyl includingacetate thewith metabolismwater to ofyield a acetate withdietary water fats. to yield a carboxylic acid plus an alcohol. Similar reac- carboxylic acidtionsplus anoccuralcohol in many. biological pathways, including the metabolism of dietary fats.O O These two …give these Acid reactants… C CH3 + H H C H + H CH3 two products. O H3C O O catalystO H3C O O These two …give these Acid reactants… C MethylCH3 + acetateH H C AceticH + acidH CHMethanol3 two products. H3C O (an ester) O catalyst H3C (aO carboxylic acid)O (an alcohol) 3 Methyl acetate Acetic acid Methanol (an ester) Rearrangement reactions(a carboxylic occur when acid) a single(an alcohol) reactant undergoes a reorganization of bonds and atoms to yield an isomeric product. An example Rearrangementis the conversionreactions occur of dihydroxyacetone when a single reactantphosphate undergoes into its constitutional a reor- isomer glyceraldehyde 3-phosphate, a step in the glycolysis pathway by ganization of bonds and atoms to yield an isomeric product. An example which carbohydrates are metabolized. is the conversion of dihydroxyacetone phosphate into its constitutional isomer glyceraldehyde 3-phosphate, a step in the glycolysis pathway by O H OH which carbohydrates2– are metabolized. 2– This reactant… O3PO C OH O3PO C O …gives this C C C C isomeric product. O HHHH H OHHH H 2– 2– This reactant… O3PO C OH O3PO C O …gives this C DihydC roxyacetone C GlyceraldehC ydisomerice product. phosphate 3-phosphate HHHH HH H

Dihydroxyacetone Glyceraldehyde phosphate 3-phosphate

80485_ch06_0149-0181j.indd 150 2/2/15 1:50 PM Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

80485_ch06_0149-0181j.indd 150 2/2/15 1:50 PM !"# !"#$%&' ( #) *+&'+,&- *. *'/#),! '&#!%,*)0

Addition reactions occur when two reactants add together to form a single product with no atoms “left over.” An example that we’ll be studying soon is the reaction of an alkene, such as ethylene, with HBr to yield an alkyl bromide.

H H H Br These two . . . add to give reactants . . . C C + H Br H C C H this product. HH H H Ethylene Bromoethane (an alkene) (an alkyl halide)

H OH H H This one Acid catalyst . . . gives these reactant . . . H C C H CC + H2O two products. H H HH

Ethanol Ethylene (an alcohol) (an alkene)

O O These two …give these Acid reactants… C CH3 + H H C H + H CH3 two products. H3C O O catalyst H3C O O

Methyl acetate Acetic acid Methanol • Rearrangement(an ester)reactions occur when a (asingle carboxylicreactant acid) undergoes(an alcohol) a reorganization of bonds and atoms to yield an isomeric product. Rearrangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product. An example An example is theis conversionthe conversionof ofdihydroxyacetone dihydroxyacetone phosphate intointo itsits constitutionalconstitutional isomer glyceraldehyde 3-isomerphosphate, glyceraldehydea step in 3-phosphate,the glycolysis a steppathway in the glycolysisby which pathwaycarbohydrates by are metabolized. which carbohydrates are metabolized.

O H OH 2– 2– This reactant… O3PO C OH O3PO C O …gives this C C C C isomeric product. HHHH HH H

Dihydroxyacetone Glyceraldehyde phosphate 3-phosphate

80485_ch06_0149-0181j.indd 150 2/2/15 1:50 PM How organic reactions occur: Mechanisms

A mechanism describes in detail exactly what takes place at each stage of a chemical transformation—which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates are for each step.

A complete mechanism must also account for all reactants used and all products formed.

All chemical reactions involve bond-breaking and bond-making. When two molecules come together, react, and yield products, specific bonds in the reactant molecules are broken and specific bonds in the product molecules are formed.

Fundamentally, there are two ways in which a covalent two-electron bond can break. A bond can break in an electronically symmetrical way so that one electron remains with each product fragment, or a bond can break in an electronically unsymmetrical way so that both bonding electrons remain with one product fragment, leaving the other with a vacant orbital.

5 !-" #$% $&'()*+ &,(+-*$). $++/&: 0,+#()*.0. !"! !-" #$% $&'()*+ &,(+-*$). $++/&: 0,+#()*.0. !"!

PROBLEM #$! PROBLEM #$! Classify each of the following reactions as an addition, elimination, substitu- Classify each of the following reactions as an addition, elimination, substitution, or rearrangement: tion, or rearrangement: (a) CH3Br ϩ KOH n CH3OH ϩ KBr (a) CH3Br ϩ KOH n CH3OH ϩ KBr (b) CH3CH2Br n H2C P CH2 ϩ HBr (b) CH3CH2Br n H2C P CH2 ϩ HBr (c) H2C P CH2 ϩ H2 n CH3CH3 (c) H2C P CH2 ϩ H2 n CH3CH3

# -#%- % HHowow O Organicrganic R eactionsReactions O Occur:ccur: M Mechanismsechanisms Having looked at the kinds of reactions that take place, let’s now see how reac- Having looked at the kinds of reactions that take place, let’s now see how reactions occur. An overall description of how a reaction occurs is called a reaction tions occur. An overall description of how a reaction occurs is called a reaction mechanism. A mechanism describes in detail exactly what takes place at each mechanismstage of a chemical. A mechanism transformation—which describes in detail bonds exactly are what broken takes and place in what at each order, stagewhich of a bondschemical are transformation—which formed and in what order, bonds and are what broken the andrelative in what rates order, are for whicheach bondsstep. A are complete formed mechanismand in what must order, also and account what the for relative all reactants rates usedare for and eachall step.products A complete formed. mechanism must also account for all reactants used and all productsAll chemical formed. reactions involve bond-breaking and bond-making. When twoAll molecules chemical comereactions together, involve react, bond-breaking and yield products, and bond-making. specific bonds When in the tworeactant molecules molecules come together, are broken react, and and specific yield products,bonds in specificthe product bonds molecules in the reactantare formed. molecules Fundamentally, are broken and there specific are two bonds ways in in the which product a covalent molecules two- areelectron formed. bond Fundamentally, can break. A therebond arecan twobreak ways in an in electronically which a covalent symmetrical two- electronway so bond that onecan electronbreak. A remainsbond can with break each in productan electronically fragment, symmetrical or a bond can waybreak so that in an one electronically electron remains unsymmetrical with each wayproduct so that fragment, both bonding or a bond electrons can breakremain in an with electronically one product unsymmetrical fragment, leaving way sothe that other both with bonding a vacant electrons orbital. remainThe symmetricalwith one product cleavage fragment, is said leaving to be homolytic, the other withand thea vacant unsymmetrical orbital. The symmetrical cleavage is said to be homolytic, and the unsymmetrical The symmetricalcleavage iscleavage said to beis saidheterolytic.to be homolytic, and the unsymmetrical cleavage is said to cleavage is said to be heterolytic. be heterolyticWe’ll. develop this point in more detail later, but note for now that the movementWe’ll develop of one this electron point in in the more symmetrical detail later, process but noteis indicated for now using that athe half- The movementmovementheaded, of orofone one“fishhook,” electronelectron arrowinin thethe symmetrical( ), whereas processtheprocess movement isis indicatedindicated of two usingusing electrons a ahalf-half in-headed curvedheaded,thearrow unsymmetrical oror “fishhook,”“fishhook”, process arrowwhile is(the indicated), whereasmovement using theof movement a twofull-headedelectrons of twocurvedin electronsthe arrowunsymmetrical in( ). processtheusing unsymmetricala full-headed processcurved is indicatedarrow. using a full-headed curved arrow ( ). Symmetrical bond-breaking (radical): A B A + B Symmetricalone bonding bond-breaking electron stays (radical): with each product. A B A + B one bonding electron stays with each product. Unsymmetrical bond-breaking (polar): A B A+ + B– + – Unsymmetricaltwo bonding bond-breakingelectrons stay with(polar): one product. A B A + B two bonding electrons stay with one product. Just as there are two ways in which a bond can break, there are two ways There areinJust whichtwo asways there a covalentin arewhich two two-electrona wayscovalent in whichtwo bond- electrona canbond form. canbond break,A bondcan there formcan formare. two in anways elec - in which a covalent two-electron bond can form. A bond can form in an elec- A bondtronicallycan form symmetricalin an electronically way if onesymmetrical electron wayis donatedif one toelectron the newis bonddonated by eachto the new tronically symmetrical way if one electron is donated to the new bond by each bond byreactanteach reactant or in anor unsymmetricalin an unsymmetrical way if bothway ifbondingboth bonding electronselectrons are donatedare donated by by reactant or in an unsymmetrical way if both bonding electrons are donated by one reactantone reactant.. one reactant. Symmetrical bond-making (radical): A B A B + Symmetricalone bonding bond-making electron is donated(radical): by each reactant. A + B A B one bonding electron is donated by each reactant. Unsymmetrical bond-making (polar): A+ + B– A B + – Unsymmetricaltwo bonding bond-makingelectrons are donated(polar): by one reactant. 6 A + B A B two bonding electrons are donated by one reactant. Processes that involve symmetrical bond-breaking and bond-making are calledProcesses radical that reactions involve. symmetricalA radical, often bond-breaking called a “free and radical, bond-making” is a neutral are called radical reactions. A radical, often called a “free radical,” is a neutral

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

80485_ch06_0149-0181j.indd 151 2/2/15 1:50 PM 80485_ch06_0149-0181j.indd 151 2/2/15 1:50 PM Processes that involve symmetrical bond-breaking and bond-making are called radical reactions.

A radical, often called a “free radical,” is a neutral chemical species that contains an odd number of electrons and thus has a single, unpaired electron in one of its orbitals.

Processes that involve unsymmetrical bond-breaking and bond-making are called polar reactions.

Polar reactions involve species that have an even number of electrons and thus have only electron pairs in their orbitals.

7 !"# !"#$%&' ( #) *+&'+,&- *. *'/#),! '&#!%,*)0

!"# !"#$%&' ( #) *+&'+,&- *. *'/#),! '&#!%,*)0 chemical species that contains an odd number of electrons and thus has a single, unpaired electron in one of its orbitals. Processes that involve unsym- chemical species that contains an odd number of electronsmetrical and bond-breaking thus has a and bond-making are called polar reactions. Polar single, unpaired electron in one of its orbitals. Processesreactions that involve involve unsym species- that have an even number of electrons and thus have metrical bond-breaking and bond-making are called polaronly electron reactions pairs. Polar in their orbitals. Polar processes are by far the more com- reactions involve species that have an even number of electronsmon reaction and thus type have in both organic and biological chemistry, and a large part of this book is devoted to their description. only electron pairs in their orbitals. Polar processes are by far the more com- In addition to polar and radical reactions, there is a third, less commonly mon reaction type in both organic and biological chemistry, and a large part of encountered process called a pericyclic reaction. Rather than explain pericy- this book is devoted to their description. clic reactions now, though, we’ll look at them more carefully in Chapter 30. In addition to polar and radical reactions, there is a third, less commonly encountered process called a pericyclic reaction. Rather than explain pericyclic reactions now, though, we’ll look at them more carefully $-% in ChapterRadical 30. Reactions

Radical reactions are not as common as polar reactions but are nevertheless $-% Radical Reactions important in some industrial processes and biological pathways. Let’s see briefly how they occur. A radical is highly reactive because it contains an atom with an odd num- Radical reactionsRadical arereactions not as common as polar reactions but are nevertheless important in some industrial processes and biologicalber pathways. of electrons Let’s (usually see seven) in its valence shell, rather than a stable, noble- briefly howA radical they occur.is highly reactive because it containsgas octet.an atomA radicalwith canan achieveodd number a valence-shellof electrons octet inin itsseveral ways. For example, the radical might abstract an atom and one bonding electron from A radicalvalence is highlyshell reactive. because it contains an atom with an odd number of electrons (usually seven) in its valence shell, ratheranother than a reactant,stable, noble- leaving behind a new radical. The net result is a radical sub- gas octet.A Aradical radicalcan can achieveachieve aa valence-shellvalence-shell octetoctet stitutionin severalin several reaction. ways.ways For. For example, the radical might example,abstract the radicalan mightatom abstractand one an atombonding and oneelectron bondingfrom electronanotherUnpaired from reactant, leaving behind aUnpairednew another reactant,radical. leavingThe net behindresult ais newa radical radical.substitution The net resultreaction is a radical. electron sub- electron stitution reaction. Rad ++BA Rad A B Unpaired Unpaired electron electron Reactant Substitution Product radical product radical

Rad ++BA Rad A B Alternatively, a reactant radical might add to a double bond, taking one Reactant Substitution Productelectron from the double bond and yielding a new radical. The net result is a radical product radicalradical addition reaction. Unpaired Alternatively, a reactant radical might add to a double bond, taking one Unpaired Alternatively, a reactant radical might add to a electron electron electron from the double bond and yielding a new radical. The net result is a Rad radical additiondouble reaction.bond, taking one electron from the double Rad + CC CC bond and yielding a new radical. The net result is a Unpaired radical addition reaction. Unpaired electron electron Reactant Alkene Addition product Rad radical radical 8 Rad + CC CC An example of an industrially useful radical reaction is the chlorination of methane to yield chloromethane. This substitution reaction is the first step in the Reactant Alkene Addition productpreparation of the solvents dichloromethane (CH2Cl2) and chloroform (CHCl3). radical radical H H Light An example of an industrially useful radical reaction is the chlorinationH C ofH + Cl Cl H C Cl + H Cl methane to yield chloromethane. This substitution reaction is the first step inH the H preparation of the solvents dichloromethane (CH2Cl2) and chloroform (CHCl3). Methane Chlorine Chloromethane H H Light H C H + Cl Cl H C Cl + H Cl H Copyright 2016 Cengage Learning.H All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Methane Chlorine Chloromethane

80485_ch06_0149-0181j.indd 152 2/2/15 1:50 PM

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#-$ Polar Reactions

Polar reactions occur because of the electrical attraction between positively polarized and negatively polarized centers on functional groups in molecules. To see how these reactions take place, let’s first recall the discussion of polar covalent bonds in Section 2-1 and then look more deeply into the effects of bond polarity on organic molecules. Most organic compounds are electrically neutral; they have no net charge, Polar reactions either positive or negative. We saw in Section 2-1, however, that certain bonds within a molecule, particularly the bonds in functional groups, are polar. Polar reactions occur Bondbecause polarityof the iselectrical a consequenceattraction ofbetween an unsymmetricalpositively and electronnegatively distribution in polarized centers on functionala bond andgroups is duein molecules to the difference. in electronegativity of the bonded atoms. Elements such as oxygen, nitrogen, fluorine, and chlorine are more electro- Most organic compoundsnegativeare electrically than carbon,neutral so; athey carbonhave atomno net bondedcharge to. Certain one of bondsthese atoms has a within a molecule, particularly the bonds in functional groups, are polar. Bond polarity is a partial positive charge (␦ϩ). Conversely, metals are less electronegative than consequence of an unsymmetrical electron distribution in a bond and is due to the difference carbon, so a carbon atom bonded to a metal has a partial negative charge (␦Ϫ). in electronegativity of the bonded atoms. Electrostatic potential maps of chloromethane and methyllithium illustrate Elements such as oxygen,these nitrogen,charge distributions,fluorine, and showingchlorine are thatmore the carbonelectro- negativeatom in chloromethanethan is carbon, so a carbon atomelectron-poorbonded to one (blue)of these whileatoms the carbonhas a partial in methyllithiumpositive charge is( �electron-rich+). (red). Conversely, metals are less electronegative than carbon, so a carbon atom bonded to a metal has a partial negative charge (�-). The carbon atom in chloromethane is electron-poor, while the carbon in methyllithium is electron-rich. ␦– ␦+ Cl Li ␦+ ␦– C C H H H H H H

Chloromethane Methyllithium 9 The polarity patterns of some common functional groups are shown in TABLE #%!. Note that carbon is always positively polarized except when bonded to a metal. This discussion of bond polarity is oversimplified in that we’ve considered only bonds that are inherently polar due to differences in electronegativity. Polar bonds can also result from the interaction of functional groups with acids or bases. Take an alcohol such as methanol, for example. In neutral methanol, the carbon atom is somewhat electron-poor because the electronegative oxygen attracts the electrons in the C ᎐ O bond. On protonation of the methanol oxygen by an acid, however, a full positive charge on oxygen attracts the electrons in the C ᎐ O bond much more strongly and makes the carbon much more electron-poor. We’ll see numerous examples throughout this book of reactions that are catalyzed by acids because of the resultant increase in bond polarity upon protonation.

A– + H ␦– H H O O ␦+ ␦+ C HA C H H H H H H

Methanol—weakly Protonated methanol— electron-poor carbon strongly electron-poor carbon

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#-$ Polar Reactions

␦– ␦+ Cl Li ␦+ ␦– C C H H H H H H

Chloromethane Methyllithium The polarity patterns of some common functional groups are shown in TABLE #%!. Note that carbon is always positively polarized except when bonded to a metal. This discussion of bond polarity is oversimplified in that we’ve considered only bonds that are inherently polar due to differences in electronegativity. Polar bonds can also result from the interaction of functional groups with acids or Some of bonds are inherently polar due to differences in electronegativity. Polar bonds can also resultbases. fromTakethe an interactionalcohol suchof functionalas methanol,groups for example.with acids Inor neutralbases. methanol, the carbon atom is somewhat electron-poor because the electronegative oxygen In neutralattractsmethanol, the electronsthe carbon in the Catom ᎐ O bond.is somewhat On protonationelectron of-poor the methanolbecause oxygenthe electronegative by oxygenan attractsacid, however,the electrons a full inpositivethe C– Ochargebond on. oxygen attracts the electrons in the C ᎐ O bond much more strongly and makes the carbon much more electron-poor. On protonationWe’ll see numerousof the methanol examplesoxygen throughoutby an thisacid, booka full of reactionspositive chargethat are oncatalyzedoxygen attracts the electronsby acids becausein the ofC– theO bondresultantmuch increasemore instrongly bond polarityand makes upon protonation.the carbon much more electron-poor. A– + H ␦– H H O O ␦+ ␦+ C HA C H H H H H H

Methanol—weakly Protonated methanol— electron-poor carbon strongly electron-poor carbon 10

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two ethyl radicals, each of which then loses a hydrogen atom to generate two molecules of ethylene. HHHH H H H H H C C 900 °C C C H H CC2 2 C C + H2 HHHH H H HH Steam cracking is an example of a reaction whose energetics are domi- nated by entropy (⌬S°) rather than by enthalpy (⌬H°) in the free-energy equa- tion ⌬G° ϭ ⌬H° Ϫ T⌬S°. Although the bond dissociation energy D for a carbon–carbon single bond is relatively high (about 370 kJ/mol) and cracking is endothermic, the large positive entropy change resulting from the fragmen- tation of one large molecule into several smaller pieces, together with the high temperature, makes the T⌬S° term larger than the ⌬H° term, thereby favoring Alkenes: Structurethe crackingand reaction.reactivity

An alkene, sometimes called an olefin, is a hydrocarbon that contains a carbon–carbon double bond. #-$ Calculating Degree of Unsaturation Because of its double bond, an alkene has fewer hydrogens than an alkane Because of itswithdouble the samebond, numberan ofalkene carbons(C —n CHn2Hn)2nhas for anfewer alkenehydrogens versus CnH2thann؉2 foran an alkane (CnH2n+2) with alkanethe same — andnumber is thereforeof carbons, referredtherefore to as unsaturatedreferred to. Ethylene,as unsaturated for example,. has the formula C2H4, whereas ethane has the formula C2H6. H H H H H C C CC H HH H H

Ethylene: C2H4 Ethane: C2H6 (Fewer hydrogens—Unsaturated) (More hydrogens—Saturated) In general, each ring or double bond in a molecule corresponds to a loss of ,two hydrogens from the alkane formula CnH2n؉2. Knowing this relationship it’s possible to work backward from a molecular formula to calculate a molecule’s degree of unsaturation—the number of rings and/or multiple bonds present in the molecule. Let’s assume that we want to find the structure of an unknown hydrocarbon. A molecular weight determination yields a value of 82 amu, which cor- 11 responds to a molecular formula of C6H10. Since the saturated C6 alkane (hexane) has the formula C6H14, the unknown compound has two fewer pairs of hydrogens (H14 Ϫ H10 ϭ H4 ϭ 2 H2) so its degree of unsaturation is 2. The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There’s still a long way to go to establish its structure, but the simple calculation has told us a lot about the molecule.

4-Methyl-1,3-pentadiene Cyclohexene Bicyclo[3.1.0]hexane 4-Methyl-2-pentyne (two double bonds) (one ring, one (two rings) (one triple bond) double bond)

C6H10

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To summarize: Add the number of halogens to the number of hydrogens. Ignore the number of oxygens. Subtract the number of nitrogens from the number of hydrogens.

PROBLEM $%! Calculate the degree of unsaturation in each of the following formulas, and then draw as many structures as you can for each: (a) C4H8 (b) C4H6 (c) C3H4

PROBLEM $%& Calculate the degree of unsaturation in each of the following formulas: (a) C6H5N (b) C6H5NO2 (c) C8H9Cl3 (d) C9H16Br2 (e) C10H12N2O3 (f) C20H32ClN

PROBLEM $%' Diazepam, marketed as an antianxiety medication under the name Valium, has three rings, eight double bonds, and the formula C16H?ClN2O. How many hydrogens does diazepam have? (Calculate the answer; don’t count hydrogens in the structure.)

H3C O N

Cl N Diazepam

$-' Naming Alkenes

Alkenes are named using a series of rules similar to those for alkanes Naming alkenes (Section 3-4), with the suffix -ene used instead of -ane to identify the functional group. There are three steps to this process. STEP 1 STEP ! Name the parent hydrocarbon Name the parent hydrocarbon. Find the longest carbon chain Find the longest carboncontainingchain containing the doublethe bond,double and namebond, theand compoundname the accordingly,compound accordingly, using the suffixusing-ene the: suf!x -ene:

CH3CH2 H CH3CH2 H CC C C !"# !"#$%&' ( #)*&+&,: ,%'-!%-'& #+. '&#!%/0/%1 CH3CH2CH2 H CH3CH2CH2 H

STEP $ Named as a pentene NOT as a hexene, since the double bond is not contained in the six-carbon chain Number the carbon atoms in the chain. Begin at the end nearer the double STEP 2 bond or, if the double bond is equidistant from the two ends, begin at the Number the carbon atoms in the chain end nearer the !rst branch point. This rule ensures that the double-bond Begin at the end nearercarbonsthe doublereceive bondthe lowestor, if thepossibledouble numbers.bond is equidistant from the two ends, begin at the end nearer the first branch point. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole CHor in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage3 Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

CH3CH2CH2CH CHCH3 CH3CHCH CHCH2CH3 56 43 12 123456 12 80485_ch07_0185-0219l.indd 189 2/2/15 1:48 PM STEP % Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of the !rst alkene carbon and placing that number directly before the parent name. If more than one double bond is present, indicate the position of each and use one of the suf!xes -diene, -triene, and so on.

CH3

CH3CH2CH2CH CHCH3 CH3CHCH CHCH2CH3 123456 123456

2-Hexene 2-Methyl-3-hexene

CH3CH2 H CC 12 CH3

CH3CH2CH2 H H2CCCH CH2 345 1234

2-Ethyl-1-pentene 2-Methyl-1,3-butadiene

CH3 CH3 CH2CH2CH3

CH3CH2CHCH CHCHCH3 H2C CHCHCH CHCH3 75 6 3124 123 465

Newer naming system: 2,5-Dimethylhept-3-ene 3-Propylhexa-1,4-diene

(Older naming system: 2,5-Dimethyl-3-heptene 3-Propyl-1,4-hexadiene)

Cycloalkenes are named similarly, but because there is no chain end to begin from, we number the cycloalkene so that the double bond is between C1 and C2 and the first substituent has as low a number as possible. It’s not neces- sary to indicate the position of the double bond in the name because it’s always

80485_ch07_0185-0219l.indd 190 2/2/15 1:48 PM !"# !"#$%&' ( #)*&+&,: ,%'-!%-'& #+. '&#!%/0/%1

STEP $ Number the carbon atoms in the chain. Begin at the end nearer the double bond or, if the double bond is equidistant from the two ends, begin at the end nearer the !rst branch point. This rule ensures that the double-bond carbons receive the lowest possible numbers.

CH3

CH3CH2CH2CH CHCH3 CH3CHCH CHCH2CH3 56 43 12 123456

STEPSTE3 P % Write theWritefull name the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the Number the substituents according to their positions in the chain, and list them alphabetically. Indicate thedoubleposition bondof bythe givingdouble thebond numberby giving of thethe !rstnumber alkeneof carbonthe first andalkene placingcarbon and placing thatthatnumber numberdirectly directlybefore beforethe theparent parentname name.. If more than one double bond is present, indicate the position of each and use one of the suf!xes If more than-diene,one -triene,double andbond sois on.present, indicate the position of each and use one of the suffixes -diene, -triene, and so on. CH3

CH3CH2CH2CH CHCH3 CH3CHCH CHCH2CH3 123456 123456

2-Hexene 2-Methyl-3-hexene

CH3CH2 H CC 12 CH3

CH3CH2CH2 H H2CCCH CH2 345 1234

2-Ethyl-1-pentene 2-Methyl-1,3-butadiene 13

CH3 CH3 CH2CH2CH3

CH3CH2CHCH CHCHCH3 H2C CHCHCH CHCH3 75 6 3124 123 465

Newer naming system: 2,5-Dimethylhept-3-ene 3-Propylhexa-1,4-diene

(Older naming system: 2,5-Dimethyl-3-heptene 3-Propyl-1,4-hexadiene)

80485_ch07_0185-0219l.indd 190 2/2/15 1:48 PM !"# !"#$%&' ( #)*&+&,: ,%'-!%-'& #+. '&#!%/0/%1

CH3

CH3CH2CH2CH CHCH3 CH3CHCH CHCH2CH3 56 43 12 123456

STEP % Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of the !rst alkene carbon and placing that number directly before the parent name. If more than one double bond is present, indicate the position of each and use one of the suf!xes -diene, -triene, and so on.

CH3

CH3CH2CH2CH CHCH3 CH3CHCH CHCH2CH3 123456 123456

2-Hexene 2-Methyl-3-hexene

CH3CH2 H CC 12 CH3

CH3CH2CH2 H H2CCCH CH2 345 1234

2-Ethyl-1-pentene 2-Methyl-1,3-butadiene

We should also note that IUPAC changed their naming recommendations in 1993 to place the locant indicating the position of the double bond immediately before the -ene suffix rather than before the parent name: but-2-ene rather than 2-butene, for instance. This change has not been widely accepted We shouldby thealso chemicalnote that communityIUPAC changed in the Unitedtheir naming States,recommendations however, so we’ll instay1993 withto place the locant indicatingthe older thebut positionmore commonlyof the double usedbond names.immediately Be aware,before though,the that-ene yousuffix mayrather than before theoccasionallyparent name, encounterfor example the newer: but -system.2-ene rather than 2-butene.

CH3 CH3 CH2CH2CH3

CH3CH2CHCH CHCHCH3 H2C CHCHCH CHCH3 75 6 3124 123 465

Newer naming system: 2,5-Dimethylhept-3-ene 3-Propylhexa-1,4-diene

(Older naming system: 2,5-Dimethyl-3-heptene 3-Propyl-1,4-hexadiene) !-" #$%&#' $()*#*+ !"! Cycloalkenes are named similarly, but because there is no chain end to For cycloalkenes,begin from, thewe doublenumberbetweenbond theC1 and cycloalkeneis C2.always As withbetween open-chainso that theC alkenes,1 doubleand Cthe 2bond newerand isthe but between notfirst yetsubstituent widely C1 has accepted naming rules place the locant immediately before the suffix in a as lowanda number C2 and theas firstpossible,cyclic substituent alkene.therefore, has asit’s lownot a numbernecessary as possible.to indicate It’s notthe necesposition- of the doublesarybond toin indicatethe name the. position of the double bond in the name because it’s always CH3 6 6 5 1 CH3 5 5 1 4 1 CH3 4 2 4 2 3 3 3 2

1-Methylcyclohexene 1,4-Cyclohexadiene 1,5-Dimethylcyclopentene Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). 14 Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning(New: reserves Cyclohexathe right to remove-1,4-diene additional content) at any time if subsequent rights restrictions require it.

For historical reasons, there are a few alkenes whose names are firmly entrenched in common usage but don’t conform to the rules. For example, the 80485_ch07_0185-0219l.indd 190 alkene derived from ethane should be called ethene, but the name ethylene 2/2/15 1:48 PM has been used for so long that it is accepted by IUPAC. TABLE #$! lists several other common names that are often used and are recognized by IUPAC. Note also that a ϭCH2 substituent is called a methylene group, a H2CPCH ᎐ substituent is called a vinyl group, and a H2CPCHCH2 ᎐ substituent is called an allyl group.

H2C H2CCH H2CCH CH2

A methylene group A vinyl group An allyl group

TABLE #$! Common Names of Some Alkenes

Compound Systematic name Common name

H2C CH2 Ethene Ethylene

CH3CH CH2 Propene Propylene

CH3 2-Methylpropene Isobutylene

CH3C CH2

CH3 2-Methyl-1,3-butadiene Isoprene

H2C CHC CH2

PROBLEM #$% Give IUPAC names for the following compounds:

(a) H3C CH3 (b) CH3

H2C CHCHCCH3 CH3CH2CH CCH2CH3

CH3

CH3CH CHCHCH CHCHCH3 CH3CH2CH2CH CHCHCH2CH3

80485_ch07_0185-0219l.indd 191 2/2/15 1:48 PM !-" #$%&#' $()*#*+ !"!

between C1 and C2. As with open-chain alkenes, the newer but not yet widely accepted naming rules place the locant immediately before the suffix in !a- " #$%&#' $()*#*+ !"! cyclic alkene. between C1 and C2. As with open-chain alkenes, the newer but not yet widely accepted naming rules place the locant immediately before the suffixCH3 in a 6 6 5 cyclic1 CH alkene.3 5 5 1 4 CH 1 3 CH3 6 6 5 4 2 1 CH3 4 2 3 5 5 1 4 12 3 3 CH3 4 2 4 2 3 1-Methylcyclohex3 ene 1,4-Cyclohexa3 diene 1,5-Dimethyl2 cyclopentene

1-Methylcyclohexene (New: Cyclohexa1,4-Cyclohexa-1,4-dienediene ) 1,5-Dimethylcyclopentene (New: Cyclohexa-1,4-diene) For historical reasons, there are a few alkenes whose names are firmly For historicalentrenchedreasons,For in commonhistoricalthere reasons,usageare a but therefew don’t arealkenes aconform few alkeneswhose to the whosenames rules. names Forare areexample,firmly firmly entrenched the in entrenched in common usage but don’t conform to the rules. For example, the commonalkeneusage derivedalkenebut don’t derived fromconform fromethane ethaneto shouldthe shouldrules be be .called Forcalledexample, ethene,ethene, but butthe the thealkene name name ethylenederived ethylene from ethane should hasbe called beenhas usedethene, been for used butso for longthe so longnamethat that itethylene is it acceptedis acceptedhas bybybeen IUPAC.used T ABLETABLEfor #$!so #$!listslong lists severalthat several it is accepted by IUPACother. commonother common names names that thatare areoften often used used and and areare recognizedrecognized by byIUPAC. IUPAC. Note Note also that a ϭCH substituent is called a methylene group, a H CPCH ᎐ sub- also that a ϭCH2 substituent2 is called a methylene group, a2 H2CPCH ᎐ sub- Note thatstituenta =CH stituentis calledsubstituent is calleda vinyl ais vinyl groupcalled group, anda , methyleneand a Ha HC2CPPCHCHCHCHgroup2 ᎐ ᎐ ,substituent substituenta H C=CH is called– issubstituent called an an called a allyl2 group. 2 2 2 vinyl groupallyl ,groupand a. H2C=CHCH– substituent called an allyl group. H2C H2CCH H2CCH CH2 H C H CCH H CCH CH A2 methylene group 2A vinyl group An2 allyl group 2 A methylene group A vinyl group An allyl group

TABLE #$! Common Names of Some Alkenes

TABLE #$!Compound Common Names of SomeSystematic Alkenes name Common name

H2C CH2 Ethene Ethylene Compound Systematic name Common name CH3CH CH2 Propene Propylene

H2C CH2 CH3 Ethene2-Methylpropene IsobutyleneEthylene CH3C CH2 CH3CH CH2 Propene Propylene CH3 2-Methyl-1,3-butadiene Isoprene CH3 2-Methylpropene Isobutylene H2C CHC CH2 15 CH3C CH2

CH3 2-Methyl-1,3-butadiene Isoprene PROBLEM #$% H C CHC CH 2 Give IUPAC2 names for the following compounds:

(a) H3C CH3 (b) CH3

H2C CHCHCCH3 CH3CH2CH CCH2CH3 PROBLEM #$% CH3 Give IUPAC names for the following compounds: (c) CH3 CH3 (d) CH3CHCH2CH3 (a) H C CH (b) CH 3CH3CH3 CHCHCH CHCHCH3 CH3CH2CH2CH3 CHCHCH2CH3 H2C CHCHCCH3 CH3CH2CH CCH2CH3

CH3

(c) Copyright 2016 CengageCH3 Learning. All RightsCH Reserved.3 May not be copied,(d) scanned, or duplicated, in whole or in part.CH Due3 toCHCH electronic rights,2CH some3 third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. CH3CH CHCHCH CHCHCH3 CH3CH2CH2CH CHCHCH2CH3

80485_ch07_0185-0219l.indd 191 2/2/15 1:48 PM