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Home , Alkene, Alkyne, Ethylbenzene, Halogenation

Unsaturated Chem101 • Contain one or more double or triple General -carbon bond.

Lecture 11 H H H Unsaturated Hydrocarbons H H C C H C C H H H H H H

University of Wisconsin-Eau Claire Chem101 - Lecture 11 2

Unsaturated Hydrocarbons Unsaturated Hydrocarbons • There are three classes of unsaturated • There are three classes of unsaturated hydrocarbons: hydrocarbons: - have at least one carbon-carbon - have at least one carbon-carbon

H H C C H C C H H H (common name)

University of Wisconsin-Eau Claire Chem101 - Lecture 11 3 University of Wisconsin-Eau Claire Chem101 - Lecture 11 4

Unsaturated Hydrocarbons Nomenclature of Alkenes • There are three classes of unsaturated • Nomenclature means a systematic set hydrocarbons: of names. - Aromatics are rings with alternating • The presence of double bonds means double and single bonds. that there is even a greater number of

H possible than for . H H • We will use the IUPAC system for naming alkenes and alkynes.

H H H

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1 Nomenclature of Alkenes Nomenclature of Alkenes • IUPAC Rules: • IUPAC Rules (con’d): 1 Count the in the longest chain that 5 Also in the prefix, identify and locate any contains the double bond to determine the groups that are attached to the root name, longest chain containing the double bond. 2 Use the ending -ene to designate the 6 Combine the prefix, root and ending names presence of a double bond. for the attached groups and the longest 3 Number the longest chain so that the chain into the complete name. carbons participating in the double bond 7 If the contains more than one have the lowest possible number. double bond, the prefixes di-, tri-, tetra 4 In the prefix, locate the double bond by before the -ene ending. using the lower of the two carbon numbers.

University of Wisconsin-Eau Claire Chem101 - Lecture 11 7 University of Wisconsin-Eau Claire Chem101 - Lecture 11 8

Exercise 12.1(p388) Exercise 12.3 (p388) Select the compounds that can be correctly called Give the IUPAC name for the following compounds: unsaturated and classify each one as an or an a. CH CH CH CH CH CH : 3 2 3

CH3 a. CH3 CH2 CH3 b. CH3 CH CH CH3

c. H C C CH CH CH CH CH CH CH CH CH 3 d. b. 3 3 CH3 CH3

CH CH CH2 3 f. c. e.

University of Wisconsin-Eau Claire Chem101 - Lecture 11 9 University of Wisconsin-Eau Claire Chem101 - Lecture 11 10

sp2 Hybridization sp2 Hybridization • The orbitals containing the valence • The 2s orbital mixes with two of the 2p in alkenes also hybridize orbitals to form three sp2 hybrid orbitals • But it is done differently than for • This leaves the remaining 2p orbital alkanes unhybridized.

University of Wisconsin-Eau Claire Chem101 - Lecture 11 11 University of Wisconsin-Eau Claire Chem101 - Lecture 11 12

2 sp2 Hybridization sp2 Hybridization • The three sp2 hybrid orbitals are oriented in the same plane and separated by 120° from one another. • The unhybridized 2p orbital is oriented at right angles to the sp2 orbitals

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sp2 Hybridization sp2 Hybridization • The three sp2 hybrid each form and σ bond to another . • The 2p orbital forms a second bond to one of the three . - This second bond is called π (pi) bond.

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σ and π Bonds σ and π Bonds • s and p atomic orbitals: • Two half-filled s orbitals overlapping to form a σ bond:

University of Wisconsin-Eau Claire Chem101 - Lecture 11 17 University of Wisconsin-Eau Claire Chem101 - Lecture 11 18

3 σ and π Bonds σ and π Bonds • A half-filled s orbital overlapping with • A half-filled p orbital overlapping with a p orbital to form a σ bond: a p orbital to form a σ bond:

University of Wisconsin-Eau Claire Chem101 - Lecture 11 19 University of Wisconsin-Eau Claire Chem101 - Lecture 11 20

σ and π Bonds σ and π Bonds • A half-filled p orbital overlapping with • A double-bond is a combination of a σ a p orbital to form a π bond: bond and a π bond:

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σ and π Bonds Geometry of Alkenes • A double-bond is a combination of a σ • This places the two carbons, and the bond and a π bond: four other atoms that these two carbons are bonded to, all in the same plane. - The bond angles between the three bonds are all approximately 120°.

University of Wisconsin-Eau Claire Chem101 - Lecture 11 23 University of Wisconsin-Eau Claire Chem101 - Lecture 11 24

4 Geometry of Alkenes Geometry of Alkenes • Unlike single bonds, there is no free • The following strategy can be applied rotation about double bonds. to determine if a double bond is • This situation can produce additional producing a pair of stereo isomers isomers. A C - These isomers are called geometric isomers. C C B D

- If A and B are different, and C and D are different, then there is a pair of geometric isomers.

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Geometry of Alkenes Geometry of Alkenes • An example of this is provided by • An example of this is provided by 2– 2–butene CH CH CH H 3 3 3 • The cis prefix is used to indicate that C C C C the chain enters and leaves H H H CH 3 on the same side of the double bond. cis-2-butene trans-2-butene • Whereas the trans prefix is used to - These are two different . indicate that the hydrocarbon chain - They have different physical properties: enters and leaves on opposite sides of {°C} {°C} Density {g/mL} cis-2-butene -139.9 3.7 0.52 the double bond. trans-2-butene -105.6 0.9 0.6

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Seeing: The Difference Physical Properties of Alkenes Between cis and trans. • The physical properties of alkenes are very similar to those of alkanes.

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5 Chemical Properties of Alkenes Chemical Properties of Alkenes • We learned that alkanes are fairly un • They are particularly susceptible to a reactive. type of reaction called the addition - Except combustion reactions, alkanes do not reaction.

undergo very many types of reactions. A B • Alkenes can also undergo combustion C C + A B C C reactions. • In addition, unlike the carbon-carbon • Addition reactions have two reactants single bonds found in alkanes, the and one product. double bonds found in alkenes are chemically reactive.

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Chemical Properties of Alkenes Chemical Properties of Alkenes • We will look at four different examples A B of addition reactions: C C + A B C C - - Addition of √ • F-F, Cl-Cl, Br-Br, I-I In this reaction A and B represent two - - Addition of atoms or two groups of atoms that are √ H-H bonded to one another by a single bond. - Addition of Hydrogen halides -A and B can be the same. √ H-F, H-Cl, H-Br, H-I - Hydration - Addition of water √ H-OH

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Chemical Properties of Alkenes Chemical Properties of Alkenes • Halogenation - Addition of halogens • Hydrogenation - Addition of hydrogen √ F-F, Cl-Cl, Br-Br, I-I - H-H Br Br H H Pt, Pd or Ni C C + C C + Br Br C C H H C C

alkene hydrogen alkene () The “Pt, Pd, or Ni” written above the arrow indicates that - This reaction is often used to test for the this reaction is catalyzed by , or presence of alkenes . √ Bromine is brown and the alkene and - This commercially important reaction is haloalkane are colorless used to convert polyunsaturated vegetable √ The loss of bromine’s brown color is a positive , which are liquid, to partially test for alkenes hydrogenated oils, which are solids.

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6 Chemical Properties of Alkenes Chemical Properties of Alkenes • Addition of Hydrogen halides • Addition of Hydrogen halides - H-F, H-Cl, H-Br, H-I - H-F, H-Cl, H-Br, H-I

H Cl Br H C C + H Cl C C

CH2 CH CH3 1-bromopropane hydrogen alkene haloalkane CH2 CH CH3 + H Br halide or - Because the being added are hydrogen H Br bromide different, it is possible to obtain two CH2 CH CH3 different products for the reaction 2-bromopropane

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Chemical Properties of Alkenes Chemical Properties of Alkenes • Addition of Hydrogen halides • Addition of Hydrogen halides - H-F, H-Cl, H-Br, H-I - H-F, H-Cl, H-Br, H-I • When there are two possible products, - For the previous example, 2-bromopropane one is usually favored greatly over the will be the major product of the reaction other. • Markovnikov’s Rule can be used to H Br Br H >> CH CH CH CH CH CH predict which product will be favored: 2 3 2 3 2-bromopropane 1-bromopropane - The hydrogen will attach itself to the carbon that has the greatest number of . -“The rich get richer”

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Chemical Properties of Alkenes Chemical Properties of Alkenes • Hydration - Addition of water • Hydration - Addition of water - H-OH - H-OH H OH O O O H OHO H2SO4 fumarse C C + H OH C C HO C C C C OH + H OH HO C C C C OH H H H H water malic acid alkene water - “fumarase” is an enzyme catalyst, which is The “H SO ” above the arrow is . It is used 2 4 able to carry out the reaction under the mild as a catalyst in the conditions found in a living cell. - This is biochemically important reaction - Malic acid is an α-hydroxy acid and is what used in the breakdown of food for obtaining makes apples sour. energy. (The genus name for apple is Malus)

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7 Chemical Properties of Alkenes Addition • Summary of alkene addition • A is a chain of monomer units reactions: - Like beads on a string. • Addition polymers are commercially very important: - - - - Polyvinylchloride (PVC) - Polytetrafluoroethylene (Teflon)

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Addition Polymers Alkynes • Though important commercially, • Alkynes contain carbon-carbon triple addition is not bonds. commonly used in biological systems. • To form a triple bond requires sp • We will encounter other types of hybridization polymerization when we discuss proteins and nucleic acids.

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Alkynes Alkynes

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8 Alkynes Aromatic compounds • Alkynes are very reactive and therefore • Often are very fragrant, hence the name not found in biological system. aromatic. • An example of an alkyne is acetylene, • It was discovered that these compounds which is used in oxy-acetylene welding are highly unsaturated hydrocarbons. torches. • H C C H However, there low reactivity is more acetylene like that of alkanes, rather than alkenes • The IUPAC system for naming alkynes or alkynes. is the same as for alkenes, but uses the • Benzene, which is representative of the -yne ending. The IUPAC name for aromatic compounds, has a molecular

acetylene is ethyne. formula of C6H6.

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Aromatic compounds Aromatic compounds • Friedrich Kekulé proposed a structure • All of the carbons in the benzene ring are for benzene in 1865. sp2 hybridized. • He proposed a rich structure with • It is now recognized that the electrons in alternating double and single bonds. the unhybridized p orbitals can travel

H H completely around the ring. - They are said to be delocalized. H H H H C C C C

C C C C H C H H C H

H H KekulŽ Structures for Benzene

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Aromatic Compounds Aromatic Compounds • The electrons in the p orbitals are • When drawing structures that contain delocalized around the ring: aromatic rings, we emphasize the delocalization by representing the π-bonds by a circle:

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9 Aromatic Compounds Aromatic Compounds

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Aromatic Compounds Aromatic Compounds • IUPAC names • Common names - When a single hydrogen is replaced the - Some aromatic compounds have common compounds is named as a derivative of names that have stuck: benzene. CH3 OH

Br CH2CH3 H H H H C C C C

C C C C H C H H C H Phenol

H H Note: Eventhough the hydrogens and carbons are not Bromobenzene shown, it is understood that they are present.

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Aromatic Compounds • The physical properties of aromatic compounds are similar to other hydrocarbons. • The chemical properties of aromatic compounds are more similar to those of the alkanes than to the alkenes. - This is because the delocalization of the p orbital electrons make the double bonds in aromatic compounds more resistant to addition reactions than the double bonds in alkenes

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