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Ch.6 Alkenes: Structure and Reactivity Alkene = Olefin

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Ch.6 Alkenes: Structure and Reactivity Alkene = Olefin Ch.6 Alkenes: Structure and Reactivity alkene = olefin H2CCH2 CH3 Ethylene α-Pinene β-Carotene (orange pigment and vitamin A precursor) Ch.6 Alkenes: Structure and Reactivity 6.1 Industrial Preparation and Use of Alkenes Compounds derived industrially from ethylene CH3CH2OH Ethanol CH3CHO Acetaldehyde CH3COOH Acetic acid HOCH2CH2OH Ethylene glycol ClCH2CH2Cl Ethylene dichloride H C=CHCl Vinyl chloride H2CCH2 2 O Ethylene oxide Ethylene (26 million tons / yr) O Vinyl acetate O Polyethylene Ch.6 Alkenes: Structure and Reactivity Compounds derived industrially from propylene OH Isopropyl alcohol H3CCH3 O Propylene oxide CH3 H3CCH CH2 Propylene Cumene (14 million tons / yr) CH3 CH3 Polypropylene Ch.6 Alkenes: Structure and Reactivity • Ethylene, propylene, and butene are synthesized industrially by thermal cracking of natural gas (C1-C4 alkanes) and straight-run gasoline (C4-C8 alkanes). 850-900oC CH (CH ) CH H + CH + H C=CH + CH CH=CH 3 2 n 3 steam 2 4 2 2 3 2 + CH3CH2CH=CH2 - the exact processes are complex; involve radical process H 900oC CH3CH2 CH2CH3 22H2CCH H2C=CH2 +H2 Ch.6 Alkenes: Structure and Reactivity • Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆So) rather than enthalpy (∆Ho) in the free-energy equation (∆Go = ∆Ho -T∆So) . ; C-C bond cleavage (positive ∆Ho) ; high T and increased number of molecules → larger T∆So Ch.6 Alkenes: Structure and Reactivity 6.2 Calculating Degree of Unsaturation unsaturated: formula of alkene CnH2n ; formula of alkane CnH2n+2 in general, each ring or double bond corresponds to a loss of two hydrogens from alkane formula degree of unsaturation: the number of rings and/or multiple bonds Ch.6 Alkenes: Structure and Reactivity unknown hydrocarbon with molecular weight 82; C6H10 corresponding alkane; C6H14 H14-H10 = H4 = 2H2 therefore, degree of unsaturation= 2 possible structures: Ch.6 Alkenes: Structure and Reactivity degree of unsaturation: containing elements other than just C, H ■ Organohalogen compounds (C, H, X, X= F, Cl, Br, I) Add the number of halogens to the number of hydrogens ; a halogen is simply a replacement of hydrogen BrCH2CH=CHCH2Br HCH2CH=CHCH2H C4H6Br2 = "C4H8" one unsaturation: one double bond or one cycle add Ch.6 Alkenes: Structure and Reactivity ■ Organooxygen compounds (C, H, O) Ignore the number of oxygens ; oxygen forms two bonds; C-C vs C-O-C or C-H vs C-O-H H2C=CHCH=CHCH2OH H2C=CHCH=CHCH2-H C5H8O= "C5H8" two unsaturation: two double bonds Ch.6 Alkenes: Structure and Reactivity ■ Organonitrogen compounds (C, H, N) Subtract the number of nitrogens from the number of hydrogens ; nitrogen forms three bonds; C-C vs C-NH-C or C-H vs C-NH2 H H NH2 H C5H9N= "C5H8" two unsaturation: one double bond and one ring Ch.6 Alkenes: Structure and Reactivity 6.3 Naming Alkenes Step 1 Name the parent hydrocarbon: Find the longest carbon chain containing the double bond and name the compound accordingly, using the suffix -ene: NOT pentene hexene Ch.6 Alkenes: Structure and Reactivity Step 2 Numbering: Begin at the end nearer the double bond or, if the double bond is equivalent from the two ends, begin at the end nearer the first branch point. This rule ensures that the double bond carbons receive the lowest possible numbers: 2 6 3 1 3 4 1 NOT 6 2 5 4 6 3 1 2 3 6 NOT 1 2 5 3 1 4 6 Ch.6 Alkenes: Structure and Reactivity Step 3 Write the full name: list substituents alphabetically ; indicate the position of double bond (the number of the first alkene carbon) immediately before the parent name ; more than one double bonds: -diene, triene... 2 3 1 1 2 3 2-Hexene 2-Methyl-3-hexene 2 1 2 4 1 3 2-Ethyl-1-pentene 2-Methyl-1,3-butadiene Ch.6 Alkenes: Structure and Reactivity cycloalkanes are named similarly, but double bond is between C1 and C2 and the first substituent has as low a number as possible ; it's not necessary to indicate the position of the double bond in the name (always C1 and C2) 1 CH3 5 1 2 4 2 1-Methylcyclohexene 1,4-Cyclohexadiene CH3 5 CH3 1 3 2 CH3 CH3 2 1 1,5-Dimethylcyclopentene NOT Ch.6 Alkenes: Structure and Reactivity Common names IUPAC name Common name Ethene Ethylene Propene Propylene 2-Methylpropene Isobutylene 2-Methyl-1,3-butadiene Isoprene 1,3-Pentadiene Piperylene Ch.6 Alkenes: Structure and Reactivity Substituent Names H H2C H2C C A methylene group A vinyl group An allyl group Br CH2 Br Μethylenecyclopentane Vinyl bromide Allyl bromide Ch.6 Alkenes: Structure and Reactivity 6.4 Electronic Structure of Alkenes • Rotation around double bond is restricted: The π-bond must break for rotation to take place around a C=C double bond - 268 kJ/mol (64 kcal/mol) is required to break the π-bond - rotational energy barrier for ethane: only 12 kJ/mol C C 90o C rotation C π-bond broken π-bond after rotation (p-orbitals are parallel) (p-orbitals are perpendicular) Ch.6 Alkenes: Structure and Reactivity 6.5 Cis-Trans Isomerism in Alkenes H CCH H3CH 3 3 X HH HCH3 cis-2-Butene trans-2-Butene cis-trans isomerism: when both carbons are bonded to two different groups BD AD these two compounds are identical; BD ADthey are not cis-trans isomers AD BDthese two compounds are not identical; they are cis-trans isomers BE A E Ch.6 Alkenes: Structure and Reactivity 6.6 Sequence Rules: The E,Z Designation cis-trans isomerism: describe the disubstituted double bond geometries ; tri-and tetrasubstituted double bonds- a general method is needed H CCHCH CH H3CCH3 3 2 2 3 HCH HCH2CH2CH3 3 cis or trans ? cis or trans ? Ch.6 Alkenes: Structure and Reactivity E, Z isomerism: a more general method for describing double-bond geometry ; E (entgegen, "opposite"); Z (zusammen, "together") High High High Low Low Low Low High Z E the higher priority groups on the higher priority groups on each carbon are on the same each carbon are on the opposite side of the double bond side of the double bond Ch.6 Alkenes: Structure and Reactivity Sequence Rule (Cahn-Ingold-Prelog rule; CIP rule) ; priority of substituents Rule 1 Considering each of the double-bond carbons separately, identify the two atoms directly attached and rank them according to atomic number. 35 17 8 7 6 1 Br > Cl > O > N > C > H Cl H Cl CH3 H C CH H3CH 3 3 (Z)-2-Chloro-2-butene (E)-2-Chloro-2-butene Ch.6 Alkenes: Structure and Reactivity Rule 2 If a decision can't be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. H H C H < C CH3 O H < O CH3 H H H CH3 CH3 H C CH3 < C CH3 C NH2 < C Cl H H H H Ch.6 Alkenes: Structure and Reactivity Rule 3 Multiple-bonded atoms are equivalent to the same number of single- bonded atoms. H H C O C O O C H H H H C C C C H H C C C C C C H C C H C C Ch.6 Alkenes: Structure and Reactivity H (E)-3-Methyl-1,3-pentadiene H3C CH3 Br (E)-1-Bromo-2-isopropyl-1,3-butadiene H O H C OH 3 (Z)-2-Hydroxymethyl-2-butenoic acid HOH Ch.6 Alkenes: Structure and Reactivity 6.7 Stability of Alkenes Relative stability from equilibrium constant: - cis-trans isomers interconvert under strong acid condition acid H3CCH3 H3C H HH catalyst H CH3 cis (24 %) trans (76%) Erel= + 2.8 kJ/mol (0.66 kcal/mol) Erel= 0.0 kcal/mol Ch.6 Alkenes: Structure and Reactivity H H H H H H H H C H CCH H H HH C H H cis trans Ch.6 Alkenes: Structure and Reactivity From heat of combustion H3CCH3 H3CH HH HCH3 o o ∆H combustion= -2685.5 kJ/mol ∆H combustion= -2682.2 kJ/mol E = +0.0 kJ/mol Erel = +3.3 kJ/mol rel Ch.6 Alkenes: Structure and Reactivity From heat of hydrogenation H3CH H3CCH3 H2 H2 CH3CH2CH2CH3 HH Pd Pd HCH3 o o ∆H hydro = -116 kJ/mol ∆H hydro = -120 kJ/mol 4 kJ/mol difference Ch.6 Alkenes: Structure and Reactivity Energy profile for hydrogenation Cis Energy Trans o o ∆G cis ∆G trans Butane Reaction progress Ch.6 Alkenes: Structure and Reactivity Stabilities of alkenes: increasing the degree of substitution leads further stabilization RR RR H R R H R H > > ~ > RR R H R H R H H H tetrasubstituted trisubstituted disubstituted monosubstituted Ch.6 Alkenes: Structure and Reactivity Explanations of alkene stabilities 1. Hyperconjugation: a stabilizing interaction between the unfilled antibonding C=C p bond and a filled C-H s bond orbital on a neighboring substituent. The more substutuents that are present, the more opportunities exist for hyperconjugation, and the more stable the alkene. bonding C-H σ orbital (filled) π* H CC σ C antibonding C-C π orbital (unfilled) Ch.6 Alkenes: Structure and Reactivity 2. Bond strength: sp2-sp3 C-C bond is stronger than sp3-sp3 C-C bond ; more highly substituted alkenes always have a higher ratio of sp2-sp3 bonds to sp3-sp3 bonds sp3-sp3 CH3 CH CH CH3 CH3 CH2 CH CH2 3 2 sp3-sp2 sp3-sp2 sp -sp Ch.6 Alkenes: Structure and Reactivity 6.8 Electrophilic Addition of HX to Alkenes • alkenes: electron rich, nucleophilic Electrophilic addition reaction: addition of electrophiles to nucleophilic alkenes Br- H Br H Br H H3CH H C 3 H C H 3 H H C H H3C H3C H 3 H carbocation intermediate The electrophile HBr is attacked by the The Br- donates an electron pair to the p-electrons of the double bond, and a positively charged carbon atom, new C-H σ-bond is formed.
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