5.8 Preparation of Alkenes: Elimination Reactions 1

b-Elimination Reactions Overview dehydrogenation of alkanes: X = Y = H dehydration of alcohols: X = H; Y = OH dehydrohalogenation of alkyl halides: X = H; Y = Br, etc. C C + X Y X Y a b 2

important economically, but rarely used in laboratory-scale syntheses Dehydrogenation limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene important economically, but rarely used in laboratory-scale syntheses 750°C CH3CH3 H2C CH2 + H2 750°C + H2 CH3CH2CH3 H2C CHCH3 3

5.9 Dehydration of Alcohols 4

Dehydration of Alcohols H2SO4 + CH3CH2OH H2C CH2 H2O 160°C OH H2SO4 + H2O 140°C (79-87%) C OH CH3 H3C CH2 H3C C H2SO4 heat + H2O (82%) 5

tertiary: most reactive OH C Relative Reactivity tertiary: most reactive R R' H OH C R H OH C primary: least reactive 6

5.10 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule 7

Regioselectivity H2SO4 + HO 80°C 90 % 10 % A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. 10

Regioselectivity CH3 CH3 CH3 H3PO4 OH + heat 16 % 84 % A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. 10

three protons on this b carbon The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH CH3 C H CH2R three protons on this b carbon 8

two protons on this b carbon The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH CH3 C H CH2R two protons on this b carbon 8

only one proton on this b carbon The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH CH3 C H CH2R only one proton on this b carbon 8

only one proton on this b carbon The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the b carbon having the fewest hydrogens. R OH CH3 C H CH2R R CH2R CH3 C only one proton on this b carbon 8

5.11 Stereoselectivity in Alcohol Dehydration 12

Stereoselectivity (25%) (75%) + OH H2SO4 heat A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other. 13

5.12 The Mechanism of Acid-Catalyzed Dehydration of Alcohols 15

1) Both reactions are promoted by acids A connecting point... The dehydration of alcohols and the reaction of alcohols with hydrogen halides share the following common features: 1) Both reactions are promoted by acids 2) The relative reactivity decreases in the order tertiary > secondary > primary These similarities suggest that carbocations are intermediates in the acid-catalyzed dehydration of alcohols, just as they are in the reaction of alcohols with hydrogen halides. 20

Dehydration of tert-Butyl Alcohol CH3 H3C CH2 H3C C H2SO4 heat + H2O first two steps of mechanism are identical to those for the reaction of tert-butyl alcohol with hydrogen halides 16

Step 1: Proton transfer to tert-butyl alcohol Mechanism Step 1: Proton transfer to tert-butyl alcohol .. H .. (CH3)3C O : + + H O H fast, bimolecular (CH3)3C H O H : + O : + H tert-Butyloxonium ion 9

Step 2: Dissociation of tert-butyloxonium ion to carbocation Mechanism Step 2: Dissociation of tert-butyloxonium ion to carbocation (CH3)3C O H : + slow, unimolecular O H : + + (CH3)3C tert-Butyl cation 9

Step 3: Deprotonation of tert-butyl cation. Mechanism Step 3: Deprotonation of tert-butyl cation. O H : CH2 + H3C C H + fast, bimolecular CH2 H3C C O H : + + H 9

react with nucleophiles lose a b-proton to form an alkene Carbocations are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols carbocations can: react with nucleophiles lose a b-proton to form an alkene 20

Dehydration of Primary Alcohols H2SO4 + CH3CH2OH H2C CH2 H2O 160°C avoids carbocation because primary carbocations are too unstable oxonium ion loses water and a proton in a bimolecular step 21

Step 1: Proton transfer from acid to ethanol Mechanism Step 1: Proton transfer from acid to ethanol .. H .. CH3CH2 O : + H O H fast, bimolecular H O H : + CH3CH2 O : + H Ethyloxonium ion 9

Mechanism Step 2: Oxonium ion loses both a proton and a water molecule in the same step. O H : H O : + CH2 + slow, bimolecular O H : + O H : + H2C CH2 + 9

5.13 Rearrangements in Alcohol Dehydration Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol. 25

Example OH H3PO4, heat + + 3% 64% 33% 26

Rearrangement involves alkyl group migration CH3 CHCH3 + C carbocation can lose a proton as shown or it can undergo a methyl migration CH3 group migrates with its pair of electrons to adjacent positively charged carbon 3% 27

Rearrangement involves alkyl group migration CH3 CHCH3 + C CHCH3 CH3 + 97% CH3 C CH3 tertiary carbocation; more stable 3% 27

Rearrangement involves alkyl group migration CH3 CHCH3 + C CHCH3 CH3 + 97% CH3 C CH3 3% 27

Another rearrangement CH3CH2CH2CH2OH H3PO4, heat CH2 CH3CH2CH + CHCH3 CH3CH 12% mixture of cis (32%) and trans-2-butene (56%) 29

Rearrangement involves hydride shift CH3CH2CH2CH2 O H + : oxonium ion can lose water and a proton (from C-2) to give 1-butene doesn't give a carbocation directly because primary carbocations are too unstable CH2 CH3CH2CH 27

Rearrangement involves hydride shift CH3CH2CH2CH2 O H + : CH3CH2CHCH3 + hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost carbocation formed by hydride shift is secondary CH2 CH3CH2CH 27

Rearrangement involves hydride shift CH3CH2CH2CH2 O H + : CH3CH2CHCH3 + CH2 CH3CH2CH CHCH3 CH3CH mixture of cis and trans-2-butene 27

Hydride Shift H H CH3CH2CHCH2 O + : + CH3CH2CHCH2 H O : 30

react with nucleophiles Carbocations can... react with nucleophiles lose a proton from the b-carbon to form an alkene rearrange (less stable to more stable) 31