1. 5.8 Preparation of Alkenes: Elimination Reactions1

2. 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

3. 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

4. 5.9 Dehydration of Alcohols4

5. 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

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

7. 5.10 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule7

8. 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

9. 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

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

11. 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

12. 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

13. 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

14. 5.11 Stereoselectivity in Alcohol Dehydration12

15. 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

16. 5.12 The Mechanism of Acid-Catalyzed Dehydration of Alcohols15

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 5.13 Rearrangements in Alcohol Dehydration
Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol. 25

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

28. 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

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

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

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

32. 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

33. 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

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

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

36. 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