Alkenes, Reactions

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Acids

Bases

Metals

Oxidation

Reduction

Halogens

R-H R-X R-OH R-O-R Alkenes

Alkenes, reactions.

Addition

ionic

free radical

Reduction

Oxidation

Substitution

Reactions, alkenes:

1. Addition of hydrogen (reduction).

2. Addition of halogens.

3. Addition of hydrogen halides.

4. Addition of sulfuric acid.

5. Addition of water (hydration).

6. Addition of aqueous halogens (halohydrin formation).

7. Oxymercuration-demercuration.

8. Hydroboration-oxidation.

9. Addition of free radicals.

10. Addition of carbenes.

11. Epoxidation.

12. Hydroxylation.

13. Allylic halogenation

14. Ozonolysis.

15. Vigorous oxidation.

1. Addition of hydrogen (reduction).

| | | |— C = C — + H2 + Ni, Pt, or Pd — C — C —

| | H Ha) Requires catalyst.b) #1 synthesis of alkanes

CH3CH=CHCH3 + H2, Ni CH3CH2CH2CH3

2-butene n-butane

Alkanes

Nomenclature

Syntheses

1. addition of hydrogen to an alkene

2. reduction of an alkyl halide

a) hydrolysis of a Grignard reagent

b) with an active metal and acid

3. Corey-House Synthesis

Reactions

1. halogenation

2. combustion (oxidation)

3. pyrolysis (cracking)

heat of hydrogenation:

CH3CH=CH2 + H2, Pt CH3CH2CH3 + ~ 30 Kcal/mole

ethylene 32.8

propylene 30.1

cis-2-butene 28.6

trans-2-butene 27.6

isobutylene 28.4

fats & oils: triglycerides

OCH2—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3

| O CH—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3

| OCH2—O—CCH2CH2CH2CH2CH2CH2CH3

“saturated” fat

OCH2—O—CCH2CH2CH2CH2CH=CH2CH2CH2CH3

| O CH—O—CCH2CH2CH2CH2CH=CH2CH2CH2CH3

| OCH2—O—CCH2CH2CH=CHCH2CH2CH3

Ω - 3 “unsaturated” oil

Saturated triglycerides are solids at room temperature and are called “fats”.

butter fat, lard, vegetable shortening, beef tallow, etc.

Unsaturated triglycerides have lower mp’s than saturated triglycerides. Those that are liquids at room temperature are called “oils”. (All double bonds are cis-.)

corn oil, peanut oil, Canola oil, cottonseed oil, etc.

polyunsaturated oils + H2, Ni saturated fats

liquid at RT solid at RT

oleomargarine

butter substitute

(dyed yellow)

Trans-fatty acids formed in the synthesis of margarine have been implicated in the formation of “bad” cholesterol, hardening of the arteries and heart disease.

2. Addition of halogens.

| | | |— C = C — + X2 — C — C —

| | X X

a) X2 = Br2 or Cl2

b) test for unsaturation with Br2

CH3CH2CH=CH2 + Br2/CCl4 CH3CH2CHCH2

Br Br 1-butene 1,2-dibromobutane

3. Addition of hydrogen halides. | | | |— C = C — + HX — C — C —

| | H Xa) HX = HI, HBr, HClb) Markovnikov orientation

CH3CH=CH2 + HI CH3CHCH3

I

CH3 CH3

CH2C=CH2 + HBr CH3CCH3

Br

Markovnikov’s Rule:

In the addition of an acid to an alkene the hydrogen will go to the vinyl carbon that already has the greater number of hydrogens.

CH3CH2CH=CH2 + HCl CH3CH2CHCH3

Cl

CH3 CH3

CH3CH=CCH3 + HBr CH3CH2CCH3

Br

CH3CH=CHCH3 + HI CH3CH2CHCH3

I

An exception to Markovikov’s Rule:

CH3CH=CH2 + HBr, peroxides CH3CH2CH2Br

CH3 CH3

CH3C=CH2 + HBr, peroxides CH3CHCH2Br

“anti-Markovnikov” orientation

note: this is only for HBr.

Markovnikov doesn’t always correctly predict the product!

CH3 CH3

CH2=CHCHCH3 + HI CH3CH2CCH3 I

Rearrangement!

Ionic electrophilic addition mechanism

1) C C + YZRDS

C C

Y

+ Z

2) C C + Z C C

YZY

mechanism for addition of HX

1) C C + HXRDS

C C

H

+ X

2) C C + X C C

HXH

why Markovinkov?

CH3CH=CH2 + HBr CH3CHCH2 1o carbocation

| H

or? CH3CHCH2 2o carbocation | more stable H

+ Br- CH3CHCH3

| Br

In ionic electrophilic addition to an alkene, the electrophile always adds to the carbon-carbon double bond so as to form the more stable carbocation.

4. Addition of sulfuric acid.

| | | |— C = C — + H2SO4 — C — C — | |

H OSO3H

alkyl hydrogen sulfate

Markovnikov orientation.

CH3CH=CH2 + H2SO4 CH3CHCH3

O O-S-O OH

5. Addition of water.

| | | |— C = C — + H2O, H+ — C — C —

| | H OHa) requires acidb) Markovnikov orientationc) low yield

CH3CH2CH=CH2 + H2O, H+ CH3CH2CHCH3

OH

Mechanism for addition of water

1)

2) C C

H OH2

C C

H

3)

C C H

C C

H OH

+ H

+

C C

H

+ H2O

C C

H OH2

RDS

| | H+ | |— C = C — + H2O — C — C — | | OH H

Mechanism for addition of water to an alkene to form an alcohol is the exact reverse of the mechanism (E1) for the dehydration of an alcohol to form an alkene.

Mechanism for dehydration of an alcohol = E1

C C

H OH

+ H C C

H OH2

1)

2) C C

H OH2

RDSC C

H

+ H2O

C C

H

3) C C + H

mechanism for addition of X2

1) C C + X--XRDS

C C

X

+ X

2) C C + X C C

XXX

How do we know that the mechanism isn’t this way?

One step, concerted, no carbocation

C C

X X

RDS

C C

X X

CH3CH=CH2 + Br2 + H2O + NaCl

CH3CHCH2 + CH3CHCH2 + CH3CHCH2

Br Br OH Br Cl Br

CH3CH=CH2 + Br--Br CHCHCH2

Br

Br H2O Cl

Some evidence suggests that the intermediate is not a normal carbocation but a “halonium” ion:

| | — C — C —

Br

The addition of X2 to an alkene is an anti-addition.

6. Addition of halogens + water (halohydrin formation):

| | | |— C = C — + X2, H2O — C — C — + HX | | OH X

a) X2 = Br2, Cl2

b) Br2 = electrophile

CH3CH=CH2 + Br2(aq.) CH3CHCH2 + HBr OH Br

mechanism for addition of X2 + H2O

1) C C + X--XRDS

C C

X

+ X

2) C CH2O + C C

XH2OX

C C

XH2O

3) C C

XHO

- H

oxymercuration-demercuration:

a) #1 synthesis of alcohols.

b) Markovnikov orientation.

c) 100% yields.

d) no rearrangements

CH3CH2CH=CH2 + H2O, Hg(OAc)2; then NaBH4

CH3CH2CHCH3

OH

alkoxymercuration-demercuration:

a) #2 synthesis of ethers.

b) Markovnikov orientation.

c) 100% yields.


CH3CH=CH2 + CH3CHCH3, Hg(TFA)2; then NaBH4 OH

CH3 CH3

CH3CH-O-CHCH3

diisopropyl ether Avoids the elimination with 2o/3o RX in Williamson Synthesis.

Ethers

nomenclature

syntheses

1. Williamson Synthesis

2. alkoxymercuration-demercuration

reactions

1. acid cleavage

hydroboration-oxidation:

a) #2 synthesis of alcohols.

b) Anti-Markovnikov orientation.

c) 100% yields.


CH3CH2CH=CH2 + (BH3)2; then H2O2, NaOH

CH3CH2CH2CH2-OH

CH3

CH3C=CH2 + H2O, Hg(OAc)2; then NaBH4

CH3

Markovnikov CH3CCH3

OH CH3

CH3C=CH2 + (BH3)2; then H2O2, NaOH

CH3

anti-Markovnikov CH3CHCH2

OH

Alcohols:

nomenclature

syntheses

1. oxymercuration-demercuration

2. hydroboration-oxidation

3.

4. hydrolysis of a 1o / CH3 alcohol

5.

6.

8.

9. Addition of free radicals.

| | | |— C = C — + HBr, peroxides — C — C — | |

H X

a) anti-Markovnikov orientation.b) free radical additionc) HI, HCl + Peroxides

CH3CH=CH2 + HBr, peroxides CH3CH2CH2-Br

Mechanism for free radical addition of HBr:

Initiating steps:

1) peroxide 2 radical•

2) radical• + HBr radical:H + Br• (Br• electrophile)

Propagating steps:

3) Br• + CH3CH=CH2 CH3CHCH2-Br (2o free radical) •4) CH3CHCH2-Br + HBr CH3CH2CH2-Br + Br• •

3), 4), 3), 4)…

Terminating steps:

5) Br• + Br• Br2

Etc.

In a free radical addition to an alkene, the electrophilic free radical adds to the vinyl carbon with the greater number of hydrogens to form the more stable free radical.

In the case of HBr/peroxides, the electrophile is the bromine free radical (Br•).

CH3CH=CH2 + HBr, peroxides CH3CH2CH2-Br

10. Addition of carbenes.

| | | |— C = C — + CH2CO or CH2N2 , hν — C — C

—

CH2

•CH2• “carbene” adds across the double bond | |— C = C — •CH2•

| | | |— C = C — + CHCl3, t-BuOK — C— C —

CCl2

-HCl

•CCl2• dichlorocarbene

| |— C = C — •CCl2•

CH3CH=CH2 + CH2N2

hv HC CH2

CH2

H3C

CHCl3, t-BuOK HC CH2

CCl2

H3C

2-butene

H3C CH

CH

CH3 + C6H5CO3HHC

HC

OCH3H3C

peroxybenzoic acid

CH3CH=CHCH3 + KMnO4 CH3CH-CHCH3

OH OH 2,3-butanediol

test for unsaturation purple KMnO4 brown MnO2

CH2=CH2 + KMnO4 CH2CH2

OH OH ethylene glycol

“anti-freeze”

13. Allylic halogenation.

| | | | | |— C = C — C — + X2, heat — C = C — C — + HX | | H allyl X

CH2=CHCH3 + Br2, 350oC CH2=CHCH2Br + HBr

a) X2 = Cl2 or Br2

b) or N-bromosuccinimide (NBS)

CH2=CHCH3 + Br2 CH2CHCH3

Br Braddition

CH2=CHCH3 + Br2, heat CH2=CHCH2-Br + HBr

allylic substitution

14. Ozonolysis.

| | | |— C = C — + O3; then Zn, H2O — C = O + O = C —

used for identification of alkenes

CH3

CH3CH2CH=CCH3 + O3; then Zn, H2O

CH3

CH3CH2CH=O + O=CCH3

15. Vigorous oxidation.

=CH2 + KMnO4, heat CO2

=CHR + KMnO4, heat RCOOH carboxylic acid

=CR2 + KMnO4, heat O=CR2 ketone

CH3CH2CH2CH=CH2 + KMnO4, heat

CH3CH2CH2COOH + CO2

CH3 CH3

CH3C=CHCH3 + KMnO4, heat CH3C=O + HOOCCH3

CH3CH=CHCH3 + KMnO4 CH3CHCHCH3

OHOHmild oxidation glycol

CH3CH=CHCH3 + hot KMnO4 2 CH3COOH

vigorous oxidation

Reactions, alkenes:

1. Addition of hydrogen

2. Addition of halogens

3. Addition of hydrogen halides

4. Addition of sulfuric acid

5. Addition of water/acid

6. Addition of halogens & water (halohydrin formation)

7. Oxymercuration-demercuration

8. Hydroboration-oxidation

9. Addition of free radicals

10. Addition of carbenes

11. Epoxidation

12. Hydroxylation

13. Allylic halogenation

14. Ozonolysis

15. Vigorous oxidation

CH3 CH3

CH3C=CH2 + H2, Pt CH3CHCH3

isobutylene CH3

“ + Br2/CCl4 CH3C-CH2

Br Br

CH3

“ + HBr CH3CCH3

Br

CH3

“ + H2SO4 CH3CCH3

O SO3H

CH3 CH3

CH3C=CH2 + H2O, H+ CH3CCH3

isobutylene OH

CH3

“ + Br2(aq.) CH3C-CH2Br OH

CH3 CH3

CH3C=CH2 + H2O,Hg(OAc)2; then NaBH4 CH3CCH3

OH

CH3

“ + (BH3)2; then H2O2, OH- CH3CHCH2

OH

CH3 CH3

CH3C=CH2 + HBr, peroxides CH3CHCH2

isobutylene Br

CH3

“ + CH2CO, hv CH3C–CH2

CH2

CH3

“ + PBA CH3C–CH2

O

CH3 CH3

CH3C=CH2 + KMnO4 CH3C–CH2

isobutylene OH OH

CH3

“ + Br2, heat CH2C=CH2 + HBr Br

CH3

“ + O3; then Zn/H2O CH3C=O + O=CH2

CH3

“ + KMnO4, heat CH3C=O + CO2

Alkenes, Reactions

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