Arenes : compounds containing both aliphatic and aromatic parts. Alkylbenzenes Alkenylbenzenes...

Arenes:

compounds containing both aliphatic and aromatic parts.

Alkylbenzenes

Alkenylbenzenes

Alkynylbenzenes

Etc.

Emphasis on the effect that one part has on the chemistry of the other half.

Reactivity & orientation

Example: ethylbenzene

EAS in the aromatic part

-CH2CH3 activates and directs ortho- & para-

CH2

CH3

CH2

CH3CH2

CH3

CH2 CH3 CH CH3

Br2, Fe Br

Br

Br2, heat

Br+ HBr

+

Free radical halogenation in the side chain

-C6H5 activates and directs benzyl

Alkylbenzenes, nomenclature:

Special names

CH3 CH3CH3

CH3

CH3

CH3

CH3

toluene o-xylene m-xylene p-xylene

others named as “alkylbenzenes”:

CHH3C CH3 CH2

H2CCH3

H2C

CHCH3

CH3

isopropylbenzene n-propylbenzene isobutylbenzene

CH2

CH2

CH3

CH3

o-diethylbenzene n-butylbenzene

Use of phenyl C6H5- = “phenyl”

CH2CH2

2-methyl-3-phenylheptane 1,2-diphenylethane

do not confuse phenyl (C6H5-) with benzyl (C6H5CH2-)

Alkenylbenzenes, nomenclature:

CH=CH2

styrene

CH2CH=CH2

3-phenylpropene(allylbenzene)

(Z)-1-phenyl-1-butene

Special name

Rest are named as substituted alkenes

Alkynylbenzenes, nomenclature:

C CH

phenylacetylene5-phenyl-2-hexyne

phenylethyne

Alcohols, etc., nomenclature:

CHH3C OH

1-phenylethanol

phenylethyl alcohol

CH2OH

benzyl alcohol

1-chloro-2-phenylethane

-phenylethyl chloride

CH2CH2-Cl

cyclohexylbenzene

phenylcyclohexane

Alkylbenzenes, syntheses:

1. Friedel-Crafts alkylation

2. Modification of a side chain:

a) addition of hydrogen to an alkene

b) reduction of an alkylhalide

i) hydrolysis of Grignard reagent

ii) active metal and acid

c) Corey-House synthesis

Modification of side chain:

Br

+ H2, Ni

+ Sn, HCl

Br

+ Mg; then H2o

ethylbenzene

Friedel-Crafts:

Ar-H + R-X, AlCl3 Ar-R + HX

Ar-H + R-OH, H+ Ar-R + H2O

Ar-H + alkene, H+ Ar-R

CH3CH3

CCH3

H3C CH3

+ H3C CCH3

BrCH3

AlCl3

+ CH3CH2-OH, H+ CH2CH3

+ CH2=CHCH3, H+CH

CH3

CH3isopropylbenzene

ethylbenzene

p-tert-butyltoluene

H+cyclohexylbenzene

H3C CH2Cl

AlCl3CH2 CH3

ortho-p-benzyltoluene

CH2Cl2, AlCl32 CH2

diphenylmethane

Friedel-Crafts limitations:

a) Polyalkylation

b) Possible rearrangement

c) R-X cannot be Ar-X

d) NR when the benzene ring is less reactive than bromobenzene

e) NR with -NH2, -NHR, -NR2 groups

polyalkylation

CH3CH3Br, AlCl3

+CH3 CH3

CH3

CH3

+

CH3CH3

H3C

+

The alkyl group activates the ring making the products more reactive that the reactants leading to polyalkylation. Use of excess aromatic compound minimizes polyalkylation in the lab.

The electrophile in Friedel Crafts alkylation is a carbocation:

R-X + AlX3 R+

R-OH + H+ R+

| |— C = C — + H+ R+

Carbocations can rearrange!

rearrangement

+ CH3CH2CH2-Br, AlCl3

CHCH3H3C

AlCl3+

C CH3H3CCH3

+

H+

isopropylbenzene

tert-butylbenzene

2-methyl-2-phenylbutane

carbocation rearrangements are possible!

CH3CCH2CH3

CH3

CH3CHCH2-BrCH3

CH3CCH2-OHCH3

CH3

n-alkylbenzenes cannot be made by Friedel-Crafts alkylation due to

carbocation rearrangements

R-X cannot be Ar-X

+ R-X, AlCl3

R

+

XAlCl3

NR

The Ar-X bond is strong and does not break like the R-X bond!

NR with rings less reactive than bromobenzene

Br

+ CH3CH2-Br, AlCl3

Br Br

CH2CH3

CH2CH3+

COOH

NO2

+ CH3-Br, AlCl3

+ CH3CH2-OH, H+

NR

NR

-CHO, -COR

-SO3H

-COOH, -COOR

-CN

-NR3+

-NO2

NR with –NH2, -NHR, -NR2

NH2

+ CH3CH2-Cl, AlCl3 NR

NH2

+ AlCl3

NH2 AlCl3

Lewis base Lewis acid deactivated to EAS

Friedel-Crafts limitations:

a) Polyalkylation

b) Possible rearrangement

c) R-X cannot be Ar-X

d) NR when the benzene ring is less reactive than bromobenzene

e) NR with -NH2, -NHR, -NR2 groups

In syntheses it is often best to do Friedel-Crafts alkylation in the first step!

Alkylbenzenes, reactions:

1. Reduction

2. Oxidation

3. EAS

a) nitration

b) sulfonation

c) halogenation

d) Friedel-Crafts alkylation

4. Side chain

free radical halogenation

Alkylbenezenes, reduction:

NR NR NR

NR

H2, Ni

H2, Ni

300oC, 100 atm.

CH3CH3

H2C

CH3

H2C

CH3

Alkylbenezenes, oxidation:

NR NR NR

NR NR

KMnO4

KMnO4

heat

CH3CH3

H2C

CH3

COOH

+ KMnO4, heat

+ KMnO4, heat

COOH

COOH

COOH+ 2 CO2

Oxidation of alkylbenzenes.

1) Syn

2) identification

C8H10:

H2C

CH3CH3

CH3

CH3

CH3

CH3

CH3 COOH

COOHCOOH

COOH

COOH

COOH

COOH

bp 136oC

bp 144oC

bp 139oC

bp 138oC

mp 122oC

mp 231oC

mp 348oC

mp 300oC

Alkylbenzenes, EAS

CH2CH3CH2CH3

CH2CH3

CH2CH3 CH2CH3

CH2CH3CH2CH3

CH2CH3CH2CH3

NO2

NO2

SO3H

SO3H

Br

Br

CH3

CH3

+

+

+

+HNO3, H2SO4

H2SO4, SO3

Br2, Fe

CH3Cl, AlCl3

-R is electron releasing. Activates to EAS and directs ortho/para

Alkylbenzenes, free radical halogenation in side chain:

benzyl free radical

CH2CH3

CH2CH3

+ Cl2, heat

+ Br2, heat

CHCH3 CH2CH2-Cl

CHCH3

Cl

+

Br

91% 9%

only

CH2CH3

benzyl free radical > 3o > 2o > 1o > CH3

CHCH3

CHCH3CHCH3 CHCH3.

.

.

.

X2 2 X.

+ X .

Alkenylbenzenes, syntheses:

1. Modification of side chain:

a) dehydrohalogenation of alkyl halide

b) dehydration of alcohol

c) dehalogenation of vicinal dihalide

d) reduction of alkyne

(2. Friedel-Crafts alkylation)

Alkenylbenzenes, synthesis modification of side chain

CHCH3

CHCH3

CHCH2

C

CH=CH2

CH

Br

OH

Cl Cl

styrene

KOH(alc)

H+, heat

Zn

H2, Pd-C

Alkenylbenzenes, synthesis Friedel-Crafts alkylation

not normally used for alkenylbenzenes.

an exception:

+ CH2=CHCH2-Br, AlCl3 CH2CH=CH2

allylbenzene

+ CH2=CH-Br, AlCl3 NR

Br

KOH(alc)conjugated with the ring

+ KOH, heat

Alkenylbenzenes, reactions:

1. Reduction

2. Oxidation

3. EAS

4. Side chain

a) add’n of H2 h) oxymercuration

b) add’n of X2 i) hydroboration

c) add’n of HX j) addition of free rad.

d) add’n of H2SO4 k) add’n of carbenes

e) add’n of H2O l) epoxidation

f) add’n of X2 & H2O m) hydroxylation

g) allylic halogenation n) ozonolysis

o) vigorous oxidation

Alkenylbenzenes, reactions: reduction

CH=CH2

CH=CH2

+ H2, Ni

+ H2, Ni, 250oC, 1,500 psi

CH2CH3

H

CH2CH3

Alkenylbenzenes, reactions oxidation

CH=CH2

CH=CH2

CH=CH2

CHCH2

COOH

CH=O

OHOH

+ CO2

+ O=CH2

KMnO4

heat

1. O3

2. Zn, H2O

KMnO4

Alkenylbenzenes, reactions EAS?

CH=CH2

electrophilic aromatic substitution

electrophilic addition

alkenes are more reactive with electrophiles than aromatic rings!

CH=CH2 + Br2, Fe CHCH2Br Br

In syntheses of alkenylbenzenes, the carbon-carbon double bond must be synthesized after any EAS reactions

CH2CH3 CH2CH3

CH=CH2

Cl

CHCH3

CH2=CH2

HF

Cl2, Fe+ ortho

CH2CH2-ClCl

Cl ClCl

Cl2, hv

KOH(alc)

p-chlorostyrene

Alkenylbenzenes, reactions side chain:

CH=CHCH3 CH2CH2CH3

CHCHCH3

CHCH2CH3

CHCH2CH3

Br

Br

Br

OSO3H

H2, Ni

Br2, CCl4

HBr

H2SO4

Benzyl carbocation

CH=CHCH3 + H+ CHCH2CH3

CHCH2CH3CHCH2CH3 CHCH2CH3

resonance stabilization of benzyl carbocation > 3o > 2o > 1o

CH=CHCH3 CHCH2CH3

CHCHCH3

CHCH2CH3

CH2CHCH3

OH

OH

Br

OH

OH

H2O, H+

Br2, H2O

1. H2O, Hg(OAc)2

2. NaBH4

1. (BH3)2

2. H2O2, NaOH

CH=CHCH3 CH2CHCH3

CH=CHCH3

CH=CHCH3

Br

O

HBr, perox.

CH2N2, hv

PBA

CH=CHCH3 + Br2, heat CH=CHCH2-Br

C CCH3

H

H

(E)-1-phenylpropene

CH3H OH

HO H

CH3HO H

H OH+KMnO4

100 syn-oxidation; make a model!

Alkynylbenzenes, syntheses:

Dehydrohalogenation of vicinal dihalides

CH=CH2 CHCH2Br

BrC CH

Br2 1. KOH

2. NaNH2

HC CH3Br

KOH(alc)

H2C CH3

CH2=CH2

HF

Alkynylbenzenes, reactions:

1. Reduction

2. Oxidation

3. EAS

4. Side chain

a) reduction e) as acids

b) add’n of X2 f) with Ag+

c) add’n of HX g) oxidation

d) add’n of H2O, H+

Alkynylbenzenes, reactions: reduction

C C CH3 + 2 H2, Ni CH2CH2CH3

+ (xs) H2, Ni heat & pressure

C C CH3 + Li, NH3

+ H2, Pd-C

anti-

syn-

Alkynylbenzenes, reactions: oxidation

C C CH3

KMnO4, heat

O3; then Zn, H2O

COOH + HOOCCH3

KMnO4

Alkynylbenzenes, reactions EAS?

C

electrophilic aromatic substitution

electrophilic addition

alkynes are more reactive with electrophiles than aromatic rings!

C + Br2, Fe C=CH

CH

CH

Br

Br

Alkynylbenzenes, reactions: side chain:

C C H C=CH

C

CCH3

Br

Br

Br

Br

Br

BrCBr

BrH

C=CH2Br

Br2

2 Br2

HBr

2 HBr

C CHH2O, H+

CCH3

O

C CH

C CH

Na

Ag+

C

C

C-Na+

C-Ag+

C CCH3

Ag+

NR, not terminal

Arenes:

alkylbenzenes

alkenylbenzenes

alkynylbenzenes

As expected, but remember that you cannot do EAS on alkenyl- or alkynylbenzenes.