Chapter IV Aromatic Compounds.pdf

CHAPTER IV

AROMATIC COMPOUNDS

AROMATIC HYDROCARBONS

Aromatic hydrocarbons may be prepared by the following methods :—

1. Wurtz - Fittig reaction. The interaction of an aryl halide, alkyl

halide and sodium gives a reasonable yield of an alkyl aryl hydrocarbon, for

example :

C 6 H 6 Br + 2Na + BrCH 2 CH 2 CH 2 CH 3 —* C 6 H 6 CH 2 CH 2 CH 2 CH 3

Two mechanisms have been proposed for the Wurtz reaction (compare Section

111,7) and for the Wurtz-Fittig reaction. According to one, sodium reacts

with the alkyl halide to produce a sodium halide and a free radical, which

subsequently undergoes coupling, disproportion a tion, etc. :

C 4 H 9 «Br + Na

C 6 H 6 Br + Na

> C 4 H, a . + NaBr

> C 6 H 5 . + NaBr

C 6 H 5 .+C 4 H 9 «.

* C 6 H 6 -C 4 H 9 «

C 6 H 5 . + C 6 H 6 •

> C 6 H 6 —C 6 H 6

C 4 H 9 «. + C 4 H 9 «.

> C 4 H 9 a —C 4 H 9 a

2CH3CHJJCH2CH2.

> CH 3 CH 2 CH = CH 2 + CH 3 CH a CH 2 CH 3

The other mechanism involves the intermediate formation of organosodium

compounds :

R—X + 2Na > R~Na+ + NaX

The products from a mixture of alkyl and aryl halides may be represented by

the following scheme :

zj*a _ + Ar—X

Ar—X * Ar Na ». Ar—Ar

The fact that n-butylbenzene can be prepared in reasonable yield by the

action of sodium upon a mixture of bromobenzene and n-butyl bromide can be

partly explained on the assumption that n-butyl bromide reacts with phenyl-

sodium more rapidly than does bromobenzene. It is interesting to note that

n-butylbenzene can be prepared either from benzylsodium and n-propyl

bromide or from phenylsodium and n-butyl bromide (Section VI,29).

2. Friedel and Crafts reaction. An alkyl halide condenses with an

aromatic hydrocarbon in the presence of anhydrous aluminium chloride to

yield, in the first instance, a hydrocarbon in accordance with the following

scheme : —

+ ""~~~

Ar— X

A1C1,

ArH -I- RX -- » ArR + HX

508

Bromobenzene n-Butyl bromide n-Butylbenzene (b.p. 182°)

The by-products are C 4 H 9 «—C 4 H f * = n-C 8 H 18 , n-octane (b.p. 125°) and

C 6 H 6 —C 6 H 6 , diphenyl (b.p. 254°), and can be readily separated by distillation.

2Na

[IV.l]

AROMATIC COMPOUNDS

509

The reaction does not, however, stop at the stage of mono-substitution since

the alkylbenzene ArR initially produced undergoes alkylation more easily

than the original hj^drocarbon ArH : mixtures of substances therefore result

and extensive purification is required in order to isolate the mono-substituted

compound. Furthermore, alkyl halides, which are capable of isomerisation,

are generally isomerised during the condensation ; thus n-propyl halides and

benzene give tso-propylbenzene, n-butyl halides yield sec.-butyl derivatives,

etc. Some mono-alkylbenzenes may be prepared by using an excess of the hydro-

carbon, which also acts as a diluent in moderating the violence of the reaction

and prevents the undue formation of poly-alkylbenzenes, for example :

Aid.

CH 3 CH 2 CH 2 C1 + C 6 H 6 (excess)

n-Propyl chloride

tso-Propylbenzene (cumene)

C 6 H 6 CH(CH 3 ) 1

A1C1.

(CH 3 ) 3 CC1 + C 6 H 6 (excess)

tert.- Butyl chloride tert.- Butyl benzene

Other catalysts which may be used in the Friedel - Crafts alkylation reaction

include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron

trifluoride, zinc chloride and hydrogen fluoride but these are generally not so

effective in academic laboratories. The alkylating agents include alkyl halides,

alcohols and olefmes.

The mechanism of the reaction is generally considered to proceed by way of

carbonium ions (alkyl cations) which attack the aromatic nucleus :

* C 6 H 6 C(CH 8 ) 3

R__X + A1C1 3

» R+[AlCl 3 Xr ^ R + + [A1C1 3 X]"~

H+ + [A1C1 3 X]~ — > HX + A1C1 3

The formation of tsopropylbenzene when n-propyl chloride is employed as the

alkylating agent is readily accounted for by the isomerisation of the alkyl

carbonium (or alkylium) ion :

CH 3 CH 2 CH 2 + ^ (CH 3 ) 2 CH +

possibly by transfer of a hydride ion from secondary carbon to primary

carbonium ion.

Two interesting applications of the Friedel and Crafts reaction to the pre-

paration of aromatic hydrocarbons will be described, viz.: —

AJCl.

C 6 H 5 CH 2 C1 + C fl H 6 (excess)

Benzyl chloride

> C 6 H 6 CH 2 C 6 H 5

Diphenylmethane

A1C1.

CHC1 3 + 3C 6 H 6 (excess)

Chloroform Triphenylmethane

By-products are formed in both preparations: thus in the former, anthracene,

and o- and p-dibenzylbenzenes are present in the fraction of high boiling point.

Diphenylmethane is more conveniently obtained by the interaction of benzyl

chloride and benzene in the presence of aluminium amalgam :

> CH(C 6 H 6 ) 3

C 6 H 6 CH 2 C1 f C 6 H 6 —5 C 6 H 6 CH 2 C e H 6 + HC1

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PRACTICAL ORGANIC CHEMISTRY

[IV,

3. Clemmensen reduction of aldehydes and ketones. Upon reducing

aldehydes or ketones with amalgamated zinc and concentrated hydrochloric

acid, the main products are the hydrocarbons (>C=O — + >CH 2 ), but

variable quantities of the secondary alcohols (in the case of ketones) and

unsaturated substances are also formed. Examples are :

HCl

C,H 5 COCH, -- > C 6 H & CH 2 CH 3

Acetophenone Ethylbonzone

C,H 6 CH 2 COCH 2 CH 3 -- - C 6 H 5 CH 2 CH 2 CH !! CH 3

Ethyl benzyl ketone

n-Butylbenzene

C 6 H 6 CH 2 CH 2 COCH 2 CH 2 CH 3 -

. C 6 H 5 (CH 2 ) 5 CH 3

The ketones are readily prepared, for example, acetophenone from benzene,

acetyl chloride (or acetic anhydride) and aluminium chloride by the Friedel

and Crafts reaction ; ethyl benzyl ketones by passing a mixture of phenylacetic

acid and propionic acid over thoria at 450° ; and n-propyl- p-phenylethylketone

by circulating a mixture of hydrocinnamic acid and n-butyrie acid over thoria

(for further details, see under Aromatic Ketones, Sections IV, 136, IV, 137 and

Purely aromatic ketones generally do not give satisfactory results : pinacols

and resinous products often predominate. The reduction of ketonic compounds

of high molecular weight and very slight solubility is facilitated by the addition

of a solvent, such as ethanol, acetic acid or dioxan, which is miscible with

aqueous hydrochloric acid. With some carbonyl compounds, notably keto

acids, poor yields are obtained even in the presence of ethanol, etc., and

the difficulty has been ascribed to the formation of insoluble polymolecular

reduction products, which coat the surface of the zinc. The addition of a

hydrocarbon solvent, such as toluene, is beneficial because it keeps most

of the material out of contact with the zinc and the reduction occurs in the

aqueous layer at such high dilution that polymolecular reactions are largely

inhibited (see Section IV, 143).

4. Wolff- Kishner reduction of aldehydes and ketones. Upon heating

the hydrazone or semicarbazone of an aldehyde or ketone with potassium

hydroxide or with sodium ethoxide solution (sealed tube), the corresponding

hydrocarbon is obtained :

RR'CO - RR'C=NNH 2 . RR'CH 2

* NaOCaH. *

The Huang - Minion modification of the reaction has the following advantages:

(i) the actual isolation of the hydrazone is unnecessary, (ii) the reaction time is

considerably reduced, (iii) the reaction can be carried out at atmospheric pressure

and on a large scale, and (iv) the yields are usually excellent. The hydrazone

is first formed in situ by refluxing a solution of the carbonyl compound in a

moderate amount of diethylene glycol or triethylene glycol with the com-

mercial 85 or 90 per cent, hydrazine hydrate and about 3 equivalents of potas-

sium hydroxide for 1 hour ; the water and excess of hydrazine are removed

by distillation until a favourable temperature for the decomposition of the

hydrazone is attained (170°-190°) and the solution is refluxed for 3-5 hours

longer.

Zn(Hg),

C 6 H 6 CHO — > C e H 6 CH 3

n-Propyl-p -phonylethylketone

n-Hexylbenzene

AROMATIC COMPOUNDS

511

The reaction is illustrated by the preparation of ethyl benzene from aceto-

phenone ; the resulting hydrocarbon is quite pure and free from unsaturated

compounds :

<W*\

CH 3 CH 3 diethyleneglycol

The disadvantages associated with the Clemmerisen reduction of carbonyl

compounds (see 3 above), viz., (a) the production of small amounts of carbinols

and unsaturated compounds as by-products, (b) the poor results obtained with

many compounds of high molecular weight, (c) the non-applicability to furan

and pyrrole compounds (owing to their sensitivity to acids), and (d) the sensi-

tivity to steric hindrance, are absent in the modified Wolff-Kishner reduction.

The mechanism of the reaction may involve the formation of an anion by the

base B, followed by the shift of hydrogen on the hydrazone anion with simul-

taneous loss of nitrogen to yield a carbanion :

RR'C=NNH 2 -f B ^ RR'C=NNH + BH +

RR'G=N— N(H} — » RR'CH + N 2

Benzyl magnesium chloride n-Propylbenzene

6. By the interaction of a Grignard reagent and an alkyl p-toluenesulphonate

for example :

C 6 H 6 CH 2 MgCl + 2p-CH 3 C 6 H 4 S0 3 C 4 H 9 * — > C 6 H 5 CH 2 (CH 2 ) 3 CH 3

RR'CH + BH+ - v RR'CH 2 + B

5. By the action of a dialkyl sulphate upon a Grignard reagent, for example :

C 6 H 5 CH 2 Cl + Mg — * C 6 H 5 CH 2 MgCl

C 6 H 6 CH 2 MgCl + (C 2 H 6 ) 2 S0 4 — * C 6 H 5 CH 2 CH 2 CH 3 + MgCl(S0 4 C 2 H 5 )

n-Butyl p-toluenesulphonate n-Amylbenzene

+ C 4 H 9 «C1 + (p-CH 3 C 6 H 4 S0 3 ) 2 Mg

For the preparation of alkyl p-toluenesulphonates, see Section IV,210.

IV,1. n-BUTYLBENZENE (Wurtz - Fittig Reaction)

Into a 1-litre round-bottomed flask, provided with a long (e.g., a 30 cm.)

double surface condenser, place 22-5 g. of clean sodium cut into small

pieces (see Section 111,7, Note 1, for experimental details concerning the

handling of sodium) and mount the flask for heating on an asbestos-

centred wire gauze. Prepare a mixture of 52 g. (35 ml.) of bromobenzene

(Section IV,18) and 51 g. (40 ml.) of n-butyl bromide (Sections 111,35

and 111,37). Add 5-7 ml. of the mixture through the condenser and

warm the flask very gently with a small luminous flame. Immediately

reaction commences (the sodium acquires a dark blue colour and

much heat is evolved), remove the flame. Introduce the remainder of

the mixture in small quantities during one hour ; shake the mixture

frequently and maintain a minute luminous flame beneath the flask.

Reflux the reaction mixture for 1—1-5 hours using a small luminous

flame ; shake the fairly solid contents of the flask from time to time.

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PRACTICAL ORGANIC CHEMISTRY

[IV,

Allow to cool and add 50 ml. of rectified spirit during 30 minutes through

a small separatory funnel fitted into the top of the condenser by means

of a grooved cork ; introduce a mixture of 25 ml. of rectified spirit and

25 ml. of water during 30 minutes, followed by 50 ml. of water. This

treatment will remove the excess of sodium. Reflux the resulting

mixture for 2-3 hours. Add 500 ml. of water and filter at the pump

from some sludge which is generally present; it is advisable to wash

the latter with a little ether. Transfer to a separatory funnel, remove

the upper hydrocarbon layer, and wash it successively with 25 ml. of

dilute sulphuric acid and 50 ml. of water ; dry over anhydrous mag-

nesium or sodium sulphate and distil (50 ml. Claisen flask and air bath,

Fig. //, 5, 3).. Collect the w-butylbenzene at 178-188° (20 g.) ; an

appreciable dark residue containing diphenyl remains in the flask. Upon

redistillation, the n-butylbenzene boils at 178-184° (1).

Note.

(1) The n-butylbenzene contains some unsaturated hydrocarbons : these can

be removed by repeated shaking with small quantities of concentrated sulphuric

acid (see Section III,7,Note 2).

IV,2. iso-PROPYLBENZENE (CUMENE)

Fit a two-litre three-necked flask with a separatory funnel, a mechanical

stirrer and a reflux condenser ; attach to the top of the condenser a tube

leading to an inverted funnel or an adapter dipping just below the surface

of a weighed quantity of water in a beaker or flask (compare Fig. //, 13, 8).

Place 700 g. (795 ml.) of dry benzene (1) and 20 g. of anhydrous aluminium

chloride (2) in the flask, and set the stirrer in motion. Add a mixture of

300 g. (342 ml.) of dry benzene and 100 g. (112-5 ml.) of n-propyl chloride

(compare Section 111,28) or 157 g. (116 ml.) of n-propyl bromide

(Section 111,35) dropwise into the flask. Warm the flask to about 80°

on a water bath ; the hydrogen halide evolved will be absorbed in the

water. When this has increased in weight by 47 g. (104 g. for hydrogen

bromide), pour the reaction mixture on to ice, remove the upper layer of

hydrocarbon, wash it successively with dilute sodium hydroxide solution

and water, and then dry with anhydrous magnesium sulphate. Distil

through a well-lagged fractionating column (compare Fig. //, 15, 5

and Fig. //, 16 y 1 ; see Sections 11,15-11,17) ; the excess of benzene

passes over first, followed by iso-propylbenzene at 151-153°. The yield

is 118g.

Notes.

(1) The moisture present in commercial benzene may be conveniently removed by

distilling off about one-tenth of the liquid ; the first fraction contains all the moisture.

It is generally unnecessary to distil the remaining liquid before use unless the

technical benzene is suspected of being highly impure.

(2) The yield of tso-propylbenzene is influenced considerably by the quality of the

anhydrous aluminium chloride employed. It is recommended that a good grade

of technical material be purchased in small bottles containing not more than

100 g. each; undue exposure to the atmosphere, which results in some hydrolysis,

is thus avoided. Sealed bottles containing the reagent sometimes have a high

internal pressure; they should be wrapped in a dry cloth and opened with care.

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