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CHAPTER IV AROMATIC COMPOUNDS AROMATIC HYDROCARBONS Aromatic Hydrocarbons May Be Prepared by the Following Methods :— 1

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CHAPTER IV AROMATIC COMPOUNDS AROMATIC HYDROCARBONS Aromatic Hydrocarbons May Be Prepared by the Following Methods :— 1 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 : C6H6Br + 2Na + BrCH2CH2CH2CH3 —* C6H6CH2CH2CH2CH3 Bromobenzene n-Butyl bromide n-Butylbenzene (b.p. 182°) The by-products are C4H9«—C4Hf* = n-C8H18, n-octane (b.p. 125°) and C6H6—C6H6, diphenyl (b.p. 254°), and can be readily separated by distillation. 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. : a C4H9«Br + Na > C4H, . + NaBr C6H6Br + Na > C6H5. + NaBr C6H5.+C4H9«. * C6H6-C4H9« C6H5. + C6H6 • > C6H6—C6H6 a a C4H9«. + C4H9«. > C4H9 —C4H9 2CH3CHJJCH2CH2. > CH3CH2CH = CH2 + CH3CHaCH2CH3 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*2Na _ + ""~~~ ArAr—— 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 : — A1C1, ArH -I- RX -- » ArR + HX 508 [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. CH3CH2CH2C1 + C6H6 (excess) » C6H6CH(CH3)1 n-Propyl chloride tso-Propylbenzene (cumene) A1C1. (CH3)3CC1 + C6H6 (excess) * C6H6C(CH8)3 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 : + R__X + A1C13 » R+[AlCl3Xr ^ R + [A1C13X]"~ R H H H+ + [A1C13X]~ —> HX + A1C13 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 : + + CH3CH2CH2 ^ (CH3)2CH 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. C6H5CH2C1 + CflH6 (excess) > C6H6CH2C6H5 Benzyl chloride Diphenylmethane A1C1. CHC13 + 3C6H6 (excess) > CH(C6H6)3 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 : C6H6CH2C1 f C6H6 —5 C6H6CH2CeH6 + HC1 510 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 — + >CH2), but variable quantities of the secondary alcohols (in the case of ketones) and unsaturated substances are also formed. Examples are : Zn(Hg), C6H6CHO — > CeH6CH3 HCl C,H5COCH, -- > C6H&CH2CH3 Acetophenone Ethylbonzone C,H6CH2COCH2CH3 -- - C6H5CH2CH2CH!!CH3 Ethyl benzyl ketone n-Butylbenzene C6H6CH2CH2COCH2CH2CH3 - . C6H5(CH2)5CH3 n-Propyl-p -phonylethylketone n-Hexylbenzene 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=NNH2 . RR'CH2 * 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. 1] 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*\ CH3 CH3 diethyleneglycol 3 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=NNH2 -f B ^ RR'C=NNH + BH + RR'G=N— N(H} — » RR'CH + N2 RR'CH + BH+ - v RR'CH2 + B 5. By the action of a dialkyl sulphate upon a Grignard reagent, for example : C6H5CH2Cl + Mg — * C6H5CH2MgCl C6H6CH2MgCl + (C2H6)2S04 — * C6H5CH2CH2CH3 + MgCl(S04C2H5) Benzyl magnesium chloride n-Propylbenzene 6. By the interaction of a Grignard reagent and an alkyl p-toluenesulphonate for example : C6H6CH2MgCl + 2p-CH3C6H4S03C4H9* — > C6H5CH2(CH2)3CH3 n-Butyl p-toluenesulphonate n-Amylbenzene + C4H9«C1 + (p-CH3C6H4S03)2Mg 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. 512 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.
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