Dictionary of Chemistry [6th Ed.]

benzenecarbaldehyde 62

b

saturated molecule, yet will not read-

ily undergo addition reactions. On

the other hand, it does undergo sub-

stitution reactions in which hydro-

gen atoms are replaced by other

atoms or groups. This behaviour oc-

curs because of delocalization of p-

electrons over the benzene ring, and

all the C–C bonds in benzene are

equivalent and intermediate in

length between single and double

bonds. It can be regarded as a reso-

nance hybrid of Kekulé and Dewar

structures (see formulae). In formu-

lae it can be represented by a hexa-

gon with a ring inside it.

benzenecarbaldehyde(benzalde-

hyde)A yellowish volatile oily liq-

uid, C 6 H 5 CHO; r.d. 1.04; m.p. –26°C;

b.p. 178.1°C. The compound occurs

in almond kernels and has an al-

mond-like smell. It is made from

methylbenzene (by conversion to

dichloromethyl benzene, C 6 H 5 CHCl 2 ,

followed by hydrolysis). Benzenecar-

baldehyde is used inÛavourings, per-

fumery, and the dyestuffs industry.

benzenecarbonyl chloride (ben-

zoyl chloride)A colourless liquid,

C 6 H 5 COCl; r.d. 1.21; m.p. 0°C; b.p.

197.2°C. It is an *acyl halide, used to

introduce benzenecarbonyl groups

into molecules. See acylation.

benzenecarbonyl group (benzoyl

group) The organic group C 6 H 5 CO–.

benzenecarboxylate(benzoate)A

salt or ester of benzenecarboxylic

acid.

benzenecarboxylic acid (benzoic

acid)A white crystalline compound,

C 6 H 5 COOH; r.d. 1.27; m.p. 122.4°C;

b.p. 249°C. It occurs naturally in

some plants and is used as a food

preservative. Benzenecarboxylic acid

has a carboxyl group bound directly

to a benzene ring. It is a weak car-

boxylic acid (Ka= 6.4 × 10 –5at 25°C),

which is slightly soluble in water. It

also undergoes substitution reactions

on the benzene ring.

benzene-1,4-diol(hydroquinone;

quinol)A white crystalline solid,

C 6 H 4 (OH) 2 ; r.d. 1.33; m.p. 170°C; b.p.

285 °C. It is used in making dyes. See

also quinhydrone electrode.

benzene hexacarboxylic acidSee

mellitic acid.

benzene hexachloride (BHC)A

crystalline substance, C 6 H 6 Cl 6 , made

by adding chlorine to benzene. It is

used as a pesticide and, like *DDT,

concern has been expressed at its en-

vironmental effects.

benzenesulphonic acidA colour-

less deliquescent solid, C 6 H 5 SO 2 OH,

m.p. 43–44°C, usually found as an

oily liquid. It is made by treating ben-

zene with concentrated sulphuric

acid. Its alkyl derivatives are used as

*detergents.

benzfuran(coumarone) A crys-

talline aromatic compound, C 8 H 6 O.

It is a heterocyclic compound hav-

ing a benzene ring fused to aÜve-

membered *furan ring.

benzil 1,2-diphenylethan-1,2-dione.

See benzilic acid rearrangement.

Kekulé structures Dewar structures

Benzene