Dictionary of Chemistry [6th Ed.]

89 B–Z reaction

b

called isobuteneor isobutylene. See

alkenes.

butenedioic acidEither of

two isomers with the formula

HCOOHC:CHCOOH. Both com-

pounds can be regarded as deriva-

tives of ethene in which a hydrogen

atom on each carbon has been re-

placed by a –COOH group. The com-

pounds show cis–trans isomerism.

The trans form is fumaric acid (r.d.

1.64; sublimes at 165°C) and the cis

form is maleic acid (r.d. 1.59; m.p.

139–140°C). Both are colourless crys-

talline compounds used in making

synthetic resins. The cis form is

rather less stable than the trans form

and converts to the trans form at

120 °C. Unlike the trans form it can

eliminate water on heating to form a

cyclic anhydride containing a

–CO.O.CO– group (maleic anhydride).

Fumaric acid is an intermediate in

the *Krebs cycle.









cis-butenedioic acid

(maleic acid)

trans-butenedioic acid

(fumaric acid)

Butenedioic acid

Butler–Volmer equation An

equation for the rate of an electro-

chemical reaction; it describes the

current density at an electrode in

terms of the overpotential. The But-

ler–Volmer equation is given by:

j = ja– jc= je[exp(1 – α)Fη/RT –

exp(– αF/RT)],

where jaand jcare the individual

cathode and anode currents respec-

tively, and jeis the equilibrium cur-

rent, called the exchange current

density. By deÜnition

je= jce= jae,

where jceis the equilibrium cathode

current and jaeis the equilibrium

anode current. F is the Faraday con-

stant, ηis the overpotential, R is the

gas constant, T is the thermodynamic

temperature, and αis a quantity

called the *transfer coefÜcient.

butterÛy effect See chaos.

butyleneSee butene.

butyl group The organic group

CH 3 (CH 2 ) 3 –.

butyl rubberA type of synthetic

rubber obtained by copolymerizing

2-methylpropene (CH 2 :C(CH 3 )CH 3 ;

isobutylene) and methylbuta-1,3-

diene (CH 2 :C(CH 3 )CH:CH 2 , isoprene).

Only small amounts of isoprene

(about 2 mole %) are used. The rub-

ber can be vulcanized. Large

amounts were once used for tyre

inner tubes.

butyraldehydeSee butanal.

butyric acidSee butanoic acid.

by-product A compound formed

during a chemical reaction at the

same time as the main product. Com-

mercially useful by-products are ob-

tained from a number of industrial

processes. For example, calcium chlo-

ride is a by-product of the *Solvay

process for making sodium carbon-

ate. Propanone is a by-product in the

manufacture of *phenol.

BZSee benzoylecgonine.

B–Z reaction (Belousov–Zhabotin-

skii reaction) A chemical reaction

that shows a periodic colour change

between magenta and blue with a pe-

riod of about one minute. It occurs

with a mixture of sulphuric acid,

potassium bromate(V), cerium sul-

phate, and propanedioic acid. The

colour change is caused by alternat-

ing oxidation–reductions in which