Dictionary of Chemistry [6th Ed.]

age. For example, the abundance of

uranium-235 in natural uranium is

0.71%. This is the natural abundance,

i.e. the abundance as found in nature

before any enrichment has taken

place.

acAnticlinal. See torsion angle.

acacThe symbol for the *acetyl-

acetonato ligand, used in formulae.

accelerantAÛammable material

used to start and spread aÜre in

cases of arson. Petrol and parafÜn are

the substances commonly used.

Traces of accelerant are detectable by

gas chromatography in forensic

work.

acceleratorA substance that in-

creases the rate of a chemical reac-

tion, i.e. a catalyst.

acceptor1.(in chemistry and bio-

chemistry) A compound, molecule,

ion, etc., to which electrons are

donated in the formation of a co-

ordinate bond. 2.(in physics) A sub-

stance that is added as an impurity to

a *semiconductor because of its abil-

ity to accept electrons from the va-

lence bands, causing p-type

conduction by the mobile positive

holes left. Compare donor.

accessory pigmentA *photosyn-

thetic pigment that traps light en-

ergy and channels it to chlorophyll a,

the primary pigment, which initiates

the reactions of photosynthesis. Ac-

cessory pigments include the

carotenes and chlorophylls b, c, and

d.

accumulator(secondary cell; stor-

age battery)A type of *voltaic cell

or battery that can be recharged by

passing a current through it from an

external d.c. supply. The charging

current, which is passed in the oppo-

site direction to that in which the

cell supplies current, reverses the

chemical reactions in the cell. The

common types are the *lead–acid ac-

cumulator and the *nickel–iron and

nickel–cadmium accumulators. See

also sodium–sulphur cell.

acenaphtheneA colourless crys-

talline aromatic compound, C 12 H 10 ;

m.p. 95°C; b.p. 278°C. It is an inter-

mediate in the production of some

dyes.

acetaldehydeSee ethanal.

acetaldolSee aldol reaction.

acetalsOrganic compounds formed

by addition of alcohol molecules to

aldehyde molecules. If one molecule

of aldehyde (RCHO) reacts with one

molecule of alcohol (R^1 OH) a hemiac-

etalis formed (RCH(OH)OR^1 ). The

rings of aldose sugars are hemiac-

etals. Further reaction with a second

alcohol molecule produces a full ac-

etal (RCH(OR^1 ) 2 ). It is common to

refer to both types of compound sim-

ply as ‘acetals’. The formation of ac-

etals is reversible; acetals can be

hydrolysed back to aldehydes in

acidic solutions. In synthetic organic

chemistry aldehyde groups are often

ac 4

a

12

Acenaphthene

 


 






 


 


 


 














 
  





  



 
 



Acetals