Modern inorganic chemistry

174 GROUP IV

H H H

Ethene can add on to certain metal salts ; it is believed that the extra

electrons of the double bond can be donated to some extent ; an

example is the compound PtCl 2 .C 2 H 4 formed with platmum(II)

chloride which has the structure

H H

\ /

C Cl

C Cl

/ \

H H

Alkynes

The essential feature of this series of hydrocarbons is the presence

of a triple bond between two carbon atoms, one a and two n:

r

"—^X Section XX

Acetylene (ethyne)—linear

This gives a linear arrangement of bonds, and alkynes, like alkenes,
are unsaturated. As might be expected, alkynes are very reactive
although certain addition reactions are unexpectedly difficult,
Terminal alkynes (ones in which the triple bond is at the end of a
carbon chain) have slightly acidic properties. Acetylene or ethyne,
C 2 H 2 , for example, reacts with an ammoniacal solution of copper(I)
chloride to give a red solid, copper(I) dicarbide, Cu 2 C 2 , which is
explosive when dry. Similarly, ammoniacal silver nitrate gives a
white solid, silver dicarbide, Ag 2 C 2. These two compounds contain
the dicarbide ion [C^C^1 ]^2 "" as does calcium 'carbide' CaC 2 , which
should really be called calcium dicarbide. All dicarbides give ethyne
when treated with a dilute acid.