Chemistry, Third edition

ALKYNES

Alkynes

The alkynes are a family of hydrocarbons that contain a carbon–carbon triple bond,

with the general formula CnH 2 n 2. The first member of the family is ethyne (old

name acetylene). Ethyne is a linear molecule:

H—CC—H

When it burns in oxygen, a great deal of heat is generated – oxy-acetylene torches

are used for cutting and welding metals:

2C 2 H 2 (g)5O 2 (g)4CO 2 (g)2H 2 O(l)

The next member of the family, propyne, is obtained by replacing one of the

hydrogen atoms in ethyne by a –CH 3 group:

CH 3 —CC—H

Alkynes are unsaturated and undergo addition reactions in a similar fashion to

alkenes. This time it is possible to obtain two addition products, depending on the

number of moles of reagent reacting with the alkyne. If one mole of hydrogen, for

example, is added to one mole of ethyne, the product is ethene:

Ni catalyst

H—CC—H(g)H 2 (g) ——H 2 C=CH 2 (g)

15O °C

Two moles of hydrogen produce ethane:

Ni catalyst

H—CC—H(g)2H 2 (g) ——H 3 C—CH 3 (g)

15O °C

17.3

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Alkynes

(i) Give the structural formula of but-1-yne, a third member of the family of alkynes.

(ii)What are the names and formulae of the two possible products formed when propyne is

reacted with hydrogen in the presence of a nickel catalyst?

(iii)Write an equation for the reaction that occurs when ethyne is reacted with excess

bromine. Name the product.

Exercise 17N

Reactions of alkenes

(i) Write an equation for the reaction of ethene with hydrogen bromide. Name the product.

(ii)Write an equation for the reaction of ethene with cold, concentrated sulfuric acid. Name

the product.

(iii)Write an equation for the reaction of propene with bromine. Name the product.

(iv)Write the structural formula of the product of the reaction of propene with cold, alkaline

potassium permanganate.

(v)Write an equation for the formation of the polymer Orlon, formed by polymerization of the

monomer CH 2 =CHCN (acrylonitrile).

Exercise 17M