5.3 Alkyne Family
1. Looking At Ethyne
This activity looks at the structure and properties of ethyne, the
simplest member of the alkyne series.
An alkyne is a hydrocarbon in which there is a triple bond
between a pair of carbon atoms in the molecule.
 Ethyne:
Structural Formula
H C C H
Molecular Formula
C2H2
 Preparation of Ethyne
CaC2 + 2H2O
(calcium carbide)
Ca(OH)2
+
C2H2
(ethyne)
 Properties of Ethyne
Property
Observation
Appearance
Colourless Gas
Solubility
Insoluble
Burning
Flammable
Reaction with Bromine
Quickly decolourises bromine
The old name for ethyne is acetylene and it is used in oxyacetylene torches for cutting steel.
2. Structures and Names
This activity is about the structures and systematic names of alkyne
molecules.
The general formula for the alkyne family is:
CnH2n-2
 But-1-yne:
Structural Formula
Molecular Formula
H H
H C C C C H
H
C4H6
H
In branched alkynes, the main chain must contain the triple
bond and the chain numbered to give the lowest position
for the triple bond.
*Branched Alkyne Structures*
3. Addition Reactions
This activity looks at some addition reactions of alkynes and how
these take place in two stages.
 Reaction of but-2-yne with chlorine.
Stage One:
H
H
H C C C C H
H
H Cl Cl H
+ Cl2
H
H C C C C H
H
but-2-yne
H
2,3-dichlorobut-2-ene
If excess chlorine is available the reaction will continue to
stage 2. Otherwise the reaction will stop here.
The product of stage one is still unsaturated and so can
add more chlorine to the double bond (stage two).
Stage Two:
H Cl Cl H
H C C C C H
H
H Cl Cl H
+ Cl2
H
2,3-dichlorobut-2-ene
H C C C C H
H Cl Cl H
2,2,3,3-tetrachlorobutane
 Reaction of propyne with hydrogen chloride:
Stage One:
H
Cl H
H
H C C C H + HCl
C C C H
H
H
propyne
H
2-chloropropene
Stage Two
H
Cl H
H Cl H
C C C H
H C C C H
H
+ HCl
H
2-chloropropene
H Cl H
2,2-dichloropropane
 Hydrogenation of 3-propylhex-1-yne:
Stage One:
CH2CH2CH 3
H
+ H2
H C C CH CH2CH2CH 3
3-propylhex-1-yne
H
H CH 2CH2CH3
C C CH CH2CH 2CH3
3-propylhex-1-ene
Stage Two:
H
H
H CH 2CH2CH3
+ H2
C C CH CH2CH 2CH3
3-propylhex-1-ene
H H CH2CH2CH3
H C C C CH 2CH2CH3
H H H
4-ethylheptane