ALKYNES

Alkynes
Chapter 7

Triple Bond Structure

Nomenclature

 IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond
4
3

2

1

2

3

4

6

7

5

1
2

3

4

6

7

5

1

3-Methyl-1-butyne

6,6-Dimethyl-3-heptyne

1,6-Heptadiyne

 Common names: prefix the substituents on the triple bond to the name “acetylene”
IUPAC name:
Common name:

2-Butyne
D imethylacetylene

1-Buten-3-yne
Vinylacetylene



Physical Properties
Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton

2-Butyne
1-Pentyne

Melting
Point
Formula
(°C)
-81
HC CH
CH3 C CH
-102
CH 3 CH2 C CH
-126
-32
CH3 C CCH3
CH 3 ( CH2 ) 2 C CH
-90

1-Hexyne

CH 3 ( CH2 ) 3 C CH

1-Octyne
1-Decyne

Name
Ethyne
Propyne
1-Butyne

Boiling
Point
(°C)
-84
-23
8

Density at 20°C
(g/mL)
(a gas)
(a gas)
(a gas)

27
40

0.691
0.690

-132

71

0.716

CH 3 ( CH2 ) 5 C CH

-79

125

0.746

CH 3 ( CH2 ) 7 C CH

-36

174

0.766

Acidity

 A major difference between the chemistry of alkynes and that of alkenes and alkanes is the acidity of the hydrogen bonded to a triply bonded carbon

the pKa of acetylene is approximately
25, which makes it a stronger acid than ammonia but weaker than alcohols Acidity

 Acetylene reacts with sodium amide to form sodium acetylide –

HC CH + pK a 25
S tronger acid NH2

S tronger base HC C- +
Weaker
base

N H3 pK a 38
Weaker
acid

 It can also be converted to its metal salt by reaction with sodium hydride or lithium diisopropylamide
(LDA)
+

Na H –
Sodium hydride

[ ( CH3 ) 2 CH] 2 N – Li +
Lithium diisopropylamide
(LDA)

Alkylation of Acetylides

 Acetylide anions are both strong bases and good nucleophiles  They undergo nucleophilic displacement reactions with alkyl halides to form new C-C bonds to alkyl groups; that is, they undergo alkylation