Lecture (7)
IV. Alkynes (Acetylenes)
1. Introduction:
The alkynes are unsaturated hydrocarbons that contain a carbon – carbon triple
bond, because of its triple bond, an alkyne has four fewer hydrogens than alkanes.
The general formula is CnH2n-2, and that for cyclic alkynes is CnH2n-4.
2. Nomenclature:
The alkynes are named according to two systems:
a) They are considered to be derivatives from acetylene by replacement of
hydrogen by an alkyl groups:
HC ≡ CH
CH3 − C ≡ CH
CH3 − C ≡ C − CH3
Acetylene
methyl acetylene
dimethylacetylene
b) IUPAC Rule:
1) In the IUPAC rule the compounds are named as alkynes in which the
final (ane) of the parent alkane is replaced by the suffix (yne).
2) Number the largest chain containing the triple bond and give it the
lowest number.
3) Multiple triple bonds are given the suffix diyne, triyne…. etc.
ex. CH3 − C ≡C-CH2 − C ≡ CH (1,4 – hexadiyne)
 Examples:
o HC ≡ CH
4
3
(ethyne)
2
1
o CH3CH2C ≡ CH
1
2
3
4
(1–butyne)
5
o CH3C ≡ C CH2CH3
(2–pentyne)
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5
o
6
CH2 CH3
│ 3 2 1
CH3CHC ≡ CCH3
4
o
Cl
Br
│
│
(4–methyl–2–hexyne)
CH3CH – CH C ≡ C CH2CH2CH3
1
2
3
4
5
6
7
8
(3-bromo-2-chloro-4- octyne)
3. Preparation of alkynes:
a. Dehydrohalogenation of dihalides:
By action of alcoholic; KOH is useful since the dihalides are readily obtained from
corresponding alkenes by addition of halogens:
Br2
KOH
alc.
CH2BrCH2Br
CH2 =CH2
ethylene
ethylenedibromide
(1,2-dibromoethane)
Br2
CH3CH=CH2
CH3CHBrCH2Br
vinylbromide
CH≡CH
acetylene
(1-bromoethene)
KOH
alc.
1,2 –dibromopropane
CH3CH=CHBr
1-bromo-1-propene
b. Thermal decomposition of ethene:
1100°C
C2H4 (g)
KOH
alc.
CH2=CHBr
C2H2 (g) + H2 (g)
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KOH
alc.
CH3C≡CH
1-Propyne
c-From methane by heating to 1500°C for a short time:
1500°C
HC ≡ CH + 3H2
2CH4
d.Dehalogenation of tetrahalides:
Br
Br
│
│
Zn dust
CH3 – C –– C – CH3
│
│
Br
Br
tetrabromobutane
∆
Br
Br
│
│
Zn dust
CH3 – C == C – CH3
∆
CH3 – C ≡ C-CH3
dibromo2-butene
2-butyne
4. Physical Properties of Alkynes:
Alkynes are insoluble in water and soluble in organic solvents of low polarity
like ether, benzene and carbon tetrachloride. They are less dense than water. Their
boiling point increase with increasing molecular weight.
5. Chemical Properties of Alkynes:
Like alkanes, alkynes undergo electrophilic addition. The carbon –carbon triple
bond is less reactive than the C = C toward electrophilic reagent.
a) Addition of hydrogen. Reduction to alkenes:
Reduction of an alkyne to the double bond stage yield either a cis- or transalkene, reduction with sodium in liquid ammonia gives trans- alkenes, and cisalkenes is obtained by hydrogenation with palladium:
Ex.1:
CH3 C ≡ C CH3
H
CH3
Na, NH3 (liq.)
C
C
CH3 42
tran - 2 - butene
H
H
H
H2 / Pd
C
C
CH3 C ≡ C CH3
CH3
CH3
Cis - 2 - butene
Ex.2:
CH3CH2C ≡ CH + H2
Pt
CH3CH2CH = CH2
1- butyne
Pt
CH3CH2CH2CH3
1- butene
n- butane
(The steps controlled by the amount of hydrogen)
b) Addition of Halogens:
Br2
CH3 C ≡ C CH3
2- butyne
CCl4
Br
CH3
│
│
Br Br
Br2
C == C
CCl4
│
│
H3C – C – C – CH3
│
│
│
│
CH3
Br
Br
Br
Trans-2,3-dibromo-2-butene
2,2,3,3-tetrabromobutane
(The steps controlled by the amount of halogen used)
c) Addition of halogen acid:
CH3 – C ≡ C – H + HBr
Propyne
(According to Markovinkov's rule)
CH3
H
│
│
C === C
│
│
Br
H
2 – Bromo1- propene
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d) Addition of water:
Obey Markovinkov's rule.
H2SO4
- C ≡ C - + H2O
O
│
║
––C –––– C ––
-C=CHgSO4
H
│
│
│
H
OH
H
Aldehyde or Ketone
Enol
A structure with (OH) attached to C=C is called enol[-ene for C=C,and ol for (OH)]
O
║
H2SO4
HC ≡ CH + H2O
HgSO4
H-C=CH
CH3CH
H OH
Vinylalcohol
Acetaldehyde(aldehyde)
O
║
H2SO4
CH3C ≡ CH + H2O
HgSO4
H3C –––– C –– CH3
CH3-C=CH2
Acetone (Ketone)
OH
e) Oxidation of Acetylenes:
With permanganate and ozone, the triple bond is broken to give carboxylic acids.
RC ≡ CR1
R – C ≡ CH
KMnO4
O3
RCOOH + R1COOH
RCOOH + HCOOH
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f) Polymerization:
Acetylene polymerizes under proper condition.
Ex.:
Cl
│
2 HC ≡ CH + HCl
CH2 = CH – C = CH2
2 – Chloro – 1, 3 - butadiene
This chlorobutadiene polymerizes to a useful synthetic rubber (neoprene).
Cl
│
N (CH2 = CH – C = CH2)
Polymerization
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Neoprene