9.5
Acidity of Acetylene
and Terminal Alkynes
H
C
C
Acidity of Hydrocarbons
In general, hydrocarbons are
exceedingly weak acids
Compound
p Ka
HF
3.2
H 2O
16
NH3
36
H 2C
CH2
CH4
45
60
Acetylene
Acetylene is a weak acid, but not nearly
as weak as alkanes or alkenes.
HC
CH
Compound
p Ka
HF
3.2
H 2O
16
NH3
36
H 2C
CH2
CH4
45
60
26
Carbon: Hybridization and Electronegativity
10-60
C
H
H
C
10-45
C
H+ +
H+ +
C
C
:
sp3
:
sp2
C
10-26
C
C
H
H+ +
C
C :
sp
Electrons in an orbital with more s character are closer to the
nucleus and more strongly held.
Sodium Acetylide
Objective:
Prepare a solution containing sodium acetylide
NaC
CH
Will treatment of acetylene with NaOH be effective?
NaOH + HC
CH
NaC
CH + H2O
Sodium Acetylide
No. Hydroxide is not a strong enough base to
deprotonate acetylene.
NaOH + HC
.. –
: +
HO
..
H
CH
C
CH
NaC
CH + H2O
..
–
+ :C
H
HO
..
stronger acid
pKa = 16
weaker acid
pKa = 26
In acid -base reactions, the equilibrium lies to
the side of the weaker acid.
CH
Sodium Acetylide
Solution: Use a stronger base. Sodium amide
is a stronger base than sodium hydroxide.
NaNH2 + HC CH
NaC CH + NH3
.. –
H 2N : +
H
C
CH
stronger acid
pKa = 26
..
H 2N
–
H + :C
weaker acid
pKa = 36
Ammonia is a weaker acid than acetylene.
The position of equilibrium lies to the right.
CH
9.6
Preparation of Alkynes
by
Alkylation of Acetylene and Terminal Alkynes
Preparation of Alkynes
There are two main methods for the preparation
of alkynes:
Carbon-carbon bond formation
alkylation of acetylene and terminal alkynes
Functional-group transformations
elimination
Alkylation of Acetylene and Terminal Alkynes
H—C
C—H
R—C
C—H
R—C
C—R
Alkylation of Acetylene and Terminal Alkynes
H—C
–
C: +
R
X
SN2
H—C
C—R + : X–
The alkylating agent is an alkyl halide, and
the reaction is nucleophilic substitution.
The nucleophile is sodium acetylide or the
sodium salt of a terminal (monosubstituted)
alkyne.
Example: Alkylation of Acetylene
HC
CH
NaNH2
HC
NH3
CNa
CH3CH2CH2CH2Br
HC
C
CH2CH2CH2CH3
(70-77%)
Example: Alkylation of a Terminal Alkyne
(CH3)2CHCH2C
CH
NaNH2, NH3
(CH3)2CHCH2C
CNa
CH3Br
(CH3)2CHCH2C
(81%)
C—CH3
Example: Dialkylation of Acetylene
H—C
C—H
1. NaNH2, NH3
2. CH3CH2Br
CH3CH2—C
C—H
1. NaNH2, NH3
2. CH3Br
CH3CH2—C
C—CH3
(81%)
Limitation
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides
undergo elimination
Acetylide Ion as a Base
E2 predominates over S N2 when alkyl
halide is secondary or tertiary
H—C
–
C:
H
C
C
X
E2
H—C
C —H +
C
C
+
: X–
9.7
Preparation of Alkynes
by Elimination Reactions
Preparation of Alkynes
by "Double Dehydrohalogenation"
H
X
H
H
C
C
C
C
H
X
X
X
Geminal dihalide
Vicinal dihalide
The most frequent applications are in preparation
of terminal alkynes.
Geminal dihalide ∅ Alkyne
(CH3)3CCH2—CHCl2
1. 3NaNH 2, NH3
2. H2O
(CH3)3CC
CH
(56-60%)
Geminal dihalide ∅ Alkyne
(CH3)3CCH2—CHCl2
NaNH2, NH3 (slow)
(CH3)3CCH
CHCl
NaNH2, NH3 (slow)
(CH3)3CC
CH
H 2O
(CH3)3CC
CNa
NaNH2, NH3 (fast)
Vicinal dihalide ∅ Alkyne
CH3(CH2)7CH —CH2Br
Br
1. 3NaNH 2, NH3
2. H2O
CH3(CH2)7C
(54%)
CH