(2) Polar additions to alkynes:
Like alkenes, alkynes undergo electrophilic addition of reagents such as
HX and X2 to the multiple bond. In the case of the triple bond the
addition takes place in two stages.
Note that both HX additions follow the Markovnikov rule:
+
C
C2 H5
H
C
X
H
C2H5
C
+
H
C
H
sp
: :
Vinyl carbocation sp-hybridised and less stable
than sp2 -hybridised
alkyl carbocations.
-
:X:
C
C2H5
+
+
H
C
C
: :
H
H
X
-
C
H
C2 H5
-
:
H
C
:X
H
C2H5
H
X
H
C
C 2H 5
C
H
H
: :
:X:
: X+
C
C2 H5
X
X
H
+
C
H
C
H
H
Although inductively electronwithdrawing the halogen atom
can stabilise the adjacent carbocation by resonance.
Terminal alkynes also undergo Markovnikov hydration in the presence
of a mercury(II) salt as catalyst - but the final product is not an alcohol:
R
C
CH
H2 O
Hg
+2
HO
H
C
R
C
H
Tautomerisation
O
C
R
Enol
CH3
Methyl
ketone
Internal alkynes are hydrated under acidic conditions without the need
for a catalyst and give mixtures of products:
CH3
C
C
H3O
C2 H5
H
C
+
C
+
C2 H5 +
CH3
C
+
H
C
CH3
C2H5
H2O - H+
H
OH
C
H2 O - H+
HO
H
C
C
CH3
C2H5
H
H C
CH3
O
CH3
C2 H5
O
H
C H
C2 H5
C
C
C2H5
C
CH3
(3) Reductions of alkynes: (a) Hydrogen and a catalyst:
HC
CH
H2 + catalyst
H2 Lindlar
catalyst
H° = -42 kcal mol -1
H2
H2C
Pd/C
CH2
H2 + catalyst
H° = -33 kcal mol -1
H3C
CH3
Lindlar catalyst = Pd/CaCO3 chemically treated to reduce reactivity
(b) Alkali metal in ammonia:
R
C
C
R
Na/Liq. NH3
2e + 2 H+  H2
Na e
R
-
C
NH3
C
R
H+
R
H
C
C
H
R
R
C
C
H
R
Radical
Radical anion
Na e
R
H
C
H
C
R
NH3
H+
R
C
H
-
C
R
Carbanion
THE CARBON-NITROGEN TRIPLE BOND - THE CHEMISTRY
OF NITRILES
Text references: McMurry (5th Edition) Chapters 21, p. 846 and
pp. 873 - 878.
Unhybridised
p-orbitals
Unhybridised
p-orbitals
C
N
sp
sp
hybrids
hybrids
Carbon: 4 valence
electrons
Nitrogen: 5 valence
electrons
Lone pair


R

C
N

180°
R
Less electronegative
C N:
+ More electronegative
Nomenclature - the systematic rules for naming nitriles:
(1) Simple nitriles are named by adding the termination nitrile to the
name of the longest carbon chain in the molecule including the nitrile
carbon (numbered as carbon 1) in the count:
C
3, 5-dimethylheptanenitrile
N
Br
C
5-bromopentanenitrile
N
(2) More complex nitriles are named from the corresponding carboxylic
acid:
O
R
CN
R
C
OH
Carboxylic acid
Nitrile
CN
CH3
CN
CH3
CN
Br
Acetonitrile
Benzonitrile
from
from
acetic acid
benzoic acid
3-bromo-2-methylcyclopentanecarbonitrile
from
3-bromo-2-methylcyclopentanecarboxylic
acid
The Preparation of Nitriles:
(1) SN2 displacement of halide by cyanide anion from primary alkyl
halides:
K+CNBr
1-bromobutane
CN
1-pentanenitrile
(2) Dehydration of (i.e. removal of water from) primary amides:
O
R
Two steps
C
R
OH
O
C
NH2
Amide
C
See CM2006
Carboxylic acid
R
O
NH2
Carboxylic acid amide
or just
Amide
SOCl2 or
POCl3
- H2O
R
C
N
Nitrile
Notes: (1) A primary amide is an amide unsubstituted on the amide
nitrogen - amides with alkyl or other substituents on N cannot be
dehydrated. (2) SOCl2 is thionyl chloride and POCl3 is phosphorus
oxychloride. Both are powerful dehydrating agents.
Reactivity of Nitriles:
R
+
C
N:
Lone-pair - protonation
can take place here.
Electron-poor site N is more electronegative
than C - easily attacked
by nucleophiles
R
C
+
N
H
R
+
C
N
H
Electrophilicity
increased
The reactions of nitriles are dominated by nucleophilic attack at the
electron-poor nitrile carbon:
(1) Hydrolysis (i.e. reaction with water) to amides and/or carboxylic
acids:
Nitrile hydrolysis can be carried out under either acidic or basic
conditions:
R
C
+ H 2O
N:
H
C
:
R
O
H
NH
R
H
C
Tautomerisation
H
:
R
O
+
+
N
NH2
H3O+
C
OH2
+
- H+
NH
R
C
OH
Subsequent hydrolysis of amide (RCONH2) to the corresponding
carboxylic acid (RCO2H) is generally rapid under acidic conditions.
(These - and similar - reactions of carbonyl compounds will be studied
in Module CM2006).
Under mild basic conditions the hydrolysis stops at the amide stage:
R
C
–
N
N
R
–:
:
:O
C
H2O
NH
R
C
OH
OH
H
Tautomerisation
NH2
R
C
O
(2) Reduction by hydride anion reagents to amines:
The reagent lithium aluminium hydride (LiAlH4, LAH) behaves
chemically as a source of hydride anion, H–, a strong reducing agent:
R
C
LAH
N
R
–
H
CH2 NH2
–
N
R
C
R
N
–
C
R
–
H
CH2 N–2
H
H
H2O
R
CH2 NH2
(3) Reaction with organomagnesium compounds, RMgX, (Grignard
Reagents) to yield ketones.
Grignard reagents, RMgX, act as a source of nucleophilic carbanions,
R– :
Simplified mechanism:
–
N
R1
C
N
–
R1
R (from RMgX)
C
H2O
NH
R1
C
R
R
Imine
Hydrolysis H3 O+
O
R1
C
R
Ketone
Organomagnesium comounds and other organometallic reagents will be
dealt with in more detail in P2 Module CM2005.