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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
C H A P T E R 6 Alkynes
Z :
R
slow
(Vinyl carbanion)
AlkyneNucleophile
R
Z
Θ
Θ
+
:
CC
R
CC
R
RRRR
HZZ
Z
Θ
+
:
fast
R
:
+ H
Z
CC CC
Θ
Ba(CN)
2
Vinyl cyanideAcetylene
H
+
CNCH
CH
CH
2
CHCN
HCN
Θ
HC
CH + CN
CN
HC
CH
CNH
HC
CH + CN
6.14 NUCLEOPHILIC ADDITION REACTIONS
Because of the greater electronegativity of the sp-hybridized carbons as compared to the sp2-hybridized
carbons, alkynes are more susceptible to nucleophilic addition reactions than alkenes like electrophilic
addition reactions; nucleophilic addition also takes place in two steps. In the rst step, a nucleophile attacks
one of the triply bonded carbons forming a vinyl carbanion:
This step is slow and hence is the rate-determining step of the reaction.
In the second step, the vinyl carbanion accepts a proton from the reagent to form the addition product.
This step being fast, therefore, does not affect the rate of the reaction.
271
Some important nucleophilic addition reactions of alkynes are given below:
1. Addition of HCN: Acetylene adds on hydrogen cyanide in the presence of barium cyanide (catalyst)
to form vinyl cyanide, used in the manufacture of synthetic rubber.
Mechanism
2. Addition of acetic acid: When acetylene is passed into warm acetic acid in the presence of mercuric
ions as catalyst, vinyl acetate is obtained.
272
Hg
2+
Acetylene
CH
CH
H
+
CH
3
OOC
Vinyl acetate
CH3 OOC
CH
2
CH
OOCCH
3
OCOCH
3
O
Vinyl acetate
Θ
H CO CH
3
HC
CH
2
+ CH
3
CO
O
Hg
2+
CHHC
HC
CH + CH
3
COO
HH
Methyl vinyl ether
CH3OH
CH3O
Θ
:
−CH3O
Θ
Θ
HCC
O + H
CH
3
CC
H
HH
CH
3
O
CC
CH 1/2 H
2
C Na +
⊕
Θ
CH + Na
CH
1/2 H
2
⊕
Θ
RC
C Na +
CH + Na
RC
liq. NH
3
Θ
⊕
RC
C Na + NH
3
193 K
RC
CH + NaNH
2
PHARMACEUTICAL ORGANIC CHEMISTRY
Vinyl acetate is used in the plastic industry.
Mechanism
3. Addition of methanol: Methanol adds to alkynes in the presence of catalytic amounts of sodium
methoxide forming vinyl ethers. This reaction is initiated by the nucleophilic attack of methoxide
ion on acetylene.
The methyl vinyl ether is useful for the manufacture of polyvinyl ether plastics.
6.15 ACIDITY OF ACETYLENIC HYDROGEN
The hydrogens attached to triply bonded carbon atom are acidic in nature, unlike their counterparts of
doubly and singly bonded carbon atoms. This is clear from the following reactions.
1. Acetylene or 1-alkynes react with sodium metal to form sodium acetylide and hydrogen, i.e.
2. Sodium acetylide can also be formed by treating such alkynes with sodamide, i.e.
C H A P T E R 6 Alkynes
−
δ
+
δ
Θ
⊕
:
CH C
+
H
HHCC
CH2CH
2
Θ
⊕
CH2CH + H
CH3 CH + H
2
Θ
:
⊕
CH3CH
3
:
H
HH
Acetylide anion
(Unshared pair
of electrons
in sp orbitals)
Vinyl anion
(Unshared pair
of electrons
in sp
2
orbitals)
Ethyl anion
(Unshared pair
of electrons
in sp
3
orbitals)
HCC
CC
H
H
CH
3
C
2 32
CH C CH CH CH CH
>
Θ ΘΘ
>
Terminal alkynes are stronger acid than ammonia but are weaker acid than water.
Let us study the cause of the acidity of such alkynes. As usual the cause of the acidic character of
terminal alkynes can be explained on the basis of (1) weakening of
C H bond and (2) stability
of the acetylide anion.
Let us elaborate these two points.
a. Weakening of C – H bond: As sp carbon is more electronegative than sp2 carbon of
3
alkene or sp
carbon of alkane due to the increased s-character in it, the electrons of C –
H bond are more strongly held by the carbon atom than by hydrogen atom. Or in a simple
language due to greater electronegativity of sp-hybridized carbon, the electron of the C – H
bond stays nearer to the carbon nucleus. Thus this hydrogen atom shows the tendency to be
liberated as proton.
b. Stability of acetylide ion: Let us compare the relative stabilities of the anions left after the
removal of a proton from acetylene, ethylene and ethane in the following equilibria.
In case of acetylide anion, the sp-hybrid orbital contains the unshared electron pair while
in ethenyl anion (vinyl) it is in sp2 and in case of ethyl carbanion is to sp3 orbital as shown
below .
Due to greater electronegativity of sp orbital, the unshared pair of electrons on acetylide
carbanion will be more tightly held to carbon nucleus than in vinyl or ethyl carbanions. Hence
relative stabilities of these three carbanions follow the sequence.
273
Therefore the equilibrium (a) will shift to the right-hand side, making acetylene a very weakly
acidic in nature.
274
Cu2C2↓ + 2NH4Cl + 2H2O
Red ppt. Acetylene
Acetylene
(copper acetylide)
Ag
2C2
↓ + 2NH4NO3 + 2H2O
(silver acetylide)
White ppt.
HC
CH + Cu
2Cl2
+ 2NH4OH
HC
CH + 2AgNO
3
+ 2NH4OH
Ag2C2 + 2HNO
3
2AgNO3 + C2H2↓
AcetyleneSilver acetylide
2CH
CH + 5O
2
4CO2 + 2H2O; H =
−1300 kJ mol
−
1
PHARMACEUTICAL ORGANIC CHEMISTRY
3. Formation of heavy metal acetylides: When acetylene is passed through an ammoniacal solution of
cuprous chloride or silver nitrate, cuprous acetylide, Cu2C2 (red), and silver acetylide, Ag2C2 (white),
is precipitated. This serves as a delicate test for acetylene.
Both these compounds, when dry, explode when struck or heated. They are decomposed by dilute
mineral acids regenerating acetylene, for example,
The formation and subsequent decomposition of acetylides can be used for the purication of
acetylene and its separation from a mixture of olens and parafn.
6.16 OXIDATION REACTION OF ALKYNES
Alkynes undergo oxidation under suitable conditions. Some of the most commonly used oxidizing agents
are oxygen, potassium permanganate and ozone. Let us now discuss the oxidation of alkynes with these
oxidizing agents.
1. Complete oxidation with oxygen (combustion): Alkynes readily burn in air or oxygen producing CO
and H
energy.
2. Controlled oxidation with oxidizing agents: Alkynes can be converted into many other products under
controlled oxidation conditions. There are two important oxidizing agents for these oxidation reactions.
These are potassium permanganate and ozone.
O. These combustion reactions are accompanied by liberation of a large amount of heat and light
2
Oxy-acetylene ame gives a very high temperature (above 3000 K) and is used for welding and
cutting of metals.
2
C H A P T E R 6 Alkynes
CC
OO
H + 4MnO
2
+ 4KOH
3CH CH + 4KMnO4 + 2H2O 3H
298–303 K
Ethyne
KMnO4, KOH
2CO2 + H2O
(O)
373−383 K
KMnO
4
, KOH
CH3COOH + CO
2
COOH
COOH
CH3C CH + 4(O)
CH
CH + 4(O)
CH2Cl
2
GlyoxalOzonideAcetylene
−50°
+ O
3
H2/Pd
or Zn/H
2
O
CHO
CHO
O
OCH
OCH
CH
CH
CH2Cl
2
CH3CC
O
2,3-Butanedione
Zn/H2O
2
or H2/Pd
223 K
Butyne-2
Ozonide
C
CH
3
C
OO
O
CH
3
CH
3
CH3 + O
3
CC
O
CH
3
3. Oxidation with potassium permanganate: Because of the presence of reactive p bonds, alkynes readily
react with KMnO4. The nature of the products obtained. However, depends upon the reaction as discussed
below.
a. Oxidation with cold KMnO4 solution: With cold dilute alkaline or neutral KMnO4 solution, alkynes
give 1,2-diketones. For example,
During this reaction, the pink colour of the KMnO4 solution is changed and a brown precipitate
of manganese dioxide is obtained. This action is, therefore, used as a test for unsaturation under
the name Baeyer’s test.
b. Oxidation with hot KMnO4: Under drastic conditions (higher temperature, high concentration of
KMnO4) even carbon–carbon triple bond is completely cleaved and the nal products formed are
carboxylic acids and carbon dioxide depending upon the position of the triple bond, i.e. C
H part of the alkyne gets oxidized to CO2 and H2O and C R part gets oxidized to RCOOH.
For example,
275
4. Oxidation with ozone: Alkynes add on ozone across the triple bond to form an ozonide, which on
reductive cleavage gives diketones.
276
Cu
Propyne
CH
3
CH
3
H3C
H2SO
4
3CH
3
C
CH
CH
CH
CuCl
+HCI
Chloroprene
CuCI
Vinyl acetylene
H
+
CHC
Cl
CH
2
CH
2
CH C
CH
2
CH CHC
CyclooctatetraeneAcetylene
HC
HC
Ni(CN)
2
Pressure
CH
CH
HC CH
HC CH
HC CH
HC CH
HC CH
HC CH
PHARMACEUTICAL ORGANIC CHEMISTRY
6.17 POLYMERIZATION
1. When passed through a red hot copper tube, acetylene polymerizes to a small extent, to give
benzene. This is also known as trimerization.
2. When passed into a solution of cuprous chloride in ammonium chloride acetylene polymerizes
to vinyl acetylene. This is also known as dimerization. These on treatment with HCl give
chloroprene.
3. Under high pressure and in the presence of nickel cyanide catalyst, acetylene tetramerizes to form
Chloroprene on polymerization produces neoprene, which is used in articial rubber.
cyclooctatetrene.
C H A P T E R 6 Alkynes
6.18 DISTINCTION BETWEEN ALKANE, ALKENE AND ALKYNE
These three types of hydrocarbons can be distinguished by the following properties:
S. no. Property Alkane (C2H6) Alkene (C2H4) Alkyne
1.
Flame observed on combustion Non-luminous Luminous Smoky (Sooty)
2. Reaction with Br
3. Reaction with cold aqueous
or alkaline KMnO
/CCl4 solution No action Orange colour is discharged Orange colour is discharged
2
solution
4
No action Pink colour is discharged
with the formation of a glycol
Pink colour discharged with
the formation of a diketone
(Baeyer’s reagent)
4. Reaction with ammoniacal AgNO
No action No action White ppt. of silver acetylide
3
solution given by terminal alkynes
only
5. Reaction with ammoniacal Cu
(given by terminal alkynes only)
No action No action Red ppt. of copper acetylide
2Cl2
Cu
2C2
MEMORY FOCUS
Reactions of Alkynes (with special reference to ethyne)•
277
278
Glyoxal
anhy. AlCl
3
(M=Pb, Ba, Hg)
M
2+
π-Complex
CC
+ M
2+
CC
Thiophene
CH
2
+ S
CH
CH
CH
CH
CH
S
Electric
C2H2 + N
2
2HCN
spark
Hydrocyanic acid
PHARMACEUTICAL ORGANIC CHEMISTRY
NOTEWORTHY POINTS
Alkynes are less reactive than alkenes towards electrophilic addition reactions (e.g. reactions with HCl, etc.).1.
Alkynes undergo nucleophilic addition reactions (e.g. reaction with H2.
undergo these reactions.
Alkynes undergo nucleophilic addition reactions in the presence of heavy metallic ions like Pb3.
2+
Ba
ions, etc. The heavy metal ions form a p-complex through co-ordination with p-electrons and loosely
held p-electron complex is displaced towards the heavy metallic ion. As a result the electron density at the
acetylenic carbon decreases and facilitates nucleophilic attack at the triply bonded carbon atoms.
O, etc.) whereas alkenes fails to
2
2+
, Hg2+,
The order of acidic nature of alkynes is:4.
CH > HC C CH3 and
HC
CH > CH2 CH2 > CH3 CH
HC
Reactions with sulphur, nitrogen and ammonia:5.
a.
b.
3
C H A P T E R 6 Alkynes
CH
2
+ NH
3
+ H
2
CH
H
Pyrrole
CH
CH
CH
CH
N
Pd/BaSO
4
cis-But-2-ene
Na/NH
3
Birch reduction
trans-But-2-ene
CH
3
CH
3
C C
CH
3
CH
3
HH
CC
CH
3
H
HCH
3
CC
Ni(CN)
2
Pressure
Cyclooctatetraene
4CH CH
CH
3
Red hot
Cu tube
CH
3
CH
3
Mesitylene
CCH3CH
3
279
c.
6.
7.
280
CH2SH
CH
2
OH
CHSH
PHARMACEUTICAL ORGANIC CHEMISTRY
8. Lewsite is a poisonous gas used in World War II. Its antidote was developed by Britain in World War II
under the name British anti-lewsite (BAL), which is
REVISION QUESTIONS
1. How are alkynes prepared? How is acetylene prepared industrially?
2. What happens when
(a) Calcium carbide is treated with water
(b) Acetylene is passed through red hot iron tube
(c) Acetylene is passed through ammoniacal silver nitrate solution
(d) Acetylene is passed through ammoniacal cuprous chloride solution
(e) Acetylene is passed through dilute sulphuric acid in the presence of mercuric sulphate
3. Write a note on acidity of 1-alkynes.
4. How will you distinguish between acetylene and ethylene?
5. How will you synthesize
(a) Acetaldehyde from acetylene?
(b) Vinyl acetate from acetylene?
(c) Vinyl methyl ether from acetylene?
(d) Acetylene from ethylene?
(e) 1-Butyne from acetylene?
(f) Acetone from propene?
6. How does acetylene react with the following reagents
(a) H2/Pd (b) H2/Pd/BaSO
(c) HBr (d) H2O/H2SO4/HgSO
4
4
(e) Na/liq NH3 (f) AgNO3/NH4OH
(g) HCN /Ba(CN)2 (h) Cu2Cl2/NH4OH