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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
+ 2 Zn
∆
Tetra halide
X
X
C C
X
X
CC
+ 2 ZnX
2
2. Nonterminal alkynes: Alkynes in which the triple bond is present in between the carbon chain,
i.e. alkynes in which both the triple-bonded carbon atoms are attached to alkyl groups are called
nonterminal alkynes. Thus,
261
CH
2-Butyne 2-Pentyne 3-Hexyne
(Dimethyl acetylene) (Ethyl methyl acetylene) (Diethyl acetylene)
— C ≡ C — CH3; CH3CH2 — C ≡ C — CH3;
3
C2H5 C ≡ C — C2H
5
6.6 COMMERCIAL METHODS OF PREPARATION OF ACETYLENE
From commercial point of view, acetylene is the most important alkyne. Two most commonly used methods
for the manufacture of acetylene are:
1. From calcium carbide (calcium acetylide): Acetylene can be prepared by the action of water on
calcium carbide or calcium acetylide.
CaC2 + 2H2O CH CH + Ca(OH)
Calcium Acetylene
acetylide
2
Calcium carbide required for the purposes is obtained by heating coke and lime at about 2500ºC
in an electric furnace.
CaO + 3C
Lime Coke
2500°C
CaC2+ CO
2. Controlled oxidation of methane at high temperature: Methane (from natural gas) is heated
briey to a very high temperature; acetylene is one of the products.
6.7 GENERAL METHODS OF PREPARATION OF ALKYNES
1. Dehalogenation of tetra halides: When treated with zinc dust, tetra halide gets dehalogenated to
form alkynes.
+ O2 2CH CH + 2CO + 10H
6CH
4
1500°C
2
262
H
KOH
NaNH
2
(alc.)
X
Br
1-Bromopropene
Propyne
1, 2-Dibromo
propane
KOH (alc.)
NaNH
2
Br
CH3CH
CH
2
C C
H
X
X
C C
H
CH3CH CHBr
C C
CH3CCH
Sodium acetylide Methyl iodide Propyne
HC C
CH
3
+ Nal
:
HC C Na⊕ + CH3l
:
Sodium acetylide Alkyl halide Propyne
C
C Na
⊕
+ RX
R + NaX
C
C
HCI + 6Ag + ICH
Heat
—
—
—
HC CH + 6AgI
Iodoform
Silver powder
33
PHARMACEUTICAL ORGANIC CHEMISTRY
2. By dehydrohalogenation of alkyl dihalides: Alkynes are obtained when vicinal dihalides (i.e.
two halogen atoms attached to adjacent carbon atoms) are treated with alcoholic caustic potash.
The reaction proceeds in two steps and under suitable conditions, the intermediate products, vinyl
halides, may be isolated.
Sodamide (NaNH2) can be used instead of potassium hydroxide in the second stage and the yields
are usually better.
3. By the reaction of sodium acetylide with primary alkyl halides: This method is used to convert
lower alkynes into higher ones. The reaction is limited to the use of primary halides because of the
tendency of secondary and tertiary halides to undergo a side reaction.
The mechanism of the reaction involves substitution of acetylide ion for halide ion by attack of
the acetylide on carbon. The acetylide ion is a strong base because it is a salt of the extremely weak
acid, acetylene.
4. By the dehalogenation of haloforms (chloroform or iodoform): Chloroform or iodoform upon
reacting with silver powder undergoes dehalogenation to form ethyne.
C H A P T E R 6 Alkynes
CHCOOK
—
—
CHCOOK
+ 2
HO
2
Electrolysis
CH
CH
+ 2C
O+ 2KOH
22
+ H
—
—
—
At cathode
At anode
5. By Kolbe’s electrolysis: Acetylene can be prepared by the electrolysis of concentrated solution of
the sodium or potassium salt of maleic acid or fumaric acid. Thus, ethyne and CO2 are evolved at
anode while H2 is liberated at cathode.
6. Synthesis from carbon and hydrogen: Ethyne can also be prepared by passing a stream of
hydrogen gas through an electric spark between two carbon electrodes. This is called Berthlot
synthesis.
6.8 PHYSICAL PROPERTIES OF ALKYNES
1. Physical state: First three members of alkynes series are gases, the next eight are liquids and still
higher ones are solids.
2. Colour and odour: These are all colourless and odourless except acetylene possessing a garlic
odour due to the presence of impurities like phosphine (PH3) and hydrogen sulphide (H2S).
3. Boiling point, melting point and density: The boiling points, the melting points, and densities
of simple alkynes are normally slightly higher than those of the alkanes and alkenes with the
corresponding carbon skeleton. This probably results from the fact that alkynes, because they contain
a triple bond, are compact, rod-like molecules. They can pack closer together in the solid and liquid
phases, which allow stronger van der Walls attractions between molecules. These properties show
the expected trend, i.e. increase with rising/molecular weights.
4. The alkynes are lighter than water.
5. Solubility: The alkynes have low polarity and are insoluble in water but they are quite soluble
in solvents of low polarity and are soluble in solvents such as ether, benzene and carbon
tetrachloride.
263
6.9 CHEMICAL PROPERTIES OF ALKYNES
Like alkenes, alkynes undergo electrophilic addition for the same reason, i.e. availability of the loosely
held p-electrons. Carbon–carbon triple bond is, however, less reactive than the carbon–carbon double bond
towards electrophilic reagents.
Alkynes also undergo nucleophilic addition reaction with an
electron-rich reagent in the presence of salts of heavy metals of
the type known to form complexes with multiple bonds.
Besides addition, alkynes undergo reactions that are due to
Whether the reaction is electrophilic or
nucleophilic (i.e. nature of reaction) depends
upon the initial attack of substrate.
REMEMBER
the acidity of a hydrogen atom held by triple-bonded carbon.
264
Vinylic carbocation
H + H
HCCX
H
⊕
. .
Θ
H + X
HCC
H X
H
HCC
YZ
or
YZ
Z Y
Y ZYZ
CC
+ Y
Z CC CC
CC
YZ
PHARMACEUTICAL ORGANIC CHEMISTRY
6.10 LOW REACTIVITY OF ALKYNES TOWARDS ELECTROPHILIC ADDITION REACTIONS
It has been found that alkynes are less reactive than alkenes towards electrophilic addition reactions. The
probable reasons for this low reactivity of alkynes may be explained as follows:
1. Formation of less stable carbonation: As we have already seen that the addition of hydrogen
halides to the triple bond occurs through electrophilic addition reaction and this addition follows
the Markownikoff’s rule. In such cases addition takes place via an intermediate carbocation, i.e.
vinylic carbocation.
Since vinylic carbocation is less stable than even the 1° alkyl carbonium ion, they should be formed
slowly and the reactivity of alkynes should be low.
2. Greater electronegativity of sp carbon than sp2 carbon: s-character is more in a sp carbon than
it is in a sp2 carbon. So the sp carbon is more electronegative than the sp2 carbon. Due to the greater
electronegativity of the sp carbon, the p-electrons are more tightly held by the carbon atom and
hence are less easily available for reaction with electrophiles.
3. Greater delocalization of p-electron cloud around the bond axis: As there is sideways
overlapping between the two p-bonds so that the four lobes of the two p-bonds merge into one
another to form a single electron cloud, which is cylindrically symmetrical about the internuclear
axis. Thus there is a greater delocalization of electrons in acetylene than it is in ethylene.
Hence acetylene should be more stable than ethylene and should be less reactive than ethylene.
6.11 ADDITION REACTIONS
Many of the reactions of alkynes are characterized by addition reactions of the triple bond. Depending upon
the conditions used, the additions can occur once or twice:
6.12 ADDITION OF HYDROGEN
Depending on the conditions and the catalyst employed, one or two molar equivalents of hydrogen will add
to a carbon–carbon triple bond.
C H A P T E R 6 Alkynes
2
3 32 3 3 3223
H ,Pt
Pt
CH C C CH H CH CH CHCH CH CH CH CH
+ → →
CH3CH
2
CH2CH
3
H
(97%)
H
Syn addition cis-3-hexene3-Hexyne
Ni2 B(P-2)
CCH2CH3 + H2
CH3CH2C
CC
Pd – CaCO
3
quinoline; or
Pd – BaSO
4
, S
cis-alkene
CCR
R + H
2
RR
HH
CC
6.12.1 Exhaustive Hydrogenation of Alkynes
When a platinum catalyst is used, the alkyne generally reacts with two molar equivalents of hydrogen to
give an alkane:
Palladium or Raney nickel can also be used as catalyst.
6.12.2 Controlled Reduction of Alkynes
However, reduction of an alkyne to an alkene can be accomplished through the use of special catalysts
or reagents. Moreover, these special methods allow the preparation of either cis- or trans-alkenes from
distributed alkynes.
1. Preparation of cis-alkene: Nickel boride (Ni2B), called P-2 catalyst, permits hydrogenation of an
alkyne in a cis
-form. Hydrogenation of alkyne in the presence of P-2 catalyst causes syn addition
of hydrogen to take place and the alkene that is formed from an alkyne with an internal triple bond
has the cis-conguration. The hydrogenation of 3-hexyne illustrates this method. The reaction takes
place on the surface of the catalyst and this accounts for the syn addition.
265
Metallic palladium deposited on calcium carbonate in the presence of lead acetate and quinoline
(known as Lindlar’s catalyst) or palladium deposited on barium sulphate conditioned by sulphur
can be used to prepare cis-alkene from disubstituted acetylenes.
2. Preparation of trans-alkene: An anti addition of hydrogen atoms to the triple bond occurs
when alkynes are reduced with lithium or sodium metal in liquid ammonia or ethylamine at low
temperature. This produces a trans-alkene.
266
CH3(CH2)
2
H
Li
C
2H5NH2
H
(CH
2)2CH3
4-Octyne trans-4-octene 52%
−78°C
C (CH
2)2CH3
CH3(CH2)2C
C
C
Br2/CCl
4
Br2/CCl
4
Propyne
trans-1, 2-
Dibromopropene
1,1,2,2-Tetra bromo
propane
Br
Br
CH
3
CCH
Br
Br
CH
3
Br
Br H
CC
CH3CCH
Cl + Cl
Cyclic halonium ion
+δ −δ
+ Cl
Cl
C
C
C
C
�
�
PHARMACEUTICAL ORGANIC CHEMISTRY
6.13 ELECTROPHILIC ADDITIONS
Electrophilic addition reactions of alkynes take place in two stages. By a proper choice of reaction
conditions, the addition can usually be stopped after the addition of one mole of the reagent. In all these
addition reactions, Markownikoff’s rule is normally followed.
As we shall see, the intermediary of cyclic cation results because the olenic intermediate is invariably
trans. Some of the important examples of electrophilic additions are given below.
1. Addition of halogens: Chlorine and bromine add on to alkynes to form dihaloalkenes or tetra halo
alkanes.
Cl
CH ≡ CH CHCl = CHCl CHCl2 — CHCl
Acetylene Acetylene dichloride Acetylene tetra dichloride
Mechanism: The mechanism of the reaction involves electrophilic addition and proceeds through the
formation of cyclic halonium ion, as shown below:
2
Cl
2
2
If instead of Br2/CCl4, bromine water is used, the reaction stops after the addition of 1 mole of
bromine. During this reaction, the reddish brown colour of Br2 is decolorized and this reaction is
used as a test for unsaturation, i.e. for double and triple bonds.
C H A P T E R 6 Alkynes
Θ
Cl
Cl
+
::
::
Cl
Cl
trans-1, 2-dichloro alkene
Tetra chloro alkane
CC
CCl2CCl
2
CCl CCl
+ Cl
2
Similarly,
C C
+δ −δ
Θ
H
+
Cyclic cation
⊕
X
X
Carbonium ion
H
X
CC
R
R
CH2RRCX
2
CH2XRC
:
Θ
X
+δ −δ
HX
:
Θ
:
R + : X :
C
RC
R + H
RCCX
The above mechanism is in keeping with the fact that
addition of halogens proceeds in anti manner.
2. Addition of halogen acids: On treatment with a
halogen acids, alkynes rst form vinyl halides followed
by alkylidene halides. The second molecule of halogen
acid adds according to the Markownikoff’s rule.
The reaction is catalysed by mercury salts.
Mechanism
267
REMEMBER
Peroxides have the same effect on the addition
of HBr to alkynes as they have on alkenes.
It may be noted that due to greater electronegativity
of halogen, one might expect the formation of a
carbonium ion during the addition of second mole of
hydrogen halide to alkenyl halide.
3. Addition of hypohalous acids: When treated with an
aqueous solution of a halogen, a product corresponding
to the addition of two moles of hypohalous acid is rst
formed. This gets decomposed immediately to give a
dihalocarbonyl compound. For example,
Learning Plus
Presence of two or more OH groups on one carbon atom make it unstable and the molecule
loses H2O molecule. Two exceptions to this rule
are chloral hydrate CCl3CH(OH)2 and the other is
carbonic acid:
HOCOH
O
PHARMACEUTICAL ORGANIC CHEMISTRY
268
O :
H
H
:
+
−H
⊕
−H
2
O
Cl
2,H2
O
Cl
+
Cl
HO
C
C
⊕
HO C
H
H
C Ð Cl
HO OHC
Cl ClC
OC
Cl ClC
C
C
H+, Hg
2+
CH
CH + H
2
O
Vinyl alcohol
HOH
HHCC
Acetaldehyde
HHO
HHCC
Mechanism
The reaction takes place as follows:
4. Addition of water (hydration of alkynes): A molecule
of water adds to the carbon–carbon triple bond in the
presence of sulphuric acid and mercuric sulphate to
form a vinyl alcohol,
1
— C C — OH
1
which readily
tautomerizes to the corresponding carbonyl derivative.
Addition of water to acetylene forms acetaldehyde
while other alkynes from ketones.
Learning Plus
Only C2H2 on addition of water gives aldehyde
and rest of all alkynes gives ketones.
C H A P T E R 6 Alkynes
H+, Hg
2+
H+, Hg
2+
CH
CH + H
2
O
RCC
R + H
2
O
Vinyl alcohol
HOH
HHCC
H OH
R RC C
Acetaldehyde
HHO
HHCC
HHO
RRCC
Hg
2+
Hg
2+
Hg
2+
+
H
⊕
Hg
⊕
−H
⊕
−Hg
2+
⊕H
+
Tautomerises
(enol)
(II)
(II)
(III)
(I)
:
H
H
: O
HOCC
C
C
C
C
C
C
HO
H
C
C
Hg
⊕
HHO
C
⊕
C
Hg
⊕
HO
C
C
H
OC
C
H
Abbreviated as Sia2BH, i.e. BH
2
CH3CH
3
CH3CH CH
This is also an electrophilic addition reaction in which mercuric ion forms a complex with the
triple bond. The reaction then proceeds as shown below:
269
Protonation of the intermediate (I) followed by loss of Hg2+ ion gives the enol (II), which
subsequently tautomerizes readily to the carbonyl compound (III).
5. Hydroboration oxidation: When a terminal or a nonterminal alkyne is treated with a sterically
hindered borane such as disiamyl borane,
a vinylic borane is obtained.
270
R CC
H + Sia
2
BH
R CC
R + Sia
2
BH
BSia
2
H
R CC
H
BSia
2
RR
H
CC
H2O2,ΘOH
Aldehyde or
ketone
(enol)
H
CCB Sia
2
H
CCOH
H
H
CCO
CH2CH
3
BSia
2
H
CH
3
COOH
CH
3CH2
3-Hexyne
CH3CH
2
CCCH
2CH3
+ Sia2BH CC
CH2CH
3
HH
CH
3CH2
CC
RCH2CH B
Gem-diborane derivativeVinylic borane
HB
RCH CH
B B
RCC
H + B
2H6
PHARMACEUTICAL ORGANIC CHEMISTRY
These vinylic boranes can then be oxidized with H2O2 in basic solution to produce aldehydes or
ketones (via the enol):
Overall, this reaction provides an anti-Markownikoff’s addition of H and OH to an alkyne,
and thus it is a convenient synthesis of aldehydes.
Hydroboration of an alkyne with an internal triple bond (nonterminal alkyne), followed by
protonolysis of the vinylic borane with acetic acid provides another method for synthesizing a
cis-alkene. An example is the preparation of cis-3-hexene from 3-hexyne.
When terminal alkynes are treated with diborane, addition occurs twice, producing a gemdiborane derivative. It is not possible to stop the addition to the state of vinylic borane.