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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 5 Alkenes
Conc. H2SO
4
∆
HOH
C C
C
C
+ H
2
O
95% H2SO
4
443 K
Ethyl alcohol Ethylene
CH
3
CH2OH
CH
2
+ H
2
O
CH
2
60% H2SO
4
373 K
Isopropyl alcohol Propylene
CH3CHOH CH
3
CH3CH CH2 + H2O
20% H2SO
4
355 K
tert-Butyl alcohol
Isobutylene
CH3COH
CH
3
CH
3
CH3CCH2 + H2O
CH
3
....Fast
H
HCC
HHH
H + H
+
O
H
O
+
H
H
H H
HCCH
5.5 METHOD OF PREPARATION OF ALKENES
The alkenes, particularly the lower members, are obtained on large scale by the cracking of higher fraction
for their preparation in the pure state. Most of these methods involve various elimination reactions and
some of these important ones are discussed below.
5.5.1 Dehydration of Alcohols
Alkenes are produced by dehydrating alcohols either by heating with an acid catalyst or by passing over
the alcohol vapours over heated alumina, Al23. The reaction for the former is discussed below:
The ease of dehydration of different types of alcohols follows the order:
Tertiary > Secondary > Primary
This is clear from the reaction conditions for dehydration of different types of alcohols as shown below:
221
tertiary as compared to secondary and primary alcohols.
Mechanism
The mechanism of acid-catalysed dehydration of an alcohol is as depicted below.
1.
222
⊕
H
H
HCC
HHH
H
O
+
H
HH
HCCH + H2O ....Slow
(rate determining)
HCC
HHH
H
HH
HCCH + H+ ....Fast
+
2.
3.
The mechanism can be explained in three steps:
1. Alcoholic group gets protonated by accepting a proton from an acid. This is a fast step.
2. The protonated alcohol in Step 2 dissociates into water and carbonium ion. This is a slow step and
hence the rate-determining step.
3. The proton gets split up to form an alkene as indicated in Step 3. This is also a fast step.
Relative stabilities of carbonium ions
1. Relative stabilities: The relative reactivities of alcohols with respect to their dehydration to alkenes
The stability of a charge system can be increased by dispersal of its charge (law of physics). Therefore,
We know that as compared to a hydrogen atom, an alkyl group has positive inductive effect, which
2. Ease of formation: As already discussed the order of dehydration of alcohols is tertiary > secondary
The ease of formation of carbonium ions, as shown below:
been established that the order of stability of carbonium ions is as follows:
Tertiary > Secondary > Primary > C+H
3
the carbonium ion.
tends to release electrons. The alkyl groups attached to electropositive carbon atom of carbonium ion
tend to release electrons, thereby reducing or dispersing its positive charge. This dispersal of positive
charge stabilizes the carbonium ions. For the same reason, a tertiary carbonium ion is more stable than
a secondary carbonium ion, which in turn is more stable than the primary carbonium ion, while C+H3
(methyl carbonium) ion with no alkyl group attached is the least stable. The stability of a carbonium
ion depends upon the tendency of the attached groups to release or withdraw electrons.
> primary. The ease with which alkenes are produced is determined by the stability of carbonium ion
formed during dehydration.
Tertiary > Secondary > Primary > C+H
3
C H A P T E R 5 Alkenes
Reactant oxygen
having full +ve change
Transition state carbon
and oxygen have partial
+ve charge
R .... OH
2
δ+δ
+
Products carbon
has full +ve charge
R+ + H2O
R
⊕
OH
2
⊕
Butanol
(n-butyl alcohol)
CH
3CH2CH2CH2
OH + H
2-Butene
(unexpected but chief product)
1-Butene
(expected product)
CHCH
3
CH3CH
CH
2
CH3CH2CH
⊕
CH3CH2CHCH2OH + H
2-Methyl-1-butanol
2-Methyl-2-butene
(unexpected but chief product)
2-Methyl-1-butene
(expected product)
CH
3
CH
3
CH3CH CCH3
CH
3
CH
3
CCH2 CH
2
This order also explains their relative stabilities. The more stable a carbonium ion the more easily it is
formed. The following reaction explains the stability of carbonium ion when alcohol is protonated during
dehydration:
+H2, which results in breaking of the carbon–oxygen bond. With the loss of a
molecule of water, the positive charge that was initially on the reactant, i.e. on oxygen atom moves to the
originally on oxygen is now divided between carbon and oxygen. Electron-releasing groups would
tend to disperse the partial positive charge developing on the carbon, thus stabilizing the transition state.
The stabilization process of transition state lowers its E
(energy of activation) and increases the speed
act
carbonium ions in transition state.
223
Orientation in dehydration of alcohols
Most often it has been found that on dehydration of alcohols, an unexpected alkene is produced, which
skeleton is changed.
1.
2.
224
+
CH
3
æ CH2 æ CH2 æ CH2 æ OH
2
+
CH
3
æ CH2 æ CH2 æ CH
2 CH3
æ CH2 æ CH CH
2
æ
æ
Butene-1
(Expected product)
++
++
rearranges
CH
3
æ CH2 æ CH2 æ CH2OH
2 CH3
æ CH2 æ CH2 æ CH2 + H2O
CH
3
æ CH2 æ CH2 æ CH2
CH3
æ CH2 æ CH æ CH
3
+
æ H
+
CH3 æ CH2 æ CH æ CH3
CH3
æ CH CH æ CH
3
æ
æ
2-Butene(stable)
CH3 æ CH2 æ CH æ CH2OH
2 CH3
æ CH2 æ CH æ CH
2
+ H
2
O
—
CH
3
—
CH
3
2-Methyl-1-butyl
carbonium ion
CH
3
æ CH2 æ CH æ CH
2 CH2
æ CH2 æ C æ CH
3
—
CH
3
—
CH
3
2-Methyl-2-butyl carbonium ion
(2º carbonium ion)
More stable
(1º carbonium)
Less stable
rearranges
The formation of an unexpected alkene during the dehydration can only be explained by considering a
rearrangement of carbonium ion, which is being explained in the following article.
Rearrangements of carbonium ions
Butene-2 is produced from n-butyl alcohol by elimination of a molecule of water from protonated alcohol, thus
forming n-butyl carbonium ion. Loss of a proton from the carbon adjacent to the positive carbon was expected
is produced from the carbonium ion is not the same carbonium ion which was initially formed from alcohol.
A similar situation exists for a number of other dehydration reactions.
The above fact can only be explained by considering a rearragement of carbonium ion. A carbonium ion
can rearrange to form a more stable carbonium ion. For example, n-butyl alcohol yields n-butyl carbonium
ion, which rearranges to form secondary butyl carbonium ion a more stable carbonium ion by shift of a
hydrogen ion to yield mainly 2-butene.
more stable one.
C H A P T E R 5 Alkenes
–C–C–
—
—
—
H
–C–C–
—
—
H
+
–C–C–
—
—
—
H
Hydride shift
–C–C–
—
—
—
R
–C–C–
—
—
R
+
–C–C–
—
—
—
R
Alkyl shift
CH –CH–C–C–H
32
—
H—H
—
H
+
CH –CH–C–C–H
32
—
H—H
+
—
H
1,2-shift
n-Butyl
Less stable
(1º carbonium ion)
sec-Butyl carbonium ion
More stable
(2º carbonium ion)
EtheneBromo ethane
∆
CH3CH
2
Br + KOH
alc.
CH
2
+ K Br + H
2
O
CH
2
Mechanism of rearrangement:
pair of electrons from an adjacent carbon atom to the carbon atom bearing the positive charge. The carbon
of electrons, it is known as a hydride shift while the migration of an alkyl group is knows as alkyl shift. This
rearrangement in which the migrating group shifts from one atom to the next atom is known as 1,2-shift.
stable carbonium ion. For example, in n-butyl carbonium ion 1,2-shift of a hydrogen takes place as shown
below to form the more stable carbonium ion.
225
The stability of carbonium ion changes in the following order:
5.5.2 Dehydrohalogenation of Alkyl Halides
Elimination of the atoms of hydrogen from the adjacent carbon atoms of an alkyl halide is called
dehydrohalogenation. Dehydrohalogenation is generally carried out by heating an alkyl halide with a strong
base such as sodium ethoxide or a concentrated alcoholic solution of potassium hydroxide. For example:
1.
Tertiary > Secondary > Primary
226
Propene1-Chloropropane
CH
3CH2CH2
Cl + KOH
alc.
∆
CH
2
+ KCl + H
2
O
CH
3
CH
∆
H
H
H
H
H
H
Br
–
δ
–δ
Transition stateBromoethane
H
H
H
CC
H
H
Br
O
HO
CC
H
H
H
H
+H
2
O + Br
2.
–
–
better nucleophile than the
ion, preferentially bring about hydrolysis of alkyl halides to alcohols. As
elimination and substitution reactions of alkyl halides compete with each other, therefore, the use of strong
bases at high concentrations and high temperature tend to favour elimination reaction over the substitution
reaction.
Mechanisms of dehydrohalogenation reaction: Two important mechanisms of elimination reactions are
being discussed here. These are E2 and E1.
E2 (elimination, bimolecular or second-order) reaction
The dehydrohalogenation of a majority of primary alkyl halides occurs by E2 mechanism. Kinetic studies of
this reaction reveals that the reaction follows second-order kinetics, i.e. rate of dehydrohalogenation reaction is
proportional to the concentration of both, the alkyl halide and the base. Thus,
Rate [Alkylhalide] [Base]
this means that E2 reaction should be a one-step process and occurs through a transition state. As the hydroxide
ion (base) begins to abstract a -hydrogen ion (hydrogen attached to carbon atom, which is next to the carbon
atom carrying the halogen atom), the carbon hydrogen bond begins to break, the carbon–carbon π-bond starts
forming and the leaving group (i.e. the halogen atom) starts leaving the α-carbon atom, taking with it electron
pair forming the carbon halogen bond. This is explained below with the help of dehydrohalogenation of
bromoethane:
2 mechanism (E for elimination, 2 for second order or bimolecular).
2 reaction occurs more readily if the leaving groups (i.e. H and X) are trans
with respect to each other rather than cis.
For example, trans-2-bromo-2-butene forms 2-butyne more readily than the cis-isomer.
C H A P T E R 5 Alkenes
2-Butyne
H
3
C
CH
3
Br
trans-2-bromo-2-butene
fast slow
H3CCH
3
HO
cis-2-bromo-2-butene
HBr
CH
3
CC
HO + H
CC
H3C
CC
C
C
H
C
C
H
+
+
X
X
Slow
(Carbonium)
CC
H
+
+H :
B
Fast
CC
:
B
E1 (elimination, unimolecular or first-order) reaction
concentration by a different mechanism known as E1 mechanism (E for elimination, 1 for unimolecular).
This mechanism occurs through a two-step process in which the rate-determining step involves only one
the base to form the alkene. The complete mechanism may be depicted as shown below:
Step 1. Dissociation of a halide to a carbonium ion by splitting a halogen anion.
(unimolecular elimination):
E
1
227
Step 2
dissociation of a halide ion. As only one molecule is involved in the formation of carbonium ion, it is
called E1 elimination.
l reaction depends upon the stability of the carbonium ion formed
during the reaction. El reactions have no stereochemical preferences since these involve carbonium ions,
which are planar species.
Orientation of E2 and E1 reactions (Saytzeff’s rule)
highly substituted alkene (i.e. having lesser number of hydrogen atoms on the doubly bonded carbon atoms)
is the major product of dehydrohalogenation. This generalization is known as Saytzeff’s rule.
228
H–O :
H–C
—
H
—
H
—
H
—
Cl
C–C–CH
3
—
H
—
H
(i)
: OH
(i )i
Alc. KOH
(i)
(i )i
CH
2
� CHCH2CH3 + KCl + H2O
1-Butene (20%)
CH
3
– CH � CH –CH3 + KCl + H2O
2-Butene (80%)
2-Chlorobutane
∆
Reactant Product Relative rates
CH
3
� CH2 � Br CH2
�
�
CH2 1.0
CH
3
� CH2 � CH2Br CH3 � CH
�
�
CH2 3.3
CH
3
� CH � CH
3
(CH3)C
�
�
CH
2
9.4
�
Br
(CH
3)3
C Br
(CH3)2C
�
�
CH
2
120
We know that greater the number of substituents across the double-bonded carbon atoms the more
stable is the alkene. Thus, in almost all the E1 and most of E2 reactions, the more highly substituted alkene
predominates.
2
may be seen that in the transition state, the double bond between carbon atoms has begun to form and,
E
be, therefore, formed more easily. As such the more stable alkene would also be formed.
Reactivities of alkyl halides in dehydrohalogenation
Saytzeff’s rule, any alkyl halide that gives a more stable (i.e. more substituted) alkene must
undergo dehydrohalogenation faster than the one with which gives a less stable (i.e. less substituted) alkene.
Tertiary (3º) > Secondary (2º) > Primary (1º) Alkyl halides
This is because branching at the carbon carrying the halogen atom increases in going from primary to
b-hydrogens
which can be abstracted by
H ions and, therefore, higher will be the probability of attack. Moreover, the
more branched alkyl halide leads to more branched carbonium ion and, therefore, more stable alkene.
Thus the stability of alkenes, not only determines the orientation of dehydrohalogenation but also the
reactivity of an alkyl halide towards elimination.
For example:
and would
act
has been found that for alkyl halides with a given alkyl group, the rate of dehydrohalogenation varies with
the nature of X in the order:
I > Br > Cl > F
C H A P T E R 5 Alkenes
CC
H3CCCCH
3
Lindlar’s catalyst or
Nickel boride
H
3
C
H
CH
3
H
H3C
CH
3
H
H
Na or Li
Liquid NH
3
2-Butyne
trans-2-butene
H
3
C
CH
3
CC
CC
⊕
+ 2Na
O
H
2
C
C
O
⊕
Na
⊕
Na
O
H
2
C
C
O
O
H
2
C
C
O
O
H
2
C
C
O
2H2O 2H + 2OH
⊕
H2CCOO
:
H
2
C
O
:
H2CO
:
H
2
C
O
:
− 2e
CH
2
+ 2CO
2
CH
2
C
O
C
O
C
O
5.5.3 By Partial or Selective Reduction of Alkynes
The controlled reduction of an alkyne to the alkene stage may brought about by catalytic reduction
reactions.
For example, catalytic reduction of 2-butyne in the presence of a palladium catalyst supported over
prepared palladized charcoal or nickel boride gives almost exclusive cis-2-butene.
trans-2-butene is the
predominant product.
229
5.5.4 Electrolysis of Sodium or Potassium Salts of Dibasic Acids (Kolbe’s Electrolytic Method)
electrolysed, an alkene (ethylene) is produced. Thus,
Sodium succinate
At anode:
230
2H + 2e H
2
⊕
CH3CH2OH
∆
HH
Br Br
H
CC
H + Zn
H
HH
H + ZnBr
2
CC
Br Br
+ I
2
+ 2NaBr
+ 2NaI
Acetone
CC
CC
Br
Br
+
2NaI
I :
+ I Br + Br
CC
CC
I + BrI I
2
+ Br
At cathode:
5.5.5 Dehalogenation of Vicinal Dihalides
When heated with zinc in alcoholic solution, a vicinal dihalide (i.e. a dihalide containing two halogens on
adjacent carbon atoms) gets dehalogenated to produce an alkene.
Dibromides also undergo the loss of a molecule of bromine (debromination) when they are treated with
a solution of sodium iodide in acetone.
Debromination by sodium iodide takes place by an E2 mechanism similar to that for dehydrohalogenation.
5.6 PHYSICAL PROPERTIES OF ALKENES
2H4 to C4H6 are
soluble in alcohol but insoluble in water; lower members of this series acts as general anaesthetics.