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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
Saturated
compound
+
XY
Alkene Attacking
reagent
CC
X
Y
CC
HH
Alkane
+ H
2
Alkene
Pt or Pd at 298 K
or Ni at 523−573 K
CC CC
Ni or Pd
or Pt
Ethylene
(Ethene)
Ethane
H
3
CCH
3
CH2 + H2
H
2
C
Ni or Pd
or Pt
Propylene
(Propene)
Propane
CH
3
CH2CH
3
CH2 + H2
CHCH
3
5.7 CHEMICAL PROPERTIES OF ALKENES
The characteristic feature of alkenes is the presence of a carbon double bond.
REMEMBER
Alkenes are more reactive
than alkanes because of the
presence of π-bond. π-bond is
weak bond and easily undergo
cleavage.
The strong (σ) bond and a weak (π) bond together form the double bond. The
π-electrons are loosely held up and are made available to the added reagent,
thus forming open-chain saturated compounds. The σ-bond remains intact
in such reactions. The typical reactions shown by alkenes are electrophilic
seek electrons and these are readily made available by loosely held electrons.
Apart from addition reactions, alkenes also exhibit substitution (due to presence of alkyl group), oxidation
and polymerization reactions.
5.7.1 Addition Reactions
reagent.
231
Addition of hydrogen (hydrogenation)
Examples:
1.
2.
Hydrogenation of alkenes is an exothermic reaction. The heat evolved when 1 mole of unsaturated
compound is hydrogenated, which is known as heat of hydrogenation.
232
Catalyst
−
(104 kcal)
∆H =
HH
(2 × 87 kcal)
(π-bond
= 40 kcal)
+
CC CC
H H
30 kcal
trans-2-butenecis-2-butene
CH
3
æ CH CH æ CH
3
28.6 kcal
CH
3
æ CH CH æ CH
3
27.6 kcal
æ
æ
æ
æ
CH3 æ CH2 æ CH CH
2
CH3 æ CH2 æ CH2 æ CH CH
2
CH3 æ CH2 æ CH CH æ CH
3
3.33 kcal
cis 28.6 kcal
trans 27.6 kcal
cis 28.6 kcal
trans 27.6 kcal
30.3 kcal
CH
3
æ CH CH æ CH
3
æ
æ
æ
æ
æ
æ
æ
æ
Heat of hydrogenation and stability of alkenes
Hydrogenation of an alkene involves the breaking of a weak π-bond (absorption of about 40 kcal), a H–H
bond (absorption of 101 kcal formation of two C–H bonds (release of about 2 × 87 kcal). The operation
brings about liberation of about 30 kcal. For example
:
compound is hydrogenated. The heat of hydrogenation for almost all alkenes is nearly 30 kcal for each
double bond but the actual value varies slightly from member to member.
Stability of alkenes: Heat of hydrogenation provides us with important information regarding stabilities
of unsaturated hydrocarbons. For example, the heat of hydrogenation for two isomeric butenes are as
follows:
As each isomer consumes 1 mole of hydrogen and produces the same product, n-butane on hydrogenation,
trans-isomer evolves 1 kcal less than the cis-isomer or in other words trans-isomer
contains 1 kcal less energy than the cistrans-isomer is more stable than the cis-isomer.
trans-isomer
as compared to cis-isomer is also due the fact that the larger substituents in a disubstituted ethylene are drawn
farther than in the cis-isomer, which results in less crowding and less van der Waals strain.
Heat of hydrogenation data reveals that the stability of an alkene is also affected by the position of the
double bond in the carbon chain. For example:
However, it should be kept in mind that the greater the number of alkyl groups attached to the doubly
bonded carbon atoms, the more will be the stability of the alkene.
The order of stability of alkenes are depicted as below:
R2C CR2 > R2C CHR > R2C CH2 > RCH CHR > RCH CH2 > CH
to lower the energy of activation (E
provides an alternate shorter route for attainment of the reaction.
act
2 CH2
C H A P T E R 5 Alkenes
Ethylene
Ethylene dichloride
Cl Cl
H2C CH2 + HCl
H
2
C CH
2
Propylene
Propylene dibromide
H
3
C CH CH
2
+ Br
2
BrBr
H3C HC CH
2
CH
2
+ Br
Br
+δ −δ
⊕
Br
slow
H2C
CH
2
+ Br
H2C
H2CCH
2
Br
Br
⊕
H2CCH
2
Br
+
Br
5.7.2 Addition of Halogens
the most reactive and iodine the least.
The reaction is carried out in presence of an inert solvent, such as CCl4 at room temperature, preferably
in cold and in the absence of light. The reaction is used for detection of a double bond as on mixing
together bromine solution in CCl4 and alkene solution in CCl4, the colour of bromine disappears. All
unsaturated hydrocarbons respond to this test.
The two-step ionic mechanism
233
As the halogen molecule comes closer to the alkene molecule, the π-electron cloud of the double bond
begins to repel the electron cloud the halogen molecule. As a result, the electrons of the halogen molecule get
displaced towards that halogen atom, which is away from the alkene molecule. This results in polarization
of the halogen molecule; the atom nearer the alkene getting a small positive charge and the other getting
a small negative charge.
The positive halogen atom of the polarized halogen molecule is immediately abstracted by alkene to form
an intermediate carbonium and a negative halide ion. This is the rate-determining step.
mechanism can be represented in a simple way as:
The electrophilic addition mechanism explained above is supported by the experimental facts. The
carbonium ion produced as a result of electrophilic attack should not only react with Br– ion but also
234
chloride besides ethylene dibromide, it also produces bromochloroethane and
compound also takes place apart from ethylene dibromide.
The combination of carbonium ions with other ions are shown below:
H2CCH
2
CH
2
Br
2
BrH2C
+
Br
−
Cl
−
I
−
CH2Br æ CH2Br
1,2-Dibromoethane
CH
2
Br æ CH2Cl
2-Bromochloroethane
CH
2
Br æ CH2Cl
2-Bromo-1-iodoethane
æ
Limitations of the above mechanism
The above mechanism involving simple carbonium ion intermediates does not explain the following two
points:
1. Stereochemistry of addition of halogens:
always give trans-dihalides (i.e. the two halogen atoms add to the planar alkene from the opposite
sides). According to the above mechanism, the addition of second halogen to the planar carbonium
2. Nonformation of rearranged products: Reactions involving carbonium ion intermediates are
accompanied by formation of rearranged products wherever possible. But there is no formation of
rearranged products during the addition of halogens to alkenes.
To overcome these two limitations of the ionic mechanism it has been suggested that the addition of
halogen to alkenes involve the formation of a cyclo-halogens ion. This mechanism is not discussed here
keeping in view the limitations of the course.
5.7.3 Addition of Halogen Acids
HX
+ HX
CC CC
Ethylene Ethyl chloride
CH2 + H
ClCH
2
CH2Cl
CH
3
Br
Propylene
Isopropyl bromide
CH3CH
CH
2
+ H
Br
CH3CH
CH
3
Learning Plus
Ethylene dibromide (EDB)
was used as pesticide and
soil fumigant. But it was
banned in 1984 in the United
States.
235
C H A P T E R 5 Alkenes
The order of reactivity of halogen acids is:
HI > HBr > HCl
Mechanism of the addition of halogen acids to symmetrical alkenes
The addition of a highly polarized reagent such as to H+d X–d
which is also the rate-determining step, involves the transfer of a proton from the halogen acid to the
alkene to form a carbonium ion. The second step involves the rapid nucleophilic attack by the halide ion
to complete the addition.
slow
⊕
H
2
C
CH
2
+ H
−δ
+δ
X
HH
H
HH + XCC
fast
HCC
H
H
+
H
X + H
XH
HH
HHCC
gives rearranged products. Thus the formation of rearrangement products
undoubtedly explain the formation of carbonium ion as an intermediate in
the addition of halogen acids to alkenes.
5.7.4 Addition of Hydrogen Halides to Unsymmetrical Alkenes: Markownikoff’s Rule
Br, in principle, can give two
products, i.e. 1-bromo propane and 2-bromo propane.
Br
(Minor) (Major)
CCH
3CH2CH2
Br + CH3CHCH
3
CH2 + HBr
CH
3
CH
However, in reality, 2-bromo propane is almost exclusive product.
Mechanism of Markownikoff’s addition
Learning Plus
Like the vicinity, vicinal comes
from the Latin word Vicinals,
which means ‘neighbouring’.
236
H3CCHCH2 + HBr
⊕
⊕
+δ +δ
Br
More stable
CH
3CH2CH2
CH3CH2CH2Br
Br
Br
Less stable
H3CCHCH
3
H3CCHCH
3
2-Methyl prop-1-ene
or Isobutylene
Tertiary butyl
carbonium ion
CH
3
H3CC
CH2 + H
l
+
CH
3
H3CC
CH3 + l
−
Tertiary butyl iodide
I
CH
3
H3CC
CH
3
CH
3
2-Methyl-but-2-ene
H3CCCH
CH
3
+ H
l
CH
3
+
H
3
CCCH
2
CH3 + l
−
2-Iodo-2-methylbutane
I
CH
3
H3CCCH
2
CH
3
(C6H5CO)2O
2
CH
2
CH
3
CH2Br
CH3CH CH2 + HBr
Propylene n-Propyl bromide
stabilizes the positive ionic charge on the attached carbon atom, so that the nucleophile will preferably
attack on that very carbon.
Thus addition of a highly polarized reagent such as H
⊕
manner that the addition of H
ion gives the more stable carbonium ion regardless of whether intermediate
+d
– Br–d to unsymmetrical alkene takes place in a
is an open carbonium ion or a bridged ion.
ome more examples are given below:
1.
2.
Thus Markownikoff’s rule may also be stated in the following form: Addition to ethylene double bond
involves the formation of a more stable carbonium ion intermediate.
Anti-Markownikoff’s addition
or anti-Markownikoff’s rule. For example:
peroxide to change the course of addition of HBr against the Markownikoff’s rule is known as the peroxide
C H A P T E R 5 Alkenes
O2R
∆ or h
ν
R ORO
R
RHO
O
H
+
Br
+
Br
H2C
2° Free radical
(more stable)
CH
2
Br CH CH
3
°
CHCH
3
Br
+
1-Bromo propane
Br
°
CH2CH
CH
3
+ H
Br
CH
2
CH2CH CH
3
H
Br
+
Br
°
Br Br
Br
°
Br
°
+
H3CCC
CH
3CH3
CH
3
C
H
3
C
CH
3
C
CH
3
CH
3
+
°°
effect
24 or H2
237
Mechanism:
place by two different mechanisms that we have already discussed and anti-Markownikoff’s addition by a
free radical mechanism. Peroxide initiates the free radical reaction mechanism.
Chain-initiating steps
1.
2.
Chain propagating steps
3.
4.
Chain terminating steps
Step 1 is the simple homolytic cleavage of peroxide molecule to produce two alkoxy free radicals. The
oxygen–oxygen bond of peroxides is weak and such reactions are known to occur readily.
Step 2 of the mechanism, abstraction of a hydrogen atom by the alkoxy radical, is an exothermic reaction
of the hydrogen atom and has a low E
Step 3
because a more stable secondary free radical is produced. Had the bromine attacked propene at the central
5.
R — O — O —R 2R — O°; DH — + 35 kcal/mol
.
act
R – O° + HBr ROH + Br
°
238
1° Free radical
H
2
CCH
Br
°
°
CH
3
Br ° + H2C
Br
+ H
2
CH —CH
3
BrH2C — CH — CH
3
CHCH
3
°
Ethylene
Ethyl hydrogen sulphate
cold
conc. acid
OSO3HCH
3
CH
2
CH
2
CH2 + HOSO2OH
OSO3H
CH
3
CH
3
CH
CH
2
+ H2SO
4
CHCH
3
H3C CH CH
3
OSO3H
H
3
C CH CH
2
HOH
+
+
+
H3C
CH
CH
3
OSO
2
O
OH
SO
2
boil
Ethanol
CH
3CH2
OH + H2SO
4
CH
2
CH
3
OSO2H + H2O
carbon atom, a less stable primary free radical would have been the result and this reaction would have
had a higher (E
act
).
Step 4 of the mechanism is simply the abstraction of a hydrogen atom from hydrogen bromide by the
radical produced in Step 3. The hydrogen abstraction produces a bromine atom that can bring about Step 3
again. Then Step 4 occurs again as a chain reaction.
Steps 5 and 6 are chain terminating steps brought about by coupling of the free radicals.
Cl, this is probably due to the fact that H Cl bond is stronger than H Br bond and is
not broken homolytically by the alkoxy free radicals. As such, free radical addition of HCl to alkene
Br bond and can be broken
molecules rather than add to the ethylenic bond.
5.7.5 Addition of Sulphuric Acid
Cold concentrated sulphuric acid forms addition product with alkenes, producing alkyl hydrogen sulphate:
Mechanism
Alkyl hydrogen sulphates on hydrolysis yield alcohols:
C H A P T E R 5 Alkenes
boil
2-Propanol
OSO
3
H
CH
3
CH
CH3 + H2O
CH
3
CHOH
CH3 + H2SO
4
H
⊕
2-Methyl propene 2-Methyl-2-propanol
C
OH
H3C
C
H
3
C
CH
2
+ H2O
H3C
H
3
C
CH
3
slow
2-Methyl propene
H3CC
CH
3
CH2 + H
H3CC
CH
3
CH
3
⊕
⊕
O
H
H
fast
H3CC
CH
3
CH
3
O
H
H3CC
CH
3
CH
3
OH
H3C
C
CH
3
CH3 +
2-Methyl-2-propanol
⊕
⊕
−
H
⊕
H
C
2
+ H2O
X Cl or Br
+
+ HX
Halohydrin
CC
XOH
CC
XX
vic-Dihalide
5.7.6 Addition of Water (Hydration of Alkenes)
directly in the presence of mineral acids to form alcohols. The addition takes place in accordance with
Markownikoff’s rule. For example:
Mechanism of hydration
The addition water to alkene in the presence of dilute acid occurs in two steps as discussed below:
239
1.
2.
5.7.7 Halohydrin Formation: Addition of Hypohalous Acids
4), the major product of the
overall reaction is not a vic-dihalide, but rather it is a halo alcohol called a halohydrium
of the solvent become reactants too.
240
CC
+
+
XX XCC
X
⊕
+
O
H
CC
X
C
O
X
H
H
H
CC
OH
X
C
⊕
–H
⊕
⊕
X2 + H2O
H
O
X + H
X
+ OHCC
X
+
C
C
OHX
+δ −δ
⊕
X
+
OH CC
O
X
H
CC
⊕
Mechanism
1.
2.
halonium ion
second step, water acts as the nucleophile and attacks the carbon of the halonium ion from the back side.
The three-membered ring opens, and a protonated halohydrin is produced or the halohydrin itself.
is the cationic part of the hypochlorous acid and on this basis, its additions to alkene can be rationalized.
5.7.8 Ozonolysis
The cleavage of the carbon–carbon double bond by ozone is completed in two stages:
1. Addition of ozone across the double bond to form an ozonide and
2. Hydrolysis of the ozonide to yield two smaller products (cleavage products).
Experimentally, alkene is dissolved in some inert solvent like carbon tetrachloride and ozone gas is
the presence of a reducing agent.