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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5852_Библиотеки_им_академика_М_И_Перельмана.pdf
C H A P T E R 5 Alkenes
CC CO + CHOCC CC
O
3
in CCl
4
OO
O
OO
O
Molozonide Ozonide
Cleavage products
(aldehydes and ketones)
H2O, Zn
(II)
O
O
O
⊕
(III)
O
O
O
⊕
(IV)
O
O
O
⊕
(I)
O
O
O
⊕
CC CC
O
3
Molozonide
(Intialozonide)
OO
O
O
O
O
⊕
Ozonide
CC
O
O
O
CC
O
O
O
+
C
O
C
OO
C
O
C
OO
⊕
Mechanism of ozonization
the other oxygen is electron rich and behaves as a nucleophile.
241
The initial ozonide, which is unstable, then rearranges spontaneously to form more stable ozonide. This
rearrangement is thought to occur through dissociation of the initial ozonide into reactive fragments that
recombine to yield the ozonide.
242
2-Hexene
(ii) H2O/Zn
(i) O
3
Butanal Ethanal
CHCH
3
CH
3CH2CH2
CH
H
C CH
3
H
CH3CH3CH2C
O + O
PropanalPropanal3-Hexene
(ii) H2O/Zn
(i) O
3
CHCH2CH3
CH
3CH2
CH
H
CCH
2CH3
H
CH3CH2C
O + O
Propanal2-Methyl-2-pentene
(ii) H2O/Zn
(i) O
3
Propanone
CH
CH
3CH2
CH
3
CH
3
C
CH
3
H
CH
3
O + O
CCCH
3CH2
CH3CHO + OCHCH3
EthanalEthanal2-Butene
(ii) Zn–H2O
(i) O
3
CHCH
3
CH3CH
CH3CH2CHO +
MethanalPropanal1-Butene
(ii) Zn–H2O
(i) O
3
HCHOCH
2
CH3CH2CH
MethanalPropanone2-Methyl propene
(ii) Zn–H2O
(i) O
3
CCH
3
CH
2
CH
3
HCHO
+
CCH
3
O
CH
3
normally zinc dust, is to check the formation of hydrogen peroxide, which if formed, would oxidize
aldehyde easily into carboxylic acids.
Application of ozonolysis in determining the position of double bond
By the process of ozonolysis, the unknown alkene is broken down into a number of smaller, more easily
unknown alkene. For example, there are three isomeric hexenes (C6H12
products give a clear picture about the position of the double bond in them.
REMEMBER
Baeyer’s reagent detects
the presence of the double
bond and ozonolysis helps in
locating the position of double
bond.
Thus the position of double bond in the above compounds is between 2:3,
the presence of a side chain.
Another example can be of butene (C4H8) as depicted below:
243
C H A P T E R 5 Alkenes
5.7.9 Hydroboration Oxidation
This is an important method for the synthesis of alcohols as
3)2, to
carbon–carbon double bond of an alkene using tetrahydrofuran
or dimethyl ether as solvent. The process of addition of diborane
to an alkene is hydroboration. The hydroboration leads to the
formation of trialkylboranes, R
3
B; these are oxidized to alcohols
in the presence of alkaline hydrogen peroxide. The reactions are
as depicted below:
1.
(a)
2CH3CH2BH
2
2CH2CH2 + (BH3)
2
(b)
(CH3CH2)2 BH
CH3CH2BH2 + CH
2
CH
2
(c)
(CH3CH2)2 BH + CH
2
CH
2
(CH3CH2)3 B
Triethyl borane
(d)
(CH3CH2)3 B + 3H2O
2
3CH3CH2OH + H3BO
3
OH
−
The complete hydroboration reaction can be written as:
6CH
2
CH2 + (BH3)
2
2(CH3CH2)3 B
3NaBH4 + 4BF
3
273 K
2(BH3)2 + 3NaBF
4
2. Preparation of n-propyl alcohol
6CH3CH CH2 + (BH3)
2
2(CH3CH2CH2)3 B
Propene
OH
−
(CH3CH2CH2)3 B + 3H2O
2
3CH3CH2CH2OH + H3BO
3
n-Propyl alcohol
3. Preparation of n-butyl alcohol
(i) (BH3)
2
n-Butyl alcohol
1-Butene
(ii) H2O2/OH
−
CH3CH2CH2CH2OH
CH3CH2CH CH
2
Learning Plus
Borane (BH3) does not exist as such at
room temperature and atmospheric
pressure. Two molecules of BH3 combine
to give diborane (B2H6), which is more
stable form.
Learning Plus
With sodium hydroxide as the base, boron
of the alkyl borane is converted to the
water soluble and easily removed sodium
salt of boric acid.
244
H2CH
2
CCH
2
BH
H
H
+
Ethylene
Borane
B
H
CH
2
H
+δ
−δ
H
H3CCH2BH
2
or
Ethyl borane
H
2
CCH
2
H
C
H
BH
H–O–O–H + OH
:
H–O–O + HO
2
Hydroperoxide anion
CH CH –B + O–O–H
32
—
CH CH
23
—
CH CH
23
CH CH –B –O–O–H
32
—
CH CH
23
—
CH CH
23
1,2 Ethyl shift
CH CH –O–B –O
32 23
CH CH
—
CH CH
23
Diethoxy ethyl borane
HO
22
—
OH
CH CH –O–B–O
32 23
CH CH
—
OCHCH
23
Triethoy borane
HO
22
—
OH
CH CH –B–O–
32 23
CH CH + OH
—
CH CH
23
Mono ethoxy diethyl borane
Hydroboration involves addition of borane, BH3 (or BH2R and BHR2 in the following stages) to double
bond with hydrogen getting attached to one doubly bonded carbon and boron to the other. The trialkyl
Mechanism of hydroboration
Hydroboration of alkenes is believed to take place by the electrophilic attack of borane, BH3. The boron
atom of BH
3
2) begins to take hydrogen atom of BH3 with its electron pair while electron-
π-electrons in the transition state. As boron gains π-electrons of the double bond,
it releases the hydrogen to C2 of ethylene. The reaction proceeds in a single step and in
transition state both
addition, the reaction proceeds in two steps.
Mechanism of oxidation of trialkyl boranes to alcohols
The mechanism for the alkaline hydrogen peroxide oxidation of alkyl boranes involves nucleophilic attack
the alcohol.
C H A P T E R 5 Alkenes
CH
3 2
CH =
12
+ B–HCH
——
H
H
CH –CH
3
CH
2
+
δ
HB–H
–
δ
—
H
Four-centred cyclic
transition state
CH –CH– CH
—
H—B–H
—
H
Mono-n-propyl borane
(CH CH CH )
3223
NaOH
3CH CH CH OH + HBO
32233
Tri-n-propyl borane n-Propyl alcohol
Boric acid
B + 3H2O
2
(CH3CH2O)3B + 3NaOH 3CH3CH2OH + Na3BO
3
Mechanism for the hydroboration of unsymmetrical alkene
Let us discuss the hydroboration of propene. The boron atom of the BH3 gets attached to the terminal
carbon atom because of two reasons:
1. The positive charge developed on C2
the transition state.
2. Because of steric hindrane, the attack of boron on less crowded and thus less hindered terminal
carbon atom is preferred.
The electrophillic attack of BF3 occurs at C1 because of electromeric effect and π-bond starts breaking
up. C2 begins to take hydrogens atom from BH3
starts gaining the π-electrons in the trainsition state. Thus a four-centred cyclic transition state is formed
as shown below:
245
From the above mechanism, it is clear that, in principle, hydroboration also follows Markownikoff’s
rule.
n-propyl borane takes place in the same manner as discussed with triethyl borane.
246
5.7.10 Hydroxylation
Conversion of an alkene to cis
potassium permanganate or with osmium tetroxide is called hydroxylation of alkenes.
Hydroxylation: Oxidation with cold alkaline KMnO
4
4 solutions, alkenes
give cis-1,2-diols also called cis-1,2-glycols. This reaction is
called hydroxylation since during this process, two hydroxyl
groups are attached to the two carbons atoms of the double
bond. For example:
()
42 2
2KMnO H O 2KOH 2MnO 3 O+ → + +
298–303 K
1,2-Ethane diol
(Ethylene glycol)
H
2
C
CH
2
OH
OH
H
2
C
C
CH2 + (O)H2O
⊕
1,2-Propanediol
298–303 K
OH OH
CH
2
+ 2KMnO4 + H2O
CH3CH CH
H
3
2
+ 2MnO2 + 2KOH
4 undergoes decolouration and a brown precipitate of
2) is formed. This reaction is therefore used as a test for unsaturation under the
name Baeyer’s test.
Mechanism of hydroxylation by cold dilute alkaline KMnO
4
4 solution occurs by a cyclic manganate ester,
which upon hydrolysis gives a cis-glycol.
CH
H
H
O
CH
O
CH
H
CH
H
O
O
O
O
Mn
+
O
Mn
O
Ethylene Cyclic magnate esterPermagnate ion
Learning Plus
Baeyer’s test is also called hydroxylation
reaction because it involves the addition of
two hydroxyl groups across the double bond.
High temperature must be avoided otherwise
the glycol formed will be further oxidized.
C H A P T E R 5 Alkenes
O
Mn
O
O
H2C
H
2
C
O
+
2H
2
O
OH
Mn
O
OH
O
+
OH
OH
H
H
2
C
H
2
C
OH
O
O
OH
OH
3
O
Mn Mn
O
O
O
+ 2MnO2 + 2OH + 2H2O
–C
–C
——
Os
Os
Os
O
OO
O
+
–C––
–C––
——
O
OO
O
:
HO
2
Osmium tetra oxide Cyclic osmate ester
–C–OH
–C–OH
——
O
O
:
+
HO
H–O
NaHSO or Na SO
323
Reduced form of osmium
MnO
4
2 and permanganate
ions.
5.7.11 Hydroxylation of Alkenes with Osmium Tetra Oxide
which can be
323
3 23 is to reduce the osmic acid
(H254
247
Stereochemistry of hydroxylation
The course of hydroxlation of alkenes by permanganate ions or osmium tetra oxide is synhydroxylation.
This can be seen, readily, when cylopentene reacts
323.
The product in either case is cis-1,2-cyclo pentane diol.
or OsO
4
44 followed
248
MnO + OH
4
Cold
OO
Mn
O
O
HO
2
OH
–
OH OH
cis-1-2-cyclopen
25ºC
Cyclopentone
OO
O
S
O
O
NaHSO
3
OH OH
OsO
4
(i) Hot KMnO
4
(ii) H
+
CH3– C – OH + CO2 + HO
2
O
Propylene
Acetic acid
(Ethanoic acid)
CH3 – CH CH
2
(ii)
(iii)
(i)
(i) Hot KMnO
4
/KOH
CH3– CH2– C – OH + H2O + CO
2
O
(ii) H
+
Propionic acid
(Propanoic acid)
CH3 – CH2 – CH CH
2
HC
3
HC
3
(i) Hot KMnO4/KOH
(ii) H
+
HC
3
C O + CO2 + H2O
HC
3
Isobutylene
(2-Methylpropene)
Acetone
(Propanone)
C CH
2
however, osmium tetra oxide is toxic and expensive. Potassium permangnate is a very powerful oxidizing
agent and, as we will see in the next section, it is easily capable of causing further oxidation of the glycol.
4. Even so, yields are sometimes very low.
5.7.12 Oxidation with Hot Alkaline KMnO4 (Oxidative Cleavage of Alkenes)
4 at 373–383 K, cleavage of the C C
2 depending upon the
nature of the alkene as shown below:
1. Terminal CH22.
C H A P T E R 5 Alkenes
(Orange colour of bromine
water is discharged)
Catalytic hydrogenation H
2
, Ni/575 K
Halogenation Br
2
/CCl
4
Addition of halogen acids HX
Addition of sulphuric acid H
2SO4
Addition of nitrosyl chloride NOCl
Addition of hypohalous acid
HOCl or Cl
2/H2
O
CH
3CH2
X
CH
3CH2
HSO
4
CH3CH2OH
H2O
Warm
Ethylenechlorohydrin
CH2CH2NO
Ethylene nitrosochloride
CI
Addition of sulphur monochloride
S2Cl
2
H2O2 (oxidation)
Cold alk. KMnO4 (KMnO4 + KOH)
Hot acidic KMnO
4
(KMnO4 + H2SO
4
)
Heating with O2 and Ag catalyst
Hydroboration 1/2B
2H6
Glycol
(Pink colour of Baeyer’s
reagent is discharged)
HCOOH
+
CO2 + H2O
(CH
3CH2)3
B
Triethylborane
Hydrolysis
3H2O
H
2
O/H
+
3CH3CH2OH
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Mustard gas
S
CH
2
CI
CH
2
CH
2
CH2CI + S
(Epoxyethane or oxirane)
O
CH3 (Sabatier–Senderen’s reaction) CH
3
Br Br
CHCH
2
OH Cl
CH
2
CH
2
CH2OH
HOCH
2
CH2OHCH2OH
CH
2
H2C
CH3CH
3
CH2CH
2
4 it is possible to locate the
position of the double bond in an unknown alkene molecule.
MEMORY FOCUS
Reactions of alkenes. As the double bond is a source of electrons, therefore, alkenes undergo
electrophilic addition reactions. Reactions of ethene are being given in detail.
249
250
H2O2 (oxidation)
Reaction with per acids
Reaction with O
3
(ii) Zn/H2O
CH
3
COOH (per acetic acid)
Combustion O2, ∆
Polymerization
Epoxyethane
2HCHO + ZnO
CO
2
+ H2O + heat + light
13.
14.
15.
16.
17.
O
CH2 + CH3COOHH2C
CH2OHHOCH
2
CH2 ]–n
–[ CH
2
.
REVISION QUESTIONS
1. How are alkenes prepared? Describe their important reactions.
2. How is ethylene prepared in the laboratory? What happens when ethylene is treated with:
(a) Br2/CCl4 (b) Dilute H2
4
(c) Concentrated H24
3 then Zn/H2
4
formula C5H10.
Hint: There are 5 alkenes of formula C5H10.
4. What happens when
(a) propene is subjected to ozonolysis?
(b) propene is treated with Cl2 at 500°C?
Hint: (a) A mixture of acetaldehyde and formaldehyde is formed.
5. (a) How will you distinguish between 1-hexene and n-hexane?
(b) How will you distinguish between propene and propane?
Ans: (a) Following tests can be used:
44
solution; n-hexene does not. The observed change will be disappearance of the purple colour
42.