CH221 CLASS 14
CHAPTER 7: ALKENES – REACTIONS AND SYNTHESIS CONTINUED
Synopsis. This class considers some further addition reactions of C=C: carbene addition, hydrogenation (reduction) and perhydroxylation (oxidation). Oxidations involving complete cleavage of C=C are dealt with next, followed by an account on polymerization.
Cyclopropane Synthesis by the Addition of Carbenes to Alkenes
Carbenes are reactive molecules containing bivalent carbon: their general formula is R
2
C. They can exist in two forms, singlet and triplet forms, whose structures are illustrated below.
Both types of carbenes are highly reactive, because the carbon atoms are two electrons short of an outer octet: carbenes are electrophiles and react with nucleophiles, including alkenes. They can be generated by a variety of methods, which give singlet or triplet species, depending on the conditions and on the substituents on the carbon atom.
E.g.
CH
2
+
N
-
N diazomethane heat or light
3
CH
2 triplet
+ N2 heat or light
CH
2
C ketene
O
3
CH
2 triplet
+ CO
CHCl
3
+ OH-
1
CCl
2 singlet
+ H2O + Cl-

The last method involves an acid base reaction between CHCl
3
and OH an example of a 1,1-( gem - or

-)-elimination reaction.
and is
: OH-
H
H2O
Cl C Cl Cl
..-
C Cl
-Cl..
C
Cl Cl
Cl Cl
Halogenated carbenes are less reactive than alkylcarbenes and are more selective (and more stereoselective) in the their reactions, one of the most important of which is cycloaddition to alkenes to give cyclopropane derivatives.
Thus dihalogenated cyclopropane derivatives can be prepared by treating alkenes with a basic solution of haloform,
E.g.
H
C
2
H
5
C C
H
CH
3
CHCl3
KOH heat
C
2
H
H
5
C
CCl
C
2
H
CH
3
Cis or Z alkene
Cis or Z cyclopropane derivative
Methylene and alkylcarbenes are highly reactive if prepared (as triplet species) by photolysis: they have low selectivity (including stereoselectivity) and hence are of limited synthetic use. Hence they are best prepared and used as
“carbenoid”
(carbene-like) species, via reactions with metals, such as in the
Simmons-Smith reaction:

CH
2
I
2
+ Zn/Cu an ether solvent
CH
3 I
CH
3
+ CH
2
ZnI
I CH
2
ZnI
Iodomethylzinc iodide
This acts as a supply of methylene carbene (CH2) but is less reactive
(and hence more selective) than the free carbene
=/
CH
3
I
CH
3
CH
2
ZnI
-ZnI2
CH
3
CH
2
CH
3
Reduction of Alkenes: Catalytic Hydrogenation
One of the best ways of reducing alkenes is via catalytic hydrogenation: reaction with H
2
in the presence of either a solid catalyst (e.g. Pd/C, Pt, PtO
2
, etc) or a liquid catalyst (a Wilkinson catalyst such as (Ph
3
P)
3
RhCl or (Ph
3
P)
3
RuCl
2
).
Hydrogenation using a solid catalyst (called heterogeneous catalytic hydrogenation) is emphasized here. The mechanism of heterogeneous catalytic hydrogenation involves adsorption/desorption processes and reactions on the catalyst surface:

The above mechanism is consistent with the observation of syn addition in many
(but by no means all) catalytic hydrogenations.
The major characteristic features of heterogeneous catalytic hydrogenation are described in the table below and these characteristics are illustrated by the examples on the next page.
Group selectivity
Reasonable. The general order of reactivity is
COCl~NO
C=C
2
>C=C>CHO>CO>aromatic
This means that C=C can be hydrogenated in the presence of CHO,
Stereoselectivity
Reasonable. Syn addition usually occurs, but not always – there is a dependence on the catalyst used.
Addition of H
2
normally occurs on the less hindered side of C=C
CO and aromatic rings, but NOT in the presence of COCl or NO
2

O
H2
Pd/C ethanol
O
Ketone function not reduced
CH
3
CH
3
H2
CH
3
Pd/C

-pinene
CH
3
CH
H
3
CH
3
H
Hydrogenation occurs on the less hinderd side
O
C
OCH
3
H2
Pd/C
Neither aromatic ring nor ester function are reduced
O
C
OCH
3
CH
3
H
H
CH
3
H2
CH
3
H2
CH
3
H
CH
3
H
Pd/C CH
3
PtO2
Trans mainly
Oxidation of Alkenes: Perhydroxylation
Cis mainly
Alkenes can be oxidized to 1,2-dihydric alcohols, known as glycols, by the action of either osmium tetroxide (OsO
4
) (followed by aqueous sodium bisulfite) or alkaline aqueous potassium permanganate. Both reactions are known as perhydroxylation reactions and both proceed via five-membered cyclic intermediates (not carbocations). Both reactions are stereoselective, involving syn addition. i. OsO4, pyridine ii. NaHSO3(aq)
OH
OH
Cyclohexene cis -1,2-cyclohexanediol
The accepted mechanism for perhydroxylation using OsO
4
is,

O
+
O
Os
O
O
Note the syn addition
O
O
Os
O
O
NaHSO3/H2O
OH
OH
A similar result, but with opposite stereochemistry, is achieved by epoxidation of the alkene, followed by acid-hydrolysis of the resulting epoxide.
RCO3H (a peroxyacid) or ROOH (a hydroperoxide)
: O :
Cyclohexene
H+
OH -H+
:
+
O H
OH trans -1,2-cyclohexanediol :
..
OH
2
Note overall anti addition attack from below
Oxidative Cleavage of Alkenes
More powerful oxidizing agents, like ozone (O
3
), cleave alkene double bonds to give a mixture of carbonyl compounds (aldehydes and ketones):

CH
3 i. O3
CH
3
C
O + O C
CH
3 ii. Zn/H+
This reaction is of rather limited synthetic use, because a larger molecule is
CH
3 fragmented to smaller ones, but it has been useful in establishing the positions of
C=C in newly discovered compounds.
Hot acidic (or neutral) potassium permanganate will also cleave alkene double bonds, but again this is of limited synthetic use, perhaps being most useful for the formation of

,

-dicarboxylic acids from cycloalkenes:
KMnO4, H+(aq)
COOH
COOH heat
Cyclohexene adipic acid (1,6-hexanedioic acid)
Note R
2
C= gives R
2
CO, RCH= gives RCOOH and CH
2
= gives CO
2
. Note also that phase transfer catalysts, such as quaternary ammonium salts or crown ethers, can greatly facilitate this reaction.
Oxidative cleavage of 1,2-Diols
Rather than use direct cleavage of alkenes (as above), it is possible to perhydroxylate alkenes (using OsO
4
, NaHSO
3
or KmnO
4
/OH ) and then cleave the resulting 1,2-diol with periodic acid (HIO
4
) or lead tetraacetate (Pb(OAc)
4
), as shown below.
OH OH
HIO4, aq THF
O O
C C C + C
E.g.
OH
HIO4, aq THF OH
O + OHC C
2
H
5
CH C
2
H
5
Like alkene cleavages, this reaction is of limited synthetic value, but has been much used to identify polyhydric alcohols and sugars.

Polymerization of Alkenes
Alkenes can successively add to each other ’s double bonds to give very large alkane molecules. This process is an example of polymerization : the alkene starting materials are called monomers and the macromolecular product is known as a polymer . Ethylene itself was the first to be polymerized, by accident, at ICI (U.K.) in 1934, by treating ethylene with a high pressure and a moderately high temperature. Polyethylene was produced in this reaction, which was later found to be a radical chain process, the initiator being oxygen in the atmosphere.
Nowadays, similar conditions are still used, except that the initiator is usually an organic peroxide, such as dibenzoyl peroxide. The type of polyethylene produced by this method is known as low-density polyethylene, as will be explained later.
The overall reaction is: dibenzoyl peroxide
1000 - 3000 atm n CH
2
CH
2
100 - 250oC
CH
2
CH
2 n
The mechanism of this reaction, like all radical chain reactions, involves initiation, propagation and termination steps.
O O
INITIATION Ph C
..
..
..
..
C Ph heat
2 Ph C
Dibenzoyl peroxide
O
PROPAGATION Ph
O
C O
.
+ CH
2
CH
2
Ph
O
C O CH
2
.
CH
2
Ph
O
C O CH
2
.
CH
2
+ CH
2
CH
2
Ph
O
C O CH
2
CH
2
CH
2
.
CH
2 etc
TERMINATION 2 R CH
2
.
CH
2
R CH
2
CH
2
CH
2
CH
2
R
Other alkenes, such as PhCH=CH
2
(styrene), CH
3
CH=CH
2
(propylene) and
CH
2
=CHCl (vinyl chloride) can be polymerized in a similar way. These reactions all progress via the generation of the more stable secondary radicals, rather than less stable primary radicals. A list of common poly(alkenes) and their main uses can be found in the textbook on page 231 (Table7.1).

R
Chain Branching during Radical Alkene Polymerization
Owing to the high reactivity of radicals, polymerization of alkenes by radical processes tends to give highly branched polymers, rather than purely linear polymers. The large number of branches on each polymer molecule prevents the molecules packing close together in the bulk material, resulting in a relatively low-density product of low crystallinity. Such polymers are generally easily moulded into packages and bottles (etc) and can be made into films and sheets.
On the other hand, they cannot be woven into fibers for making clothing and ropes. Other kinds of polymerization processes are used to make poly(alkenes) with these properties.
Branching can occur in two main ways:
Short-Chain Branching

CH
CH
2
H
.
CH
2
CH
2
CH
2
Intramoleculer hydrogen abstraction R
.
CH
H
CH
CH
2
2
CH
2
CH
2
Growing polymer chain
CH
2
.
CH
2 etc
R
CH
2
CH
2
CH CH
2
H
CH
2
CH
2
CH
2
Short chain branch

Long-Chain Branching long-chain branch
CH
2
CH CH
2
CH
2
CH
2
.
CH
2
.
CH CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
.
CH CH
2
CH
2
H
CH
2
Intermolecular hydrogen abstraction
CH
2
CH
2
H
CH
2
Separate growing polymer chains
Cationic Polymerization
Alkenes can be polymerized by repeated electrophilic addition reactions across
C=C. The electrophiles, as cations, are generated in situ by the action of protic or
Lewis acids: n CH
2
C
CH
3
BF3, trace of
H2O
CH
3
C
CH
3
-80oC
CH
2
Isobutylene (methylpropene)
CH
3 n polyisobutylene
The best monomers are those most able to stabilize a positive charge by resonance (i.e. those that form secondary or tertiary carbocations), like isobutylene.
The mechanism of polymerization is believed to be as follows.

BF
3
+ H
2
O
(trace)
H+ + BF3OH-
H+ + CH
2
C polyisobutylene
CH
3
CH
3
+
C
CH
3
CH
3 CH
3 tert -butyl cation electrophilic addition
CH
3
CH
2
C
CH
3
2nd monomer molecule further electrophilic additions of further monomers
CH
3
CH
3
CH
3
C CH
2
+
C
CH
3
CH
3 dimeric 3o cation