(5) Free-radical addition of hydrogen halides to alkenes - AntiMarkovnikov formation of alkyl halides:
Carbon free radicals are very reactive intermediates with one unpaired
electron in the carbon valence shell. They will be studied in depth in
Module CM3001.
Free radical reactions usually require a small amount of an initiator - a
compound which readily generates free radicals - in order to get started:
RCH
CH2
ROOR (Initiator)
HX
RCHH
CH2X
Mechanism:
RCH
ROOR (Initiator)
CH2
HX
RO
RO•
RCH2
OR
H
Mild
Heat
X
CH2X
2 RO•
ROH + X•
2°
RCH
CH2
•
RCH
+ •X
C X
H2
Most stable radical
•
CH2
RCHX
•
RCH
RCH2
C X + HX
H2
CHAIN REACTION
CH2X
1°
+ X•
Note the use of single-headed 'fishook' arrows to indicate movement of
a single electron.
(6) Oxidation of alkenes with per-acids - formation of epoxides:
RCH
CH2
RCO2OH
R usually = 3-Cl-C6H4
O
RCH
CH2X
Epoxide
The reaction is a concerted syn- addition of an oxygen atom to the
double bond - the C-C -bond is broken and the two C-O bonds are
formed simultaneously - so that the stereochemistry of the alkene is
preserved in the epoxide.
RCH
CH2
O
RCO2OH
RCH
Epoxide
or
oxirane
R usually = 3-ClC6 H4
O
C
H
O
:
O:
:
O
:
R
CH2
CH3
H C
CH3
CH3
C
CH3
H
+ RCO2 H
(7) Oxidation of alkenes with potassium permanganate - formation of
cis- 1,2-diols (glycols):
RCH
CH2
(i) KMnO4
CH2
Glycol
(1,2-diol)
:
H
H
O:
O
O
-O
O
Mn
Mn
-O
OH
RCH
(ii) NaOH, H2 O
syn- addition
O
HO
Permanganate
anion
O
H2 O, NaOH
H
HO
H
OH
cis - diol
(8) Oxidation of alkenes with ozone - cleavage of the double bond via
the formation of ozonides and their subsequent reduction:
O:
O:
etc.
O
+
:
:
-
:
H3C
:
-O
:
:
O
RCH=O + O=CH2
:
:
(ii) Zn, CH3CO2 H
: +
O:
:
-
CH2
:
RCH
(i) O3
:
O
: +
O:
syn- addition
H3C
O
Molozonide
:
H
O
O
O
O
O:
H3 C
H
Zn
CH3 CO2H
O
H3 C
O
H
O
Ozonide
Notice that in the reduction step of ozonolysis:
(i) an alkene with an unsubstituted carbon atom yields formaldehyde
(ii) an alkene with a monosubstituted carbon atom yields an aldehyde
(iii) an alkene with a disubstituted carbon atom yields a ketone
Hence these reactions can be useful for the characterisation of
molecules of unknown structure which contain carbon-carbon double
bonds.
(9) Catalytic reduction of alkenes to alkanes:
Alkenes are not spontaneously reduced to alkanes when treated with
hydrogen. However in the presence of a transition metal catalyst the
reaction proceeds efficiently - and usually with cis- stereospecificity - at
room temperature and with either 1 At or elevated pressures of
hydrogen.
Catalysts can either be soluble in the reaction mixture (homogeneous)
or insoluble (heterogeneous).
One of the most important of the homogeneous catalysts is
(PPh3)3RhCl, tris(triphenylphosphine)rhodium(I) chloride ('Wilkinson's
catalyst').
Typical heterogeneous catalysts are palladium on charcoal (Pd/C) or
platinum oxide (PtO2, 'Adam's catalyst').
CH3
CH3
CH3
H2, PtO2
CH3
H
H
Note stereospecific cis-addition of hydrogen
Reduction of alkenes with the aid of homogeneous catalysts such as
Wilkinson's Catalyst will be studied in depth as part of the Module
CM4104.
THE CARBON-CARBON TRIPLE BOND - THE CHEMISTRY
OF ALKYNES
Text references: McMurry (5th Edition) Chapter 8.
Electronic structure of the carbon-carbon triple bond:
Unhybridised
p-orbitals


C
H


C
C
H

sp
180°
hybrids
H
C
C
H
Nomenclature - the systematic rules for naming alkynes:
(1) Find the longest chain of carbon atoms that includes the triple bond
- then name as for the corresponding saturated hydrocarbon (alkane) but
use the termination -yne rather than -ane:
H
C
C
H
CH3
CH C
C
CH3 CH2
4-methyl-2-hexyne
C
CH3
2-Butyne
Ethyne (Acetylene)
CH3
C
CH3
CH3
CH3 CH2
CH CH2 C
CH
4-methyl-1-hexyne
The Preparation of Alkynes:
Remember how we generated carbon-carbon double bonds by
elimination of HX from an alkyl halide:
H
C C
  X
Base
- HX
C
C
-
+ B:H+ + X
A similar double -elimination of HX from a 1,2- or vicinial alkyl
dihalide generates a triple bond:
H
H
C
X
C
X
2 Base
- 2 HX
C
C
+ 2 B:H+ + X
If we remember that 1,2-dihalides are themselves prepared by the
electrophilic addition of a halogen to an alkene we have a sequence of
reactions for converting double to triple bonds:
Br
Br2
1-pentene
Br
NaNH2
NH3 liq.
- 2 HBr
1-pentyne
The double dehydrohalogenation of an 1,2-dihaloalkene proceeds via a
vinyl halide - hence vinyl halides themselves are useful precursors for
alkynes:
Br
NaNH2
NH3 liq.
Br
- HBr
Br
- HBr
Reactions of Alkynes:
(1) C-H Acidity:
Base
A vinyl
bromide
Unlike p- and d-orbitals, s-orbitals do not have a node (i.e. a region of
zero electron density) at the nucleus. In fact s-orbital electron density
actually penetrates right into the atomic nucleus. The amount of selectron density involved is tiny but enough to produce observable
effects. One significance of this effect is that s-electrons - or electrons
in orbitals with a high % s-character - are attracted to and stabilised by
the positively charged nucleus more strongly than electrons in orbitals
with little or no s-character. This, in turn, influences the acidity of C-H
bonds.
R
H
+ H+
R
Carbanion
C
C
Alkyl
sp3
C
C
Alkynyl
sp
Vinyl (or alkenyl)
sp2
25% s
C
33% s
25% s
Basicity of carbanion
H
C
C
C
C
C
H
H
pKa - 60
pKa - 44
pKa - 25
Acidity of hydrocarbon
Of all hydrocarbons, the sp terminal C-H bonds in alkynes are the most
easily deprotonated:
R
C
C
+
H + Na NH 2
-
Amide anion a strong base
R
C
C Na+ + NH3
Acetylide or
alkynide anion
Acetylide anions are both good nucleophiles and strong bases:
CH3
C
C
–
+
H
R1
C
SN 2
Br
D
1°
R1
CH3
C
C
+ Br
C H
-
D
H
CH3
C
C
-
+
2°
H
SN 2
Br
H
E2
- HBr
H
H
C
H
C
CH3