Brown's GCE Chemistry Revising
Advanced Level Organic Chemistry
Revision Notes PART 10
Summary of organic reaction mechanisms -
A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
10.2 Some reaction mechanisms of ALKANES
The reaction mechanisms of alkanes
Part 10.2 ALKANES - introduction to the
reaction mechanisms of alkanes.
Free radical chlorination/bromination to give halogenoalkanes (haloalkanes,
Free radical thermal cracking to give shorter alkanes and alkenes
Ionic catalytic cracking to give shorter alkanes and alkenes Revision
include full diagrams and explanation of the mechanisms of alkanes and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained.
Introduction to alkane reactivity
are not very reactive molecules. Most
reactions require some energy input to initiate a reaction e.g.
high temperature and catalyst for cracking, uv light for chlorination or a spark
to ignite them (initiating free radical reactions).
main reasons account for this lack of reactivity compared to most
other homologous groups of organic molecules.
C-C (bond enthalpy 348 kJ mol-1) and C-H
(bond enthalpy 412 kJ mol-1) bonds are very strong so bond fission does not
readily happen. The carbon atom radius is small, giving a short and
strong bond with other small atoms. Therefore the reactions
will tend to have high activation energies resulting
in slow/no reaction.
and hydrogen have similar electronegativities, so there is
no polar bond giving a slightly positive carbon (Cδ+) which
can be attacked by electron pair donating nucleophiles.
C-C and C-H bonds are single covalent and no region of
particularly high electron density
susceptible to attack by electron pair accepting
electrophiles. [e.g. like a double C=C bond see
alkenes which are highly reactive despite the fact the
C=C double bond has]
10.2.2 The free radical chlorination/bromination
mechanism of alkanes
What is the reaction
mechanism of chlorine reacting with alkanes like methane and ethane
reaction is: R3C-H + Cl2
==heat/uv==> R3C-Cl + HCl
The reaction is
initiated by higher temperatures e.g. 250-400oC or uv
light at room temperature.
If other hydrogen
atoms are available on the original hydrocarbon then polysubstituted
chloroalkanes will be formed
=> chloromethane => dichloromethane => trichloromethane =>
form initially 1-chloropropane or 2-chloropropane
1,1- or 1,2- or 1,3- or 2,2-dichloropropanes etc. etc.!
==> CH3CH2CH2Cl or CH3CHClCH3
or CH3CHClCH2Cl or ClCH2CH2CH2Cl
and ultimately CCl3CCl2CCl3
You can write a
similar set of equations for ethane, butane and pentane etc...
mechanism 6 -
free radical chlorination of an alkane,
when all R's are H, CR3H = methane
is the initiation
step when the chlorine molecule is split into two chlorine atoms/radicals
by homolytic bond fission by the impact-absorption of the
ultraviolet photon. Its quantum of energy, E=hv, must
be great enough to break the Cl-Cl bond.
means the original
pair of (Cl-Cl) bonding electrons is split between the
two radicals formed.
illustrates how to use
half-arrows to show a homolytic bond fission step
The red dots represent
the unpaired electron on the
free radical and the half-arrows show the individual electron
breaking of the Cl-Cl bond in the chlorine molecules begins the
reaction because it is the weakest of the bonds of any reactant
molecule involved. Bond enthalpies/kJmol-1:
Cl-Cl = 242, C-C = 348, C-H = 412, and even the new bond
formed, C-Cl, is 338.
radicals are highly reactive species with an unpaired electron
and tend to form a new bond as soon as is
possible by e.g. in this case by ...
(2) and (3)
are chain propagation steps, because as well as producing one
of the reaction products, a new free radical is also produced to
continue the reaction, which is why such reactions are sometimes
referred to as 'chain reactions'.
(4) to (6)
are three possible chain
termination steps which remove the highly reactive free radicals
as two unpaired electrons form a new bond, in this case single C-C
mechanism for bromination is similar.
alkane is methane, traces of ethane are found in the final
mixture of products. This provides evidence for a mechanism
involving a methyl radical.
free radical thermal cracking
What is the reaction
mechanism of hydrocarbon alkanes being cracked to form alkenes,
smaller alkanes and hydrogen.
hydrocarbons are heated to a high temperature (450-900oC,
with/without superheated steam) they are thermally decomposed
'cracked' to form mainly alkanes of lower C number, alkenes of
equal or smaller C number and hydrogen.
31 - free radical thermal cracking of alkanes
temperature is high enough, the kinetic energy of the particles is
sufficient to cause bond fission on collision, and this initiates a
free radical chain reaction.
initiation step when a C-C bond in an alkane molecule is split
into two alkyl free radicals
by homolytic bond fission. This means the original C-C bonding
electron pair is split between the two alkyl radicals formed.
bonds will break first in the initiation step, so the C-C
bond (bond enthalpy 348 kJmol-1) will tend to break
first as the C-H bond is stronger (bond enthalpy is 412 kJmol-1).
red dots represent the
unpaired electron on the free radical.
radicals are highly reactive species with an unpaired electron
and tend to form a new bond as soon as is possible, in this
case by ...
(2) to (6)
are chain propagation steps,
because as well as a product, a free radical is also produced to
continue the chain reaction
and lead to, in this case, a variety of other products.
(7) and (8)
are two possible chain
termination steps which remove the highly reactive alkyl free
radicals. The unpaired electrons from the two radicals 'pair up' to
form a new bond.
ionic mechanism for catalytic cracking
occurs at lower temperatures
over a ceramic/zeolite catalytic material based on silica and/or
aluminium oxide (I've called it Z in the mechanisms). The mechanism
is believed to be ionic and some examples of reaction steps are
given below. Z represents a matrix element of the
alumino-silicate catalyst and can act in a variety ways e.g. Lewis
acid or base, Bronsted-Lowry acid or base. R1,
R2, R3 = alkyl
can occur via an alkane or an alkene previously formed from an
example shows chain scission forming a 'smaller' alkene and a
'smaller' carbocation to continue the chain.
lower alkene or alkane is formed.
ASSOCIATED ADVANCED ORGANIC CHEMISTRY PAGES
All Advanced Organic
of Organic Functional Groups
Quiz on Organic Structure Recognition
Summary of organic
chemistry functional group tests
and bond angles of simple organic molecules
An ALIPHATIC ORGANIC STRUCTURE and NOMENCLATURE BUMPER multiple choice QUIZ
Type in name
(short answer) BUMPER ALIPHATIC ORGANIC
keywords phrases: reaction conditions formula
intermediates organic chemistry reaction mechanisms free radicals initiation step propagation
steps termination steps R3C-H + Cl2 ==heat/uv==> R3C-Cl + HCl [mechanism 6] R =
alkyl e.g. CH3, CH3CH2 etc. or aryl e.g. C6H5, CH3C6H4 etc.CH4 ==> CH3Cl ==>
CH2Cl2 ==> CHCl3 ==> CCl4 CH3CH2CH3 ==> CH3CH2CH2Cl or CH3CHClCH3 => CH3CH2CHCl2
or CH3CHClCH2Cl or ClCH2CH2CH2Cl or CH3CCl2CH3 etc. and ultimately CCl3CCl2CCl3
H3C. + .CH3 ==> H3C-CH3 CH3CH3 CH3CH2CH3 ==> CH4, CH2=CH2, CH3CH3, CH2=CHCH3, H2
CH3CH2CH3 ===> CH4 + CH2=CH2 CH3CH2CH3 ===> CH2=CHCH3 + H2 2CH3CH2. ==> CH2=CH2
+ CH3CH3 R1-CH2-CH2-R2 + Z+ ==> R1-CH2-CH+-R2 + HL (ii) R1-CH=CH-R2 + HZ ==>
R1-CH2-CH+-R2 + Z- R1-CH2-CH+-R2 ==> R1+ + CH2=CH-R2 R1+ + Z- ==> R3CH=CH2 + ZH
R1+ + ZH ==> R1H + Z+ (CH3)3C-CH2-C(CH3)=CH2 + HZ ==> (CH3)3C-CH2-C+(CH3)2 + Z-
(CH3)3C-CH2-C+(CH3)2 ==> (CH3)3C+ + CH2=C(CH3)2 (CH3)3C-CH2-C(CH3)=CH2 +
(CH3)3C+ ==> (CH3)3C-CH2-C+(CH3)2 + CH2=C(CH3)2 (CH3)3C+ + Z- ==> CH2=C(CH3)2 +
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