Brown's GCE Chemistry Revising
Advanced Level Organic Chemistry
A Level Revision Notes PART 10
Summary of organic reaction mechanisms
A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
10.3 Reaction mechanisms of ALKENES
Electrophilic addition of
Part 10.3 ALKENES - introduction to the
reaction mechanisms of alkenes.
Electrophilic addition of hydrogen bromide [HBr(conc. aq) and
HBr(g/non-polar solvent)] to form halogenoalkanes. These revision
include full diagrams and explanation of the ionic electrophilic
addition reaction mechanisms of alkenes and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained.
Alkenes are reactive molecules,
particularly when compared to alkanes.
reactive towards electron pair accepting electrophiles
because of the high density of negative electron charge associated
electrons of the double bond (diagram below)
bonds of the C=C double bond are shown in the diagram
The sigma bond is present between
all the atoms in organic molecules. However in alkenes, the double
bond consists of a sigma bond (σ)
and a pi bond (π).
Two electrons are in the molecular orbital of the sigma bond which
is directed linearly between the two carbon atoms.
BUT, the other two electrons of
the C=C double bond are in the pi orbitals which lie above and below
the plane of the sigma bonds of the >C-C< system (1 electron pair
It is the electrons of these
pi orbitals that are susceptible to electrophilic attack by an
electron pair acceptor - an electrophile.
However alkenes can
also readily undergo free radical reactions e.g. their
peroxide catalysed polymerisation to form a poly(alkane) and these
reactions also involve the interaction of free radicals with the
electrophilic addition reactions of alkenes are compared with the nucleophilic addition to carbonyl
compounds in the aldehydes and ketones section.
10.3.2 The electrophilic addition of hydrogen bromide to alkene
The organic synthesis of bromoalkanes by reacting hydrogen bromide with alkenes
Examples of the
addition of hydrogen bromide to alkenes
What is the reaction mechanism
for the addition of hydrogen bromide to an alkene?
Does the mechanism
change if the solvent is changed?
Do the products of
the reaction depend on the solvent used?
Can isomeric products
be formed in the addition of hydrogen bromide to an alkene?
If a liquid
alkene is mixed with, or a gaseous alkene is bubbled through,
concentrated hydrobromic acid, HBr(aq)
(hydrogen bromide dissolved in water) a bromoalkane
overall reaction: R2C=CR2
+ HBr ==> R2CH-CBrR2
hydrogen bromide is a strong acid i.e. completely ionises to
give the oxonium ion and bromide ion.
- electrophilic addition of hydrogen bromide to an alkene in aqueous
In the acid
solution via step
the H3O+ or oxonium ion
(hydrated proton) is the 'attacking electrophile' and
protonates the alkene to form the intermediate positive
carbocation R2CHCR2+. The
oxonium ion is an electrophile because it accepts a pair of
electrons from the alkene
bond to form the new C-H bond.
(2) the (already present) negative bromide ion rapidly
combines with the carbocation to form the bromoalkane product.
The bromide ion donates a pair of electrons to form the new C-Br
high concentration of water present, a water molecule could
also interact with the carbocation to eventually form a
small amount of the alcohol R2CHCR2OH,
this again provides evidence of an ionic mechanism.
mechanism 3 -
electrophilic addition of hydrogen bromide to an alkene in
In this case,
for step (1),
attacking electrophile is the already polarised hydrogen bromide
which splits heterolytically to protonate the alkene,
forming the carbocation and a bromide ion. The HBr molecule is
an electrophile because it accepts a pair of electrons from the
bond to form the new C-H bond.
(2) the bromide ion formed in step (1) rapidly combines
with the carbocation to form the bromoalkane. The bromide ion
donates a pair of electrons to form the new C-Br bond.
for an IONIC MECHANISM
is a general comment for all the electrophilic addition
reactions of alkenes.
reaction is carried out in the presence of other negative
ions e.g. chloride ion from adding sodium chloride salt to
an aqueous reaction mixture, then some chloroalkane is
produced via step (2).
symmetrical alkene is when the atoms/groups are the same
on either side of the C=C double bond.
or but-2-ene CH3-CH=CH-CH3
means which ever way round the HX addition takes place
onto the double bond, you always get the same product.
alkene is when the atoms/groups are NOT the same on either
side of the C=C double bond e.g.
or but-1-ene CH2=CH-CH2-CH3
means that when addition to the double bond with a
non-symmetrical reagent itself, e.g. like H-X,
you have the
possibility of two different isomeric addition
+ H-X ==> CH3-CHX-CH3 or
Which begs the questions, which isomer
predominates? and why?
isomer is likely to predominate for adding a non-symmetrical
reagent to a non-symmetrical alkene and the rule can be
stated in various ways but the IUPAC definition of 1997
For the heterolytic addition of a polar molecule to an alkene
(or alkyne), the more electronegative (nucleophilic
like OH- or Br- etc.) atom (or part)
of the polar molecule becomes attached to the carbon atom
bearing the smaller number of hydrogen atoms [or you can
say the least electronegative (most electrophilic like Br+
or H+ etc.) will attach to the carbon atom bonded
with the most H atoms). BUT the
'rule' only applies to the ionic mechanism, you can get
the opposite effect in free radical addition in the presence
orientation of the products from non-symmetrical
addition (HX or Br2(aq) see later) is
governed by the stability of the carbocation
intermediate formed by the protonation of the alkene by
the attacking H-X electrophile, and explains the
order of carbocation stability is tertiary >
secondary > primary, because alkyl groups give a
slight electron donating inductive effect (+I)
via the attraction of the positively charged carbon
atom. This spreads the positive charge of the
carbocation and gives the carbocation more stability
by lowering its potential energy. It is a general rule
of physics that spreading out electric charge lowers the
potential energy and increases the stability of a
most stable carbocation will be the one most likely to
exist with a sufficient life-time to be hit by the
electron pair donating ion (e.g. X-) or any
other electron pair donor, including water (see
addition of bromine water).
The positive carbon of the most stable carbocation, has
attached to it the most alkyl groups and the least
HX to a non-symmetrical alkene you would expect the
major isomer to be e.g.
from propene, CH3CH=CH2
you expect mainly CH3CHX-CH3
from methylpropene, (CH3)2C=CH2
you expect mainly (CH3)2CX-CH3
from 2-methylbut-2-ene you expect mainly (CH3)2CXCH2CH3
> and some (CH3)2CHCHXCH3
you expect mainly
happens in terms of optical
isomers/activity if the product has a chiral carbon*?
example of a chiral carbon results from when four
different atoms or groups (a to d) is bonded to the same
carbocation i.e. *Cabcd
the carbocation has a plane of symmetry due to the
trigonal planar arrangement of the bonds around the
carbon atom carrying the positive charge.
symmetrical arrangement means that if the product is
potentially optically active, a racemic mixture will be
formed because the e.g. bromide ion, can add with equal
probability on both sides of the carbocation.
result in equal quantities of the optical isomers
(enantiomers), giving an optically inactive racemic
on adding HX, will give a racemic mixture of the
optical isomers of CH3*CHXCH2CH3
with some XCH2CH2CH2CH3 which
is incapable of optical isomerism because it does not
have a chiral carbon.
radical addition of hydrogen bromide
mixed halogen compounds (inter-halogen
compounds), such as iodine(I) chloride ICl, will also add to the
alkene double bond.
+ ICl ==> CH3CHI-CH2Cl or
Markownikoff rule 2-chloro-1-iodopropane should be the
principal product because chlorine is more electronegative
than iodine, so think of it as the addition of Iδ+-Clδ-.
10.3.2 The electrophilic addition of
bromine to alkene
now on separate page
10.3.5 The electrophilic addition of conc. sulphuric
acid to alkene now on separate page
10.3.6 The acid catalysed electrophilic addition of
water to alkene now on
10.3.7 The free radical addition polymerisation
of an alkene now on
keywords phrases: electrophile
mechanism steps reagents reaction conditions formula intermediates organic chemistry reaction mechanisms steps
electrophilic addition of hydrogen bromide hydrobromic acid to alkenes ethene
propene butene R2C=CR2 + HBr ==> R2CH-CBrR2 HBr(g/aq) + H2O(l) ==>
H3O+(aq) + Br-(aq) R2CH-CR2+ + Cl- ==> R2CH-CR2Cl ethene H2C=CH2 or but-2-ene
CH3-CH=CH-CH3 propene CH3-CH=CH2, methylpropene (CH3)2C=CH2 or but-1-ene
CH2=CH-CH2-CH3 CH3-CH=CH2 + H-X ==> CH3-CHX-CH3 or CH3-CH2-CH2-X CH3CH+CH3 (sec)
> CH3CH2CH2+ (prim) (CH3)2C+CH2CH3 (tert) > (CH3)2CHC+HCH3 (sec) CH3CHX-CH3
CH3CH2-CH2X (CH3)2CH-CH2X (CH3)2CXCH2CH3 > and some (CH3)2CHCHXCH3 from
but-1-ene, CH2=CHCH2CH3 CH3-CHXCH2CH3 XCH2-CH2CH2CH3
and Organic Synthesis INDEX