
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
A Level Revision Notes PART 10
Summary of organic reaction mechanisms
Help in revising organic chemistry - A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
Part 10.4 Halogenoalkanes
Reaction
with water and hydroxide ion
Part 10.4 HALOGENOALKANES - introduction to
the mechanisms of halogenoalkanes
(haloalkanes, alkyl halides).
Nucleophilic substitution by water and nucleophilic substitution by the hydroxide
ion. These revision
notes
include full diagrams and explanation of the nucleophilic substitution
reaction mechanisms of
halogenoalkanes (haloalkanes) and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained
when halogenoalkanes react with water and alkalis to give alcohols on hydrolysis.
Water, amines and hydroxide ion are typical electron pair donating
nucleophiles that can attack a partially positive carbon atom of a
carbon-halogen bond e.g. C-Cl, C-Br and C-I.
Sub-index for
this page
10.4.1
Introduction
to halogenoalkane reactivity
10.4.2
Nucleophilic substitution of halogenoalkane
by
hydroxide ion
(hydrolysis with OH-)
10.4.3
Nucleophilic substitution of halogenoalkane by direct hydrolysis with
water
10.4.4
Extra comments on hydrolysing of halogenoalkanes with
water or alkaline hydroxides
See also
Part 3
Index of
ALL revision notes on halogenoalkanes
Part 4
Index
of all revision notes on the physical and chemical properties of alcohols
[SEARCH
BOX]
10.4 HALOGENOALKANES
(old names 'haloalkanes'
or 'alkyl halides')
10.4.1 Introduction to
halogenoalkane reactivity
-
Halogenoalkanes
owe their reactivity, especially compared to the unreactive alkanes, to
two principal reasons.
-
R3C-X = halogenoalkane/haloalkane/alkyl halide/halogenated alkane
etc. X = halogen e.g. Cl, Br or I
-
The
carbon-halogen bond is polar, Cδ+-Xδ-
due to the difference in
electronegativity between carbon and the halogen.
-
The
Cδ+
carbon is then susceptible to nucleophilic attack by electron pair
donor neutral molecules (e.g. the nucleophiles :NH3, H2O:)
or ions (e.g. :OH-, -:CN).
-
The
carbon-halogen bond is usually the weakest bond
in the molecule and
significantly weaker than the carbon-carbon or carbon-hydrogen
bonds.
-
Average bond
enthalpies/kJmol-1: C-C 348, C-H 412, both relatively
high requiring high activation energies for reaction.
-
Average bond
enthalpies/kJmol-1: C-Cl 338, C-Br 276, C-I 238,
generally lower resulting in lower activation energies.
-
A comparison
of aliphatic halogenoalkanes and aromatic halides is dealt with in
FURTHER COMMENTS.
-
IMPORTANT NOTE on
structure classification
-
In the mechanism
diagrams you will see part of the molecular structure shown as R3C
-
PLEASE do not
assume this means a tertiary (tert) halogenoalkane (haloalkane).
-
R3C-
is used repeatedly to minimise the number of graphic images
needed.
-
In general a
halogenoalkane (haloalkane) has the structure R3C-X
where R = H, alkyl or aryl.
-
A primary
halogenoalkane (haloalkane) can be shown as RCH2-X
where R = H, alkyl or aryl.
-
A secondary (sec)
halogenoalkane (haloalkane) can shown as R2CH-X where
R = alkyl or aryl.
-
A tertiary (tert)
halogenoalkane (haloalkane) can be shown as R3C-X
where R = alkyl or aryl.
Remember:
A neutral or negative
nucleophile, Nuc: or Nuc:-, is an
electron pair donor that can attack an electron deficient partially/wholly
positive carbon atom to form a new C-Nuc bond.
10.4.2 Nucleophilic substitution of halogenoalkane by
hydroxide ion
(hydrolysis with OH-)
Organic synthesis of alcohols from halogenoalkanes (haloalkanes,
alkyl halides) by reaction with sodium/potassium hydroxide
-
What is the reaction
mechanism for the hydrolysis of halogenoalkanes/haloalkanes?
-
e.g. for the reaction hydrolysis with NaOH(aq):
[see mechanisms 1, 2, 33 and 34 below]
-
R3C-X
+ OH-
==> R3C-OH + X-
-
The
halogenoalkane is usually refluxed with aqueous sodium hydroxide,
NaOH(aq), but some RX molecules are reactive enough to
hydrolyse when just mixed with water (see
further down).
-
The reaction
can proceed by two different mechanisms,
which can occur simultaneously!, and are sometimes referred
to as SN1 'unimolecular' and SN2
'bimolecular', BUT these 'molecular' terms are based on
kinetic studies of the reaction and refer to the overall order of
the reaction (see reaction kinetics).
The vast majority of reaction steps occur via bimolecular
collisions, even if its only with the solvent (as in the case of
step (1) of the SN1 mechanism below), so beware of
terminology.
mechanism 1 -
nucleophilic substitution of a halogenoalkane by hydroxide ion
(SN1
'unimolecular' via carbocation)
Three diagram
'styles' are shown below for the SN2
bimolecular mechanism that does NOT involve a carbocation.
style (a)
mechanism 2 - nucleophilic
substitution of a halogenoalkane by hydroxide ion
(SN2 'bimolecular')
simplest representation of the one
step mechanism
style (b)
mechanism 34 -
nucleophilic substitution of a halogenoalkane by hydroxide ion
(SN2
'bimolecular')
showing the intermediate transition
state ('activated complex')
style
(c)

mechanism 33 -
nucleophilic substitution of a halogenoalkane by hydroxide ion
(SN2
'bimolecular')
showing the intermediate transition
state ('activated complex') in a 3D representation - note the inversion
of the crucial carbon atom, if its an asymmetric carbon atom you don't
produce the optical (R/S) stereoisomers you might expect!
-
SN2
'bimolecular', a
one step bimolecular collision mechanism
[mechanisms 2, 34 and 33 above]
-
The Cδ+-Xδ-
bond is polar
because of the difference in electronegativity between carbon (2.1)
and chlorine (3.0), so the electron rich nucleophile, the
hydroxide ion, attacks the slightly positive carbon.
-
The nucleophile
acts as an electron pair donor (Lewis base) to bond with the
Cδ+
carbon to make the C-O bond in the newly formed C-OH alcohol group.
-
Simultaneously
the chlorine atom is ejected,
taking with it the C-X bond pair, so forming the chloride ion on
expulsion.
-
This mechanism
is most likely with primary halogenoalkanes.
Tertiary halogenoalkanes tend to react by the SN1 mechanism
involving a carbocation, secondary halogenoalkanes react via both
mechanisms (see extra discussion points).
Diagram styles
and explanation:
-
Style (a)
Does not show the 'activated complex' or 'transition state' at the
point where the hydroxide ion ('incoming') and the chloride ion
('outgoing') are 'half-bonded' to the central carbon atom of the
functional group.
-
Style (b)
Shows a simplified version of the 'activated complex' or 'transition
state'. The
blue
....
represent 'half-bonds' (not weak inter-molecular forces) in the sense
that the hydroxy/hydroxide group is 'coming in' and the
chloro/chloride group is 'going out'!
-
The change
can also be represented as a simple
reaction progress-profile diagram 41.
-
Note that 'activated complex' or 'transition state' is not the
same as an intermediate like a carbocation which is a definite
entity in its own right, however short its lifetime.
-
Style (c)
Shows the reaction in terms of the stereochemistry. This is not a
particularly important 'style' unless the original halogenoalkane is
chiral (see note on chirality
below).
Also read the FURTHER COMMENTS for this reaction
10.4.3 Nucleophilic substitution of halogenoalkane by
direct hydrolysis with water
mechanism 10 -
nucleophilic substitution of a halogenoalkane by water
(SN1
unimolecular via carbocation)
mechanism 35
nucleophilic substitution of a halogenoalkane by water
(SN2 bimolecular)
Further notes
10.4.4 Extra comments on the hydrolysis of
halogenoalkanes with water or alkaline hydroxides
-
FURTHER COMMENTS on these
halogenoalkane (haloalkane) nucleophilic substitution reactions
-
Comparison of
aliphatic halogenoalkane and aromatic halide hydrolysis.
-
Halogenoalkanes hydrolyse much more readily than aryl halides
which are aromatic compounds with a halogen atom directly bonded
to the benzene ring. The carbon-halogen bond is stronger and
less polar in aromatic compounds compared to halogenoalkanes.
This is because in the aromatic halogen compounds there is some
overlap/delocalisation of the lone pairs of electrons of the
chlorine with the
π
electrons of the benzene ring. This makes the aromatic ring
C-Hal bond stronger and less easy to break, hence less
polar
and less susceptible to nucleophilic attack.
-
However,
if the halogen is in an alkyl side chain off the benzene ring,
hydrolysis will readily take place.
-
e.g.
refluxing chloromethylbenzene (phenylchloromethane, above left) with aqueous/ethanolic sodium hydroxide
gives phenylmethanol.
-
C6H5CH2Cl
+ NaOH
==> C6H5CH2OH
+ NaCl
-
or
or Whereas
hydrolysis of any of three other isomeric
aromatic halogen compounds, namely
chloro-2/3/4-methylbenzene (above left) where the Cl atom is
attached to the ring, is very difficult to achieve to
produce a phenol (where the OH hydroxy group is directly
attached to a benzene ring) by hydrolysis.
-
ClC6H4CH3
+ NaOH
===>
HOC6H4CH3 + NaCl
-
is
not impossible, BUT, NOT EASY and
couldn't be done in a school laboratory! though it can be brought
about at higher temperatures and high pressures than normal reflux conditions in
a school laboratory, but in an
'industrial' sealed reaction vessel context!
-
Rate of
reaction, OH-
versus H2O:
-
Only
applies to the SN2 mechanism:
Irrespective of the structure of the halogenoalkane, the
hydrolysis will be faster with aqueous sodium hydroxide than
just water because the hydroxide ion is a more powerful
nucleophile. Although they are both electron pair donors, the
hydroxide ion carries a full negative charge compared to the
electrically neutral water molecule.
-
The rate
determining formation of the carbocation in the SN1
mechanisms means the rate is not affected by using alkali or
just water (read about the kinetics below).
-
Reaction
kinetics: The
possibility of two reaction mechanisms has consequences for the rate
expressions when the rates of halogenoalkane (RX)
nucleophilic substitution reactions are studied.
-
The SN1
mechanism is referred to as a 'unimolecular',
despite it being a two/three step mechanism of bimolecular
collisions, because the rate is only dependent on one reactant, the
R3C-X is shown in step (1), but it still has to
collide with the solvent!
-
[see
mechanism 1
or mechanism 10 and
reaction profile 42]
-
Experimental
results produce the overall 1st order rate expression:
rate = k1[RX]
-
This is
because the activation energy of the 1st step, forming the
carbocation by heterolytic bond fission, is so high, that the
speed is relatively low, so step
(1) alone determines the speed of the reaction. This is
referred to as the rate determining step
(or rds in shorthand!). Step
(2) has a much lower activation energy and is much
faster (see reaction profile diagram 42).
You would register zero order for the order of reaction with
respect to sodium hydroxide (or more specifically, the hydroxide
ion concentration).
-
The SN2
mechanism is referred to as a 'bimolecular'.
-
[see
mechanisms 2/33/34 or mechanism
35 and reaction profile 41]
-
Experimental
results produce the overall 2nd order rate expression:
rate = k2[RX][OH]
-
This is
because it is a one step mechanism involving the bimolecular
collision of the two reactant molecules/ions (see also
reaction profile diagram 41). The rate depends on both the
halogenoalkane and hydroxide ion concentrations (individually,
1st order with respect to both).
-
The relative
reactivity of the C-X bond, where X = F, Cl, Br or I.
-
In general the
reactivity order is C-I > C-Br > C-Cl > C-F.
-
This is
due to the decrease in bond enthalpy as the halogen atom radius
increases the bond gets longer and weaker, i.e. easier to
form the carbocation in the SN1 mechanism or easier
to release a X- ion from the 'activated complex' in
the SN2 mechanism.
-
This bond
strength factor overrides any increase in bond polarity, which
on face value, since the order of increasing polarity is: C-F >
C-Cl > C-Br > C-I, which would suggest an increase in
susceptibility to nucleophilic attack at the delta positive
carbon, but that's not what is observed!
-
Carbon chain
structure and relative reactivity.
-
Con-current
nucleophilic substitution AND elimination reactions are
discussed at the end of the section on the elimination mechanism where
things get even more complicated.
-
Some points concerning the
STEREOCHEMISTRY of the reaction :-
-
What happens if the
original haloalkane has chirality?
-
AND what is the optical
activity of the product?
-
If the haloalkane
has three different R groups on the carbon of the C-Hal bond, i.e.
RR'R''CX, then there are four different groups bonded to the carbon
of the C-Hal bond. Therefore the molecule is chiral and can
exhibit optical isomerism
(non-super imposable mirror image forms). If the initial
halogenoalkane is an optical isomer, the stereochemical consequences
depend on which mechanism by which the halogenoalkane reacts. The
results can be very complex and their full explanation goes beyond
this level. However there are two product formation trends.
-
-
(Apologies for
repeating diagrams but it helps to appreciate these
stereochemical points)
-
In the SN1
carbocation mechanism (e.g.
mechanism 1 above), the three bonds of the R groups of
the carbocation formed in step (1), are in a trigonal planar
arrangement >C-. This means the nucleophile (e.g. OH-
or H2O)
can attack the carbocation with equal probability on each side.
-
This results in a tendency for a racemic mixture to form,
theoretically an optically inactive mixture of equal amounts of the
two R/S stereoisomers.
-
(What you actually get
in practice is a significant reduction in optical activity in the
product)
-
For an optically
active halogenoalkane reactant, the considerable reduction in
optical activity in a nucleophilic substitution reaction is
evidence of the SN1 unimolecular mechanism i.e. the
formation of a trigonal planar carbocation.
-
-
However, in
the case of the SN2 mechanism (e.g.
mechanism 33 above), racemisation
does NOT take place and chirality and optical activity is
completely preserved in the molecule, BUT inversion takes place
i.e. the absolute 3D configuration of the product is completely
opposite to that of the reactant.
-
Stereochemically the most successful line of attack for SN2
substitution, is if the nucleophile hits the carbon of the C-Hal
bond on the opposite side to the halogen atom. The result has
been likened to an umbrella being blown inside out in a gale!
The three single bonds for the -CRR'R'' are pushed through and
so the configuration inverted! [see mechanism 33 style (c)]
-
For an optically
active halogenoalkane reactant, the retention of complete
optical activity in a nucleophilic substitution reaction is
evidence of the SN2 bimolecular mechanism.
-
Reaction progress
profiles
to show the progress of SN1 and SN2 nucleophilic
substitutions

Reaction progress profile for the
SN1 hydrolysis of halogenoalkanes with the hydroxide
nucleophile OH-
e.g. for mechanism diagram 1 above, note that step 1 is the unimolecular
rate determining step, with the largest activation energy Ea1.
-
Reaction profiles
above:
-
Diagram 45
corresponds to mechanism 1 and diagram 42
corresponds to mechanism 10.
-
The 'progress'
of an SN1 reaction for e.g. tertiary halogenoalkane
hydrolysis with sodium hydroxide or water, can be represented by a
'double/triple hump' diagram where the troughs represents the formation
of the intermediates. The 2nd activation energy, Ea2,
is much smaller than the 1st activation energy, Ea1,
because, unlike the 1st step, no bond is broken in the 2nd step and the
water/hydroxide ion readily bonds to the carbocation. Ea3
is also small for the water hydrolysis because protons are readily
transferred in acid-base reactions.
Reaction progress profile for the
SN2 hydrolysis of halogenoalkanes with the hydroxide
nucleophile OH-
e.g. for mechanism diagram 2 above, note that step 1 is the bimolecular
rate determining step, single activation energy of Ea.
-
Reaction
profile - diagram 41 above (corresponds to
mechanism diagrams 2/33/34) The 'progress' of an SN2
reaction
e.g. for primary halogenoalkane hydrolysis with sodium
hydroxide, can be represented by a 'single hump' diagram
where the peak represents the formation of 'activated complex'
or 'transition state', in which the 'outgoing' Cl and the
'incoming' OH are 'half-bonded' with the functional group carbon
atom.
SN1
hydrolysis of a halogenoalkane with H2O (e.g. for
mechanism 10 above, note the triple hump!)
Step 1 is the unimolecular rate determining step,
with the largest activation energy Ea1

Reaction profile 45b for the SN2
hydrolysis of halogenoalkanes directly with water (for mechanism diagram 35
above)
Step 1 is the bimolecular rate determining step,
with the largest activation energy Ea1.
10.4.3 The nucleophilic substitution of halogenoalkane by
cyanide ion now on separate page
10.4.4 Nucleophilic substitution of a halogenoalkane with
ammonia or primary aliphatic amine
now on separate page
10.4.5 The
elimination of hydrogen bromide from a bromoalkane
now on separate page
TOP OF PAGE
APPENDIX
COMPLETE MECHANISM
and Organic Synthesis INDEX
(so far!)
hydrolysis mechanism
hydroxide ion or water with haloalkanes halogenoalkanes
for AQA AS chemistry, hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes
for Edexcel A level AS chemistry, hydrolysis mechanism hydroxide ion or
water with haloalkanes halogenoalkanes for A level OCR AS chemistry A,
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for OCR Salters AS chemistry B,
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for AQA A level chemistry, hydrolysis
mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for A level Edexcel A level chemistry,
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for OCR A level chemistry
A, hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for A level OCR Salters A
level chemistry B hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes for US Honours grade 11 grade 12
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for
pre-university chemistry courses pre-university A level revision
notes for hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes A level guide
notes on hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for schools colleges academies science course tutors images
pictures diagrams for hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes A level chemistry revision notes on
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for revising module topics notes to help on understanding of
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes university courses in science
careers in science jobs in the industry laboratory assistant
apprenticeships technical internships USA US grade 11 grade 11 AQA A
level chemistry
notes on hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes Edexcel
A level chemistry notes on hydrolysis mechanism hydroxide
ion or water with haloalkanes halogenoalkanes for OCR A level chemistry
notes WJEC A level chemistry notes on hydrolysis mechanism
hydroxide ion or water with haloalkanes halogenoalkanes CCEA/CEA A level
chemistry notes on hydrolysis mechanism hydroxide ion or
water with haloalkanes halogenoalkanes for university entrance examinations
keywords phrases: reaction conditions formula
intermediates organic chemistry reaction mechanisms nucleophilic substitution R3C-X + OH-
==> R3C-OH + X- R3C-X + 2H2O ==> R3C-OH + X- + H3O+ C6H5CH2Cl + NaOH ==>
C6H5CH2OH + NaCl ClC6H4CH3 + NaOH ==> HOC6H4CH3 + NaCl (CH3)3CBr > (CH3)2CHBr >
CH3CH2Br > CH3Br (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+ hydrolysis
mechanism hydroxide ion or water with haloalkanes halogenoalkanes
for AQA AS chemistry, hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes
for Edexcel A level AS chemistry, hydrolysis mechanism hydroxide ion or
water with haloalkanes halogenoalkanes for A level OCR AS chemistry A,
hydrolysis mechanism hydroxide ion or water with haloalkanes halogenoalkanes for OCR Salters AS chemistry B,
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for AQA A level chemistry, hydrolysis mechanism hydroxide ion or
water with haloalkanes halogenoalkanes for A level Edexcel A level chemistry,
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for OCR A level chemistry
A, hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for A level OCR Salters A
level chemistry B hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes for US Honours grade 11 grade 12 hydrolysis
mechanism hydroxide ion or water with haloalkanes halogenoalkanes for
pre-university chemistry courses pre-university A level revision
notes for hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes A level guide
notes on hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for schools colleges academies science course tutors images
pictures diagrams for hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes A level chemistry revision notes on
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes for revising module topics notes to help on understanding of
hydrolysis mechanism hydroxide ion or water with haloalkanes
halogenoalkanes university courses in science
careers in science jobs in the industry laboratory assistant
apprenticeships technical internships USA US grade 11 grade 11 AQA A
level chemistry
notes on hydrolysis mechanism hydroxide ion or water with
haloalkanes halogenoalkanes Edexcel
A level chemistry notes on hydrolysis mechanism hydroxide
ion or water with haloalkanes halogenoalkanes for OCR A level chemistry
notes WJEC A level chemistry notes on hydrolysis mechanism
hydroxide ion or water with haloalkanes halogenoalkanes CCEA/CEA A level
chemistry notes on hydrolysis mechanism hydroxide ion or
water with haloalkanes halogenoalkanes for university entrance examinations how
do you explain the reactivity of xyz towards nucleophiles like the hydroxide
ion? how do bond strength and bond polarity affect the reactivity of xyz?, give
the mechanism of hydrolysis of xyz with sodium hydroxide, describe the
difference between an unimolecular SN1 mechanism and a bimolecular SN2
mechanism, why can't aryl chlorides like chlorobenzene be readily hydrolysed by
sodium hydroxide solution? how does the relative stability of carbocations
affect the rate of nucleophilic attack on xyz? how does the stability of
carbocations affect the mechanistic steps of the hydrolysis of xyz with sodium
hydroxide?
TOP OF PAGE
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