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Revising organic chemistry
Doc Brown's
Summary of organic reaction mechanisms
Part II Halogenoalkanes
Revision
notes
include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained.
Revision notes for GCE Advanced
Subsidiary Level AS Advanced Level A2 IB
Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters
Chemistry CIE Chemistry revising courses for pre-university students
(equal to US grade 11 and grade 12 and Honours/honors level courses)
HALOGENOALKANES
(old names 'haloalkanes'
or 'alkyl halides')
Halogenoalkanes
owe their reactivity, especially compared to the unreactive alkanes, to
two principal reasons.
RX = halogenoalkane/haloalkane/alkyl halide/halogenated alkane
etc.
-
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. :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.
Nucleophilic substitution of halogenoalkane by
hydroxide ion
(hydrolysis with OH-)
-
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')
style (b) 
mechanism 34 -
nucleophilic substitution of a halogenoalkane by hydroxide ion
(SN2
'bimolecular')
style
(c)

mechanism 33 -
nucleophilic substitution of a halogenoalkane by hydroxide ion
(SN2
'bimolecular')
-
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
*
mechanism index

mechanism 10 -
nucleophilic substitution of a halogenoalkane by water
(SN1
unimolecular via carbocation)

mechanism 35
nucleophilic substitution of a halogenoalkane by water
(SN2 bimolecular)
-
SN2
'bimolecular', a
two step bimolecular collision mechanism but NOT involving a
carbocation.
[mechanism 35 above]
-
Step
(1): 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 water molecule, attacks the slightly
positive carbon. The
nucleophilic water acts as an electron pair donor (Lewis base) to
bond with the 'delta positive' carbon to give the C-O bond of the
protonated alcohol. Simultaneously the chlorine atom is ejected,
taking with it the C-X bond pair, so forming the chloride ion on
expulsion. Step (1)
is the rate determining step (rds) and the rate effectively only
depends on the halogenoalkane concentration.
-
In step
(2)
another water molecule rapidly accepts the proton from the protonated
alcohol to leave the free alcohol product.
-
The reaction
profile would be similar to diagram 45
-
This mechanism
is most likely with primary halogenoalkanes,
but very slow if at all with water, much faster with the hydroxide
ion, a more powerful nucleophile (negative ion as well as an
electron pair donor). Tertiary halogenoalkanes tend to react
by the SN1 mechanism involving a carbocation, secondary
halogenoalkanes react via both mechanisms (see
extra discussion points).
-
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
∏
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 the 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 EASY!
though it can be brought about at higher temperatures and high pressures
than normal reflux conditions in the school laboratory, and in an
'industrial' sealed reaction vessel.
-
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 becoming less polar 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,
that is an optically inactive mixture of equal amounts of the
two optical isomers.
-
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.
SN1 with OH-
SN1
with H2O
-
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
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.
The nucleophilic substitution of halogenoalkane by
cyanide ion

mechanism 36 -
nucleophilic substitution of a halogenoalkane by cyanide ion
(SN1
'unimolecular' via carbocation)

mechanism 8 -
nucleophilic substitution of a halogenoalkane by cyanide ion
(SN2
'bimolecular')
Nucleophilic substitution of a halogenoalkane with
ammonia or primary aliphatic amine

mechanism 37
- nucleophilic substitution of a halogenoalkane by ammonia
(SN1 unimolecular via carbocation)
-
SN1
unimolecular, a
three step ionic mechanism via carbocation formation
[mechanism
37 above]
-
In step
(1) the
Cδ+-Brδ-
polar bond of the halogenoalkane splits heterolytically to
form a carbocation and a free halide ion (e.g. chloride or bromide)
and this is a reversible reaction.
-
In step
(2) the nucleophilic
electron pair donating ammonia molecule rapidly adds to the
carbocation to give the protonated amine product R3C-NH2+.
-
In step
(3)
one of the excess ammonia molecules can remove a proton to leave the
primary amine product.

mechanism 9 -
nucleophilic substitution of a halogenoalkane by ammonia
(SN2 bimolecular)
-
SN2
'bimolecular', a
two step mechanism [mechanism 9
above]
-
Step
(1)
the Cδ+-Brδ-
bond is polar, so the electron rich nucleophile, the
ammonia molecule, attacks the slightly positive carbon. The
nucleophile acts as an electron pair donor (Lewis base) to bond with
the 'positive' carbon. Simultaneously the bromine atom is ejected,
taking with it the C-Br bonding pair of electrons, so forming the
bromide ion.
-
In step
(2)
one of the excess ammonia molecules can remove a proton to leave the
primary amine product.
-
Further substitution
can take place because the
amine product itself is a nucleophile.
-
e.g. if you start
with bromomethane and react it with ammonia, the following products
can be formed ..
-
CH3Br
+ NH3
==> [CH3NH3]+Br-
-
the primary amine,
methylamine, CH3NH2, as its bromide
salt
-
CH3NH2
+ CH3Br
==> [(CH3)2NH2]+Br-
-
the secondary
amine, dimethylamine, (CH3)2NH,
as its bromide salt
-
(CH3)2NH
+ CH3Br
==> [(CH3)3NH]+Br-
-
the
tertiary amine, trimethylamine, (CH3)3N,
as its bromide salt
-
With a primary aliphatic amine a secondary aliphatic amine is formed:
[mech's
38
and 11 below]
-
The reaction of a
halogenoalkane and excess primary amine can be written as ...
-
R3C-Br
+ 2R'-NH2 ==> R3C-NHR' + [R'-NH3]+Br-
-
The
halogenoalkane is heated with excess concentrated
ethanolic solution of the primary amine in a sealed vessel to
form a secondary amine.
-
The
secondary amine product is freed by adding strong alkali
e.g. aqueous sodium hydroxide, NaOH(aq).
-
[R3C-NH2R']+
+ OH- ==> R3C-NHR' + H2O
-
The mechanisms for the reaction of primary amines with halogenoalkanes are
essentially the same as for ammonia above.
So all the general comments for ammonia apply here too, so I will not
repeat them, where it says ammonia, just say
primary amine! In step
(3)
one of the excess primary amine molecules can remove a proton to leave
the primary amine product.

mechanism 38 -
nucleophilic substitution of a halogenoalkane by a primary amine
(SN1 unimolecular via carbocation)

mechanism 11 - nucleophilic substitution of a halogenoalkane by a
primary amine (SN2 bimolecular)
The
elimination of hydrogen bromide from a bromoalkane

mechanism 26
elimination of HBr from a halogenoalkane by hydroxide ion
(E2 'bimolecular')

mechanism 27 -
elimination of HBr from a halogenoalkane by hydroxide ion
(E1 'unimolecular')
-
E1 mechanism,
two step
process via carbocation formation. [mechanism
27 above]
-
In step
(1) the
polar and weakest bond, Cδ+-Brδ-,
breaks heterolytically to form a carbocation and bromide ion.
-
In step
(2)
the strongly basic and nucleophilic hydroxide ion abstracts a
proton from the carbocation to form water and simultaneously the C-H
bond pair (bottom left) shifts to complete the C=C double bond of the
alkene.
-
FURTHER COMMENTS
-
Con-current
nucleophilic substitution AND elimination reactions. RX =
halogenoalkane/haloalkane/alkyl halide/halogenated alkane.
-
i.e. nucleophilic
substitution to give an alcohol versus elimination to give an alkene.
-
With
halogenoalkanes of at least C2, the use of strong
alkali like sodium hydroxide can also produce alkene products by
an elimination whose mechanism
is discussed above.
-
Elimination E2
is favoured
by ethanol solvent and substitution is favoured by
water/aqueous media.
-
One explanation
is that in ethanol, the ethoxide ion is formed, and is a stronger
base than water, hence better at abstracting a proton in the E2
mechanism and competes with the nucleophilic attack of the OH-
ion at the Cδ+-Xδ-
bond in the SN2 mechanism.
-
Also, although ethanol
is polar, it is less polar than water and a more polar solvent favours
substitution. A lower temperature also favours elimination and ethanol
boils at 79oC, 21oC lower than water (bpt. 100oC).
-
Elimination E1
is more favoured by increase in alkyl groups attached to the
carbon of the C-halogen bond, therefore yield of alkene
increases in the order tert RX
>sec RX > prim RX
-
This is due to the
with the greater stability of the intermediate
carbocation which is more likely to be formed in the order tert RX >
sec RX > prim RX (as is the preference of the SN1
mechanism!),
-
so carbocation
stability is tert R3C+ > sec R2CH+ > prim
RCH2+ (R = alkyl),
-
therefore increasing
stability of the carbocation, increases the chance of proton loss from the carbocation to give the alkene (see
carbocation stability discussion).
-
e.g. (i) after
refluxing ethanolic sodium hydroxide (CH3CH2OH/NaOH),
bromoethane (prim) gives 1% ethene, whereas 2-bromopropane (sec) gives 80%
propene. (ii) At 80oC in aqueous-ethanol/NaOH
2-bromo-2-methylpropane (tert) gives 19% methylpropene and
2-bromopropane (sec) gives 5% propene. Note in the 2nd example the
presence of water significantly reduces the alkene yields, but the
tert yield is greater than the sec.
-
Elimination E2
is favoured by an even stronger base, particularly in the less
polar ethanol, than sodium
hydroxide e.g. like potassium
hydroxide since in the E2 mechanism the hydroxide ion abstracts
a proton from the halogenoalkane.
-
See comments
under using ethanol solvent, and probably more significant for prim >
sec > tert halogenoalkanes.
-
Its worth noting that
with weak bases like ammonia very little elimination occurs.
-
Summing up: The
'classic' conditions to maximise the yield of an alkene are refluxing the
halogenoalkane with ethanolic potassium hydroxide. Compared to water, it
involves the less polar ethanol, a lower reaction temperature. The method
also uses the strongest 'common' base and further more, the yield
of alkene will increase for tert RX > sec RX > prim RX.
-
-
Copyright Dr W P Brown 2000-2010
All rights reserved on the revision notes pages, quizzes, worksheets, x-words
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