Lesson 4 halogenoalkanes

Advanced Higher Chemistry
Unit 3
Halogenoalkanes
Halogenalkanes
Also known as haloalkanes or alkyl halides
 Organic compounds containing halogens are rare
in the natural world i.e. most are synthetic.
 Important uses include

 in medicine, e.g. chloroform (trichloromethane)
 in agriculture as pesticides
 in plastics e.g. PVC, PTFE
 as solvents e.g. carbon tetrachloride
 Unfortunately they are implicated in
environmental damage to the planet, notably in the
overuse of pesticides and in damage to the ozone
layer.
Classification of halogenoalkanes
Classified as
primary
(1°)
secondary (2°) tertiary (3°)
H

R
R
C
H
X
R
C
H
R
X
R
C
X
R
Primary – the carbon atom carrying the halogen has only one alkyl group or
two hydrogen atoms attached to it.
Secondary – the carbon atom carrying the halogen has two alkyl groups or
one hydrogen atom attached to it.
Tertiary – the carbon atom carrying the halogen has three alkyl groups or no
hydrogen atoms attached to it.
Nomenclature of halogenoalkanes





The more complicated the molecule, the greater the
possibility there is for structural isomerism.
The presence of a halogen atom is shown by the
appropriate prefix: fluoro-, chloro-, bromo-, or iodo-.
If the molecule contains more than one halogen atom of
the same type this is shown by the prefixes di-, tri-, tetra, etc.
The position of the halogen is shown by a number in
front of the prefix.
The substituents are listed in alphabetical order e.g.
dibromo comes before chloro because ‘b’ comes before
‘c’. Prefixes such as di and tri are ignored.
Examples
2,3-dichloro-3-methylpentane
1,2-dichloropropane
3-bromo-2-methylpentane
Exercise

Now complete the exercise on page 18 of
your unit 3(b) notes.
Bonding in Halogenalkanes

All bonds in halogenalkanes are sigma bonds
(see Bonding in Alkanes).
Synthesis of Halogenalkanes
See Alkenes – Hydrogen halide addition for
monohalogenalkanes.
 See ‘Alkenes – Halogenation’ for
dihalogenalkanes.

Reactions of Halogenalkanes

Two main types of reaction
 Nucleophilic Substitution
 Elimination


The C-X bond is fairly polar, due to the difference in
electronegativity between carbon and the halogens.
Reactivity seems to be related to the bond strength since the
order of reactivity is generally
R-I > R-Br > R-Cl > R-F
weakest
strongest
bond
bond
and to the position of the carbon-halogen bond within the
molecule.
Halogenalkanes - Nucleophilic Substitution

The following terms are often used when discussing
substitution reactions –
Y +
R3C-X  R3C-Y +
X
NUCLEOPHILE


SUBSTRATE
PRODUCT
LEAVING
GROUP
Because the C-X bond is polar, with the C carrying a partial
positive charge, the C will be susceptible to attack by
nucleophiles.
If the C-X bond breaks heterolytically, and X- ion will be
formed. Cl-, Br- and I- are all stable ions and are regarded as
good leaving groups i.e. the presence of these atoms in a
molecule will facilitate the heterolytic cleavage of a bond.

Experimental evidence has shown that there
are two possible mechanisms for nucleophilic
substitution reactions
 The SN2 Reaction
 The SN1 Reaction
Halogenalkanes - The SN2 Reaction
E.g. – Hydrolysis of bromoethane, a primary
halogenalkane, in an aqueous alkali solution.
-
-
C2H5Br(l) + OH (aq)  C2H5OH(aq) + Br (aq)

A study of the reaction kinetics show that the reaction
is first order with respect to (w.r.t.) hydroxide ions and
first order w.r.t. bromoethane.
i.e.

-
Rate = k[C2H5Br][OH ] (see Unit 2)
This means the Rate Determining Step (RDS) must
involve a bromoethane molecule and a hydroxide ion
SN2 – The Mechanism
Transition State
•
The nucleophilic hydroxide ion approaches the partial positive carbon (from
the opposite side of the bromine atom).
• A bond begins to form between the oxygen and carbon atoms, at the
SAME time the carbon-bromine bond weakens.
• A transition state will form with a ½ O-C bond and ½ C-Br bond, only
IF there was enough energy in the collision.
• The O-C bond forms completely, the C-Br bond breaks completely
NB - If the initial halogenalkane is chiral (see later) this causes an
inversion of chirality. For this reason the 3-D representation
of this mechanism IS IMPORTANT!!
S
Substitution
N
Nucleophilic
2
RDS involves 2
particles
Halogenalkanes - The SN1 Reaction

E.g. – Hydrolysis of 2-bromo-2-methylpropane, a
tertiary halogenalkane, in water.
(CH3)3CBr(l) + H2O(l)  (CH3)3COH(aq) + HBr(aq)

A study of the reaction kinetics show that the reaction is
first order w.r.t. the halogenoalkane but zero order w.r.t.
water.
i.e. Rate = k[(CH3)3CBr(l) ]

This means the Rate Determining Step (RDS) must
involve only the halogenalkane.
SN1 – The Mechanism
1)
2)
The C-Br bond breaks heterolytically forming a
planar carbocation (stabilised by the electron
donating effect of the alkyl groups, see later slide)
and a bromide ion.
The nucleophilic O atom on the water can then
attack the +ve carbon atom and form the alcohol.
NOTE
If the halogenalkane is chiral (see later), the product
will be a racemic mixture (see later) as the
intermediate carbocation is planar and can be
attacked from either side. For this reason the 3-D
representation of this mechanism is NOT
important!!
S
Substitution
N
Nucleophilic
1
RDS involves 1
particle
SN1 or SN2 Hydrolysis?

SN1 favoured by –
 Tertiary halogenalkanes
(carbocation stabilised by alkyl groups)
 Highly polar solvents

SN2 favoured by –
 Primary and secondary halogenalkanes
 Presence of OH- ions (i.e. alkaline solution)
Stability of carbocations

The order of stability of carbocations is:
primary < secondary < tertiary

Alkyl groups have a tendency to push electrons
towards a neighbouring carbon atom hence, in a
tertiary carbocation the three alkyl groups help
stabilise the positive charge on the tertiary
carbon atom. A primary carbocation has only
one alkyl group so will therefore be much less
stable.
Halogenalkanes – Importance of Substitution

Synthesis of
 Specific Alcohols (hence ketones,
aldehydes and alkanoic acids)
 Amines (using ammonia)
 Synthesis of ethers
 Synthesis of nitriles
Halogenalkanes – Synthesis of Alcohols
R-X  R-OH
 Alcohols can then be oxidised to aldehydes or
ketones. Aldehydes can then be oxidised to
form alkanoic acids.
 See SN1 and SN2 mechanism for specific
examples.
Halogenalkanes – Synthesis of Amines
Alkylammonium ion
(intermediate)
• The polarity of the N-H bond and the lone pair of
electrons allow ammonia to act as a nucleophile.
• The ammonia molecule attacks the slightly
positive carbon atom, displacing the halide ion.
Removal of the hydrogen ion then produces the
amine.
Halogenalkanes – Synthesis of Ethers
Especially
for unsymmetrical ethers.
e.g. C2H5O- Na+ + BrCH3  C2H5OCH3 + Na+ BrReaction is carried out at low temperature, otherwise
elimination reaction may dominate (due to the alkoxide
ion being a base as well as a nucleophile)
NOTE All nucleophiles are bases and vice versa
Sodium ethoxide is produced by the reaction of
sodium with a dry sample of alcohol.
e.g. Na + C2H5OH  C2H5O- Na+ + H2
Halogenalkanes – Synthesis of Nitriles
e.g.
CH3CH2CH2I + K+CN-  CH3CH2CH2CN + I CN- is the cyanide ion.
 Reaction is carried out under reflux.
 Reaction is useful as it extends the carbon chain.

Nitriles can then be converted into alkanoic acids or
amines.
Halogenalkanes – Elimination Reaction




Halogenalkanes will form alkenes in the presence of a
strong base.
This involves the removal (i.e. elimination) of a
hydrogen halide.
e.g. CH3CH2CH2Br
 CH3CH=CH2 + HBr
Nucleophiles are bases and vice versa, so in a
reaction there will be elimination and substitution
reactions occurring at the same time.
The reaction conditions will determine which process
dominates.
•
There are two possible mechanisms, E1 and E2.
•
The mechanism that dominates will depend on the
strength of the base and the environment of the
halogen atom (1°, 2° or 3°)
Halogenalkanes – E1 Reaction
• Favoured for tertiary halogenalkanes due to
stabilisation of the carbocation by the electron
donating effect of the alkyl groups.
• Only 1 particle involved in the RDS.
Halogenalkanes – E2 Reaction
• 2 particles involved in the RDS.
Exercise

Complete the exercise on page 24 of your
Unit 3(b) notes.