Advanced Higher Chemistry

Unit 3

Halogenoalkanes

Halogenalkanes Also known as haloalkanes or alkyl halidesOrganic 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 C X

H

R

R C X

H

R

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.

Examples2,3-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 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!!

• A bond begins to form between the oxygen and carbon atoms, at the SAME time the carbon-bromine bond weakens.

Substitution

Nucleophilic

RDS involves 2 particles

SN

2

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) 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.

2) The nucleophilic O atom on the water can then attack the +ve carbon atom and form the alcohol.

NOTEIf 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!!

Substitution

Nucleophilic

RDS involves 1 particle

SN

1

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 ofSpecific Alcohols (hence ketones,

aldehydes and alkanoic acids)Amines (using ammonia)Synthesis of ethersSynthesis 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

• 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.

Alkylammonium ion(intermediate)

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 versaSodium ethoxide is produced by the reaction of sodium with a dry sample of alcohol. e.g. Na + C2H5OH C2H5O- Na+ + H2

Halogenalkanes – Synthesis of Ethers

Halogenalkanes – Synthesis of Nitrilese.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.