Topic 10.5: Halogenoalkanes

10.5.1 Describe, using equations, the substitution reactions of halogenoalkanes with sodium hydroxide

Halogenoalkanes can be classified according to their structure as primary, secondary or tertiary.

This class of structure plays an important role in the reactions of the halogenoalkanes. Halogenoalkaenes contain polar carbon - halogen bonds that make them easy to convert to other products

Halogenoalkanes will undergo nucleophilic substitution (Sn) reactions. This involves the attack of a negatively charged / neutral electron rich species (called a nucleophile) on the slightly positive carbon atom bonded to the halogen

This causes the polar carbon - halogen bond to break so that the halogen is substituted by the nucleophile

The are two mechanisms for nucleophilic substitution reactions, called SN1 and SN2. This stands for substitution nucleophilic first or second order.

10.5.2 Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms

Tertiary halogenoalkanes react in a SN1 mechanism

The rate of this reaction is dictated by one factor, the concentration of the halogenoalkane

Only the halogenoalkane is involved in the rate expression and therefore involved in the rate determining step

This is therefore a first order reaction that is unimolecular process with a molecularity of one

The first stage of the mechanism involves the breaking of the carbon - halogen bond to give an intermediate carbocation

This is bond breaking is called heterolytic fission. This is a slow process and so is the rate-determining step

The second stage involves the nucleophile bonding with the carbocation intermediate. This happens very quickly. It therefore does not appear in the rate expression

Note: To gain all the marks off this questions, this is directly pulled off the answers of past papers

SN1:
Curly arrow showing Cl leaving;
Representation of tertiary carbocation;
curly arrow going from lone pair / negative charge on O in OH- to C+;
Do not allow arrow orginating on H in OH-;
Formation of organic product
(CH3)3COH and CL-


SN2 mechanism has primary halogenoalkanes reacting this way.

The rate of reaction is dictated by two factors. By both the concentration of the halogenoalkane and the concentration of the nucleophile

Both the halogenoalkane and the nucleophile are involved in the rate expression and therefore both are involved in the rate determining step

This is a second order reaction, bimolecular process with a molecularity of two.

This mechanism involves the simultaneous attack of the nucleophile and loss of the halogen

An activated complex is formed during the process

Note: To gain all the marks off this questions, this is directly pulled off the answers of past papers

SN2:
Curly arrow going from lone pair / negative charge on O in OH- to C;
Do not allow curly arrow originating on H in OH-:
Curly arrow showing Br leaving;
Accept curly arrow either going from bond between C and Br to Br in bromoethane or in the transition state;
Representation of transition state showing negative charge, square bracket and partial bonds;
Do not penalize if HO and BR are not a t 180 to each other
Do not award M3 if OH---C bond is represented
Formation of organic product CH3CH2OH and Br-