THE NUCLEOPHILIC SUBSTITUTION REACTIONS BETWEEN HALOGENOALKANES AND HYDROXIDE
THE NUCLEOPHILIC SUBSTITUTION REACTIONS BETWEEN HALOGENOALKANES AND HYDROXIDE IONS 2. 11 f. describe the mechanisms of the substitution reactions of halogenoalkanes and recall those in 1. 7. 2 e and 1. 7. 3 e Connector: 1. Describe the reagents, conditions required, and the product formed, when 1 -bromopropane undergoes a nucleophilic substitution reaction with hydroxide ions. 2. Sketch the apparatus used for above the reaction. 3. Write the chemical equation for this reaction. Crowe 2009
If a halogenoalkane is heated under reflux with a solution of sodium or potassium hydroxide, the halogen is replaced by -OH and an alcohol is produced. Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture.
The mechanism of the reaction between a primary halogenoalkane and aqueous hydroxide ions This is an example of nucleophilic substitution. Because the mechanism involves collision between two species in the slow step of the reaction, it is known as an SN 2 reaction.
The mechanism of the reaction between a tertiary halogenoalkane and aqueous hydroxide ions 1. The mechanism involves an initial ionisation of the halogenoalkane: followed by a very rapid attack by the hydroxide ion on the carbocation (carbonium ion) formed:
The mechanism of the reaction between a tertiary halogenoalkane and aqueous hydroxide ions 2. followed by a very rapid attack by the hydroxide ion on the carbocation (carbonium ion) formed: This is again an example of nucleophilic substitution.
The mechanism of the reaction between a tertiary halogenoalkane and aqueous hydroxide ions 3. The slowest step in a reaction sequence controls the overall rate of a reaction. This time the slow step of the reaction only involves one species - the halogenoalkane. It is known as an SN 1 reaction.
The SN 2 bimolecular nucleophilic substitution reaction (Primary halogenoalkanes) primary halogenoalkane Be very careful when you draw the transition state to make a clear difference between the dotted lines showing the half -made and half-broken bonds, and those showing the bonds going back into the paper.
The SN 1 unimolecular nucleophilic substitution reaction (Tertiary halogenoalkanes)
Evidence for SN 1 and SN 2 reactions 1. • What happens to the rate of reaction when the concentration of bromoalkane A is changed? • What happens to the rate of reaction when hydroxide ion concentration is changed? • SN 1 or SN 2?
Evidence for SN 1 and SN 2 reactions 2. 1. What happens to the rate of reaction when the concentration of bromoalkane B is changed? 2. What happens to the rate of reaction when hydroxide ion concentration is changed? 3. SN 1 or SN 2?
Revision questions - Other mechanisms 1. Describe the mechanism of the chlorination of methane. a. What conditions are required for the reaction to take place? b. What name is given to this type of reaction? 2. Describe the mechanism of the reaction between a. Bromine and ethene b. Hydrogen bromide and ethene c. What name is given to this type of reaction? 3. Describe the mechanism of the reaction between hydrogen bromide and propene. a. What name is given to this type of reaction?
The mechanism of the chlorination of methane two chlorine radicals
Chain propagation reactions 1. H 3 C H + Cl CH 3 + H-Cl Homolytic fission CH 3 + Cl Cl CH 3 Cl + Cl chlorine radical regenerated
Chain propagation reactions 2.
Chain termination reactions These are reactions which remove free radicals from the system without replacing them by new ones. 2 Cl CH 3 + Cl + CH 3 Cl 2 CH 3 Cl CH 3
The electrophilic addition of bromine
Explaining the addition mechanism of HBr and propene secondary carbocation – more stable Markovnikov addition H goes to side with most H primary carbocation – less stable Which mechanism has the more stable carbocation? The more stable the carbocation, the faster it forms and so the faster the reaction occurs.
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