Nucleophilic substitution and elimination Flashcards
What are nucleophiles?
species that donate a pair of electrons to form a covalent bond
What are the three types of nucleophilic substitution and the conditions required?
-with NaOH/KOH (heated with aqueous) to form alcohols
-with NaCN/KCN (ethanolic and heated) to form nitriles
-with with 2NH3 (heat under pressure and excess ammonia in a sealed contained) to form primary amines
What happens during nucleophilic substitution?
-the lone pair on the nucleophile forms a covalent bond with the slightly positive carbon atom
-the C-H bond breaks by heterolytic fission
-the nucleophile bonds in its place and a halide ion is formed
What is the process of halogenoalkanes with aqueous hydroxides sometimes alternatively called?
hydrolysis
How do you name a nitrile after the nucleophilic substitution of CN?
you include the CN carbon when naming it, and add nitrile on the end
Why is the nucleophilic substitution of CN important?
it provides a way of increasing the length of the carbon chain
What can happen to the nitrile formed after the nucleophilic substitution of CN?
nitriles are readily hydrolysed in acid solution to form carboxylic acids
- add 2H2O and HCl
Explain the role of ammonia in the two steps of the nucleophilic substitution reaction
-first step it acts as a nucleophile
-second step it acts as a base (accepts a H+)
Why do we use excess ammonia?
to prevent successive substitution (reduces the chance of further reaction of the primary amine with halogenoalkanes to produce secondary/tertiary amines)
What is different about halogenoalkanes reacting with hydroxide ions?
both substitution and elimination reactions can occur concurrently
What are the respective products of a substitution and an elimination reaction?
substitution produces alcohols, elimination produces alkenes
What are the products of an elimination reaction?
water and a halide ion
What is the difference in the role of the OH ion in the respective substitution and elimination reactions?
in substitution it acts as a nucleophile, in elimination it acts as a base
What conditions make it more likely for a substitution reaction to occur?
-less branching
-primary halogenoalkanes
-using water as the solvent
-lower temperatures
What conditions make it more likely more an elimination reaction to occur?
-increased branching
-tertiary halogenoalkanes
-using ethanol as the solvent
-higher temperatures
