9701 · 15.1
Halogenoalkanes — FAQ
Frequently asked questions for 9701 Halogenoalkanes. Direct answers first, then deeper explanation — then practise with marking.
Why does an SN1 reaction produce a racemic mixture?
The SN1 mechanism proceeds via a planar carbocation intermediate. This flat structure can be attacked by the incoming nucleophile from either the top face or the bottom face with equal probability. If the carbon atom is a chiral centre, this leads to the formation of both enantiomers in equal amounts, resulting in a 50:50 mixture called a racemic mixture.
What is the role of ethanol as a solvent in elimination reactions?
Ethanol is a less polar solvent than water. It prevents the hydroxide ions from being heavily solvated (surrounded by solvent molecules), which makes them act as a stronger base. A stronger base is more effective at removing a proton, favouring the elimination pathway over the substitution pathway.
Why are fluoroalkanes so unreactive in nucleophilic substitution?
The C-F bond is the strongest of all the carbon-halogen bonds due to the small size and high electronegativity of fluorine, leading to significant orbital overlap. A large amount of energy is required to break this bond (high bond enthalpy), so the activation energy for substitution is very high, making the reaction extremely slow.
Can secondary halogenoalkanes undergo both SN1 and SN2?
Yes, secondary halogenoalkanes are intermediate. They can undergo both SN1 and SN2 mechanisms, often simultaneously. The exact pathway and product distribution depend on the specific conditions: the strength of the nucleophile, the solvent, and the temperature.