Halogenoalkanes — common mistakes

When drawing mechanisms, curly arrows must be precise. They must start from a source of electrons (a lone pair or a bond) and point to the destination (an atom or a space where a bond will form). Always include all partial charges (δ+, δ−) and full ionic charges where appropriate. Marks are frequently lost for inaccurate arrows.

Be extremely precise with reaction conditions in your exam answers. 'Warm, aqueous NaOH' leads to substitution to form an alcohol. 'Hot, ethanolic KOH' leads to elimination to form an alkene. Mixing these up is a common mistake that will lose you marks.

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.

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.

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.

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.