Question

Flask A - mix of benzophenone + alumunium isopropoxide + isopropanol
Flask B - mix of benzophenone + methanol + sodium borohydride

Each of product dissolved in acetone and added by a drop of potassium permanganate

1. which one has larger amount of brown precipitate and why
2. is brown precipitate implies an oxidation reaction and shows that alcohol was likely present?

Can you explain it detail.

Answer 1

1. Potassium permanganate oxidises alcohols and is itself reduced. The brown precipitate is the reduced form of permanganate.

Potammisum permangante oxidises primary alcohol ( methanol) to carboxylic acid and it oxidises secondary alcohol ( isopropanol) to ketone. The oxidation of primary alcohol consumes more permanganate as primary alcohol is oxidised to aldehyde in the first step and then it is oxidised to acid. Where as the oxidation of secondary alcohol stops at first step i.e to carbonyl (ketone).

As primary alcohol consumes two equivalents of potassium permanganate for complete oxidation, more brown precipitate forms here. So, flask B produces more brown precipitate.

2. Brown precipitate does not always imply that alcohol was present. Because aldehydes can also be oxidised by permanganate to give the brown precipitate.