Question

How to prepare penta bromobenzene from aniline?

Answer 1

Hint: As we know that the amine group in the aniline acts as electron- donating group and hence it can easily allow bromine to add up at ortho and para positions respectively. Then, to activate meta positions, we will require amine to convert into electron withdrawing group which can be done through acylation reaction. And then we will remove this group by diazotization process followed by final bromination to form penta bromobenzene.

Complete step by step answer: The preparation of tribromobenzene from aniline is done in the following manner:-

-We know that amine ($-N{{H}_{2}}$ ) group is an electron donating group which will activate the ring at ortho and para position with respect to the substituent. That means this ring will act as a nucleophile and easily attack $B{{r}_{2}}$ without any requirement of the catalyst such as $FeB{{r}_{3}}$. The 3 Br atoms add up as shown below:-

-The next thing we need to do is activate the meta site of the ring and it can be done by converting the amine group into electron withdrawing group with the help of Friedel – Craft Acylation.

-To limit the steric hindrances we will hydrolyze it in acidic medium and diazotize the amine ($-N{{H}_{2}}$ ) group to convert it into $-{{N}^{+}}\equiv N$. This can be removed by using hypophosphorous acid (${{H}_{3}}P{{O}_{2}}$) and replace it with H.

-At last we can just add one more Br by adding $B{{r}_{2}}$ in the presence of $FeB{{r}_{3}}$ and we will get our desired product as follows:-

This way we can prepare Pentabromobenzene from aniline.

Note: As we can observe that, in the third step we would require a catalyst such as $FeB{{r}_{3}}$ even after 4 electron withdrawing groups. This is due to steric hindrances which would not allow the addition of Br atom to the molecule and hence we can assume that the yield of the final product tends to be poor.
-For diazotization, we can also use nitrous acid ($HN{{O}_{2}}$) instead of sodium nitrate in HCl or any other mineral acid.