what are tarry oxidation products??

The word tarry is used for something which stays temporarily at its place.

In context of amines, if you proceeds for direct nitration of aniline, then you will end up with a mixture of aniline and nitrobenzene.

For two reasons, direct nitration of aniline is avoided.

1. By direct nitration of aniline, we will obtain a mixture ofo, m and p nitro derivative of aniline. So it will not be a useful reaction from quality point of product.

2. Second reason is that nitric acid is not only a nitrating agent but also act as a strong oxidizing agent. So it is not useful reaction, as it often gives tarry oxidation products along with some nitration products.

Therefore, to avoid these problems, first amino group of aniline is protected by acetylation and then its nitration is done.