You can't use 'however' and 'whereas' in the same way. Follow these general rules in order to avoid grammar mistakes:
Use 'however' (followed by a comma) at the beginning of a sentence to contrast with the previous sentence:
Some people believe that schools are responsible for the behaviour of their students. However, others argue that discipline is the responsibility of parents.
Use 'whereas' (after a comma) to contrast two ideas in the same sentence:
Some people believe that schools are responsible for the behaviour of their students, whereas others argue that discipline is the responsibility of parents.
You can use 'on the other hand' or 'by contrast' in the same way as 'however'. You can use 'while' instead of 'whereas'.
On the other hand和by contrast的用法和however相同，而while和whereas的用法一样。
2. while, whilst, whereas
While and whilst are the same, but whilst is a bit more formal (according to one famous grammar book).
Whereas is always used for contrast. While/whilst can be used for contrast, or to mean "at the same time".
As can mean "because" or "exactly at the moment when".