Assigning the IUPAC name of this compound Going by the rules that were given to me by my professor:


I first decide the seniormost group and its parent chain. In this case, cyclohexene carbonitrile.
Then, I decide direction to traverse the ring to assign locants by the first point of difference. Accordingly, I move towards bromine.


The final name I obtain is thus "2-bromo 3-hydroxy cyclohex-4-enecarbonitrile". The answer given in my workbook however is "6-bromo 5-hydroxy cyclohex-3-enecarbonitrile". Apparently, they decided to traverse the ring in reverse direction.

Obviously, this must be some special case. Please explain the exact rule involved here. Thanks!
This is because alkene has higher preference than alkyl halides. So when counting clockwise position of ene is less than that when counting anticlockwise. Hence the answer is as stated.

Check out Rule 6.2 of basic principles on Wikipedia.