Causally asking help,
If you go to London, purchase things and bring me.
Situation is I don't know if he is going or not. I'm not stress him to go. If he go, then I'm asking help to purchase things and bring them to me.
I am trying to use the words "whenever you go", but in the dictionary, 'whenever' means 'every time that' and I'm asking for one time help.
Your question is hard to understand, @loops, but I think you are asking for a word that means "if you go to London, then that time ... " as opposed to "every time you go to London..."
The answer is that in most varieties of English there is not one word that conveys the meaning. I would say
If you happen to go London, ...
or
If you go to London, ...
("When you go to London, ..." would imply that I know for sure that you are going there).
However, there are some varieties of English where "whenever" can be used in this way. So to me
Whenever you go to London, ...
means "every time you go". But I have heard people from Northern Ireland, for example, using it to mean exactly what you want to say.
