[vanklik]

I can not understand what the difference between substituent and functional group... May be someone could help me with it please? 

Thanks for your *delete me*

[stewie griffin]

I would say that all functional groups are substituents, but not all substituents are functional groups. The term "Functional group" should be used when the group is reactive and can therefore be used to functionalize the molecule further (for example, alcohol, ketone, alkene, etc). So an alcohol can be called a functional group or a substituent.... cause it's both. However a substituent could also be something like a tert-butyl group hanging off of a ring. The tert-butyl here is not really a functional group b/c you can't do synthetic reactions at that site. But it is a substituent on the ring. Does that make any sense?

[vanklik]

I would say that all functional groups are substituents, but not all substituents are functional groups. The term "Functional group" should be used when the group is reactive and can therefore be used to functionalize the molecule further (for example, alcohol, ketone, alkene, etc). So an alcohol can be called a functional group or a substituent.... cause it's both. However a substituent could also be something like a tert-butyl group hanging off of a ring. The tert-butyl here is not really a functional group b/c you can't do synthetic reactions at that site. But it is a substituent on the ring. Does that make any sense?

Yes, that's perfect! Thank you!