[Aakash]

I wanted definition of functional group and I got that functional group is a group of atoms that decides the properties of a compound. But don't other groups of atoms like CH₃ in an organic compound also affect the property of the compound?

[Kate]

Well, I think it's probably because every single organic molecule out there has carbon and hydrogen atoms, it wouldn't make sense to call groups such as -CH3 a functional group.

Besides, molecules with just carbons and hydrogens with sigma bonding between them aren't reactive in light of the common organic reactions. They mostly react through radical reactions.

[discodermolide]

Of course CH₃ is a functional group. Think about toluene!

[Kate]

Oops. I stand corrected then, sorry.

[discodermolide]

You should also think about pericyclic reactions, Diels-Alder reactions for example are not radical reactions, but they have reactants containing only C-H and C-C bonds.

[Kate]

You should also think about pericyclic reactions, Diels-Alder reactions for example are not radical reactions, but they have reactants containing only C-H and C-C bonds.

We don't study pericyclic reactions in the 2 organic chem's my chemistry degree offers.

And I edited my other post to include sigma bonding only.

[discodermolide]

That's not good, read up about the Diels-Alder reaction, one of the most important for forming rings that is known.

[Kate]

Yes, I know it isn't good. 

Anyway, I just looked up the Diels-Alder reaction and it occurs between a conjugated diene in a s-cis conformation and a dienophile, with the formation of 2 sigma bonds that close the ring and cleavage of 1 pi bond in the diene and 1 in the dienophile.

[Aakash]

so..

[zsinger]

So,
A functional group lets a chemist know what is likely to happen based on reactivities of reactants being mixed together.  For instance, carbonyl creates an electropositive carbon center that is amenable to attack by a suitable Nucleophile.  Wikipedia "functional groups".  There ya go!
       -Zack

[Aakash]

thanks.