What Are Alkyl Groups and How Do They Relate to Organic Chemistry?

[MycelliumMan]

I'm a bit confused about what alkyl groups are in organic chemistry. I thought functional groups by definition where groups of atoms which contained at least 1 element other than carbon or hydrogen which were connected to the carbon skeleton of the molecule.

What are alkyl groups then? I read about them on wikipedia and they mentioned things about free radicals. What the hell have free radicals got to do with this?

I'll use methionine as an example
http://upload.wikimedia.org/wikipedia/commons/2/23/Methionin_-_Methionine.svg
at the left side of that molecule there is a methyl group attached to the Sulfur atom. I only know this because I read an example they give in a book. First of all what makes this a methyl "group"? Is it a group because its not actually part of the carbon chain but instead its just a carbon atom separated from the main chain by that sulfur atom? Where do free radicals come into this? Theres no lone electron attached to that carbon atom seeing as its saturated by those hydrogen atoms.

Am I getting something mixed up here or have I got the right idea as to what alkyl groups are? Are they just groups of carbon atoms that are separated from the main chain?

Another thing does "R" always refer to an alkyl group or can it refer to any elements? For example what makes a ketone group and different from a plain old carbonyl group or an aldehyde? Can ketones only consist of carbon chains along with they're carbonyl group or can you have ketones which consist of any elements?

 

[LtStorm]

Functional group don't have to contain atoms other than carbon or hydrogen. An alkyl is the 'most simple' functional group, one or more single-bonded carbons with one degree of unsaturation from hydrogen (such as a methyl group). I guess it's not really directly stated that alkyl groups are made up of the same stuff as the carbon backbone they're attached to, but, that's how it goes.

When you have a compound like, say, 2-ethylpropane, the 'ethyl' in that would be an alkyl group attached to the second carbon of the propane chain. So that's pretty much the whole of it; alkyl groups are just carbon and hydrogen functional groups with only single bonds that are attached to the carbon backbone in organic compounds. Mind you, if they're attached by a double or triple bond they become alkenyl or alkynyl groups.