Question

What is a functional group?

Answer 1

`"..a site of differing electronegativity along an hydrocarbyl chain.."`

Explanation:

Given an hydrocarbyl chain......

`H_2C-CH_2CH_2CH_2C-`

Along the chain we can substitute any hydrogen atom for a functional group, and so introduce a site of different reactivity. Substitute `H` for `OH` you gets an alcohol; substitute `H` for `X`, you get a halide; substitute with `NHR` you get an amine. On 2 adjacent carbon centres, unsaturation can also be introduced to give an olefin, i.e. `-CH=CH-`, or an acetylene, `-C-=C-`, sites of unsaturation.

The chemistry of the carbon chain is pretty limited (you can burn it of course but that's the end of the line); substitute functionality into the chain, and you can make anything, hormones, pheromones, stimulants, drugs. Organic chemistry is not going to go out of business given the tremendous variety of the carbon chain functionalized by heteroatoms.