[qwerty44]

Question:
What is the substituent on the following compound?
CH3CH2CH2CH2Cl

Answer: Cl

Please explain.

[Mitch]

What a vague question.

[qwerty44]

Is a substituent something that replaces hydrogen? So would a methyl group be a substituent?

[kremar]

Usually you talk of a substituent when you have a series of compounds in the (very generic) form of R-X, where R is always the same but X varies from each compound within the series.

In that case you refer to X as a substituent, and speak of the effects that different X's have on the chemical and phisical properties of R-X as "substituent effect". The compound with X=H is usually refered as the "parent compound" or the "parent chain".

Thou in organic chemistry a substituent is usually a group replacing an hydrogen attached to a carbon atom, this can be further generalized if it helps you explain a given trend.
For example, you could define a series of compounds where R = "HO" and X = H, CH3, COCH3, and analyze how the substituent affects the acid behavor of each of the three compounds (water, methanol and acetic acid).

[organoman]

Normally hydrocarbons (compounds containing carbon and hydrogen only) are considered as parent organic compounds and any other atom or group replacing hydrogen forms some sort of derivative of these compounds. such atoms or functional groups are known as substituents.
Eg.
Parent compound           Derivative                substituent
Ethane                         Chloroethane            Chlorine atom
Cyclohexane                  nitrocyclohexane       nitro group
benzene                        Benzoic acid             -COOH group
note: in last example -COOH (including carbon) is considered as substituent.
Sometimes even a hydrocarbon attached to a common hydrocarbon especially aromatic may be considered as substituent.
eg. In Toluene methyl group can be considered as substituent on benzene ring.

[english]

Yeah pretty vague.  "Substituent" usually refers to a functional group, or a bonded atom or group that is notably different from the rest of the compound.