1 A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule.

here are the solutions:

2.1 A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule.

2.2
2.2.1 $\boxed{\text{A}}$
2.2.2 $\boxed{\text{C}}$
2.2.3 $\boxed{\text{B}}$

2.3
2.3.1 The IUPAC name of compound B is 2-methylpropanal.
2.3.2 The IUPAC name of compound C is 4-methylpent-2-yne.
2.3.3 The IUPAC name of compound D is 2,3-dibromo-3-ethylpentane.

2.4
2.4.1 The structural formula of compound A (2-methylpentan-3-one) is:

$$\boxed{CH_3CH(CH_3)COCH_2CH_3}$$

2.4.2 The name of the reaction that occurred to produce compound E (Ethyl butanoate) is esterification.
2.4.3 The structural formula of the carboxylic acid needed to produce compound E (Ethyl butanoate) is:

$$\boxed{CH_3CH_2CH_2COOH}$$