[deutdeut]

1. Is phenol a homologous series? What are the functional groups involved?

2. If the organic compound has 2 or more different functional groups, which homologous series should they be? For example, 2-oxopropanoic acid, should it be carboxylic acid or just ketone? How can I determine the group that it belongs?

3. For the compund 2-phenylbutane, should it be classfied as aromatic hydrocarbon or just an alkane?

[Yggdrasil]

1. Phenol can refer to the actual compound (C₆H₅OH) or compounds with a phenolic functional group.  So yes, phenols can be considered a homologous series.  The functional group is an alcohol (-OH) on an aromatic ring.

2. You should take the functional group which encompasses the most functional groups.  For example, you should not consider a carboxylic acid an alcohol or a ketone even though it contains an -OH and a carbonyl group.  In chemistry, the whole is more than the sum of its parts.  A carboxylic acid has different properties than the combination of a ketone and an alcohol.  For example, a carboxylic acid has an acidic proton where neither an alcohol nor a ketone contains any significantly acidic protons.  Similarly, a phenol is contains both an alcohol functional and an aromatic functional group.  Together, these two functional groups give phenols different properties than both alcohols and aromatics.  For example, the OH on the phenol has a relatively low pKa (~9-10 vs. ~13-14 for an alcohol).

3.  I would consider this an aromatic hydrocarbon.  In general, if an alkane has any functional groups attached, you would not consider it an alkane, unless the alkane is very long (for example, in fatty acids).

[deutdeut]

So, for but-2-enoic acid and 3-oxobutanoic acid, they are both considered to be carboxylic acid?

And 2-phenylbutane, as the name suggests, shouldn't it be an alkane? Because I know that for aromatic hydrocarbon, usually are the alkyl groups substituting the benzene ring, isn't it?

[Yggdrasil]

To tell the truth, I had never heard of the term homologous series before and I had just skimmed the wikipedia article on homologous series before my first post.  After reading more carefully about homologous series, I think I may have to revise some of my answers.

1.  I'm not sure whether phenols can be considered a homologous series.  It depends on the series of phenols which you use to construct your series.

2.  2-oxopropanoic acid belongs to neither the carboxylic acid nor the ketone homologous series because it cannot be obtained from a member of either series by homologation.  I don't think its useful to try to classify complex molecules with multiple functional groups into homologous series.

3.  2-phenylbutane has an aromatic part and an alkane part.  Technically it's correct IUPAC name would be 2-butylbenzene, which makes its parent compound the aromatic hydrocarbon.  In other words, don't put too much weight on what the name says.  A name is just a name.  A rose by any other name would smell as sweet (or I guess a chemist would say "a thiol by any other name would smell as putrid")