Why do the three molecules below
all belong to the same homologous series? (A) They are all hydrocarbons. (B) They have increasing numbers of
carbon atoms. (C) The ratio of hydrogen atoms to
carbon atoms is two to one. (D) They are all unsaturated. Or (E) they all contain the same
A homologous series is defined as a
family of compounds that have the same functional group and general formula, with
each compound in the series differing by a structural unit. Looking at the three molecules, we
can see that each contains a carbon-carbon double bond. We can also see that each structure
differs from the next by a single CH2 structural unit. If we look at the molecular formula
of each molecule, we can see that each molecular formula fits the same general
formula C𝑛H2𝑛. All together, these pieces of
information tell us that all three molecules belong to the alkene homologous
With this information in mind,
let’s take a look at the answer choices. Answer choices (A) and (B) do
describe the alkene homologous series. However, these statements also
apply to the members of the alkane homologous series. As such, we cannot use these
statements to justify why the three molecules given in the question all belong to
the same homologous series.
Answer choice (C) states that the
ratio of hydrogen atoms to carbon atoms is two to one. Looking at the general formula for
alkenes, we can see that this is the case, as the number of hydrogen atoms in an
alkene is twice the number of carbon atoms. But this is also true of members of
the cycloalkane homologous series. So answer choice (C) cannot be the
answer to this question.
Answer choice (D) states that all
of the molecules in this series are unsaturated. Molecules are considered
unsaturated when they contain double or triple bonds. This statement can be used to
describe alkenes as well as other homologous series, like alkynes. Therefore, answer choice (D) is not
the answer to the question.
This leaves us with answer choice
(E). The molecules in the question all
belong to the same homologous series because they all contain the same functional
group, a carbon-carbon double bond. We can see that all of the members
of the alkyne homologous series contain the same functional group, a carbon-carbon
triple bond. This further shows that members of
the same homologous series contain the same functional group.
So the three molecules shown in the
question all belong to the same homologous series because they all contain the same
functional group: answer choice (E).