A Homologous Series is a group of organic chemical compounds, usually listed in order of increasing size, that have a similar structure (and hence also similar properties) and whose structures differ only by the number of CH2 units in the main carbon chain. Taken verbatim from What is a Homologous Series ?
Therefore, acetic and butyric (ethanoic and butanoic) acids satisfy these conditions. (CH3COOH and CH3-CH2-CH2-COOH)
Please note, that they differ by CH2-CH2 group!
Acetone and acetaldehyde (propanone and ethanal) also differ by CH2, but they belong to different types of organic compounds (they have different functional groups), so they do not have similar properties. (CH3-CO-CH3 and CH3-COH)
1-propanol and 2-propanol are isomers of the same alcohol.
Ethanol or propanal are again different types of organic compounds (different functional groups), so they do not have similar properties.
However, propanal and acetaldehyde (ethanal) belong to the homologous series. They differ by CH2 and have similar properties. This pair is not in your question, but it might be informative to think of it.
Answers
Explanation:
A homologous series in organic chemistry is a group of organic compounds (compounds that contain C atoms) that differ from each other by one methylene (CH2 ) group. For example, methane, ethane, and propane are part of a homologous series. The only difference among these molecules is that they have different numbers of CH2 groups.
Each member of a homologous series is called a homologue, which can also be spelled ''homolog.'' For example, methane and ethane are homologues and belong to the same homologous series. They differ from each other by one CH2 group. The formula of methane is CH4 and the formula of ethane is C2 H6.
Chemical Properties
Homologues have similar chemical properties because the functional group of the molecule does not change. Remember, the functional group is just the area of the molecule that is reactive or can change in chemical reactions. Because the functional group is the same for each molecule in a homologous series, the homologues have similar reactivities. Some examples of functional groups in homologous series are alcohols (-OH), carboxylic acids (-COOH), aldehydes (COH), ketones (C=O), and amines (N), among many others. So when you are looking to identify homologues, the functional group will be the same. The only thing that will change is the length of the carbon chain