Acetic acid is one of the limiting carboxylic acids. Accordingly, other saturated carboxylic acids can be homologues of acetic acid. Their common property is the presence of a carboxyl group, which just defines them as organic acids.
The concept of homology in chemistry
In organic chemistry, the properties of a particular compound are usually determined by one or more functional groups contained in it. So, for example, the properties of alcohols are due to the presence of a hydroxyl group -OH, aldehydes and ketones - a carbonyl group -CO. The functional groups are attached to the carbon skeleton of the molecule. And since carbon has the ability (on which all organic chemistry is based) to form long stable chains of linked atoms, the same group can attach to molecules of different sizes and form compounds that are similar in chemical properties, but because of the difference in size and quantity. carbon atoms are otherwise dissimilar. The set of connectionsdiffering from each other by a certain number of groups -CH2-, is called a homological series, the group itself -CH2- is called a homological difference, and compounds in row - homologues. The simplest example of a homologous series is the series of saturated hydrocarbons (alkanes).
Using elementary arithmetic, it is easy to verify that any two of these compounds differ from each other by nCH2 groups.
It is also important to pay attention to the first, that is, the simplest member of the homologous series. In the case of alkanes, this is methane: it contains only one carbon atom and has all the basic properties of alkanes. However, sometimes carbon alone is not enough. For example, in the series of alkenes, the simplest compound is ethene (which, by analogy with ethane, has two carbons), to create a carbon-carbon double bond characteristic of alkenes, at least two atoms C.
Homologous series of saturated carboxylic acids
Ethanoic (common name - acetic) acid belongs to the class of limiting carboxylic acids. Its properties are determined by the functional group -COOH, also called carboxyl.
The molecular formula of acetic acid is CH3COOH, or C2H4O 2. You can add new pieces -CH2- to it to get larger molecules: homologues of acetic acid with a carbon chain of three, four, ten and even thirty atoms long. However, in this case, it is also possible to “take away” one homologous link from acetic acid:then we get methane, or acetic acid HCOOH. Despite the fact that the only carbon belongs to the functional group, formic acid also belongs to the class of carboxylic acids and is the simplest compound of their homologous series.
Changing properties in a homologous series
The closest homologues of acetic acid are methane acid HCOOH and propanoic (or propionic) acid C2H5COOH. All three compounds are liquids under normal conditions, methane and ethanoic acids are volatile, with a pungent odor. Limit carboxylic acids with a carbon chain length of 4 to 24 atoms are the so-called saturated fatty acids isolated from natural oils and fats. There are also larger acids - they, as a rule, are part of waxes or fats of animal origin. Higher carboxylic acids are solids.
