Useful Notes on Functional Groups of Compounds

The organic compound that occur in protoplasm generally contain certain groups of atoms that are always linked together and are attached as a group, be covalent bonds, to the molecule of which they are a part. Such groups function as distinct entities.

They are spoken of as radicals or functional groups, and they have an electric charge, positive or negative. Their charge is derived from the fact that when the covalent bonds that hold such a group to its molecule are broken, paired, shared electrons are separated.

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Both the molecular residue and the separated group then have unshared and unpaired electrons and are “free” radicals or ions. These, like electrons, under normal conditions exist free only theoretically or under special conditions.

They tend to combine immediately with something else. The combining valence of a group, radical or ion depends on the net charge of the group. Examples from inorganic chemistry are the ammonium “radical” (⁺NH₄), valence 1; the sulfate radical (=SO₄), valence 2; the phosphate radical (=PO₄), valence 3. A free organic radical would be the methyl group: -CH₃.

In organic chemistry different substances often have similar chemical potentialities because they contain the same functional groups. For example, glucose, a sugar, has at one end of the molecule hydroxyl (-OH) groups, an arrangement that is characteristic of alcohols or organic bases’.

Glucose therefore, with its curious structure, has the combining properties both of an alcohol and of an aldehyde because its molecule contains those functional groups. Glucose is sometimes called an aldose (aldehyde-sugar). In a glucose isomer, fructose (fruit sugar), the carbonyl oxygen is attached to a carbon atom within the carbon chain rather than at the end of it:

When arranged in this way the group is called keto group, and sugars containing it are called ketoses. Note that the carbon atoms in the glucose chain are numbered, for reference, from 1 through 6.

If we oxidize the aldehyde group of glucose it changes to the carboxyl group. This is characteristic of all organic acids. These ionize to yield a hydrogen ion.

By thus replacing the aldehyde group in glucose with the cartoxyl group we form the monocarboxylic sugar acid, gluconic acid. If we also oxidize the alcoholic group at the other end of the glucose molecule we form a dicarboxylic sugar acid (a saccharic or glyceric acid).

Any basic alcoholic hydroxyl group usually reacts readily with any acidic carboxyl group to form an ester or organic salt, plus water. The formation of an ester is analogous to the reaction between the hydroxyl ion of an alkali (e.g., Na⁺OH~) and the hydrogen ion of an acid (e.g, H⁺Cl^–) to form the salt, NaCl, and H₂O.

This illustrates a difference between the organic hydroxyl group and the inorganic hydroxyl ion. It also illustrates the principle that the way an atom or group act depends to a great extent on what other atoms it is combined with.

Using the glucose molecule again as our model, we may replace the hydroxyl group of carbon atom 2 with an amino group (NH2), producing glucosamine:

This is an amino sugar and is found in the polysaccharide chitin, the rigid, structural substance in the skeletons of insects and Crustacea and in the cell walls of fungi, protozoa and some other protists.

The large chemical family of amines, of which glucosamine is only one example, may be thought of as ammonia molecules (NH₃) with one or more of the hydrogen atoms replaced by organic groups (e.g., methyl amine: H₃C—NH₂).

The amino group (-NH₂) is the ammonia residue. If the amino group is attached to the alpha carbon atom of a carboxylic acid, such as propionic acid, we have an alpha amino acid, alanine:

Since all amino acids have at least one basic amino group and at least one acidic carboxylic group, they can form bonds between the acidic and basic groups of adjacent amino acid molecules, producing long chains or polymers called polypeptides, the basic of protein structure. The bonds between the amino groups and the carboxylic groups are called peptide bonds.

Other molecules with both acidic and basic groups may react similarly. For example, the molecules of a fatty acid the beta- hydroxybutyric acid (P-hba), having acidic carboxyl groups and basic hydroxyl groups, can join, forming ester bonds between the carboxylic and hydroxyl groups of adjacent molecules. Long chains of the polymer, poly-p-hba, are formed:

Another important functional group in protoplasmic substances in the sulfhydryl or mercapto group (-SH). It may be thought of an analogous to hydroxyl group in sulfur.

The -SH group is found in the amino acids cystine, cysteine and methionine, all of which are extremely important in the formation of enzymes and other essential structures in the cell. Other important functional groups will be mentioned in the appropriate place.