Aromatic Hydrocarbons, Alkyl Halides, and Alcohols: Properties and Nomenclature

Posted on Jan 20, 2025 in Chemistry

Aromatic Hydrocarbons

Aromatic hydrocarbons are benzene derivatives. Benzene is characterized by unusual stability, which is given by the particular arrangement of conjugated double bonds. They are named because of their strong, usually pleasant, odors. The generic name of the mono- and polycyclic aromatic hydrocarbons is “arene,” and radicals derived from them are called “aryl” radicals. All of them can be considered derivatives of benzene, which is a cyclic molecule, hexagonal-shaped, with a bond order intermediate between a single bond and a double bond. Experimentally, it is found that the six bonds are equivalent; hence, the benzene molecule is represented as a resonant structure between the two formulas proposed by Kekule in 1865, according to the following schedule:

  • When there is only one radical on the benzene ring, that radical is named first, followed by the word “benzene.”
  • If there are two radicals, their relative position within the benzene ring is indicated by the numbers 1,2, 1,3, or 1,4, taking the number 1 for the most important substituent. However, in these cases, the prefixes “ortho,” “meta,” and “para” are still used to indicate the same positions for the second substituent.
  • In the case of more than two substituents, they are numbered so that they receive the lowest locants, and are sorted alphabetically. If you have several choices, the alphabetical order of preference of the radicals will decide.
  • When the benzene radical acts on another chain, it is named “phenyl.”

Alkyl Halides

These are hydrocarbons containing halogen atoms in their molecule: R-X, Ar-X. Although not proper hydrocarbons, not being composed of only hydrogen and carbon, they are considered derivatives thereof with regard to their nomenclature and formulation.

  • They are named by first citing the halogen followed by the name of the hydrocarbon, indicating, if necessary, the position of the halogen in the chain, knowing that the double and triple bonds have priority over the halogen in the allocation of numbers.
  • If the same halogen is repeated, the prefixes di, tri, tetra, etc., are used.
  • When all the hydrogens of a hydrocarbon are replaced by a halogen, the prefix “per” is prefixed to the name of the halogen.
  • If there are no unsaturations, numbering starts from the end carbons having a halogen closest; otherwise, the unsaturations are preferred. Write the halogens with their locants, preceded by the name of the corresponding hydrocarbon.

Alcohols

Their structure is similar to that of hydrocarbons, in which one or more hydrogen atoms are substituted by “hydroxyl” groups, -OH. They are named as the hydrocarbons from which they come, but with the ending “-ol,” and indicating with a locant number (the lowest possible) the position of the alcoholic group. According to the position of the carbon that sustains the -OH group, alcohols are called primary, secondary, or tertiary. If the molecule has more than one -OH group, the endings “-diol,” “-triol,” etc., are used, with numbers indicating the positions where these groups are. There are important polyols like glycerin (propanetriol), glucose, and other carbohydrates. When the alcohol is not the main function, it is named as “hydroxy-“, indicating the corresponding locant number.