Nomenclature and Isomerism Study Guide

Structural Isomerism

Isomers are defined as different compounds with the same molecular formula. 2-Methylbutane has two other isomers with the molecular formula C₅H₁₂. Each compound must have a unique systematic name. Because molecules can be drawn many different ways, a name can confirm whether the molecule with the same molecular formula is an isomer or the same molecule drawn differently.

Some of the ways to draw 2-methylbutane (i.e., the same molecule drawn differently) are as follows:

Alkenes and Alkynes

The nomenclature of alkenes and alkynes follows the same rules as alkanes, except the double or triple bond must be numbered. The multiple bond is numbered on the first number to which it is assigned. Also, because double bonds have a rigid configuration, they can exhibit a cis or trans isomerism. A cis structure is one with substituents on the same side of the double bond, and the trans is one with the substituents are on opposite sides of the double bond.

Example:

Name the following molecules:

Solution:

Stereoisomerism

Stereoisomers are isomers with the same connectivity but a different three-dimensional structure. Your hands are stereoisomers, mirror images but nonsuperimposable. They have the same connectivity, but you cannot put your left glove on your right hand. Bromochloroiodomethane is one of the simplest molecular models. Notice that the carbon atom has four different groups attached, which is called a stereocenter or stereogenic carbon.

Functional Groups

So far, only hydrocarbons have been discussed in this lesson. Oxygen, nitrogen, and the halogens are elements commonly found in organic molecules. The exact bonding and arrangement of these elements in compounds are functional groups (see Table 12.3). The functional group of an organic compound determines the specific physical and chemical properties. For example, most esters have a fruity fragrance, most haloalkanes have a density greater than 1 (the density of water), and alcohol reacts readily with alkali metals.

Example:

Identify the functional groups in phenylalanine (an amino acid and component of aspartame and proteins) and vanillin (vanilla scent):

Solution:

Practice problems for these concepts can be found at - Nomenclature and Isomerism Practice Questions