MCAT Organic Chemistry Review

Isomers

2.1 Structural Isomers

Structural isomers are the least similar of all isomers. In fact, the only thing that structural isomers (also called constitutional isomers) share is their molecular formula, meaning that their molecular weights must be the same. Aside from this similarity, structural isomers are widely varied, with different chemical and physical properties. For example, five different structural isomers of C6H14 are shown in Figure 2.2. Each of these molecules looks completely different, but has the same number of carbon and hydrogen atoms.

Figure 2.2. Structural (Constitutional) Isomers of C6H14

Physical and chemical properties are prime MCAT material and are often tested in the context of isomerism. Physical properties are characteristics of processes that don’t change the composition of matter, such as melting point, boiling point, solubility, odor, color, and density. Chemical properties have to do with the reactivity of the molecule with other molecules and result in changes in chemical composition. In organic chemistry, the chemical properties of a compound are generally dictated by the functional groups in the molecule.

KEY CONCEPT

Physical properties: no change in composition of matter; examples include melting point, boiling point, solubility, odor, color, density.

Chemical properties: reactivity of molecule, resulting in change in composition; generally attributable to functional groups in the molecule.

MCAT Concept Check 2.1:

Before you move on, assess your understanding of the material with these questions.

1.    What property or properties do structural isomers have in common?

2.    Of the compounds cyclopropanol, 2-propanol, acetone, and prop-2-ene-1-ol, which are structural isomers of each other?

3.    What are physical properties? Give three examples of physical properties.

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4.    What are chemical properties?