Question 18 - Chemical and Physical Foundations of Biological Systems Practice Test for the MCAT

Aromatic compounds possess a 𝞹 system of exceptional stability. They are defined as cyclic, planar compounds with a number of conjugated 𝞹 electrons equal to 4n+2, where n is a whole number (this is known as Hückel’s rule). Aromatic compounds have low reactivity and relatively high melting and boiling points. If a compound is cyclic and planar, and has a delocalized 𝞹 system that does not obey Hückel’s rule, it is called antiaromatic.

Benzene is the most well-known aromatic compound. Its chemical structure is shown on the figure below:

The presence of substituents on benzene can increase or decrease its reactivity towards electrophilic aromatic substitution. If the substituent is an electron donor, reactivity increases, but if the substituent is an electron acceptor, reactivity decreases. Substituents with resonance have a stronger influence on reactivity than substituents with inductive effect.

Four different organic compounds labeled from A to D are studied. Their chemical structures are shown below: