MCAT Organic Chemistry Review

Carboxylic Acids

Introduction

Carboxylic acids, with both carbonyl and hydroxyl groups, are some of the most reactive organic molecules you’ll encounter on Test Day. As we will see in this chapter, these molecules can react as acids (as their name suggests), nucleophiles, and electrophiles, and are integral to many biological processes. Carboxylic acids are found in soaps, oils, preservatives, skin care products, clothing, and—most importantly for the MCAT—amino acids. Carboxylic acids often have strong, unpleasant odors. For example, acetic acid (ethanoic acid) is the main ingredient in vinegar; propionic acid (propanoic acid) gives Swiss cheese its smell; butyric acid (butanoic acid) is found in rancid butter and body odor.

So what makes carboxylic acids so interesting and versatile? First, they’re acids, so they like to give away protons—particularly because when they do so, the remaining negative charge resonates between two oxygen atoms, making the anion very stable. This makes carboxylic acids some of the most acidic compounds encountered in organic chemistry, with pKa values between 3 and 6. Compare this with alcohols, which have an average pKa around 17. Carboxylic acids are also excellent at hydrogen bonding, which results in large intermolecular forces and high boiling points. Finally, carboxylic acids are ubiquitous in nature and are synthesized by all living organisms.