Isomers for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about isomers, plus practice questions and answers
(Note: This guide is part of our MCAT Organic Chemistry series.)
Table of Contents
Part 1: Introduction to isomers
Part 2: Structural isomers
Part 3: Chirality
a) Chiral centers
b) R, S configuration
c) E, Z configuration
d) Diastereomers and enantiomers
e) Conformational isomers
Part 4: In biological systems
a) Amino acids
b) Carbohydrates
c) Boat and chair conformations
Part 5: High-yield terms
Part 6: Passage-based questions and answers
Part 7: Standalone questions and answers
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Part 1: Introduction to isomers
Isomers are compounds with the same chemical formula but different arrangements of atoms. Often, different isomers will behave differently. This is especially important in considering how chemicals react with one another. For instance, it is important to keep isomers in mind when creating drugs so that the chemical interacts with the body as desired--without any lethal side effects.
This study guide will take you on a tour of the different types of isomers, along with examples and naming conventions. Throughout this guide, several key terms are highlighted in bold. At the end of this guide, there are also several MCAT-style practice questions for you to test your knowledge against.
Let’s get started!
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Part 2: Structural isomers
Recall that covalent bonds form when one or more electron pairs are shared between atoms. Covalent bonds arrange atoms in space and influence the overall geometry of molecules. (For more information on this topic, be sure to refer to our guide on bonds and interactions.)
Hence, covalent bonds are crucial in any discussion of isomers, because isomers arise from different spatial connectivities of atoms in three-dimensional space.
Chemical structures should always be considered in their three-dimensional configurations. In other words: the bonding of individual atoms within a molecule must be taken into account. Structural isomers arise from forming covalent bonds in different arrangements despite having the same number and types of atoms. As a result, structural isomers have different structures in three-dimensional space due to the unique arrangements of atoms by covalent bonds.
Figure: Examples of structural isomers, butane and isobutane, both of which share the same chemical formula but take on different atomic connectivities
The spatial arrangement of atoms in three-dimensional space is important when discussing isomers. It can be difficult to depict these arrangements on a two-dimensional sheet of paper. By convention, bonds that are drawn on a wedge are said to be “coming out of” the page. Bonds that are drawn on a dash are said to be “going into” the page. Bonds that are drawn with a solid line are in the same plane as the page.
Figure: The structure of cysteine, drawn using dashes and wedges.
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