Isomers on the MCAT

Watch out, isomers will try to trick you!

During your MCAT study, you may come across two structures that have the same number of atoms and thus, the same formula. When analyzing these, you may think - oh these are the same, BUT that is not always the case.

If two structures have the same number of atoms BUT are arranged differently, they will behave differently and, therefore are NOT the same compound.

Isomers are super important to understand, so let us dive a little further into looking at key terms, definitions, and topics that will be important for the MCAT regarding isomers.

Let’s get started!

Isomers on the MCAT: What You Need to Know

Isomers are covered in introductory organic chemistry. 

Introductory organic chemistry accounts for 15% of the content covered in the Chemical and Physical Foundations of Biological Systems and 5% of the content covered in the Biological and Biochemical Foundations of Living Systems.

It’s hard to predict the exact number of questions about isomers that will appear on the MCAT. However, you can expect it to appear in both the Bio/Biochem and Chem/Phys sections. 

Luckily for you, this review will give you a concise yet comprehensive review of isomers that you are required to understand. 

Let’s get started!

Important Sub-Topics - Isomers

1. Overview & Breakdown of Isomers

Isomers are compounds that have the same chemical formula but different arrangements of atoms. This difference in arrangement causes the compounds to behave differently even if they look identical, which is important to understand. 

There are two major types of isomers – stereoisomers and constitutional isomers.

(Coming Soon!) Full Study Notes : Overview and Breakdown of Isomers

For more in-depth content review on the overview and breakdown of isomers, check out these detailed lesson notes created by top MCAT scorers. 

2. Structural (Constitutional) vs. Stereoisomers

Isomers can be classified into two main categories: constitutional isomers and stereoisomers. The primary distinction between the two lies in their atomic arrangements. 

Constitutional isomers share the same molecular formula but have different atomic arrangements, whereas stereoisomers have identical molecular formulas and atomic arrangements, but differ in their spatial arrangement.

The organization of atoms within a molecule determines the specific pattern in which they are bonded together. Consequently, constitutional isomers exhibit distinct functional groups and side groups owing to their differing atomic arrangements.

Covalent Bond: Stereochemistry of Covalently Bonded Molecules

Stereochemistry is the study of the three-dimensional arrangement of atoms within molecules. In covalent bonding, stereochemistry refers to the spatial arrangement of atoms around a central atom or a double bond.

The stereochemistry of covalent bonding plays an important role in determining the properties and reactivity of molecules. For example, the different stereoisomers of a molecule may have different biological activities or pharmacological properties. Additionally, the stereochemistry of a reaction can affect the outcome of chemical reactions, as certain stereoisomers may be more reactive or less reactive than others.

(Coming Soon!) Full Study Notes : Structural (Constitutional) v.s. Stereoisomers

For more in-depth content review on the structural (constitutional) v.s. stereoisomers, check out these detailed lesson notes created by top MCAT scorers. 

3. Stereoisomers: Enantiomers

Enantiomers are a type of stereoisomer that exist in pairs, where one is the mirror image of the other. These pairs are also known as optical isomers and are distinguished by their non-superimposable nature. This means that the two enantiomers cannot be overlapped with each other. 

(Coming Soon!) Full Study Notes : Stereoisomers: Enantiomers

For more in-depth content review on stereoisomers: enantiomers, check out these detailed lesson notes created by top MCAT scorers. 

4. Stereoisomers: Diastereomers

Diastereomers are a type of stereoisomer that differ from enantiomers as they do not mirror one another. 

Unlike enantiomers, diastereomers can be distinguished by their non-superimposable nature, as well as their dissimilar physical properties and reactivity. These differences may include variations in their melting points, boiling points, and densities.

The main difference between diastereomers and enantiomers is that enantiomers are mirror images of each other and have identical physical and chemical properties, while diastereomers are not mirror images and have different physical and chemical properties.

(Coming Soon!) Full Study Notes : Stereoisomers: Diastereomers

For more in-depth content review on stereoisomers: diastereomers, check out these detailed lesson notes created by top MCAT scorers. 

5. Naming Different Configurations

Naming isomers can become complex and may require a detailed understanding of organic chemistry nomenclature rules.

a. Naming structural isomers:

The naming of structural isomers is based on IUPAC rules and is dependent on the type of isomerism present. For example, chain isomers have a different naming system than positional isomers.

There are 3 steps to naming according to IUPAC:

  1. Prefix indicated by the branch group, which is the alkyl group, attached to the longest carbon chain.

  2. The root name shows the number of carbon atoms in the longest carbon chain.

  3. The suffix shows the homologous series.

b. Naming Enantiomers:

The naming of enantiomers uses the R/S system, which assigns a priority to each substituent group based on its atomic number. The R configuration refers to the clockwise arrangement of groups around a chiral center, while the S configuration refers to the counterclockwise arrangement.

c. Naming Diastereomers:

They can be named using a combination of the E-Z and R/S systems, depending on the type of stereochemistry present. For example, (Z)-2-bromo-3-chlorobut-2-ene can be considered (2Z,3R)-2-bromo-3-chlorobut-2-ene.

(Coming Soon!) Full Study Notes : Naming of Different Configurations

For more in-depth content review on naming of different configurations, check out these detailed lesson notes created by top MCAT scorers. 

Important Definitions and Key Terms - Isomers

Key Term


Chiral Atom/Stereocenter

Carbon atoms that are bonded to 4 different types/groups of atoms.


International Union of Pure and Applied Chemistry. IUPAC is the universally-recognized authority on chemical nomenclature and terminology.

Atomic Number

The number of protons in the nucleus of an atom of a chemical element. It is represented by the symbol "Z" and determines the chemical properties of an element.


Two structures cannot be overlapped with each other.


Compounds that do not have the same molecular formula.


Compounds that have the same molecular formula.

Additional Reading Links (Coming Soon!) – Study Notes for Isomers on the MCAT

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