Reactions and Separations for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about reactions and separations, plus practice questions and answers
(Note: This guide is part of our MCAT Organic Chemistry series.)
Table of Contents
Part 1: Introduction to reactions and separations
Part 2: Organic reactions
a) Nucleophilic attack
b) Electrophilic attack
c) Saponification and esterification
d) Amino acid synthesis
Part 3: Key reagents in organic chemistry
a) Oxidizing and reducing agents
b) Tollens’ reagent and Benedict’s reagent
c) Heat and light
Part 4: Separation techniques
a) Polyacrylamide gel electrophoresis (PAGE)
b) Extraction
c) Distillation
d) Chromatography
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 reactions and separations
Organic chemists use their knowledge of functional groups and reactions to synthesize products. How are these products separated from the reactants and purified?
In this article, we’ll cover all that you’ll need to know about reactions and separation techniques for the MCAT. From identifying key reagents to understanding the fundamentals of high yield experimental techniques, you will be ready for whatever the MCAT has in store for you on reactions and separations on test day.
Throughout this guide, several key terms are marked in bold. At the end of this article, there are also several practice questions for you to test your knowledge against.
Let’s get to it!
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Part 2: Organic reactions
a) Nucleophilic attack
Nucleophiles are species that are especially attracted to positively charged or polarized species. These have high electron density due to the presence of pi bonds and/or electron lone pairs. Since nucleophiles will donate their electrons to form covalent bonds, they are considered Lewis bases.
An Sn1 reaction, or unimolecular nucleophilic substitution reaction, is carried out through two steps. First, a carbocation is formed after a leaving group departs. This is an ionic compound that contains a positively charged carbon. Next, the nucleophile attacks the carbocation. This results in a racemic mixture, or mixture of R and S stereoisomers because the attack can occur on either side of the carbocation.
Since a higher electron density can better stabilize a carbocation’s positive charge, Sn1 reactions tend to prefer more substituted carbons. Additionally, Sn1 reactions favorably occur in protic solvents.
Figure: The SN1 mechanism. In the first step, the leaving group leaves, which results in the formation of a carbocation. In the second step, the nucleophilic attack occurs.
Note that the “S” in the name “Sn2” refers to the substitution that occurs. “N” refers to the nucleophilic attack, while “1” refers to the presence of one and only one species during the rate-limiting step.
Sn2 reactions, or bimolecular nucleophilic substitution reactions, are carried out in one step and they tend to proceed in an aprotic solvent. Essentially, the departure of the leaving group and the nucleophilic attack occur simultaneously. As seen in the figure below, this is a back-sided attack that inverts the stereochemistry of the reactants.
Figure: The SN2 reaction.
Since both steps of this reaction occur simultaneously, the Sn2 mechanism can also be described as concerted. Note that the rate-limiting step of this reaction involves two species, as indicated with the “2” in “Sn2.”
b) Electrophilic attack
Electrophiles are species that love electrons and negatively charged species. They may be positively charged or positively polarized. For example, a carbonyl carbon does not have a positive charge. However, due to its bond with the more electronegative oxygen atom, it is positively polarized. This makes it electrophilic. Additionally, since electrophiles accept electron pairs, they are considered Lewis acids.
Heterolysis refers to the cleavage of a covalent bond that results in one of the atoms referred to as the leaving group taking both bonded electrons. Weak bases, such as the conjugate bases of strong acids, are good leaving groups because they can stabilize the negative charge from the electrons.
For more information on the functions of electrophiles and nucleophiles, be sure to refer to our guide on the fundamentals of organic chemistry.
c) Saponification and esterification
Saponification and esterification are two key reactions you should be familiar with for test day. For more information on the structure and function of fatty acids, be sure to refer to our guide on lipids and membranes.
Figure: The basic structure of a triglyceride.
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