Types of Reaction Mechanisms in Organic Chemistry

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  • 0:03 Reactions in Organic Chemistry
  • 0:31 Sn1 Reactions
  • 1:20 Sn2 Reactions
  • 2:09 E1 Reactions
  • 3:13 E2 Reactions
  • 3:39 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we'll learn about the general reaction mechanisms in organic chemistry. We'll also learn how to identify when the reactions will be used and explore the mechanisms for each.

Reactions in Organic Chemistry

There are a lot of reactions in organic chemistry, and it can seem daunting to remember them all. But if you can remember the four main types of reactions, then you can generally know how any reaction will proceed. What's important is knowing when each type of reaction will occur.

The four main types of reactions in organic chemistry are substitution 1 (Sn1), substitution 2 (Sn2), elimination 1 (E1), and elimination 2 (E2).

Sn1 Reactions

Sn1 reactions are substitution reactions that use a weak nucleophile or base. This means that these reactions will occur given two conditions: you have a weak nucleophile or base, and there is a good leaving group or molecular fragment on the carbon chain.

Sn1 reactions have three steps:

  1. The leaving group leaves (carbocation forms)
  2. Nucleophile attacks the carbocation
  3. Nucleophile is deprotonated

Sn1 reaction

In this example, the iodine is the leaving group. Once it leaves, a secondary carbocation is created. (Note that this reaction cannot occur if a primary carbocation is created.) Next, the water can attack the carbocation after which the water is deprotonated. The result is an alcohol that replaced the iodine.

Sn2 Reactions

Sn2 reactions are also substitution reactions, but they require a strong nucleophile. An Sn2 reaction will also occur if the leaving group is a primary group instead of an Sn1 reaction. Sn2 reactions occur in a single, concerted step: the nucleophile attacks the alpha carbon while it kicks off the leaving group.


In this example, the X generally represents any halide or any good leaving group. The electrons from the nucleophile attack the alpha carbon, which then kicks off the leaving group. Since this is a good nucleophile, it already has a negative charge and won't need to be deprotonated.

Here we also end up with an alcohol that replaced the leaving group, but this reaction makes a primary alcohol possible. If a strong nucleophile is used, this reaction will occur with primary, secondary, or tertiary leaving groups.

E1 Reactions

E1 reactions are elimination reactions that use a weak nucleophile or base. Elimination reactions are different from substitution reactions in that the leaving group is taken off and nothing replaces it. This allows a double bond to form (or a triple bond if a double bond already exists).

E1 reactions occur in two steps:

  1. The leaving group leaves, creating a carbocation
  2. The beta carbon gets deprotonated and the double bond forms

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