Organic Chemistry Review Sn1, Sn2, E1, E2 Reactions

Organic Chemistry may have a bad reputation,,, but its actually quite simple. Something I notice alot of students struggle with in Ochem 1 is Sn1, Sn2, E1 and E2 reactions.
First off, Lets define two types of reactions, A) substitution reactions and B) Elimination reactions
Substitution reactions occur when an Alkyl Halide reactes with a nucleophile in a way that the nucleophile replaces the halogen attatched to the carbon strand/ring.
Substitution reactions can be split into two groups: A) Sn1 reactions and B)Sn2 reactions
Things to remember about Sn1 reactions:
1) Sn1 reactions are two step reactions, meaning the bond breaks between the carbon and the halogen, and afterward the nucleophile attatches itself to the positive carbon(electrophile). Because there is an intermediate product (the carbon strand without the halogen) the stability of this itermediate product will determine the speed of the Sn1 reaction. Stability of the intermediate product is determined by how many R groups are attatched to it, meaning a tertiary alkyl halide will definatly go through an Sn1 reaction, while a secondary alkyl halide may go through an Sn1 reaction, and a primary alkyl halide will not go through an Sn1 reaction.
2)The Sn1 reaction results in a racemic mixture as its product.
3)The Sn1 reaction exhibits first order kinetics, meaning that only the concentration of the alkyl halide effects the rate of reaction (concentration of nucleophile has no effect)
4)favored by polar protic solvents
Things to remember about Sn2 reactions:
1)The Sn2 reaction is a one step reaction meaning that the bond between the halogen and carbon breaks as the bond between the nucleophile is created. Because of this, limiting steric strain will speed up the Sn2 reaction. If the nucleophile is “bulky” the reaction will be too slow, and if the alkyl halide is bulky, the reaction will not start. For Sn2 reactions, primary alkyl halides are the best candidates for Sn2 reaction, secondary alkyl halides can go through Sn2 reactions, and tertiary alkyl halides do not go through Sn2 reactions.
2)The stereochemistry of an Sn2 reaction is very specific in that the nucleophile always attacks from the opposite side of the halogen. This results in the inversion of configuration at a stereogenic center.
3)The Sn2 reaction exhibits second order kinetics, meaning that the concentration of the alkyl halide and the nucleophile both effect the rate of reaction. If you double concentration of both, you quadruple you reaction speed.
4) favored by polar aprotic solvents
Elimination reactions occur when a base attacks a hydrogen attatched to a carbon which is diagnal from the halogen. After the hydrogen leaves, the carbon forms a double bond with a the carbon bonded to the halogen, which in turns releases the halogen.
Elimination Reactions can be split into two groups:A)E1 Reactions, B)E2 Reactions
Things to remember about E1 reactions
1) similar to Sn1 reactions, E1 reactions are a two step reaction, and exhibit first order kinetics in the same fashion.
2)Weak bases favor the E1 reaction.
3)more substituted halides react fastest, meaning tertiary alkyl halides react the fastests, while primary halides rarely react, (if at all)
4)Polar protic solvens that solvate the ionic intermediates are needed
Things to remember about E2 reactions
1) similar to Sn2 reacitons, E2 reactions are a one step reaction, and exhibit second order kinetics in the same fashion
2)Strong Bases favor the E2 reaction
3)more substituted halides react fastest, meaning tertiary alkyl halides react the fastests, while primary halides rarely react, (if at all)
4) favored by polar aprotic solvents

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3 Responses

  1. Thank you very much. Very useful summary.

  2. Good post, I think that the E1, E2 SN1 and SN2 reactions are in my list of the 11 important organic reactions which chemistry undergrads need to know.

  3. [...] Organic Chemistry Review Sn1, Sn2, E1, E2 Reactions April 20093 comments 3 [...]

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