Chem 214 (Substitution reactions)

What are the general steps in an Sn1 reaction?

A carbocation and a nucleophile, which then result in a substitution of the desired group.

What happens in an Sn2 reaction?

Direct substitution takes place.

What acronym do we use and what are each of its components?

SCENE, S=Sterics, C=carbocation, E=Electrophile, N=Nucleophiles, E=Environment

What does the S in scene tell us?

Sterics, with a primary leaving group, the reaction will often be Sn2. With a secondary leaving group, the reaction can either be Sn1/Sn2. And with a tertiary leaving group, the reaction will be Sn1. This is because tertiary leaving groups are too sterically hindered to allow for the from the back insertion needed for an Sn2 reaction.

What does the C in Scene tell us?

Carbocation, it tells us that the presence of a carbocation intermediate means the reaction is Sn1.

What does E1 in Scene tell us?

Electrophile, less about differentiating between Sn1 and Sn2, more about determining whether the reaction is a substitution reaction. Identifying electron-deficient species (full/partial positive charges) attacking a substrate to replace an existing atom or group, rather than just adding across a double bond.

what does N tell us in Scene?

Nucleophiles, we look at the types of nucleophiles associated with each type of reaction.

Sn1 = generally weak nucleophiles

1. alcohols

2. Thiols

3. Halides

Sn2= Generally strong nucleophiles

1. Carbon

2. Sulfur

3. Phosphorous

4. Nitrogen

5. Oxygen

What does E2 tell us and what is the acronym associated with that?

E2 tells us about the Environment, we use STAB to remember the factors.

What does STAB tell us?

Solvent

Temperature

Acid vs Base

What are the solvents associated with Sn1 and Sn2 reactions?

Sn1 = protic, polar solvents

(alcohols, etc)

Sn2 = Aprotic solvents (want to avoid protonating the nucleophile, but they can be polar)

Acetone, DMF, DCM, DMSO

What temperature is associated with Sn1 and Sn2 reactions?

Sn1 = Heated

Sn2 = Room Temperature

What does the presence of Acid and Base tell us about a reaction?

Acid = Sn1 reaction

Base = Sn2 reaction (base either is the nucleophile, makes the nucleophile or makes us consider whether it is an elimination reaction.)