Nucleophilic substitution reactions: Sn1 vs Sn2

 SN1 and SN2 are substitution reactions. One atom or group is replaced, or substituted, by another.

SN1 reaction mechanism

In the SN1 reaction, the first step is the generation of a carbocation by the dissociation of the leaving group. The carbocation intermediate is sp2 hybridized, and so has a planar geometry. This allows the next step, the attack of the nucleophile on the carbocation, to happen on either side of the plane, resulting in a racemic mixture of products.

SN2 reaction mechanism

In the SN2 reaction, the entire reaction takes place in a single step, starting with the attack of the nucleophile on the central carbon. The nucleophilic attack leads to a transition state where the nucleophile and the leaving group are both loosely associated with the central carbon, and any negative charge is distributed between the nucleophile and the leaving group. To complete the reaction, the nucleophile leaves, and stereochemistry at the central carbon is inverted. The molecule appears to be flipped inside out, like an umbrella flipped the wrong way in a rainstorm.

SN1 or SN2?

So you have a substituted alkane, and you have a nucleophile. How do you know whether an SN1 or SN2 reaction will occur?

There are a number of factors that influence this, which are summarized in the table below.

	Sn1	Sn2
Substrate	Tertiary, allylic, benzylic halides (stable carbocations)	Primary substrate
Leaving Group	Stable anions	Stable anions, weak bases (NOT F-, OH-, OR-, NH2-)
Nucleophile	Non-basic, neutral (must have unshared e- pair)	Basic, negative
Solvent	Polar solvents, protic or aprotic	Polar aprotic solvent (no –NH or –OH groups), non-polar
Stereochemistry of product(s)	Racemic mixture	Inverted (backside attack of Nu-)

What makes a good nucleophile?

Nucleophilicity and basicity are positively correlated. OH- is a stronger base and better nucleophile than CH3COO-, which is more basic and nucleophilic than H2O.

Nucleophilicity increases going down the periodic table. I- > Br- > Cl-, and SH- > OH-.

Negative nucleophiles are more reactive than neutral ones. Usually. So, Sn2 reactions are usually carried out under basic conditions.

Solvents: Polar aprotic vs. Polar protic

Polar aprotic solvents (ex. CH3CN, DMF, DMSO) are good for Sn2 reactions because they are able to dissolve the nucleophilic salts, but tend to solvate (surround) the cations of these salts rather than the nucleophilic anion, separating the anion from its cation and destabilizing it, causing it to be more reactive. Polar protic solvents (solvents with –OH and –NH groups) stabilize the nucleophile, making them less reactive in Sn2 reactions. Non-polar solvents are ok for Sn2; they do not stabilize the cation, but they also do not stabilize the nucleophile, so they do not decrease the nucleophile’s reactivity.

Polar solvents are good for Sn1 reactions because they stabilize the carbocation intermediate formed in the rate-limiting step of the reaction. Non-polar solvents do not offer this stabilization, and so Sn1 reactions do not occur under those conditions.

Aprotic solvents (solvents with no available H⁺), most polar to least polar

Name	Dielectric polarization
Hexane	1.9 (non-polar)
Benzene	2.3
Diethyl ether	4.3
Chloroform	4.8
Hexamethylphosphoramide (HMPA)	30
Dimethylformamide (DMF)	38
Dimethylsulfoxide (DMSO)	48 (very polar)

Protic solvents (solvents with an available H⁺), most polar to least polar

Name	Dielectric polarization
Acetic acid	6.2 (less polar)
Ethanol	24.3
Methanol	33.6
Formic acid	58.0
Water	80.4 (very polar)