Which solvents favor S N 1 reactions and which favor SN 2 reactions?

Name: Problem 32, Chapter 7: Organic Chemistry
Uploaded: 2022-09-06T20:51:32-08:00
Duration: 3 min 18 s
Channel: Molly Cary
Description: Which solvents favor S N 1 reactions and which favor SN 2 reactions?

Which solvents favor S N 1 reactions and which favor SN 2...

Key Concepts

Reaction Mechanisms

In nucleophilic substitution reactions, the SN1 mechanism involves a two-step process where the leaving group departs to form a carbocation intermediate before the nucleophile attacks, making the rate dependent solely on the substrate. Conversely, the SN2 mechanism is a one-step concerted process where the nucleophile attacks the substrate simultaneously as the leaving group departs, resulting in inversion of configuration. Understanding these mechanisms is essential to predict the effects of various reaction conditions.

Polar Protic Solvents

Polar protic solvents, which have hydrogen bonding capabilities (such as water and alcohols), are known to stabilize both carbocations and anions. This stabilization lowers the energy barrier for the rate-determining step in SN1 reactions, making these solvents favorable for mechanisms that proceed via carbocation intermediates.

Polar Aprotic Solvents

Polar aprotic solvents (like acetone, acetonitrile, and dimethyl sulfoxide) do not form strong hydrogen bonds with anions, which leaves nucleophiles relatively free and more reactive. This enhanced nucleophilicity makes them ideal for SN2 reactions, where a strong and unencumbered nucleophile is critical for the concerted displacement mechanism.

Solvent Effects on Reaction Pathways

The choice of solvent plays a crucial role in determining whether an SN1 or SN2 mechanism is favored. In SN1 reactions, solvents that can stabilize ionic intermediates (polar protic solvents) are preferred, while in SN2 reactions, solvents that enhance nucleophilic strength by minimizing solvation of the nucleophile (polar aprotic solvents) are ideal. This solvent effect is a fundamental concept in physical organic chemistry related to reaction kinetics and mechanism control.

Transcript

00:04 We are given the following for solvents and asked to determine if they would favor an sn1 mechanism or sn2 mechanism.

00:15 So right out of the gate, i just noticed that a and c are the same.

00:19 I'm guessing that's an error in the book, so i'm just going to cross out c because they're the exact same molecule.

00:26 So back to the problem.

00:30 When we're determining which mechanism, sn1 or sn2, a solvent is going to favor, we're really asking, is it polar protic or polar a -protic? and we know that prodic solvents tend to favor sn1, and a -product favor sn2.

00:58 That's because prodic solvents are better able to solvents the unstable adions and cations that are formed during an sn1 mechanism.

01:11 There's a lot more ions just happening in the system.

01:15 And so prodig solvents are better able to do that.

01:18 They're better able to stabilize everything.

01:21 A -protic, on the other hand, favor an sn2 mechanism because sn2 mechanisms need a strong nucleophile.

01:29 A -protic solvents don't solvate anions very well.

01:34 Which means that they're going to allow those strong nucleophiles more space.

01:39 They won't be as hindered and they're going to be better able to attack our electrophile.

01:45 So what we're really looking at here is if these are protic or a protic...

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