Comments on: Deciding SN1/SN2/E1/E2 – The Solvent

By: James Ashenhurst

James Ashenhurst — Fri, 29 Aug 2025 15:20:35 +0000

In reply to Jamie. Fixed. Thanks!

By: Jamie

Jamie — Wed, 27 Aug 2025 09:13:05 +0000

Excellent article as always James. A tiny error for you to correct – your nucleophile NaOCH3 in the 1st section should be named sodium methoxide (not potassium).

By: James Ashenhurst

James Ashenhurst — Wed, 09 Jul 2025 20:07:30 +0000

In reply to Is.

No, no, no.

CH3O(-) is a strong base in CH3OH (pKa of 15). I don’t know where you get the idea that it’s not.

By: Is

Is — Wed, 09 Jul 2025 17:44:08 +0000

In a polar protic solvent like ch3oh, och3- should be weak and do sn1 in 2 degree.. but answer is sn2..how is this possible.. pls help

By: Matthew Pierce

Matthew Pierce — Fri, 01 Nov 2024 18:51:00 +0000

Great update! Thank you so much! It is really interesting to see how, in reference 5, changing the solvent from a protic solvent to DMSO gives a tiny increase in amount of elimination with isopropyl bromide + methoxide. AND in this very same example the temperature is actually lower than the comparison example with protic solvent.

Some common wisdom with these reactions seems to be based on a game of telephone that has been going on for 150 years.

By: James Ashenhurst

James Ashenhurst — Wed, 15 Feb 2023 02:38:39 +0000

In reply to Aayushi. Usually in the Williamson the substrate is a primary alkyl halide. When the substrate is secondary, elimination will compete.

By: Aayushi

Aayushi — Tue, 14 Feb 2023 18:00:37 +0000

Relating to the last sentence I said… Alcohol Is a polar protic solvent… (but it’s less polar than water… and the textbook kept saying that a less polar solvent favours elimination…)
My last sentence more fittingly refers to the Williamson synthesis.
Sorry for the several comments, I am trawling this site for my chemistry final in 2 weeks.

By: Aayushi

Aayushi — Sun, 12 Feb 2023 14:05:09 +0000

Oh a couple of other people have asked about the alcoholic and aqueous KOH already-
I’ve found the source with the explanation:
https://flic.kr/p/2ogxqFf
The book says that alkoxide ion prefers to be a base rather than a nucleophile for steric reasons, as opposed to OH- which can go in for an SN2 easier.
The textbook is Pradeep’s New Course Chemistry for Class 12
So yeah, exception to polar protic solvents favouring E2 over SN2.

By: Aayushi

Aayushi — Sun, 12 Feb 2023 13:17:39 +0000

The second reaction tripped me up because ethoxide ion in ethanol is the usual reagent for the Williamson ether synthesis, which is an SN2 reaction.
(Not on topic, but my textbook also had something confusing about whether it’s better to have a tertiary alkyl halide substrate or a tertiary alkoxide nucleophile for that SN2 reaction…I may ask about it on the comments section of that page)
But as you say it’s not that the SN2 reaction won’t occur at all, it’s that the E2 reaction will occur in greater proportion, I guess. The Williamson page said how we get alkoxide ion from alcohol in situ with Na or NaH, and if the alkoxide ion is given beforehand, we could use a polar aprotic solvent to prevent elimination from competing.
Also, in my course we hear that aqueous KOH leads to an SN2 reaction while alcoholic KOH leads to an E2 reaction. I heard the explanation is that OH- is a good nucleophile for an SN2 reaction, and when there’s alcohol and KOH the OH- can deprotonate the alcohol to get alkoxide ion, which is a stronger base than a nucleophile, so takes a proton for an E2 reaction. Does this hold up?

By: James Ashenhurst

James Ashenhurst — Wed, 20 Jul 2022 18:14:56 +0000

In reply to J. Fluoride ion (not HF) is perfectly capable of acting as a nucleophile - e.g. TBAF (tetrabutylammonium fluoride)