Hello,

Thank you for your question, it is a good one.

Since I have not been provided with a page number for this question, I'm going to assume that you are attempting the question on page 80 of the EK study guide with the answer on page 113.

I also want to add that I believe this question and answer is poorly worded and unspecific so you may find this answer long. This is to provide context and detail and hopefully answer your question about nucleophilicity in general.

Let's start with the question:

1. The EK study guide has not specified an electrophile, and as you will see, this is going to be of great importance. There are two types of electrophiles that we can consider for this question which will give different rankings! The two types of electrophiles we will consider are:

a. the proton (H+)
b. an Sn2 type substrate (like CH3-LG where LG is a good leaving group)

Now, I'll address their answer, as well as your comments about nucleophilicity:

1. they state "ethanol is more basic". This is correct if we consider BRONSTED BASICITY. Recall that a Bronsted base will accept a proton with the following equilibrium:



Since the O-H bond is more polarized than the S-H bond, the oxygen atom will have a larger delta minus and will therefore attract the positively charged proton more readily, rendering the ethanol more BRONSTED basic than the ethanthiol.

2. However, Bronsted basicity doesn't not tell a complete store for nucleophilicity. Lewis basicity also plays a large role. Remember that a Lewis base is defined as an electron pair donor, and a better Lewis base is a better nucleophile. Better Lewis bases are usually larger (electrons are more diffuse) and usually less electronegative (electrons are more freely donated). For this reason, ethanethiol will usually be a better LEWIS base than ethanol, in the reactions you are required to know for the MCAT.

3. Not required to answer this question, but still of importance for nucleophiles, is solvent considerations (polar protic which can hydrogen bond and polar aprotic which cannot hydrogen bond) and nucleophile size (bulkier nucleophiles tend to be poorer nucleophiles).


All this being said, I believe the question that the EK study guide is trying to ask is considering the proton as the electrophile (consideration a. when discussing the question). In this scenario, BRONSTED basicity is the most important and their ranking stands.

However, if we were considering an Sn2 reaction, their ranking would be incorrect as the thiol would certainly be the better nucleophile than the ethanol (and pretty much equivalent in strength to the phenoxide).

I hope this clarifies things. Please feel free to follow up with any questions you may have, I will be sure to check back tomorrow.

Excellent work in your careful reading!

Hello,

Thank you for your question. As Emma explained the two definitions are not exactly the same, they're like two sides of the same coin so to speak. On the MCAT, it's not going to be one definition vs the other. The way I reason out the question on the EK book is that the O is more electronegative so it carries more electron density and it's more willing to go and attack an electrophile (that's what nucleophiles do) because it's smaller in size. When comparing O and S you're really comparing electronegativity and atom size. The more spread out the electron density (such as in a larger atom) the more stabilized it is and hence will be less reactive.

Bottom line is you want to determine which atom is more likely to go and react more willingly and it's often going to be balancing out multiple things to make a final decision. Remember, on the MCAT, always try and use the strategy of elimination. It will most likely lead you to a 50/50 answer if you know which are the most reactive (charged nucleophiles) and least reactive (neutral ones).

Hope this helps!

Fatemah

I had the same question and enjoyed reading this explanation. Thank you!

To the question above by Abadryon, I believe that prioritizing the idea that nucleophilicity is proportional to Lewis base strength is a good idea. This is the case where the thiol is the stronger nucleophile. Considering Bronsted Lowry is like imagining an acid/base reaction. Most of the reactions requiring a nucleophile (nucleophilic substitutions and additions) in the MCAT from my experience will fall under the former consideration of Lewis bases instead.

This is my rationale. Please, if someone knows more or something different, chip in so that I do not confuse anybody.

Thanks!