Welcome Guest Search | Active Topics |

Tag as favorite
Electronegativity and Nucleophilicity
Abadryon
#1 Posted : Sunday, July 02, 2017 10:12:16 PM
Rank: Newbie

Groups: Registered
Joined: 5/2/2017
Posts: 2

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi all,

I just wanted to inquire about these two concepts: From my understanding, a nucleophilicty is associated with a greater negative charge on a compound, primarily dictated by lone pairs or a high presence of Pi electrons (through triple/double bonds). Nucleophiles want to "donate"/"share" these electrons to the electrophiles in nucleophilic substitutions. Therefore, it is clear that electronegativity can come into hand, whereas the more electronegative (more greedy for electrons) an atom is, the less nucleophilic it is since it does not want to share its electrons. However, this logic was unclear when I stumbled upon a question comparing ethanol and ethanethiol in terms of their nucleophilicity. The book said that ethanol is more nucleophilic than ethanethiol because oxygen is more electronegative than sulfur, making ethanol more basic?? Wouldn't that make ethanol less nucleophilic since oxygen would not want to donate its electrons? I tried to reason it out by saying that perhaps when ethanol is deprotonated (to form its conjugate base), the more electronegative oxygen may be able to retain its negative charge better than when ethanethiol is deprotonated therefore making ethanol possess a greater negative charge (stronger nucleophile).

Can someone confirm my reasoning?
emmadavy
#2 Posted : Monday, July 03, 2017 9:23:43 PM
Rank: Instructor

Groups: Registered
Joined: 6/9/2016
Posts: 1

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
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:

B + H+ goes to BH+


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!
Abadryon
#3 Posted : Monday, July 03, 2017 11:47:02 PM
Rank: Newbie

Groups: Registered
Joined: 5/2/2017
Posts: 2

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi Emma,

Thank you very much for the clarifications!

So from my understanding, BRONSTED basicity deals with an atom's willingness to accept protons, whereas LEWIS basicity involves an atom's willingness to donate electron pairs?


I just wanted to further inquire that, for the purposes of the MCAT, which definition (Lewis or Bronsted) should we apply when ranking nucleophilicity or even electrophilcity? I find that these two theories make the acid-base definitions very ambiguous and can be confusing at times.
fatemahhabib
#4 Posted : Tuesday, July 11, 2017 6:22:41 PM
Rank: Instructor

Groups: Registered
Joined: 6/16/2017
Posts: 1

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
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
Nicole_5521
#5 Posted : Friday, May 21, 2021 11:59:05 PM
Rank: Advanced Member

Groups: Registered
Joined: 4/30/2020
Posts: 71

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hello, I've also been confused about this question.

Does this mean that depending on how you look at the molecules, ethanol could be a better nucleophile than ethanethiol, and ethanethiol could be a better nucleophile than ethanol?

If something like this were to show up on the MCAT, how would you go about choosing the correct answer?

Thanks!
Aleksandra_6728
#6 Posted : Monday, June 14, 2021 10:50:17 PM
Rank: Newbie

Groups: Registered
Joined: 5/16/2021
Posts: 8

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
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!
INSTR_Molly_129
#7 Posted : Saturday, July 10, 2021 7:01:37 PM
Rank: Advanced Member

Groups: Registered
Joined: 5/23/2021
Posts: 68

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi all,

I approached this differently than what had previously been discussed. I used only the Lewis model as its generally (in my opinion) the most useful model for MCAT.

When considering basicity vs. nucleophilicity, its not just about how easy it is to donate an electron (electronegativity considerations), but also, how easy is it for the atom to support a negative charge.

In basicity, we consider how well an atom would be able to support extra electron density as a base. The better it is able to support it, the worse it is as a base. As we go across the periodic table within the same row, the electronegativity increases from left to right. This helps to stabilize the electron density. Therefore, basicity would decrease going from left to right. When going down a column, there is a drastic increase in radii as we go from top to bottom, therefore resulting in much more stabilized electron density at the bottom of the column, since it is able to spread that negative charge across a much larger surface area. Therefore, basicity decreases from top to bottom.

In this case, OH is the better base compared to SH. This is corroborated by pKa values (OH has a pKa of ~15 while SH has a pka of ~10).

In nucleophilicity, we consider how tightly the atom holds onto its electrons. The stronger it holds on to its electrons, and the less nucleophilic it would be. As we go from left to right on the row, the electronegativity increases. Therefore, nucleophilicty decreases from left to right. When we look within the column, the top-most atom would be the worst nucleophile since its going to hold on to its electron so tightly that it will never give it up. When we consider how easy it is for electrons to be picked off (to be donated) since its sooooo far from that positive core that holds electrons tightly, we will see that nucleophilicity increases from top to bottom.

In this case, SH is the better nucleophile since it is much larger and therefore will be able to hold on to its electrons less than oxygen would.

In general, basicity decreases down a column, but nucleophilicity increases down a column. This is the reason why OH is a better base, but SH is a better nucleophile.

Hope this clarifies your questions!

Cheers,

Molly
Users browsing this topic
Guest (4)
Tag as favorite
You cannot post new topics in this forum.
You cannot reply to topics in this forum.
You cannot delete your posts in this forum.
You cannot edit your posts in this forum.
You cannot create polls in this forum.
You cannot vote in polls in this forum.

Clean Slate theme by Jaben Cargman (Tiny Gecko)
Powered by YAF | YAF © 2003-2009, Yet Another Forum.NET
This page was generated in 0.145 seconds.