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Factors Affecting SN2 Reactions
#1 Posted : Monday, June 22, 2020 11:36:18 PM
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I'm just reviewing the factors affecting SN2 rxn's and am a little confused on the trends that make a Nucleophile stronger/better;

It says that Nucleophilicity generally parallels basicity [when comparing species with the same attacking ion] - what does the second part of this sentence actually mean? From that I would gather all the trends we look for in Acid/Base chem surrounding what makes a SA would just be reversed to find a SB. My interpretation then would be that we are looking for unstable molecules.

- Why do we then say that Nucleophilicity usually Increases down a column of the periodic table? Doesn't Acidity increase down a column due to electron charge being more spread out? Thus making a weaker Nucleophile? In med reels as well Mike just said to refer to stability trends in acid base chem, which seems to contradict the earlier statement of nucleophilicity paralleling basicity. On pg. 85 it specifically says that size predominates over EN within a group, which adds to this contradcition because this would indicate that if Nucleophilicity was following basicity it would be weaker going down a group. On pg.87 however, it says leaving a -ve charge on a more EN is more favourable than leaving one on a less EN so this seems to align with this statement?

- Why is it the opposite then for halides?

Overall I'm just a little confused with tying Nucleophilicity to Basicity and would appreciate any help :)
#2 Posted : Thursday, June 25, 2020 10:12:03 PM
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There are a lot of conflicting contextual rules, I will try to put out a set here that reconciles this, framed according to periodic trends.

1. Across a row, nucleophilicity decreases with electronegativity, as more electronegative elements want to give away electrons less. (This is the same as saying nucleophilicity increases with basicity, as more electronegative elements are less basic).

Rule 1 assumes that your atoms are in the same ionization state.

For example, we would predict from this trend that Na would be the most nucleophilic of its row. In fact, it is so nucleophilic that you only tend to find it as Na+ in Ochem reactions. Because Na+ has already given its electrons away, it is not nucleophilic at all.

In short, you have to compare neutral atoms with neutral atoms, you can't compare OH- to NH3 as easily in this case.

2. Down a column, we no longer consider acidity/basicity as atom size is more important than electonegativity. We just consider if the atom is smaller or larger and ignore everything else.

The bigger an atom is, the more likely it will bump into an electrophile at the right orientation, resulting in a reaction. This is for example why S is more nucleophilic than O.

We usually consider these two rules for the common atoms in biology, for example N, C, O, S, P, and a couple extra elements. These are the rules I would want to have down for the MCAT

Another quick note about 1 and 2 is we usually only use one rule at a time - note in review question on page 163 we compare N C and O at the same ionization state (-1). If you had to compare N and S you would have to weigh the importance of rule 1 and 2, which isn't really possible to do except by experiment and is therefore outside of the scope of things you would be expected to know for the MCAT

3. Halides are complicated exceptions and solvent dependant. This is more complicated and less useful for the MCAT than rules 1 and 2

Most nucleophiles you look at don't have a full blown negative charge on them. For F, Cl, Br and I they will bear this full blown negative as nucleophiles, so how nucleophilic they are depends on how well they stabilize negative charge. For the cases of negative charge, because they are all so far on the right we consider their electronegativity about the same and consider size more significant.

Because F- stabilizes negative charge the least, being smallest, it is the most intrinsically reactive, and therefore nucleophilic. However, it is also the most basic. That means in a polar protic solvent, it will hydrogen bond a lot, preventing it from acting as a nucleophile.

What I am trying to do with the framework is tell you when to use one of the conflicting rules, just as in periodic trends.

This is a lot to digest, I hope this framework helps. On the bright side, if you really absorb this framework, identifying nucleophiles gets a lot easier. I would try to use this framework and practice it on examples instead of memorizing.

Please let me know if you have any questions.

#3 Posted : Friday, June 26, 2020 5:03:47 PM
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Hello, I tried posting this reply but the Forum was down! There's a great reply already but if it helps, here's some additional information:

Nucleophilicity is a subset of basicity. Most nucleophiles are also good lewis bases (electron pair donors).

[when comparing species with the same attacking ion] - in the example given, the attacking atom is always oxygen. This part is saying nucleophilcity is the same as basicity when looking at the same atom.

Here's a link that may help clarify with some great diagrams: http://www.chem.ucalgary...arey5th/Ch08/ch8-6.html

As for SA and SB, the key characteristic to look at is how much they ionize in solution, strong species ionize completely (100%) and weak species don't. The conjugate of a strong acid will be a weak base. But conjugates of weak acids do have some basicity (and vice versa) and the relative strength between a weak acid/base and its conjugate can be compared.

Nucleophilicity is a kinetic concept, it describes how good a nucleophile at reacting - how fast it is, how little energy it requires. Basicity is a thermodynamic concept, it tells you about the stability of the product. For example, F- is more basic than I- but in a protic solution it is less nucleophilic since the hydrogens form a tight shell around the F- and prevent it from actively interacting with other species.

Here are the trends and why:
- L to R on periodic table nucleophilicity decreases and basicity decreases --> since increasing electronegativity decreases the lone pair availability.

- Top to bottom on periodic table nucleophilicity increases since because of size and polarizability --> electron density of larger atoms is more readily distorted since the electrons are further from the nucleus. However, basicity decreases (acidity increases down a group) because polarizability is much less important for bond formation to the very small proton.

Hope this helps!
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