strongest nucleophile

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A. No, because fluorine is very electronegative. Electron density is pulled from the lone pair, making it less nucleophilic.
B. No, because, the hydroxide anion is relatively stable. Oxygen is quite electronegative (see above).

So it's C or D. That should be clear right away.

Carbon is less electronegative than nitrogen, so in CH3- all the electron density will be on the carbon. It will be VERY interested in removing its negative charge, i.e., it will be looking for a proton or other cation. Recall that although NH2- is a very bad leaving group, it still is a leaving group, which shows it can exist. To my knowledge, you will never find CH3- as a leaving group. That's one reason alkanes are essentially nonreactive.
 
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Why are "F-" and the "B" in the second choice bolded?

I'd say D. Grignard reagents are really strong nucleophiles, size is irrelevant because all negative atoms here are in the same row, and the least electronegative will most easily attack electrophiles.
 
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I guess i should have given more detail. Anyways, this was a question from the mcat question of the day. I answered Flourine because it was electronegative. I thought that Electronegative contributed to Nucleophilicity? So my first instinct was to go with the most electron negative being Fluorine but it turns out that the answer is CH3. Thanks guys.
 
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gettheleadout said:
Why are "F-" and the "B" in the second choice bolded?

I'd say D. Grignard reagents are really strong nucleophiles, size is irrelevant because all negative atoms here are in the same row, and the least electronegative will most easily attack electrophiles.
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Format error. So how can you tell a good nucleophile? I know you can tell a good living group by electronegative right?
 
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In aprotic solvents, nucleophicity correlates to base strength:
CH3- > NH2- > OH- > F-

No way, that AAMC would ask this question without specific whether it's in protic or aprotic solvents, b/c it wouldn't sense.

Seriously, sloppy question. Well, I guess as gettheleadout, size is irrelevant for these particular atoms.
 
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RickTaylor said:
A. No, because fluorine is very electronegative. Electron density is pulled from the lone pair, making it less nucleophilic.
B. No, because, the hydroxide anion is relatively stable. Oxygen is quite electronegative (see above).

So it's C or D. That should be clear right away.

Carbon is less electronegative than nitrogen, so in CH3- all the electron density will be on the carbon. It will be VERY interested in removing its negative charge, i.e., it will be looking for a proton or other cation. Recall that although NH2- is a very bad leaving group, it still is a leaving group, which shows it can exist. To my knowledge, you will never find CH3- as a leaving group. That's one reason alkanes are essentially nonreactive.
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So does that mean that the less electronegative an atom is, the better a nucleophile? Thanks for the response 🙂
 
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Wow, this was the mcat question of the day and people got it owned by it. Im surprised. Theres a specific periodic trend for this; nucleophile strength increases from right to left and top to bottom on the periodic table. Thats all you really needed to know.
 
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I memorized it as such; nucleophile trends == atomic radius trends.

But then there's been huge discussion on here as to aprotic and protic solvents.
 
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I think what is tricky about this question is that the trend you stated above is slightly altered in protic solvents.

So in aprotic solvents, basicity is directly related to nucleophicity. But when it comes to the most electronegative small anions, that trend is reversed in protic solvents. For those anions, in protic solvents, nucleophicity is actually I-> Br- > Cl-> F-. You can see, just for these electronegative anions, the "trend" has reversed itself in protic solvents.

However, it's still the same for CH3-> NH2 > OH- > and that's something I myself just learned with this question.

You could memorize your stated trend and be mostly fine...but you'd get it wrong if you applied that to an SN1 reaction that asked you to rank, F-, Cl- , Br- and I-...To be honest, it's hard for me to explain and I'm sure someone can do a better job.
 
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Just to make sure I got it..
CH3- is a stronger nucleophile because it is the one least stable with a negative charge?
Thank you!
 
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The way you're supposed to remember it is the least electronegative is the best nucleophile. It hates those negative charges, and wants to form a bond--fast. C>N as a nucleophile, at the least.
 
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Solvent shouldn't matter. Strongest is strongest, but best for a reaction in a certain solvent is different.
 
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what confused me was the bold. I thought OP was telling that A was the answer which seemed wrong...

CH3- is such a strong base that it trumps the others regardless of which solvent they are in.
 
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pfaction said:
So what's your classification, MedPR?
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What do you mean? I don't have any shortcuts to identifying which is stronger, I just know that the relationship between nucleophile A and nucleophile B does not change with the solvent. If A is stronger than B, it is always stronger than B, regardless of the solvent.
 
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MedPR said:
What do you mean? I don't have any shortcuts to identifying which is stronger, I just know that the relationship between nucleophile A and nucleophile B does not change with the solvent. If A is stronger than B, it is always stronger than B, regardless of the solvent.
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I know this is a couple of months old, but considering it kept coming up every time I searched for nucleophilicity trends and we love telling people to use the search function, I thought a few serious misconceptions should be corrected for those currently studying.

MedPR's sentiments are actually incorrect, organic chemistry is all about context. Something can be a nucleophile in one context and an electrophile in another, an acid in one, and a base in another. The context always matters. In the case of the question above, in the same row/period the trend is the same in all contexts for the purpose of the MCAT, so this question could be answered without context; however, if the atoms were in different rows context IS needed.

IN THE SAME ROW/PERIOD, and in the same ROW only, nucleophilicity almost always follows the base trend (from right to left) meaning the LESS electronegative the BETTER nucleophile only in the same row. The fact is regardless of solvent for MCAT intents and purposes.

In the same COLUMN/GROUP, an UNCHARGED nucleophile will follow the trend for RADIUS. As the atom gets LARGER, nucleophilicity INCREASES. Meaning nucleophilicity increases DOWN a column with an uncharged nucleophile (helpful knowledge when you are trying to pick a nucleophile rather than a base to favor Sn over E). This is also true of a CHARGED nucleophile in POLAR PROTIC solvent (since the polarity of the solvent partially neutralizes/stabilizes the charge, allowing the typical trend of INCREASING DOWN a group).

In the case of an APROTIC solvent, the trend is reversed only in a COLUMN/GROUP. In this case, since the charge is not neutralized/stabilized, the smaller atoms will make the greatest nucleophile (same as base). or INCREASE UP a GROUP.

So overall, an uncharged, or partially neutralized (protic solvent) will trend down and to the left; while a charged nucleophile in aprotic solvent will proceed up and to the left.
 
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impact2d said:
I know this is a couple of months old, but considering it kept coming up every time I searched for nucleophilicity trends and we love telling people to use the search function, I thought a few serious misconceptions should be corrected for those currently studying.

MedPR's sentiments are actually incorrect, organic chemistry is all about context. Something can be a nucleophile in one context and an electrophile in another, an acid in one, and a base in another. The context always matters. In the case of the question above, in the same row/period the trend is the same in all contexts for the purpose of the MCAT, so this question could be answered without context; however, if the atoms were in different rows context IS needed.

IN THE SAME ROW/PERIOD, and in the same ROW only, nucleophilicity almost always follows the base trend (from right to left) meaning the LESS electronegative the BETTER nucleophile only in the same row. The fact is regardless of solvent for MCAT intents and purposes.

In the same COLUMN/GROUP, an UNCHARGED nucleophile will follow the trend for RADIUS. As the atom gets LARGER, nucleophilicity INCREASES. Meaning nucleophilicity increases DOWN a column with an uncharged nucleophile (helpful knowledge when you are trying to pick a nucleophile rather than a base to favor Sn over E). This is also true of a CHARGED nucleophile in POLAR PROTIC solvent (since the polarity of the solvent partially neutralizes/stabilizes the charge, allowing the typical trend of INCREASING DOWN a group).

In the case of an APROTIC solvent, the trend is reversed only in a COLUMN/GROUP. In this case, since the charge is not neutralized/stabilized, the smaller atoms will make the greatest nucleophile (same as base). or INCREASE UP a GROUP.

So overall, an uncharged, or partially neutralized (protic solvent) will trend down and to the left; while a charged nucleophile in aprotic solvent will proceed up and to the left.
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Can you give me an example of two nucleophiles, A and B, where A is stronger than B in protic, but B is stronger than A in aprotic?
 
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MedPR said:
Can you give me an example of two nucleophiles, A and B, where A is stronger than B in protic, but B is stronger than A in aprotic?
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Example:

Nucleophile A: I-
Nucleophile B: F-

A is stronger in a protic solvent because of its large size. It will not "stick" to the solvent molecules as well as the smaller sized nucleophile (polarity argument here); therefore, it can serve as a better attacker to an electron deficient atom.

However, in the gas phase (or another aprotic solvent), F- is the stronger nucleophile because of its small size. The small size causes its charge to be extremely condensed, and in organic chemistry, the lack of ability to spread out the charge = highly unstable/reactive.
 
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CodeRedDew said:
Example:

Nucleophile A: I-
Nucleophile B: F-

A is stronger in a protic solvent because of its large size. It will not "stick" to the solvent molecules as well as the smaller sized nucleophile (polarity argument here); therefore, it can serve as a better attacker to an electron deficient atom.

However, in the gas phase (or another aprotic solvent), F- is the stronger nucleophile because of its small size. The small size causes its charge to be extremely condensed, and in organic chemistry, the lack of ability to spread out the charge = highly unstable/reactive.
Click to expand...

Exactly, neutral and protic follow the trend of nucleophilicity increasing down a group (I->F-). Aprotic charged follow the base trend F->I-. I like to think of the standard neutral nucleophile trend as down and to the left, and protic solvent makes charged nucleophiles "kind of neutral". If they are charged and not "neutralized" they follow the base trend.

Sent from my ADR6400L using SDN Mobile
 
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