What is the best Nucleophile?

[bharat008]

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I was doing this question in destroyer and I didn't really get it..

What is the best Nucleophile?
A) AlCl3
B) H2O
C) C2H5OH
D) CH3SH
E) CH3COOH

AlCl3 is lewis acid so it's a electrophile since acids are positively charged.= not an answer
H20 is weak base and weak nucleophile. = Not an answer

CH3COOH is resonance stabilized so that would decrease the nucleophilicity since it would stabilize the negative charge on the oxygen atoms..

How do i compare between C2H5O- and CH3S-.. I thought that C2H5O- is a stronger nucleophile compared to CH3S- b/c nuleophilicity increases as you go up the periodic table and to the left in aprotic solvent.

The main question is how do you compare them??
The answer is CH3S- since it's less electronegative than oxygen so it will be able to share the electrons much more easily with electrophile.

The explanation makes sense, but in the chad's video he said that nucleophilicity increases up the column and to the left in aprotic solvent..
In protic solvent it increases down the column and still to the left..

😕😕

 

[Tyrangiel]

I don't think this is a complete answer by any means (and maybe not even a correct one!), but how I think of it is that the electronegativity of the O in the ethanol leads it to be a poor nucleophile because it is more likely to have the H deprotonated than to attack something. On the other hand since the S is less electronegative it has greater electron density in the H attached to it and the lone pairs on it are more likely to attack another electrophile.

 

[bharat008]

I don't think this is a complete answer by any means (and maybe not even a correct one!), but how I think of it is that the electronegativity of the O in the ethanol leads it to be a poor nucleophile because it is more likely to have the H deprotonated than to attack something. On the other hand since the S is less electronegative it has greater electron density in the H attached to it and the lone pairs on it are more likely to attack another electrophile.

Since O is smaller in radius compared to S; the O would have higher electron density per unit of square area. Therefore, smaller nucleophiles are better at making covalent bonds compared to larger nucleophiles who share their electrons around the large atom and stabilizes the negative charge; if you stabilize nucleophile then you make it more stable and it would be less reactive..

 

[bharat008]

😕

 

[Tyrangiel]

Not quite sure what your second post is asking?

 

[Bis-GMA111]

an alcohol is a weak nucleophile and weak base (R-OH) and usually goes through SN1...RO- refers to an alkoxy group which is a strong nucleophile...C2H5OH is ethanol so I think you got the groups mixed up?

 

[pksatodia]

I was doing this question in destroyer and I didn't really get it..

What is the best Nucleophile?
A) AlCl3
B) H2O
C) C2H5OH
D) CH3SH
E) CH3COOH

I am not sure about this answer but I might be wrong. Answer is D b/c its Primary carbon and C is Secondary carbon. I think for nucleophile primary is better then secondary or might be other way around which makes the answer wrong
Chad talks about it in O1.4 or O2.4 or O2.5

 

[bharat008]

I am not sure about this answer but I might be wrong. Answer is D b/c its Primary carbon and C is Secondary carbon. I think for nucleophile primary is better then secondary or might be other way around which makes the answer wrong
Chad talks about it in O1.4 or O2.4 or O2.5

Both are primary nucleophiles. One has methyl group while other has ethyl; still just 1 R group..

 

[pksatodia]

Both are primary nucleophiles. One has methyl group while other has ethyl; still just 1 R group..

😱 you are right
me going back to study basic chem

 

[Golfguy]

It seems like a strange one.

CH3SH is a stronger acid, and thus has a weaker conjugate base.

The negative charge of the conjugate base is able to stabilize the charge over a greater area.

I think Orgoman's explanation is correct. The S atom doesn't hold onto the electrons as tightly, thus is a better nucleophile. Another example would be p-aminophenol. The NH2 group acts as the nucleophile in acetaminophen synthesis.

However, Kaplan says that basicity is king in aprotic solvents. So, it would appear that ethanol is the stronger base, and thus the stronger nucleophile.

 

[recyrb]

ask chad and let us know please!

 

[invictusx]

I was doing this question in destroyer and I didn't really get it..

What is the best Nucleophile?
A) AlCl3
B) H2O
C) C2H5OH
D) CH3SH
E) CH3COOH

AlCl3 is lewis acid so it's a electrophile since acids are positively charged.= not an answer
H20 is weak base and weak nucleophile. = Not an answer

CH3COOH is resonance stabilized so that would decrease the nucleophilicity since it would stabilize the negative charge on the oxygen atoms..

How do i compare between C2H5O- and CH3S-.. I thought that C2H5O- is a stronger nucleophile compared to CH3S- b/c nuleophilicity increases as you go up the periodic table and to the left in aprotic solvent.

The main question is how do you compare them??
The answer is CH3S- since it's less electronegative than oxygen so it will be able to share the electrons much more easily with electrophile.

The explanation makes sense, but in the chad's video he said that nucleophilicity increases up the column and to the left in aprotic solvent..
In protic solvent it increases down the column and still to the left..

😕😕

The better nucleophile would be CH3SH because S is less electronegative than oxygen. This means that electrons will not be tightly held together and will rather donate them. Also, the question never told us which solvent the nucleophile was in. Oxygen would rather KEEP its electrons.

 

[bharat008]

The better nucleophile would be CH3SH because S is less electronegative than oxygen. This means that electrons will not be tightly held together and will rather donate them. Also, the question never told us which solvent the nucleophile was in. Oxygen would rather KEEP its electrons.

So, if you are comparing the nucleophiles in a column then SIZE predominates everytime, no matter what solvent it is.. Could we assume that??

 

[Golfguy]

Just checked my Bruice textbook.

It depends on the solvent:

Polar Protic: The more polarizable species (greater in size, and a weaker base) is more nucleophilic because it interacts weakly with protic solvents, whereas strong bases interact stongly.

Polar aprotic: The stronger the base is the better the nucleophile.

So, unless the questions specifies a polar protic solvent, it is wrong.

 

[invictusx]

So, if you are comparing the nucleophiles in a column then SIZE predominates everytime, no matter what solvent it is.. Could we assume that??

Nope! Solvent definitely affects nucleophilic strength. What you learned from Chad's is correct.

 

[casper22]

I dont think this has anything to do with the solvent...

Comparing C and D basically comes to which one can can stabilize the negative charge better...Oxygen or Sulfur?

Since they both are in the same column, size places an important role - the trend is going from top to bottom, the ability to stabilize the charge increases...in other words, as Sulfur is bigger than Oxygen, the charge placed on sulfur is spread over a larger volume than when placed on oxygen and hence sulfur can stabilize the negative charge better...

 

[bharat008]

I dont think this has anything to do with the solvent...

Comparing C and D basically comes to which one can can stabilize the negative charge better...Oxygen or Sulfur?

Since they both are in the same column, size places an important role - the trend is going from top to bottom, the ability to stabilize the charge increases...in other words, as Sulfur is bigger than Oxygen, the charge placed on sulfur is spread over a larger volume than when placed on oxygen and hence sulfur can stabilize the negative charge better...

Why does nucleophilicty increases as you move up the periodic table?
Reason being that as you move up the atom gets smaller in size and it would make it harder for the atom to stabilize the negative charge. The harder it gets the better the nucleophile since its really UNSTABLE. In this case; S is larger atom compared to O, therefore it will stabilize the negative charge on S; making the nucleophile really stable and decreasing its reactivity compared to other one.

Depends on how you think about it. IF you think that since S is less electronegative than O it would be easier for it to share electrons; then why can't we apply the same concept when comparing F- and Cl-. Btwn F- and Cl-; F- is a better nucleophile compared to Cl- even though it's most electronegative and smaller in size..

So, I came down to these;;; if you are comparing the nucleophiles in the same column except HALOGENS then size predominates.. If you are comparing it in same ROW then BASICITY predominates more than size..
I hope this assumption would be okay?? 👍

 

[aliss4]

Hmmmmmmmm! which number is that in the destroyer? Something is not right here. The Pka of CH3SH is about 10 which makes its conjugate base weaker than C2O5OH's conjugate base (pka of C2H5OH=15). Even in Ochem textbook it says RS- is a better nucleophile than CH3O-( or C2H5O-) in METHANOL solvent( which makes sense) but here????

Oh I get it; the author indirectly is telling us reaction is happening in a protic solvent since CH3SH and C2H5OH are both polar. Then S- is better nucleophile than Oxygen since is larger and can escape better from solvent molecules.

 

[bharat008]

Hmmmmmmmm! which number is that in the destroyer? Something is not right here. The Pka of CH3SH is about 10 which makes its conjugate base weaker than C2O5OH's conjugate base (pka of C2H5OH=15). Even in Ochem textbook it says RS- is a better nucleophile than CH3O-( or C2H5O-) in METHANOL solvent( which makes sense) but here????

Oh I get it; the author indirectly is telling us reaction is happening in a protic solvent since CH3SH and C2H5OH are both polar. Then S- is better nucleophile than Oxygen since is larger and can escape better from solvent molecules.

It's question #73 on 2009 Destroyer.
The author can't tell us indirectly like that unless it was a reaction.. It would scare the hell out of me if they decide to imply indirectly on real DAT..

let's hope they don't ask this kinda double standard questions..

 

[aliss4]

I know! either way, the question doesn't give us enough information. I am pretty sure on real DAT, they would give us some hints.

 

[Stacker]

THE END TO ALL ANSWERS:

Please stop arguing after my post.
I really hope this clears everything out.

Why is the S compound a better answer than ethanol.

1) Let me define electronegativity: of an element) Tending to acquire electrons and form negative ions in chemical reactions.

2) Size - Generally, the more linear and/or smaller the nucleophile, the more nucleophilic it will be. This is because it can react at more sites and will not be sterically hindered if it is smaller or linear.

3) Electronegativity- The more electronegative an atom is, the less nucleophilic it will be. This is because more electronegative atoms will hold electron density closer, and therefore will be less likely to let that electron density participate in a reaction. We see this in calculations and experiments that show nucleophilicity decreases as you get closer to fluorine on the periodic table (C > N > O > F)

4) Polarizability- The more polarizable an atom is, the more nucleophilic it will be. Polarizability is defined as the ability to distort the electron cloud of an atom, which allows it interact with a reaction site more easily. Generally, polarizability increases as you travel down a column of the periodic table (I > Br > Cl > F)


VERY Good nucleophiles
HS-, I-, RS-

Good nucleophiles
Br-, HO-, RO-, CN-, N3-

Fair nucleophiles
NH3, Cl-, F-, RCO2-

Weak nucleophiles
H2O, ROH

VERY weak nucleophiles
RCO2H

***Don't thank me all at once. S compound is better.

 

[Noelle23]

In one of Chad's O)-chem video's (2.6) he talks about Strong Bases (weak Nuc) have negative on oxygens and strong Nuc (weak Base) have negatives on things other than oxygens.

Strong Nuc (Weak Base)

Cl-
Br-
I-
CN-
N3-

Strong Base (Weak Nuc)

Anything with a negative Oxygen

 

[bharat008]

In one of Chad's O)-chem video's (2.6) he talks about Strong Bases (weak Nuc) have negative on oxygens and strong Nuc (weak Base) have negatives on things other than oxygens.

Strong Nuc (Weak Base)

Cl-
Br-
I-
CN-
N3-

Strong Base (Weak Nuc)

Anything with a negative Oxygen

Wrong. Anything that has a negative on Oxygen is a STRONG NUCLEOPHILE and STRONG BASE. It all comes down to what substrate you have. If methyl or primary then SN2 is favored; secondary or tertiary then E2 is favored..👍

 

[Noelle23]

Wrong. Anything that has a negative on Oxygen is a STRONG NUCLEOPHILE and STRONG BASE. It all comes down to what substrate you have. If methyl or primary then SN2 is favored; secondary or tertiary then E2 is favored..👍

Thats only true is it's RO-

Like NaOEt or NaOCh3 whith R groups attached. I referring to no R groups attached.