O-chem question

ssodamy · Jun 30, 2011

Regarding the better nucleophile

Which is the best nucleophile?
a)H2O
b)AlCl3
c)C2H5OH
d)CH3SH
e)CH3COOH

As I know, in general oxygen is more nucleophilic than sulfer in aprotic solvent while in protic solvent, sulfer is more nucleophilic than oxygen.

The thing is that this question does not mention about the solvent but the answer is d.

Please help me about this question.

yappy · Jun 30, 2011

Bump: I dont understand this either.

I thought you can eliminate H20 because it rarely disassociates. ALCl3 is an inert solvent....

I dont understand how to rank C, D, & E? I thought the general trend of nucleophiles was up and to the left in aprotic solvents.

ssodamy said:
Regarding the better nucleophile

Which is the best nucleophile?
a)H2O
b)AlCl3
c)C2H5OH
d)CH3SH
e)CH3COOH

As I know, in general oxygen is more nucleophilic than sulfer in aprotic solvent while in protic solvent, sulfer is more nucleophilic than oxygen.

The thing is that this question does not mention about the solvent but the answer is d.

Please help me about this question.

rockclock · Jun 30, 2011

Page 2: http://www.freelance-teacher.com/organic_chemistry_sn2_sn1_e2_e1.pdf

this guy has a video series on youtube, too. really good stuff...

woox · Jun 30, 2011

OH is a bad leaving group so A, C, E are eliminated...AlCl3 is a ionic compound

leaves D

RING12 · Jun 30, 2011

ssodamy said:
Regarding the better nucleophile

Which is the best nucleophile?
a)H2O
b)AlCl3
c)C2H5OH
d)CH3SH
e)CH3COOH

As I know, in general oxygen is more nucleophilic than sulfer in aprotic solvent while in protic solvent, sulfer is more nucleophilic than oxygen.

The thing is that this question does not mention about the solvent but the answer is d.

Please help me about this question.

know this for nucleophilicity:
1. nucleophilicity increases as negative charge increase. ex) NH2- is more nucleophilic than NH3.

2. Nycleophilicity increases going down the periodic table with a group. ex) F-<cl-<Br-<I-.
3. Nuclephilicity increases going left in the periodic table across a period. ex) NH2- is more nucleophilic than OH-.

I hope that helps you. 😉

ssodamy · Jun 30, 2011

rockclock said:
Page 2: http://www.freelance-teacher.com/organic_chemistry_sn2_sn1_e2_e1.pdf

this guy has a video series on youtube, too. really good stuff...

Thanks it was helpful.

ssodamy · Jun 30, 2011

woox said:
OH is a bad leaving group so A, C, E are eliminated...AlCl3 is a ionic compound

leaves D

thanx

ssodamy · Jun 30, 2011

RING12 said:
know this for nucleophilicity:
1. nucleophilicity increases as negative charge increase. ex) NH2- is more nucleophilic than NH3.

2. Nycleophilicity increases going down the periodic table with a group. ex) F-<cl-<Br-<I-.
3. Nuclephilicity increases going left in the periodic table across a period. ex) NH2- is more nucleophilic than OH-.

I hope that helps you. 😉

thanx

ssodamy · Jun 30, 2011

yappy said:
Bump: I dont understand this either.

I thought you can eliminate H20 because it rarely disassociates. ALCl3 is an inert solvent....

I dont understand how to rank C, D, & E? I thought the general trend of nucleophiles was up and to the left in aprotic solvents.

whatever, thanx for replying

AllahIsGreater · Jun 30, 2011

"never bet against SH" -- Dr. Romano

yappy · Jul 1, 2011

Why whatever? What I said was true. BTW alot of what these other posters have said is false. Nuc: follows the base trend (in aprotic enviroment) - therefore it increases going up a group and to the left on the periodic table. And in those above examples OH would not be the LG; H would be to create an OR nuc:.

To challange that: Are they saying the H on the carboxlic acid would not donate the acidic H? lol.

ssodamy said:
whatever, thanx for replying

Troyvdg · Jul 1, 2011

AllahIsGreater said:
"never bet against SH" -- Dr. Romano

+1

aprotic solvents only increase nucleophilicity! it doesn't decrease the nucleophilicity of already good nucleophiles like SH

RING12 · Jul 1, 2011

yappy said:
Why whatever? What I said was true. BTW alot of what these other posters have said is false. Nuc: follows the base trend (in aprotic enviroment) - therefore it increases going up a group and to the left on the periodic table. And in those above examples OH would not be the LG; H would be to create an OR nuc:.

To challange that: Are they saying the H on the carboxlic acid would not donate the acidic H? lol.

and increases going down a group not up a group!!

yappy · Jul 1, 2011

Well lets hash this out.

chad + some other resources have said that when evaluating nucleophile strength in protic vs aprotic enviroments - the nucleophile is stronger going up a group in an aprotic enviroment and stronger going down a group in a protic enviroment.

This is because the larger nucleophiles (going down) are not influenced by the protic solvent due to their size.

Does anyone have any good sources that say the same or different?

I really want to know if I'm wrong and where i'm misunderstanding this concept.

Thanks

RING12 said:
and increases going down a group not up a group!!

RING12 · Jul 1, 2011

yappy said:
Well lets hash this out.

chad + some other resources have said that when evaluating nucleophile strength in protic vs aprotic enviroments - the nucleophile is stronger going up a group in an aprotic enviroment and stronger going down a group in a protic enviroment.

This is because the larger nucleophiles (going down) are not influenced by the protic solvent due to their size.

Does anyone have any good sources that say the same or different?

I really want to know if I'm wrong and where i'm misunderstanding this concept.

Thanks

yes, what you are saying is compeletly right. but in general like here that doesnt say any thing about the solution you should look down the group. chad explanation made me confuse too. what I said about thoes three rules I actually didnt make them up. I got them from priceton review mcat organic chemistry.View attachment photo (5).JPG

yappy · Jul 1, 2011

So if I'm understanding right..

The most negative is the best nucleophile.

Nucleophilicity increases from right to left.

And within a group it increases going down the group.

Thanks Ring. I'll investigate it a little more and make sure i fully understand it.

RING12 said:
yes, what you are saying is compeletly right. but in general like here that doesnt say any thing about the solution you should look down the group. chad explanation made me confuse too. what I said about thoes three rules I actually didnt make them up. I got them from priceton review mcat organic chemistry.View attachment 17247

flin5845 · Jul 1, 2011

LetsGo2DSchool · Jul 3, 2011

RING12 said:
yes, what you are saying is compeletly right. but in general like here that doesnt say any thing about the solution you should look down the group. chad explanation made me confuse too. what I said about thoes three rules I actually didnt make them up. I got them from priceton review mcat organic chemistry.View attachment 17247

I don't know if one can completely trust Princeton Review. If they were that good, then you would hear about more people using their book on the forums.

Troyvdg · Jul 3, 2011

LetsGo2DSchool said:
I don't know if one can completely trust Princeton Review. If they were that good, then you would hear about more people using their book on the forums.

these three guidelines are in any organic chemistry textbook 👍

ssodamy · Jul 6, 2011

yappy said:
Well lets hash this out.

chad + some other resources have said that when evaluating nucleophile strength in protic vs aprotic enviroments - the nucleophile is stronger going up a group in an aprotic enviroment and stronger going down a group in a protic enviroment.

This is because the larger nucleophiles (going down) are not influenced by the protic solvent due to their size.

Does anyone have any good sources that say the same or different?

I really want to know if I'm wrong and where i'm misunderstanding this concept.

Thanks

Thank you for the interposing an objection. Truth be told I also got the concept from Chad video. Now it became obvious. 👍

ssodamy · Jul 6, 2011

RING12 said:
yes, what you are saying is compeletly right. but in general like here that doesnt say any thing about the solution you should look down the group. chad explanation made me confuse too. what I said about thoes three rules I actually didnt make them up. I got them from priceton review mcat organic chemistry.View attachment 17247

Thank you!!!!

Now it became clear!!!👍

JustwantDDS · Jul 6, 2011

someone asked this on chad's forums here is his reply:

"The vertical trend for nucleophile strength depends on the type of solvent.

In protic solvents, nuc strength increases going down a group on the periodic table (so SH- is stronger in protic solvents).

But in aprotic solvents, nuc strength increases going up a group on the periodic table (so OH- is stronger in aprotic solvents).

The difficulty is that many professors and textbooks only present the trend for protic solvents and make it sound as if it's an absolute rule for all situations even though it's not. Because of this, if you do see a question that doesn't specify the type of solvent then I would use the trend for protic solvents to answer that one.

Hope this helps!"

ATPham · Jul 6, 2011

woox said:
OH is a bad leaving group so A, C, E are eliminated...AlCl3 is a ionic compound

leaves D

can someone comment on this please? im confused about how leaving group is related to something being a better nucleophile

ssodamy · Jul 8, 2011

JustwantDDS said:
someone asked this on chad's forums here is his reply:

"The vertical trend for nucleophile strength depends on the type of solvent.

In protic solvents, nuc strength increases going down a group on the periodic table (so SH- is stronger in protic solvents).

But in aprotic solvents, nuc strength increases going up a group on the periodic table (so OH- is stronger in aprotic solvents).

The difficulty is that many professors and textbooks only present the trend for protic solvents and make it sound as if it's an absolute rule for all situations even though it's not. Because of this, if you do see a question that doesn't specify the type of solvent then I would use the trend for protic solvents to answer that one.

Hope this helps!"

Good!!! It's helpful!! Thanx👍

Expert · Jul 8, 2011

ssodamy said:
Regarding the better nucleophile

Which is the best nucleophile?
a)H2O
b)AlCl3
c)C2H5OH
d)CH3SH
e)CH3COOH

As I know, in general oxygen is more nucleophilic than sulfer in aprotic solvent while in protic solvent, sulfer is more nucleophilic than oxygen.

The thing is that this question does not mention about the solvent but the answer is d.

Please help me about this question.

Since this question does not mention anything about a solvent, you can probably assume that any solvent information is not necessary in figuring out the correct answer.

Nucleophiles are typically bases, with negative charges and/or lone pairs. Strong nucleophiles have electron donating groups.

CH3COOH is an acid, and therefore not a nucleophile.
AlCl3 is a lewis acid (think EAS reactions), and also not a nucleophile.
H20 and C2H5OH both contain an -OH group, which has lone pairs making them nucleophilic. But C2H5OH has an an alkyl (ethyl) group attached to the -OH, making it more electron donating (and nucleophlic) than H2O, which has no electron donating groups.

So now you are left with C2H5OH and CH3SH. Both oxygen and sulfur contain lone pairs. But oxygen is more electronegative than sulfur, so oxygen holds it electrons more tightly than sulfur, making it less nucleophilic. The answer turns out to be CH3SH.

needzmoar · Jul 8, 2011

ssodamy said:
As I know, in general oxygen is more nucleophilic than sulfer in aprotic solvent while in protic solvent, sulfer is more nucleophilic than oxygen.

Question really bothered me so here's my attempt.

long version: Sulfur, is the better nucleophile between neutral oxygen and neutral sulfur. Why is this so you may ask? Well it has to do with the energy of the transition state, which affects the rate of attack, which is, by definition, determines if it is a strong nucleophile ore not. Sulfur's electron cloud is larger than oxygen's due to it's valence electrons being loosely held. This allows for a more easily polarized electron cloud which results in a greater partial sigma bond overlap in the transition state, which lowers the activation energy, which increases the rate, which increases the nucleophilicity.

short version: nucleophilicity increases as you go down and to the left, as many have already stated.

Troyvdg said:
+1

aprotic solvents only increase nucleophilicity! it doesn't decrease the nucleophilicity of already good nucleophiles like SH

This is true. true dat. I don't think I could have given a more concise answer than that. Reaction type and solvent information were not given, so just follow the general trend.

Barking Lamb · Jul 8, 2011

Consider this problem intuitively. 'O' holds its electrons much closer to its nucleus than Sulfur. This means sulfur's electrons are making a bigger cloud! A bigger electron cloud is more effective at reaching out and grabbing a nucleus. Sulfur, therefore, must be a better nucleophile.

JustwantDDS · Jul 8, 2011

👍👍

Barking Lamb said:
Consider this problem intuitively. 'O' holds its electrons much closer to its nucleus than Sulfur. This means sulfur's electrons are making a bigger cloud! A bigger electron cloud is more effective at reaching out and grabbing a nucleus. Sulfur, therefore, must be a better nucleophile.

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