DAT Destroyer OC 2012 #26

[Meat Gyver]

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Quick request for clarification:

Which is the best nucleophile?

H20
AlCl3
C2H5OH
CH2SH
CH3COOH

Answer: CH2SH

I understand that S is less electronegative than O thus it will be a better nucleophile. However, having watched Chads video on nucleophilic strength in both aprotic and protic solvents, the trend for a nucleophile in aprotic solution would favor oxygen (C2H5OH) since reactivity increases as you go up the periodic table. Yet, in protic solvent, the oxygen would be stabilized much better, thus sulfur (CH2SH) would be a better nucleophile. Of course, no mention of protic or a protic is given, so I just based my answer on electronegativity because that trend remains constant between both type of solvents.

I just wanted to get some ideas of how everyone else would approach this question. Also if it was mentioned that the nucleophile was in aprotic solvent, would it change the answer?

Thanks!

 

[patricklin27]

I dont get how ch2sh is a better nucleophile. S is a larger atom than O.. So wouldnt that make Sulfur a less nucleophilic atom than oxygen?

 

[patricklin27]

If they don't say anything, do we just assume there are in a protic solvent?

 

[Meat Gyver]

I certainly hope the real DAT would clarify that question better. Or atleast keep the relative sizes the same and just vary electronegativity.

 

[DrRoyal Pains]

I ran into this problem on the 2013 edition. bump.

 

[roquer2]

Funny. I made a thread about this specific question a while ago. Check it out and hope it helps.

http://forums.studentdoctor.net/showthread.php?t=1003289

 

[patricklin27]

Funny. I made a thread about this specific question a while ago. Check it out and hope it helps.

http://forums.studentdoctor.net/showthread.php?t=1003289

im still confused as to what the right explanation for this question. I was always taught that if youre comparing two atoms in the same column, you look at size difference and if you are comparing two atoms in the same row, you look at EN. When in doubt, size difference is always more important than EN.

It doesnt make sense b/c Sulfur is a bigger atom than Oxygen, so its negative charge is going to be spread out more and thus its nucleophilicity is going to be less than oxygen.

 

[patricklin27]

im guessing you dont look at size difference when comparing nuclephilicity?

 

[roquer2]

im still confused as to what the right explanation for this question. I was always taught that if youre comparing two atoms in the same column, you look at size difference and if you are comparing two atoms in the same row, you look at EN. When in doubt, size difference is always more important than EN.

It doesnt make sense b/c Sulfur is a bigger atom than Oxygen, so its negative charge is going to be spread out more and thus its nucleophilicity is going to be less than oxygen.

"K.I.S.S."

For the DAT, I'll trust Chad's vids/words on concepts you have to know over Destroyer, for the most part.

For a regular gen chem class, I'll go with Destroyer and whatever specifics are mentioned.

That's just me though.

 

[patricklin27]

"k.i.s.s."

for the dat, i'll trust chad's vids/words on concepts you have to know over destroyer, for the most part.

For a regular gen chem class, i'll go with destroyer and whatever specifics are mentioned.

That's just me though.

kiss?

 

[roquer2]

kiss?

"Keep it simple, stupid"

Not calling you stupid, it's just an expression I'm guessing you haven't heard of, lol.

 

[bluemoon27]

yea i remember chad saying this in the video! bahaha

 

[lowkeydr]

I had to think about this question for a bit when I encountered it.
H20
AlCl3
C2H5OH
CH2SH
CH3COOH
Basically you know that nucleophiles get stronger as you go left and up a periodic table on aprotic solvents. CH2SH is Aprotic.. so its like sitting on fairly high nucleophile list
However, nucleophiles get stronger as you got left and DOWN on protic solvents. H2O, C2H5OH, CH3COOH are all protic, so they are actually on the weak side of nucleophiles.

and of course AlCl3 is an electrophile.

This is the logical conclusion I made... If I'm wrong someone correct me please!

 

[Dotoday]

Do not worry about a solvent, just go with electronegativity. The solvent is important during the reaction.

Sent from my SPH-L710 using Tapatalk

 

[patricklin27]

So size difference doesnt play a part in nucleophilicity?

 

[Illfavor]

IN GENERAL, nucleophilicity mirrors basicity.(it's probably not even clear saying that! it's murky haha) Thus, atoms with in the same column with equal charges and lone pairs are discriminated based on SIZE. The larger atom is more polarizable, which makes attack and bonding easier. Furthermore, the less electronegative atom is less able to stabilize electrons and is more reactive. Sulfer, case closed.

 

[patricklin27]

IN GENERAL, nucleophilicity mirrors basicity. Thus, atoms with in the same column with equal charges and lone pairs are discriminated based on SIZE. The larger atom is more polarizable, which makes attack and bonding easier. Furthermore, the less electronegative atom is less able to stabilize electrons and is more reactive. Sulfer, case closed.

no i believe you got it wrong. For larger atoms, the negative charge is spread out more, so the negative charge is more stable and less reactive. Larger atoms have more polarizability, but that doesn't make it more basic or nucleophilic. higher polarizability just means that its electron cloud can be distorted easier and has nothing to do with attracting a positive charge or hydrogen.

 

[Illfavor]

no i believe you got it wrong. For larger atoms, the negative charge is spread out more, so the negative charge is more stable and less reactive. Larger atoms have more polarizability, but that doesn't make it more basic or nucleophilic. higher polarizability just means that its electron cloud can be distorted easier and has nothing to do with attracting a positive charge or hydrogen.

From the UC Davis Chem Wiki:

"What happens as we move up and down a column when considering uncharged nucleophiles? It turns out that, in the case of uncharged nucleophiles, size dictates nucleophilicity. This is because larger elements have bigger, more diffuse, and more polarizable electron clouds. This cloud facilitates the formation of a more effective orbital overlap in the transition state of bimolecular nucleophilic substitution (SN2) reactions, resulting in a transition state that is lower in energy and a nucleophilic substitution that occurs at a faster rate."

It even says just below that on the page Se>S>O

 

[patricklin27]

From the UC Davis Chem Wiki:

"What happens as we move up and down a column when considering uncharged nucleophiles? It turns out that, in the case of uncharged nucleophiles, size dictates nucleophilicity. This is because larger elements have bigger, more diffuse, and more polarizable electron clouds. This cloud facilitates the formation of a more effective orbital overlap in the transition state of bimolecular nucleophilic substitution (SN2) reactions, resulting in a transition state that is lower in energy and a nucleophilic substitution that occurs at a faster rate."

It even says just below that on the page Se>S>O

well thats weird.. im guessing if something is uncharged, then nucleophilicity increases down a column and if the molecule is charged, then basicity decreases down a column and nucleophilicity decreases/increases (depending on solvent)?

at least thats what we learned in chad's videos. im confused.

 

[Moka11]

Doesn't chad say it makes a difference if its protic or not? That determines if we go up or down.

 

[Illfavor]

Doesn't chad say it makes a difference if its protic or not? That determines if we go up or down.

There is a "general" trend and a solvent specific trend. In "general," nucleophilicity increases down a column.