Rank Order of SN2 Reactivity: CH3-Cl vs CH3-CO-CH2-Cl

[Saitama]

Homework Statement

The decreasing order of rate of SN2 reaction is:
a)CH₃-Cl
b)CH₃-CO-CH₂-Cl

Homework Equations

The Attempt at a Solution

I have been trying hard to find the reason why i am wrong. It's obvious that less hindrance, more reactivity towards SN2. Using the same logic, the answer should be a>b but it is b>a. Now i don't understand where i went wrong. There is more hindrance in b than a.

 

[Saitama]

Hello?? Would someone like to help?

 

[AGNuke]

Which hindrance are you talking about? Steric?

Steric hindrance will matter if the attacked carbon is secondary or tertiary. Since the attacked carbon in both the cases are primary, steric hindrance is out of question.

Now, try forming the substrate, i.e. the carbocation, which is formed after the chloride atom leaves in both the cases.

For the hint, more stable carbocation will be lax in proceeding in S_N2 reaction than less stable one.

 

[Saitama]

Which hindrance are you talking about? Steric?

Yes, i am talking about Steric.

Now, try forming the substrate, i.e. the carbocation...

Hey, wait, i think there is no carbocation formation in the SN2 reaction mechanism.

 

[AGNuke]

A substrate is formed, when neither the attacking group is bonded completely, or the leaving group leaves it completely. This substrate is like a forming carbocation.

This substrate has two ways to go - either trace back or advance. If the carbocation, which is "forming" is stable, the reaction will proceed forward, if not, it will trace back.

It is true, that carbocation independently is not formed, but the substrate is nonetheless under scrutiny for determining the answer.

This is like formation of substrate in Mercuration-Demercuration process, if you had read it.

 

[chemisttree]

Which hindrance are you talking about? Steric?

Steric hindrance will matter if the attacked carbon is secondary or tertiary. Since the attacked carbon in both the cases are primary, steric hindrance is out of question.

Methyl chloride is not primary. Chloroacetone is bulkier around its reaction center than methyl chloride so steric hindrance is different for these two substrates. But there is something special about chloroacetone that has to do with the stability of the transition state.

Now, try forming the substrate, i.e. the carbocation, which is formed after the chloride atom leaves in both the cases.

We're talking SN2 here. There is no carbocation formed after the chloride leaves.

For the hint, more stable carbocation will be lax in proceeding in S_N2 reaction than less stable one.

You have this backwards.

 

[AGNuke]

Methyl chloride is not primary. Chloroacetone is bulkier around its reaction center than methyl chloride so steric hindrance is different for these two substrates. But there is something special about chloroacetone that has to do with the stability of the transition state.

Point taken.

We're talking SN2 here. There is no carbocation formed after the chloride leaves.

I never mentioned independent carbocation. I only said that to determine the answer, making the substrate as independent carbocation can help.

I mentioned the substrate stage. And I also gave an example which I believe he had read (if he is also pursuing for JEE)

You have this backwards.

Figures. I myself got doubts soon afterwards. Maybe I jotted my notes incorrectly.

As a matter of fact, I believe CH₃-(C=O)-CH₂⁺ is less stable as electromeric effect of carbonyl group will be disastrous.

 

[chemisttree]

I never mentioned independent carbocation. I only said that to determine the answer, making the substrate as independent carbocation can help.

I mentioned the substrate stage. And I also gave an example which I believe he had read (if he is also pursuing for JEE)

Right, but if you take your analogy a little further, solvents that stabilize this theoretical 'independent cation' better should lead to faster reaction rates. This is counter to what is known about the reaction, so using analogies has its risks, especially for the first time student wading through all of this nomenclature, energy diagrams, steric discussions, etc. I prefer to discuss it in a more precise manner.

Figures. I myself got doubts soon afterwards. Maybe I jotted my notes incorrectly.

As a matter of fact, I believe CH₃-(C=O)-CH₂⁺ is less stable as electromeric effect of carbonyl group will be disastrous.

By 'electromeric effect' I assume that you mean inductive effect. Yes, the inductive effect destabilizes any cationic character at the reaction center but more important is the adjacent pi electron system to the developing sp2-like transition state.

 

[Saitama]

My book says nothing about a positive charge being developed on carbon during the SN2 mechanism. I am using the book Organic Chemistry by Morrison and Boyd.

Methyl chloride is not primary. Chloroacetone is bulkier around its reaction center than methyl chloride so steric hindrance is different for these two substrates. But there is something special about chloroacetone that has to do with the stability of the transition state.

Thanks for the reply chemisttree! I was waiting for you to jump in.
Well, i was thinking about the same thing. It seems like that the stability has to do with the carbonyl group attached. If i draw the transition state,

Nu is the attacking nucleophile.
You say that it has to do with the adjacent pi electron system, i don't see what it can do here. I can only think of the inductive effect which stabilizes the negative charge developing on Nu and Cl.

 

[AGNuke]

Substrate. The substrate plays the most important part in determining the rate of the reaction. ... Other reasons affecting S_N2 due to substrate involve stability of carbocation formed after removing the leaving group i.e. ease of forming the carbocation, more stable the carbocation is more is the probability for reaction to undergo S_N1 mechanism.

The substrate which is formed, Carbon has partial positive charge. If not, how come two δ- groups are attached to a Single Carbon without carbon having δ+ charge? See water molecule for a change.

According to excerpt I posted, Carbonyl group is an electron withdrawing group. So it seems like withdrawing electrons from electron deficient group is not a good idea, so the reaction tries to avoid this, thus the molecules goes into S_N2 reaction.

 

[Saitama]

The substrate which is formed, Carbon has partial positive charge. If not, how come two δ- groups are attached to a Single Carbon without carbon having δ+ charge?

According to my book, no charge develops on the carbon. You can check the book at google books. Check the Organic Chemistry, 6/e by Morrison and Boyd and see page 222.

 

[AGNuke]

Damn. I need that book ASAP.

It says that if the rate of attack is equal to rate of leaving, the carbon atom does not accumulate considerable polar charge.

Now read Page 245

In Contrast, the nature of alkyl group of the substrate exerts a profound effect on which mechanism (S_N1 or S_N2) is to be followed. In R, two structural factors are at work: Steric hindrance, which determines the ease of back-side attack (thus facilitating S_N2) and ability to accommodate a positive charge, which determines the ease of heterolysis (thus facilitating S_N1)

 

[chemisttree]

Well, i was thinking about the same thing. It seems like that the stability has to do with the carbonyl group attached. If i draw the transition state,

Nu is the attacking nucleophile.
You say that it has to do with the adjacent pi electron system, i don't see what it can do here. I can only think of the inductive effect which stabilizes the negative charge developing on Nu and Cl.

You will note that the transition state you have illustrated here has three groups aligned roughly equidistant in a plane and the incoming and leaving groups aligned along an axis orthogonal to that plane. This is a description of an sp2 hybridized reaction center. The adjacent pi system (C=O) can conjugate with this reaction center. How might this conjugation affect the energy of the transition state? What does delocalization do to the energy of the transition state?

BTW the difference between the rate of reaction of chloroacetone relative to methyl iodide is on the order of ten thousand.

 

[chemisttree]

The substrate which is formed, Carbon has partial positive charge. If not, how come two δ- groups are attached to a Single Carbon without carbon having δ+ charge? See water molecule for a change.

According to excerpt I posted, Carbonyl group is an electron withdrawing group. So it seems like withdrawing electrons from electron deficient group is not a good idea, so the reaction tries to avoid this, thus the molecules goes into S_N2 reaction.

Remember we started with a neutral molecule and a negatively charged nucleophile. That negative charge of the nucleophile is smeared out between approaching and leaving group. The positive charge resides wholly on the counterion of the nucleophile, which you haven't shown. In the case where a neutral molecule nucleophile approaches, the corresponding positive charge develops on the attacking nucleophile. The substrate stays remarkably neutral.

 

[AGNuke]

The substrate stays remarkably neutral.

Yes, that seems the case. What if the rate of attacking isn't equal to rate of leaving. Like in this case, due to steric reasons, attacking may be bit slower...?

I studied some graphs of nucleophilic substitution rate of methyl, ethyl, sec. propyl and tert. butyl. They said that due to steric hindrance, sec.propyl and tert.butyl's substrate experiences charge, and they undergoes S_N1 substitution, rather than S_N2 substitution as in methyl and ethyl case.

I was taught (yesterday only) that if we see something causing the difference in rate of attacking and leaving (steric hindrance) we must try forming carbocation. If it is stable, then S_N1, else, S_N2. Would be enough for questions asked in JEE.

God knows what I am drinking right now...

 

[chemisttree]

If the rate of attack and the rate of leaving were not the same we would have two cases.
1. Rate of attack is faster than rate of leaving. Since we are bringing electrons with a nucleophile, the reaction center briefly contains too many electrons (?) and we are dealing with carbanions (?), 5 bonds to carbon and so forth and thus any discussion of carbocation stability is utter fantasy as is this possibility.

2. Rate of attack is slower than rate of leaving. Since the bond breaking process is occurring faster than the bond making process, we are left with an electron deficient reaction center... a carbocation. It's stability governs the reaction order (substitution, nucleophilic, unimolecular) so we aren't really talking about SN2 reactions at all.

 

[AGNuke]

2. Rate of attack is slower than rate of leaving. ... It's stability governs the reaction order (substitution, nucleophilic, unimolecular) so we aren't really talking about SN2 reactions at all.

So, I conclude that it can deviate from S_N2 mechanism.

 

[chemisttree]

SN1 reactions are defined as having the rate determining step being the generation of the charged intermediate. If the question were asking about SN1 reactions we would be discussing the stability of charged intermediates and leaving groups breaking their bond with the substrate faster than the attacking nucleophile forming a new bond.

Good discussion but for a different problem.

 

[AGNuke]

Believe me, our exam loves to fiddle with our minds over such a small things. And since the exam is now subjective instead of objective, it is more difficult, with writing a sentence of wisdom. Everything must be crystal clear. Thanks for that.

 

[Saitama]

Woah! Seems like a lot of discussion happened here.

The adjacent pi system (C=O) can conjugate with this reaction center.

I don't get it, what do you mean by conjugate with the reaction center?

I am not sure but are you talking about the resonance effect here?