Bulky Leaving Group???

[greenseeking]

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Equilibrium question:

If in one reaction R group of the phosphine is methyl what is observed when the metyl is replaced by ethyl?

a. reation rate increases with the ethyl because steric hindrance of the leaving group has increased
b. reaction rate decreases with the ethyl because steric hindrance of the leaving group has increased
c. reaction rate increases with ethyl b/c steric hindrance of the LG decreased
d. rxn rate decreases b/c steric hindrance of the LG has decreased.

Answer is A but chose B. How does a bulkier LG help with the reaction rate? wouldn't it be harder for the nucleophile to attack the LG if there is so much steric hindrance?

Are they saying that "bulkier" LG would be a more stable in the solution??

EDIT: Making the LG more bulkier would make it a more BASIC LG. Aren't good LG's weak bases? Therefore, shouldn't it be A?

 

[MedPR]

First thing, the nucleophile doesn't attack the leaving group.
Second thing, better leaving group = faster reaction because the leaving group must leave before the reaction is complete (this is true for SN1 and SN2). Steric hindrance = bad = nobody likes it = everyone wants to split up (leave).

 

[greenseeking]

First thing, the nucleophile doesn't attack the leaving group.

you're right there. thanks.

Second thing, better leaving group = faster reaction because the leaving group must leave before the reaction is complete (this is true for SN1 and SN2).

yes agreed that better LG=faster reaction. but how does adding a bunch of alkyl groups make it a better leaving group? Also wouldn't the nucleophile would have a harder time attacking the M b/c of steric hindrance?

 

[pm1]

yes agreed that better LG=faster reaction. but how does adding a bunch of alkyl groups make it a better leaving group? Also wouldn't the nucleophile would have a harder time attacking the M b/c of steric hindrance?

The way I think of it is by realizing that a leaving group will have a negative charge.
If the leaving group is more bulky, the "bulkiness" will stabilize the anion through induction, thus making it a better leaving group.

And a better leaving group would always increase the reaction rate, because, as MedPR said, a leaving group needs to leave before the rxn goes to completion. I think you might be confusing it with bulkiness of the substrate.

Anyone, please correct me if I am wrong.

 

[greenseeking]

The way I think of it is by realizing that a leaving group will have a negative charge.
If the leaving group is more bulky, the "bulkiness" will stabilize the anion through induction, thus making it a better leaving group.

And a better leaving group would always increase the reaction rate, because, as MedPR said, a leaving group needs to leave before the rxn goes to completion. I think you might be confusing it with bulkiness of the substrate.

Anyone, please correct me if I am wrong.

OK so I guess there is a difference between the bulkiness of the LG and bulkiness of substrate.... I think I was not distinguishing between the two.

 

[Silo004]

I think this is wrong pm1, b/c methyl groups stabilize negative charge better than ethyl groups. It's the opposite of positive charges.

 

[greenseeking]

The way I think of it is by realizing that a leaving group will have a negative charge.
If the leaving group is more bulky, the "bulkiness" will stabilize the anion through induction, thus making it a better leaving group.

And a better leaving group would always increase the reaction rate, because, as MedPR said, a leaving group needs to leave before the rxn goes to completion. I think you might be confusing it with bulkiness of the substrate.

Anyone, please correct me if I am wrong.

Also, doesn't adding on more alkyl groups destabilize the LG b/c it makes it a stronger base. I remember that electron donating groups increases the basicity and best LG's are weak bases.

 

[Silo004]

Also, doesn't adding on more alkyl groups destabilize the LG b/c it makes it a stronger base. I remember that electron donating groups increases the basicity and best LG's are weak bases.

But are you adding on more alkyl groups? I don't think so. It seems to me that you are adding larger alkyl groups.

Sterics and electronics don't always have to agree and can complicate things, right?

I would just limit the conversation to sterics for simplicity's sake.

 

[CodeRedDew]

Also, doesn't adding on more alkyl groups destabilize the LG b/c it makes it a stronger base. I remember that electron donating groups increases the basicity and best LG's are weak bases.

Having methyls on a substituent vs. ethyls doesn't make a group more electron-donating. They're both the same in terms electron donating capability (hyperconjugation).

Secondly, I believe the reaction increases because ethyl causes a greater chelating effect which in turn increases the rate of the reaction (according to the last sentence). Chelation effects may cause the molecule to establish an orientation that is more favorable for the nucleophile to attack.


Sent from my iPhone

 

[pm1]

But are you adding on more alkyl groups? I don't think so. It seems to me that you are adding larger alkyl groups.

Sterics and electronics don't always have to agree and can complicate things, right?

I would just limit the conversation to sterics for simplicity's sake.

Sorry for being wrong guys and thanks for correcting me!

I was actually thinking of more alkyl groups instead of larger groups.

So now I am confused. Why is the ethyl group a better leaving group than the methyl group?

 

[Mornhavon]

.

 

[greenseeking]

I don't really understand your question, OP

Maybe this will help:

http://chemwiki.ucdavis.edu/Organic_Chemistry/Reactions/SN2_Reaction/Leaving_group

awesome website thanks... i understand now. i must have forgotten some of the basics of ochem! (took it more than 5 years ago!)

 

[drechie]

very confusing thread. can someone offer the consensus on how to approach this question and what to consider ( i was stuck on the electron donating capabilities of methyl vs ethyl). What should we be thinking of