Dat Destroyer - OChem - Question 31

[bangity]

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Hi, I got the reaction to the point where the cyclopentane ring with ethyl + OC2H5 attached to C1 and OTs attached to C2.

The next reagent NaOCH3 is a very strong base (unhindered) or even if hindered it would be E2 as well. SN2 is only for primary and methyl for such a strong base. and the good leaving group OTs leave. I don't understand why it is SN2. Shouldn't it be E2?

Can someone explain why OC2H5 isn't the leaving group instead of OTs?

Thanks!

 

[bluesolic]

Hi, I got the reaction to the point where the cyclopentane ring with ethyl + OC2H5 attached to C1 and OTs attached to C2.

The next reagent NaOCH3 is a very strong base (unhindered) or even if hindered it would be E2 as well. SN2 is only for primary and methyl for such a strong base. and the good leaving group OTs leave. I don't understand why it is SN2. Shouldn't it be E2?

Can someone explain why OC2H5 isn't the leaving group instead of OTs?

Thanks!

I don't understand your question 100% but the one in C1 group is not a leaving group al all. and NAOCH3 is good nuc and it will drive SN2 since there is super good leaving group OTs in C2.

OTs is one of the best leaving group in the universe..


if there is Br , for example, then there will be SN2 and E2 since Br is located in secondary carbon.

Even Iodine at C2, it will probably drive SN2 ( I assume 80% SN2, 20% E2 ) since Iodine is also super good leaving group

 

[R1Guy]

Hi, I got the reaction to the point where the cyclopentane ring with ethyl + OC2H5 attached to C1 and OTs attached to C2.

The next reagent NaOCH3 is a very strong base (unhindered) or even if hindered it would be E2 as well. SN2 is only for primary and methyl for such a strong base. and the good leaving group OTs leave. I don't understand why it is SN2. Shouldn't it be E2?

Can someone explain why OC2H5 isn't the leaving group instead of OTs?

Thanks!

Hey, could you take a picture of this and post it? I will be glad to do my best to help you, but I would like to see the way it is written in destroyer.

 

[bangity]

Hi, this is the attachment. Hope it doesn't do anything with copyright. So what will be a good case for E2 in a similar example where there is a strong base? Is it a not so good leaving group? and


why CH3CH2O- a bad leaving group? In nucleophilic substitution, when we attach the carbonyl carbon (C=O) by a good nucleophile, doesn't the e- come back down from O and kick away such group?

 

[biomolecmed]

Otos is a great leaving group. NaOch3 although is a strong base can also be a good nucleophile. OC2H5 is a much worse leaving group. Basically, there is nothing wrong with E2, but Sn2 is fastest and easiest, therefore in this case works best. Hope this helps.

 

[bangity]

Otos is a great leaving group. NaOch3 although is a strong base can also be a good nucleophile. OC2H5 is a much worse leaving group. Basically, there is nothing wrong with E2, but Sn2 is fastest and easiest, therefore in this case works best. Hope this helps.

So in general, SN2 is faster than E2 when there is a strong base and good leaving group?

Thanks!

 

[R1Guy]

Hi, this is the attachment. Hope it doesn't do anything with copyright. So what will be a good case for E2 in a similar example where there is a strong base? Is it a not so good leaving group? and


why CH3CH2O- a bad leaving group? In nucleophilic substitution, when we attach the carbonyl carbon (C=O) by a good nucleophile, doesn't the e- come back down from O and kick away such group?

It looks like you had two questions 1. why is OTs the leaving group and not C2H50-. OTs (the tosylate) is an EXCELLENT leaving group. On the other hand, the C2H5O- is a poor leaving group because once it leaves it is basic (b/ we have a neg. charge on the oxygen that is NOT STABILIZED BY RESONANCE). When a tosylate leaves, we still have a neg. charge on the oxygen, but it is stabilized by resonace across THREE oxygens atoms (so it is nice and stable and happy once it leaves!) So long story short, OTs is an excellent leaving group.

Your second question is why is this SN2 instead of E2, given that it is secondary with a relatively strong base. And the answer is when you have a secondary with a strong base/strong nucleophile, you will get BOTH SN2 and E2 competing with eachother. I see you took a pic of the solution, but in the actual problem did he list any reaction conditions/solvents for that last step? When you have a secondary with a strong base/strong nucleophile, an alcohol solvent and heat will help favor E2, whereas a polar aprtotic solvent (like DMSO or Acetone) will help favor SN2. If he did not list any reaction conditions, and his answer choices included both SN2 and E2 products, it's not the best question in my opinion b/ you need more info than that. Since his solution manual is showing sn2, my guess is that in the problem under the last arrow he also included a polar aprotic solvent.

Hope that helps.

 

[bluesolic]

So in general, SN2 is faster than E2 when there is a strong base and good leaving group?

Thanks!

No, SN2 and E2 is nothing to do with speed,

come on man, you gotta know this concept ,


SN2 works only with NUC

E2 works only with BASE

CH3CH2O - is strong NUC & BASE ok?

so it can be either SN2 OR E2

but here is the thing,


There are competition between few things here.

first, it is secondary OTs = SN2 and E2

Second, OTs is super good leaving group = SN2 favor.

Third , CH3CH2O- is strong base and nuc = SN2 and E2


I have three SN2, and two E2

SN2 wins


very bad way to explain but it is all competition between reactions in Ochem

so you can see SN2 wins here

 

[bluesolic]

It looks like you had two questions 1. why is OTs the leaving group and not C2H50-. OTs (the tosylate) is an EXCELLENT leaving group. On the other hand, the C2H5O- is a poor leaving group because once it leaves it is basic (b/ we have a neg. charge on the oxygen that is NOT STABILIZED BY RESONANCE). When a tosylate leaves, we still have a neg. charge on the oxygen, but it is stabilized by resonace across THREE oxygens atoms (so it is nice and stable and happy once it leaves!) So long story short, OTs is an excellent leaving group.

Your second question is why is this SN2 instead of E2, given that it is secondary with a relatively strong base. And the answer is when you have a secondary with a strong base/strong nucleophile, you will get BOTH SN2 and E2 competing with eachother. I see you took a pic of the solution, but in the actual problem did he list any reaction conditions/solvents for that last step? When you have a secondary with a strong base/strong nucleophile, an alcohol solvent and heat will help favor E2, whereas a polar aprtotic solvent (like DMSO or Acetone) will help favor SN2. If he did not list any reaction conditions, and his answer choices included both SN2 and E2 products, it's not the best question in my opinion b/ you need more info than that. Since his solution manual is showing sn2, my guess is that in the problem under the last arrow he also included a polar aprotic solvent.

Hope that helps.

Even with the protic solvent, this reaction goes to SN2 because of OTs. ( I think with heat, then it will go little more of E2 like half half, but never go E2 completely.. )

and also it doesn't matter which solution you are using in THIS CASE. you are right that alcohol solvent make Nuc slower but this doesn't really make change. ( well, using DMSO will speed up to SN2 , yeah, because it will go SN2 anyways, but change from SN2 to E2 doesn't really depend on the solvent , but yeah sure alcohol will make nuc slower.. )

 

[bangity]

It looks like you had two questions 1. why is OTs the leaving group and not C2H50-. OTs (the tosylate) is an EXCELLENT leaving group. On the other hand, the C2H5O- is a poor leaving group because once it leaves it is basic (b/ we have a neg. charge on the oxygen that is NOT STABILIZED BY RESONANCE). When a tosylate leaves, we still have a neg. charge on the oxygen, but it is stabilized by resonace across THREE oxygens atoms (so it is nice and stable and happy once it leaves!) So long story short, OTs is an excellent leaving group.

Now i understand this well. Thanks 🙂

To Bluesolic: this is actually easier to understand.

 

[bluesolic]

Now i understand this well. Thanks 🙂

To Bluesolic: this is actually easier to understand.

Yes, he explained really well , I can see.

But you shoud be careful, putting alcohol solvent doesn't make it to E2. it doesn't matter,it will go SN2

just becareful on that kind of problem later, don't put it E2 even though they ask you after putting protic solvent.

 

[bangity]

No, SN2 and E2 is nothing to do with speed,

come on man, you gotta know this concept ,


SN2 works only with NUC

E2 works only with BASE

CH3CH2O - is strong NUC & BASE ok?

so it can be either SN2 OR E2

but here is the thing,


There are competition between few things here.

first, it is secondary OTs = SN2 and E2

Second, OTs is super good leaving group = SN2 favor.

Third , CH3CH2O- is strong base and nuc = SN2 and E2


I have three SN2, and two E2

SN2 wins


very bad way to explain but it is all competition between reactions in Ochem

so you can see SN2 wins here

Since it is secondary, can we have carb+? and go SN1? So very good leaving group (halide) or OTs will favor SN and if secondary, it competes between SN2 and E2?

What do you think of this Decision Tree I found?

http://chemistry.umeche.maine.edu/CHY251/subelim.html

 

[bluesolic]

Since it is secondary, can we have carb+? and go SN1? So very good leaving group (halide) or OTs will favor SN and if secondary, it competes between SN2 and E2?

It depends,

You got strong BASE & NUC ( E2 & SN2 favor type attacker ) either way possibly happens ( depends on which molecule it is attacking )


now, you have molecule that will be attacked by the "strong BASE & NUC" molecule..


so, look at the molecule . not the attacker.

, it has secondary with good leaving group .


normally, secondary will drive both

but in this case, even though it is secondary, it is really good leaving group so the rate of the SN2 rxn is similar to primary halide ( ex . Br )

that's why it will drive to SN2.


now, if same question that has Br attached on Secondary ,

Br is good leaving group , However, it is not super mega good leaving group as OTs

SO, there will be E2 and SN2 mixture.


now, if same questions that has Iodine attached on Secondary,

Iodine is super mega good leaving group ( not as strong as OTs but, it is much more strong good leaving group then Br ) so, it will favor to SN2.


how to figure out which is good leaving group is ,,,,

imagine that leaving group could be stable after it's taken off from the molecule...

OTs, is super stable after it's taken out

Iodine is also very stable after it's taken out same as Br ( but Br is not stable as Iodine )


you know the differences ?



I am not a good teacher so This might not be good explaination.

 

[bluesolic]

Since it is secondary, can we have carb+? and go SN1? So very good leaving group (halide) or OTs will favor SN and if secondary, it competes between SN2 and E2?

What do you think of this Decision Tree I found?

http://chemistry.umeche.maine.edu/CHY251/subelim.html

SN1/E1 is when the attacker is not either base or nuc ( useless )

then nothing happens first because attacker is useless , too shy to drive something.


so, the rxn doesn't happen until the target molecule start to change itself,

like,,, leaving groups taken out from the target molecule.

and now there is carb+ and then now the useless attacker can slowly attacker the target molecule and that's SN1/E1 ( happens together )


Now the solvent comes very important here,

because once the target molecule become carb+ , the solvent MUST be protic ( stabilize the ion ) otherwise target molecule CAN'T become carb+ and it means NO SN1/E1

 

[bangity]

Thanks. I understand SN1/E1 well now. But when I look at all the flow chart online, None of them state that this should be SN2 and instead the majority is E2. I do understand stability of anion by resonance.

So the better the leaving group, majority will shift toward SN2? Not so good leaving group and and weak base (good nuc) will drive E2?

 

[bluesolic]

Thanks. I understand SN1/E1 well now. But when I look at all the flow chart online, None of them state that this should be SN2 and instead the majority is E2. I do understand stability of anion by resonance.

So the better the leaving group, majority will shift toward SN2? Not so good leaving group and and weak base (good nuc) will drive E2?

leaving group doesn't decide that much, but it contributes alot though..

Attacker ( <-- you now might understand what this is .. ) usually decide SN2 or E2

ok, I think I am confusing you but there is nothing is sure in Ochem, Everything is contributing rxn in some way.


so, base on your question,

better the leaving group , majority will shift toward SN2 ? - it depends, yes, good leaving group HELP the rxn to SN2, this is just one of the requistes.. which means , you have good leaving group but super bad Nuc, then the rxn will not go SN2 but SN1/E1

Not so good leaving group with weak base ( good nuc ) = SN2 or No rxn (depends on how bad the leaving group is )

remember stronger base = more E2

stronger Nuc = more SN2


good leaving group w/ attacker ( SUPER BAD Nuc / Strong base )

= E2 ( you got it ? )

good laving group w/ useless attacker

= SN1/E1


good leaving group w/ attacker ( good nuc / weak base )

= SN2


after you get this ideas, then primary, secondary, tertiary concept come in ( but this concept is pretty easy)


and lastly, the solution concept comes in , ( this concept is not really important to figure out which rxn happens , but you need to know the relationships btwn solutions and molecules work )

like I said, I am not a good teacher, hope other person give you better explain

 

[bangity]

It is easy to understand now. But the problem with the question above is.

Good leaving group + Good Nuc favor SN2 (strong base favor E2) => I don't know for sure it is E2 or SN2 because the attacker is both strong plus great leaving group.

 

[bluesolic]

It is easy to understand now. But the problem with the question above is.

Good leaving group + Good Nuc favor SN2 (strong base favor E2) => I don't know for sure it is E2 or SN2 because the attacker is both strong plus great leaving group.

Yes, first when you see the attacker, you might think " ok, it can be SN2/E2 "

so now, you look at the leaving group, OTs is super good leaving group , so it will go SN2

The reason they put OTs and the answer is SN2 is that , they try to confuse you once putting strong nuc/base but they put OTs to make the question no error.


If they put Br not OTs, then this question is suck quesion because Br is good leaving group but it is not extreme good leaving group so the rxn will be SN2/E2 ( plus, it is secondary so definately E2 also favored)

so, putting OTs in the question is really smart and good question,

because people might be confused like you did, " strong base/nuc with secondary? , well, it is SN2/E2 !!! well, E2 is even more favored !!! because it is secondary!!! " BUT it is NOT, because of OTs is super mega power good leaving in the galaxy, it is SN2.

IF OTs is located in tertiary, with same condition, then, yes it will drive E2.

Hope you get the idea, let me know if you have another question.

 

[bangity]

Yes, first when you see the attacker, you might think " ok, it can be SN2/E2 "

so now, you look at the leaving group, OTs is super good leaving group , so it will go SN2

The reason they put OTs and the answer is SN2 is that , they try to confuse you once putting strong nuc/base but they put OTs to make the question no error.


If they put Br not OTs, then this question is suck quesion because Br is good leaving group but it is not extreme good leaving group so the rxn will be SN2/E2 ( plus, it is secondary so definately E2 also favored)

so, putting OTs in the question is really smart and good question,

because people might be confused like you did, " strong base/nuc with secondary? , well, it is SN2/E2 !!! well, E2 is even more favored !!! because it is secondary!!! " BUT it is NOT, because of OTs is super mega power good leaving in the galaxy, it is SN2.

IF OTs is located in tertiary, with same condition, then, yes it will drive E2.

Hope you get the idea, let me know if you have another question.

Thank you very much. Things are getting clearly. So one last question, If the condition are the same meaning

Strong Base + Good Nucleophile => SN2/E2

Substrate: Very Good leaving Group (OTs) => SN2 >> E2

Substrate: primary/secondary with very Good leaving Group => SN2 >>> E2

Substrate: If Tertiary with very Good leaving group + Good attacker => E2 (due to hindered => it will be hard to come in and attack the Carbon bearing the leaving group => Attack the alpha-H instead (easier) and drive E2)?


So Basically, Very Good Leaving Group (TOs) ==> they leave so fast that the attacker (strong) can come in and substitute.
With Good Leaving Group (X-) ==> They leave not so fast ==> Attacker (being so strong) would says "I rather attack the alpha H because Leaving group is kind of still there"?

 

[bluesolic]

Thank you very much. Things are getting clearly. So one last question, If the condition are the same meaning

Strong Base + Good Nucleophile => SN2/E2

Substrate: Very Good leaving Group (OTs) => SN2 >> E2 ( DEPENDS ON CHARACTERISTIC OF ATTACKER , I ASSUME YOU UNDERSTAND THIS THROUGH)

Substrate: primary/secondary with very Good leaving Group => SN2 >>> E2 (PRIMARY = SN1 (UNLESS THE ATTACKER IS MEGA POWER BASE/ SUCK NUC), SECONDARY = DEPENDS ON ATTACKER BUT WITH LEAVING GROUP LIKE OTS?, YES SN2 IS FAVORED)

Substrate: If Tertiary with very Good leaving group + Good attacker => E2 (due to hindered => it will be hard to come in and attack the Carbon bearing the leaving group => Attack the alpha-H instead (easier) and drive E2)? YES

(TERTIARY , ALWAYS E2 OR SN1/E1, NO SN2)

So Basically, Very Good Leaving Group (TOs) ==> they leave so fast that the attacker (strong) can come in and substitute.
With Good Leaving Group (X-) ==> They leave not so fast ==> Attacker (being so strong) would says "I rather attack the alpha H because Leaving group is kind of still there"? YES, KIND OF, ATTACKER WILL TRY TO KICK THE LEAVING GROUP OUT BUT IF THE LEAVING GROUP NEVER LEAVES, THEN IT WILL GO E2 OR NO RXN.

 

[bangity]

Thank you BUT How can PRIMARY GOES SN1? That's impossible?

 

[bluesolic]

Thank you BUT How can PRIMARY GOES SN1? That's impossible?

sorry, that's SN2 not SN1. Primary is SN2 favored.

hope it helps

 

[bangity]

Thanks. things are very clear now. I will try to do more problem. Thank you for all your help.