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Pembleton · 07-28-2004, 12:41 PM

What makes the leaving group spontaneously leave?

And no alkenes on the exam means no need to study E1 and E2 reactions, dienes or alkene additions? right?

DieselPetrolGrl · 07-28-2004, 12:47 PM

wheer did u hear that there are no alkenes on the exam?? dies alder is my fav rxn..god let me luck out with just that for orgo...
sn1 leaving group isnt important and i think it leaves bc of the polar protic solvent (water/ethnol) someone help/ clarify me ....thanks

Pembleton · 07-28-2004, 12:54 PM

Check out this PDF. It mentions no alkenes on the exams. I wasn't sure if we would still need to have some grasp of the mechanics though.

http://www.aamc.org/students/mcat/topics.pdf

BisMuOxo · 07-28-2004, 01:01 PM

Quote:

Originally Posted by Pembleton

What makes the leaving group spontaneously leave?

In a particular solvent at a particular temperature there is some equilibrium concentration of each species. One species may be neutral and another a carbocation for example. This question is like asking what in neural water makes some H+ and some OH- form.

Pembleton · 07-28-2004, 01:27 PM

Ahh, now I understand. Basically I need to apply my Gchem knowledge to Orgo. They really should integrate the two better.

Thanks.

shaq786 · 07-28-2004, 09:41 PM

A good leaving group is one that can have its conjugate base stabalized by resonance. If there is no resonance characteristics on the leaving group then a good leaving group is generally a weak base. Just like Br, Cl, and I are weak bases becuase their conjugate acids are strong.

But just like someone said, it also depends on polar protic solutions(or aprotic, I'm not sure) becuase these solutions can crowd around carbocations and reduce their activation energy, causing them to react with another nucleophile.

07-29-2004, 01:06 AM

Just to reiterate what everyone else said:

SN1 works with polar protic solvents; those capable of hydrogen-bonding.

As for what makes a good leaving group, it depends on the conjugate base of the acid. The conjugate base of a very strong acid (HI, HBR, HCl in that order) have very little to none in basic properties. So, to give you an idea, those would be best. Something like an amine though, RNH2, that would be a conjugate base of an extremely weak acid, and would therefore be a very good base. It can also be stabilized by resonance due to the lone pair on the nitrogen atom; this would make for a bad leaving group.

Also, just to note, the conjugate bases of weak acids are weak bases. If you had to compare a bunch of different compounds though, then generally, it's an inverse relationship; the stronger the acid, the weaker the conj. base, and vice-versa.

DaveinDallas · 07-29-2004, 07:05 AM

Well, what about the electronegativity of the particular species. I mean, this thing isn't just merrily sitting out there, sipping iced tea and than wham, bam, thank you ma'am it up and leaves. I seem to recall somewhere that polarity of the species was also an influencing factor as well as if there were any species nearby that would draw electrons and thus weaken the bond, so that when the environment is correct, it will leave with another species. Kind of like what makes it a strong acid in the first place - how willing is it to deprotonate and lose that H+. Well, IIRC, if there's a halogen neaby, that thing is ready to do the deed.

07-28-2004, 12:41 PM	#1
Pembleton Senior Member Status: Medical Student Join Date: Jun 2002 Posts: 695	SN1 leaving group SDN Members don't see this ad. What makes the leaving group spontaneously leave? And no alkenes on the exam means no need to study E1 and E2 reactions, dienes or alkene additions? right?

07-28-2004, 12:47 PM	#2
DieselPetrolGrl twisterChamp&so much more Join Date: Jul 2003 Location: In a chocolate hole - kicked the white rabbit out last week. Posts: 1,888	wheer did u hear that there are no alkenes on the exam?? dies alder is my fav rxn..god let me luck out with just that for orgo... sn1 leaving group isnt important and i think it leaves bc of the polar protic solvent (water/ethnol) someone help/ clarify me ....thanks __________________ guju and mush...someday they will be my bestest friends and we will live in a carved giant coconut.... and host lovely macadamia and crushed baboa leave fiestas... where all of mcat thread groupies will come and party like its 1899.

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07-28-2004, 12:54 PM	#3
Pembleton Senior Member Status: Medical Student Join Date: Jun 2002 Posts: 695	Check out this PDF. It mentions no alkenes on the exams. I wasn't sure if we would still need to have some grasp of the mechanics though. http://www.aamc.org/students/mcat/topics.pdf

07-28-2004, 01:27 PM	#5
Pembleton Senior Member Status: Medical Student Join Date: Jun 2002 Posts: 695	Ahh, now I understand. Basically I need to apply my Gchem knowledge to Orgo. They really should integrate the two better. Thanks.

07-28-2004, 09:41 PM	#6
shaq786 Senior Member Join Date: Jul 2004 Posts: 312	A good leaving group is one that can have its conjugate base stabalized by resonance. If there is no resonance characteristics on the leaving group then a good leaving group is generally a weak base. Just like Br, Cl, and I are weak bases becuase their conjugate acids are strong. But just like someone said, it also depends on polar protic solutions(or aprotic, I'm not sure) becuase these solutions can crowd around carbocations and reduce their activation energy, causing them to react with another nucleophile.

07-29-2004, 01:06 AM	#7
Assembler Guest Posts: n/a	Just to reiterate what everyone else said: SN1 works with polar protic solvents; those capable of hydrogen-bonding. As for what makes a good leaving group, it depends on the conjugate base of the acid. The conjugate base of a very strong acid (HI, HBR, HCl in that order) have very little to none in basic properties. So, to give you an idea, those would be best. Something like an amine though, RNH2, that would be a conjugate base of an extremely weak acid, and would therefore be a very good base. It can also be stabilized by resonance due to the lone pair on the nitrogen atom; this would make for a bad leaving group. Also, just to note, the conjugate bases of weak acids are weak bases. If you had to compare a bunch of different compounds though, then generally, it's an inverse relationship; the stronger the acid, the weaker the conj. base, and vice-versa.

07-29-2004, 07:05 AM	#8
DaveinDallas Senior Member Join Date: Jan 2004 Posts: 373	Well, what about the electronegativity of the particular species. I mean, this thing isn't just merrily sitting out there, sipping iced tea and than wham, bam, thank you ma'am it up and leaves. I seem to recall somewhere that polarity of the species was also an influencing factor as well as if there were any species nearby that would draw electrons and thus weaken the bond, so that when the environment is correct, it will leave with another species. Kind of like what makes it a strong acid in the first place - how willing is it to deprotonate and lose that H+. Well, IIRC, if there's a halogen neaby, that thing is ready to do the deed.