SN2 Rxn Question

[DentalKitty]

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Hi guys, I'm just starting my review of Organic. I'm confused about leaving groups and nucleophiles. Here is my question:

Will 2-bromobutane undergo SN2 with I- in an aprotic solvent?

The answer says yes, and also talks about the inversion of the stereochemistry (which I get). However, I thought that Iodine is a better leaving group and Bromide is a better nucleophile. So wouldn't the rxn not take place? Or am I getting confused and applying rules for protic/SN1? Does the strength of the nucleophile and leaving group reverse in an aprotic solvent? Any help explaining this would be appreciated. Thanks!

 

[dantheman2007]

All you have to know is that a number of conditions must be present for a rxn to proceed either via SN1 or SN2. Your problem contains a secondary halide and aprotic solvent - conditions suitable for SN2 to take place where inversion will occur.
If your problem listed t-butyl- a tertiary halide in CH3OH - a protic solvent the rxn will proceed SN1 and racemization will occur
Hope this Helps

best,

dantheman2007

 

[ATLATLATL]

ok so lemme ask ya'll something....with SN2 considering it's aprotic and SN1 is polar protic...
wat are some good examples...
gosh i really hope this exam doesn't go in depth with wat r good leaving groups and so forth....pain in the BUTTTT...
but yea if ya'll don't mind giving me some common examples for both..
cuz check this out...i'm thinking polar protic...would be anything like water...hmmm ethanol...methanol...hmmm watelse tho?
and polar aprotic....acetone maybe? 😱

 

[dantheman2007]

all those are good examples

 

[jdmsamurai]

ok so lemme ask ya'll something....with SN2 considering it's aprotic and SN1 is polar protic...
wat are some good examples...
gosh i really hope this exam doesn't go in depth with wat r good leaving groups and so forth....pain in the BUTTTT...
but yea if ya'll don't mind giving me some common examples for both..
cuz check this out...i'm thinking polar protic...would be anything like water...hmmm ethanol...methanol...hmmm watelse tho?
and polar aprotic....acetone maybe? 😱

keyword: protic..
protic = anything that can form H bonds...and its usually H-O or H-N...so as a rule of them if something has a HO or HN its probably protic and if something that isnt then is aprotic

 

[jdmsamurai]

isnt I- a weaker base then Br-? so then it will not cause an SN2 reaction. usually in a SN2 rxn a stronger base does a "back side" attack on the alkane. the weaker base is the best leaving group and since I- is a weaker base then it tends to stay as the best leaving group.

 

[DailyDrivenTJ]

isnt I- a weaker base then Br-? so then it will not cause an SN2 reaction. usually in a SN2 rxn a stronger base does a "back side" attack on the alkane. the weaker base is the best leaving group and since I- is a weaker base then it tends to stay as the best leaving group.

I would NOT compare the nucleophilicity to bascity. You can have a strong base and won't have a good nucleophile and vise versa.

Substitution reactions calls for nucleophile, not base. Strong bases remove proton.

 

[jdmsamurai]

I would NOT compare the nucleophilicity to bascity. You can have a strong base and won't have a good nucleophile and vise versa.

Substitution reactions calls for nucleophile, not base. Strong bases remove proton.

Nucleophiles
"In other words, the stronger the base, the stronger the nucleophile. For example nucleophilic strength decreases in the order:

RO- > HO- > RCO2- > ROH > H20"

quoted from kaplan bb...

 

[DailyDrivenTJ]

One of the primary example of strong base is not a good nucleophile is tertiary butoxide while it one of the strongest base, it does not go through nucleophilc substitution or addition. While there is some correlation between the nucleophilicity and basicity, it is correlation is not always true.

I would not believe everything Kaplan book says, they have rather quick and dirty explanations. Go back to your textbook if you really want to learn this stuff.

 

[jdmsamurai]

One of the primary example of strong base is not a good nucleophile is tertiary butoxide while it one of the strongest base, it does not go through nucleophilc substitution or addition. While there is some correlation between the nucleophilicity and basicity, it is correlation is not always true.

I would not believe everything Kaplan book says, they have rather quick and dirty explanations. Go back to your textbook if you really want to learn this stuff.

the reason t-but oxide isnt a good nucleophile isnt because of its basicility. it is because of its steric hinderance!!!....SN2 reactions does a backside attack on the alkane. you go back to your textbook if you want to learn that stuff. i have an ochem book with me.

quote from "organic chemistry 4th edition by paula yurkanis bruice"

Basicity is a measure of how well a compound shares it lone pair with a proton. The STRONGER the base, the better it shares its electrons....STRONGER BASES ARE BETTER NUCLEOPHILES.......NOTE THAT from NH2- > HO- > F- that NH2- is the best nucleophile because its the STRONGER BASE

NUCLEOPHILITY is the measure of how readily a compound is able to attack an electron deficient atom...

so back to your "wonderful example" of t-butxoxide...even though its the stronger base...its a worst nucleophile due to steric hinderance compared to ethoxide ion....


now if you think the course book is wrong too...you must figured out something new we didnt know genius

and back to the I- and Br-...the reason i said it wouldnt do sn2 is because I- is a weaker leaving group, but in reality their pka's are very similiar so it would cause a reversible reaction where I- displaces Br- and Br- displaces I- all day long

 

[DailyDrivenTJ]

Well, why don't you stop feeding words in my mouth and read what I typed in. I don't appreciate that at all.

Stand behind what I said and I am firm at that.

Didn't I say basicity is not always the indication of good nucleophile and presented an information/example? You agreed that tBuO- is "a worst nucleophile", WTF? Do you see some contradiction here??

So in what I have typed in, was there a wrong information? I don't see any.

Get that straight and discredit me. 🙂 You owe me an apology.

 

[DentalKitty]

Thanks for the help guys, I won't read into the question so much next time I guess.