Dat Destroyer Road Map 5

[Dusk]

All the other road maps are relatively straight forward.. But there's something wrong with either 5 or me (probably me)

First question,
2-bromobutane is reacted with CH3SH/DMSO to give a butane with S-CH3 attached at the 2 carbon. Ok, that's fine and dandy, but why is it an SN2 reaction?? CH3SH is not a strong base, so wouldn't it do SN1?

Second question,
2-bromo-2-methylbutane is reacted with CN- / D to give 2-methyl-2-butene product through an E1 mechanism...

According to Chad, isn't CN- a strong nuc, but a weak base? So shouldn't the prefered reaction be SN2? But it is a tertiary halide, does that take preference over the fact that CN- is a good nuc?

Someone please save me.

 

[rccostner]

i believe you are right on both questions. Dr. Romano tends to be a little confusing with some of his OChem explanations and not very thorough on a lot of the confusing reactions.

 

[Demps]

I think for the first question, DMSO is aprotic so SN2 is favored over SN1. I'm not sure about the 2nd question.

 

[Demps]

Can you say the second question would give mixture of E1 and SN1 products? I'm confused with this question too

 

[Dusk]

Thanks for the input guys.

Demps, even though DMSO is aprotic and SN2 is technically favored over SN1, I learned to not look at protic/aprotic solvents to see if rxn goes which way.

If you have an protic solvent, you can do E1,E2,SN1,SN2, but if you have an aprotic solvent, there's no way you'll do E1 or SN1...

That's why I'm pretty confused on my first question.

And I'm pretty lost on the second question, I'm guessing since SN2 is impossible with tertiary halide, SN1 or E1 would be the two products? And the roadmap just showed the E1....

 

[Demps]

Let's just hope if we encounter such question, the choice only has E1 or SN1 products specifically lol

 

[JustwantDDS]

Thanks for the input guys.

Demps, even though DMSO is aprotic and SN2 is technically favored over SN1, I learned to not look at protic/aprotic solvents to see if rxn goes which way.

If you have an protic solvent, you can do E1,E2,SN1,SN2, but if you have an aprotic solvent, there's no way you'll do E1 or SN1...

That's why I'm pretty confused on my first question.

And I'm pretty lost on the second question, I'm guessing since SN2 is impossible with tertiary halide, SN1 or E1 would be the two products? And the roadmap just showed the E1....

you just said it yourself
if you've got aprotic theres no way ull do sn1....
thats why its not sn1, its sn2

 

[Demps]

you just said it yourself
if you've got aprotic theres no way ull do sn1....
thats why its not sn1, its sn2

 

[Dusk]

you just said it yourself
if you've got aprotic theres no way ull do sn1....
thats why its not sn1, its sn2

that just... blew my mind..
lol wow

But another question comes to mind, so it does sn2, and it can't do sn1, but it also won't do E2 because CH3SH is a weak base, therefore I'm guessing sn2 works because CH3SH is a good nuc?

 

[wired202808]

that just... blew my mind..
lol wow

But another question comes to mind, so it does sn2, and it can't do sn1, but it also won't do E2 because CH3SH is a weak base, therefore I'm guessing sn2 works because CH3SH is a good nuc?

First question is actually very easy, the bromine is on a secondary carbon, and its in Aprotic solution (DMSO) this guarantees that it has to be SN2 since E2 is not a choice (E2 can only occur on a secondary carbon if you have a bulky base) and since DMSO is Aprotic it CANNOT be SN1 or E1 because they use Protic solvents.

Second question the Br is on a tertiary carbon meaning it can only be a choice between E2, SN1 or E1, since CN- is weak base it has to be E1.

In addition PLEASE PLEASE PLEASE save yourself the headache and remember that you must first determine what carbon (primary, secondary or tertiary) before trying to figure out Sn1 or SN2 or E1 or E2.

Remember

Strong Base/Nucleophile
Primary - Sn2 or E2 (with bulky base ONLY!)
Secondary - Sn2 or E2 (with bulky base ONLY!)
Tertiary - E2 ONLY!

Weak Base/Nucleophile
Primary - nothing CANNOT HAPPEN; therefore it must be SN2 unless its a bulky base and then would be E2
Secondary - Sn1 or E1 (racemization)
Tertiary - Sn1 or E1 (racemization)