Achiever Question, Number 99

[CANgnome]

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This is from Achiever 2, Q99 ochem.

The answer is A. I chose E, because of what i wrote down.

Williamson ether synthesis is possible.

At the same time, isn't E2 also possible? To produce CH3OH and Propene?

 

[UndergradGuy7]

This is from Achiever 2, Q99 ochem.

The answer is A. I chose E, because of what i wrote down.

Williamson ether synthesis is possible.

At the same time, isn't E2 also possible? To produce CH3OH and Propene?

E2 prefers tertiary > secondary > primary carbons.

 

[CANgnome]

oooh i get it

the alpha carbon is the determining carbon for E2/SN2. In this case, the alpha carbon is a primary alkyl halide, so E2 is not preferred. Is this correct?

 

[nainaaz]

To promote Elimination (E2), the base should readily abstract a proton but not readily attack a carbon atom. A bulky strong base such as t-butoxide (-OC(CH3)3), enhances elimination.

To promote substitution, we need a good nucleophile with limited basicity, a highly polarizable species that is conjugate base of a strong acid (Br-, I-).

Hope that clarifies your question...🙂

 

[RCT PC CRN]

The question says MAJOR so the only answer would be A.

If it doesn't say MAJOR then E2 also possible and the answer could be debatable but here MAJOR resolves it to Sn2 only. IMO, question like this always will have MAJOR or MINOR to point you to definitive answer.

 

[UndergradGuy7]

oooh i get it

the alpha carbon is the determining carbon for E2/SN2. In this case, the alpha carbon is a primary alkyl halide, so E2 is not preferred. Is this correct?

Yes the carbon is primary (the carbon that connects to I), since it only connects to one other carbon. So you would look for this to check if Sn2 or E2 is better. You would look at the nucleophile too to determine if Sn1/E1 vs Sn2/E2.