destroyer ochem #62

[NOORALLAH]

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I am I getting this wrong or the answer is wrong... I think the answer is D but it's saying C.... primary halides are more reactive in E2 right????

 

[needzmoar]

I am I getting this wrong or the answer is wrong... I think the answer is D but it's saying C.... primary halides are more reactive in E2 right????

The SN2 product is favoured if the reactant is a primary halide.

Could you post the question?

 

[NOORALLAH]

It says arrange the fallowing in order of decreasing reactivity to E2... most reactive first
primary alkyl bromide, primary alkyl chloride, cyclic alkyl bromide (so secondary), cyclic alkyl iodide (secondary), and tertiary alkyl iodide

I have primary alkyl chloride >primary alkyl bromide>cyclic alkyl iodide> cyclic alkyl bromide>tertiary alkyl iodide

The answer is saying totally the opp!!!!

 

[needzmoar]

It says arrange the fallowing in order of decreasing reactivity to E2... most reactive first
primary alkyl bromide, primary alkyl chloride, cyclic alkyl bromide (so secondary), cyclic alkyl iodide (secondary), and tertiary alkyl iodide

I have primary alkyl chloride >primary alkyl bromide>cyclic alkyl iodide> cyclic alkyl bromide>tertiary alkyl iodide

The answer is saying totally the opp!!!!

SN2 is the only mechanism that favours 1>2>3, the rest (SN1,E1,E2) favours 3>2>1

Just memorize the preference for SN2 and the rest will just be the opposite.

 

[NOORALLAH]

I thought that E2 and SN2 have the same trend.... won't it make sense that a bulky strong acids favor the least sub alky halide... like primary halides!!! can you explain the E2 trend...

 

[needzmoar]

I thought that E2 and SN2 have the same trend.... won't it make sense that a bulky strong acids favor the least sub alky halide... like primary halides!!! can you explain the E2 trend...

Sure.

Firstly, for elimination reactions, the base is reacting with a hydrogen that's on an adjacent carbon from the halide. Secondly, a bulky strong base is sterically hindered. We don't expect to see an SN2 product because it's a poor nucleophile due to the "bulkiness".The preference for elimination reactions stems from the stability of the alkene. A more stable alkene is formed from a tertiary halide than from a primary halide.

edit:exception

A primary halide with a small, unhindered strong base will result in the SN2 product. (e.g. williamson ether synth.)

 

[NOORALLAH]

Thanks for elaborating! 🙂