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DAT Ochem Question

themuffinman11

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So for this question, I thought it would be B because I was thinking that since the nucleophile is neutral, it would be weak so SN1 is favoured, but the answer says how it will go SN2. Does the degree of substitution of the carbon take priority over nucleophile strength for determining whether it would be SN1 or SN2?
 

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averageasian

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look at the halide: it is primary and paired with ethanol so you're only route is thus SN2 . Forming a carbocation here isn't really the best idea. And as you can see in your product, no hydride shift occur to offset the formation of a primary carbocation. The primary halide was simply swapped out for the nucleophile via an SN2 mechanism.

Choice D is t-butoxide, a strong base used for e2 elimination. It is clearly not in your reactants and thus will not be included in the rate law and determine the rate of the reaction. Remember that for SN2, the only things affecting rate are k (temp, catalysts etc.), conc. of nucleophile, and conc. of electrophile.
 
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themuffinman11

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look at the halide: it is primary and paired with ethoxide so you're only route is thus SN2 . Forming a carbocation here isn't really the best idea. And as you can see in your product, no hydride shift occur to offset the formation of a primary carbocation. The primary halide was simply swapped out for the nucleophile via an SN2 mechanism.

Choice D is t-butoxide, a strong base used for e2 elimination. It is clearly not in your reactants and thus will not be included in the rate law and determine the rate of the reaction. Remember that for SN2, the only things affecting rate are k (temp, catalysts etc.), conc. of nucleophile, and conc. of electrophile.

But it's ethanol not ethoxide, it's a weak nucleophile, but yeah regardless of SN1 or SN2, I can see why D would be right cause that gives the elimination product
 
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averageasian

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But it's ethanol not ethoxide, it's a weak nucleophile, but yeah regardless of SN1 or SN2, I can see why D would be right cause that gives the elimination product
good catch on that. but remember since your halide is ALWAYS in the rate law, whether it is primary secondary or tertiary plays a major role into what mechanism will occur. Yes the nucleophile is important but equally so is the electrophile and its a major tell about what will happen.
 
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