Which statements are true for SN2 reactions of haloalkanes? (a) Both the haloalk
ID: 946891 • Letter: W
Question
Which statements are true for SN2 reactions of haloalkanes? (a) Both the haloalkane and the nucleophile are involved in the transition state. (b) The reaction proceeds with inversion of configuration at the substitution center. (c) The reaction proceeds with retention of optical activity. (d) The order of reactivity is 3° > 2° > 1° > methyl. (e) The nucleophile must have an unshared pair of electrons and bear a negative charge. (f) The greater the nucleophilicity of the nucleophile, the greater is the rate of reaction.
Explanation / Answer
a) True. Technically, an SN2 reaction means; S stands for substitution, N for nucleophilic, and the 2 is because the initial stage of the reaction involves two species - the haloalkane and the nucleophilic ion.
b) True. Be very careful when you draw the transition state to make a clear difference between the dotted lines showing the half-made and half-broken bonds. In a S2N reaction the molecule is inverted during the reaction - rather like an umbrella being blown inside-out.
c) True. In the case of the SN2 mechanism, racemisation does NOT take place and chirality and optical activity is completely preserved in the molecule, But inversion takes place. So, For an optically active halogenoalkane reactant, the retention of complete optical activity in a nucleophilic substitution reaction is evidence of the SN2 mechanism.
d) False. This mechanism is most likely with primary halogenoalkanes. Tertiary halogenoalkanes tend to react by the SN1 mechanism involving a carbocation, secondary halogenoalkanes react via both mechanisms. So the order for SN2 is more likely as follows: 1°>2°>3°.
e) True. We'll talk about this mechanism through using an ion as a nucleophile, because it's slightly easier. And all halo must have at least one pair of electrons or a negative charge but not necessaraly together.
f) False. Just realise that the bigger or greater is the nucleophile, the stronger the bond. So if it is harder to break, the slower the reaction will be.
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