N1 Mechanism - The WikiPremed MCAT Course"> S<sub class="intersub">N</sub>1 Mechanism - The WikiPremed MCAT Course

Integrated SequencePhysics Chemistry Organic Biology

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Work, Energy, and Power


The Chemical Bond



Chemical Kinetics


Intramolecular Cationic Rearrangements

Reactions of Alkyl Halides

The substitution and elimination reactions are MCAT favorites. Let us discuss SN1 substitution. SN1 substitution occurs on hindered substrates that form stable carbocations, with the leaving group departing with its bonding pair in the rate determining step.

The departure of the leaving group is the rate determining step in SN1 substitution, so SN1 substitution kinetics are first order in alkyl halide.

With limiting (pure) SN1 substitution, the nucleophile attacks the trigonal planar carbocation from the two sides at equal rates, forming two enantiomers at equal rates, giving racemic products. In most cases, however, due to either nucleophilic assistance in carbocation formation or due to the incomplete independence of the carbocation from the leaving group ion, only partial racemization is realized.

Because the SN1 mechanism passes through a carbocation intermediate, product can form with an altered carbon skeleton due to carbocation rearrangement.

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