Question

Six halides: 1-chlorobutane, 2-chlorobutane, Allyl chloride, 2-chloro-2-methylpropane, 1-chloro-2-methylpropane, 2-bromobutane.

SN2: w/ sodium iodide in acetone
SN1: w/ silver nitrate in ethanol

Explain the order of reactivity under SN2 and SN1 conditions by considering alkyl halide structure, nature of the learoup, temperature, solvent, etc.
Please be descriptive and answer them correctly.

Answer 1

​The reactions that occur are SN2 substitutions in which iodide ion is the nucleophile; the order of reactivity is primary > secondary > tertiary.
With the reagent, primary bromides give a precipitate of sodium bromide in about 3 min at room temperature, whereas the primary and secondary chlorides must be heated to about 50C before reaction occurs. Secondary and tertiary bromides react at 50C, but the tertiary chlorides fail to react in a reasonable time.

​hence the order for SN2 is: Allyl chloride> 1 chlorobutane> 1-chloro-2-metylpropane> 2- bromobutane> 2- chlorobutane>2- chloro-2-metylpropane

​Such a reaction will be of the SN1 type. Tertiary halides are more reactive in an SN1 reaction than secondary halides, which are in turn more reactive than primary halides. Differing rates of silver halide precipitation would be expected from halogen in each of these environments, namely, primary < secondary < tertiary. Alkyl bromides and iodides react more rapidly than chlorides, and the latter may require warming to produce a reaction

in a reasonable period. Allylic and benzylic halides, even when primary, show reactivities as great as or greater than

tertiary halides because of resonance stabilization of the resulting allyl or benzyl carbocations

​hence the order for SN1 is: Allyl chloride>2- chloro-2-methylpropane> 2- bromobutane> 2- chlorobutane>1-chloro-2-metylpropane> 1- chlorobutane

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