Order the reactivity of the compounds evaluated in the \"Effect of Structure of

Question

Order the reactivity of the compounds evaluated in the "Effect of Structure of the Alkyl Halide" experiment from slowest to fastest? Were these results consistant with the expeceted results? Discuss why your results were or were not in agreement with the expected results.

procedure: in all three test tubes 2mL of 15% sodium iodide in acetone: test tube 1: 2 drops of 1-bromobutane, test tube 2: 2 drops of 2-bromobutane, test tube 3: 2 drops of 2-bromo-2-methylpropane.

experiment: test tube 1: cloudy with precipiate at first, at 15 minutes clear with precipitate. test tube 2: clear with yellow tint. test tube 3: yellow, dark yellow at 15 minutes.

Explanation / Answer

This is an example of an SN2 (substitution nucleophilic bimolecular reaction).

The reactivity of alkyl halides would depend on the steric hindrance around the carbon atom having the leaving group. The transition state of SN2 reaction has 5-bonds to it, thus more the bulk around this carbon atom higher would be the energy required to form the transition state and hence least favorable the reaction would be for SN2. The reactivity order for alkyl halides in an SN2 reaction is : 1o > 2o > 3o.

We have a primary 1-bromobutane, a seconday 2-bromobutane and a tertiary 2-bromo-2-methylpropane alkyl halides as the starting material. The reaction would thus be slowest in case of tertiary 2-bromo-2-methylpropane alkyl halide and fastest for 1-bromobutane with NaI as the reagent in the reaction.

The results are consistent with the expected order for the alkyl halides. NaBr is insoluble in acetone and forms precipitate. NaI in acetone is soluble and gives yellow coloration.

In test tube 1 : Formation of a clear solution with precipitate indicates formation of alkyl iodide and NaBr (insoluble precipitate)

In test tube 2 : Only a small amount of alkyl halide reacts to give NaBr which dissoves initially to give a clear solution. the remaining unreacted NaI in solution gives yellow tint.

In test tube 3 : No reaction occured. The color of solution is due to NaI only.

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