Organic Chemistry I: Studying Sn1 Reactions (nucleophilic substitution at satura

Question

Organic Chemistry I: Studying Sn1 Reactions (nucleophilic substitution at saturated carbon)

Purpose of the experiment: Investigate some factors that influence the rate of Sn1 reactions.

In this experiment, it was noticed that the solvent polarity had the greatest impact on the rate of Sn1 reaction. This was observed in the reaction of 2-bromo-2-methylpropane + 40% 2-propanol, which reacted faster comparing to the other parts of the experiment. Please explain why the solvent polarity had the most pronounced effect on the observed times.

Thank You In Advance!

Explanation / Answer

Some of the most important information concerning nucleophilic substitution and elimination reactions of alkyl halides has come from studies in which the structure of the alkyl group has been varied. As noted earlier, the carbon bonded to the halogen is sp3 hybridized in all the alkyl halides discussed here. If we examine a series of alkyl bromide substitution reactions with the strong nucleophile thiocyanide (SCN–) in ethanol solvent, we find large decreases in the rates of reaction as alkyl substitution of the alpha-carbon increases. Methyl bromide reacts 20 to 30 times faster than simple 1º-alkyl bromides, which in turn react about 20 times faster than simple 2º-alkyl bromides, and 3º-alkyl bromides are essentially unreactive or undergo elimination reactions. Furthermore, -alkyl substitution also decreases the rate of substitution, as witnessed by the failure of neopentyl bromide, (CH3)3CCH2-Br (a 1º-bromide), to react.
Alkyl halides in which the alpha-carbon is a chiral center provide additional information about these nucleophilic substitution reactions. Returning to the examples presented at the beginning of this section, we find that reactions 2, 5 & 6 demonstrate an inversion of configuration when the cyanide nucleophile replaces the bromine. Other investigations have shown this to be generally true for reactions carried out in non-polar organic solvents, the reaction of (S)-2-iodobutane with sodium azide in ethanol being just one example ( in the following equation the alpha-carbon is maroon and the azide nucleophile is blue). Inversion of configuration during nucleophilic substitution has also been confirmed for chiral 1º-halides of the type RCDH-X, where the chirality is due to isotopic substitution.

Thus, hydrolysis of tert-butyl chloride in a mixed solvent of water and acetonitrile gives a mixture of 2-methyl-2-propanol (60%) and 2-methylpropene (40%) at a rate independent of the water concentration. The alcohol is the product of an SN1 reaction and the alkene is the product of the E1 reaction. The characteristics of these two reaction mechanisms are similar, as expected.

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