Effects of a New Set of Computed Additive Parameters on the Rate of SN2 Substitution Reactions
JOURNAL OF EDUCATION AND SCIENCE,
2010, Volume 23, Issue 2, Pages 7-15
A guide by the idea that the rate of SN2 substitution reactions are highly affected by the charge of the atoms at or near to the reaction center together and the bond length between carbon atom and the leaving group. Also the total strain energy of reacting molecules has great effects on the reaction rate. The present work employed high level ab nitio calculation (RHF/3-21G) in order to calculate the above parameters for a set of fifteen bromoalkyl amines undergoing. SN2 replacement reaction of the bromide ion by hydrogen thiosulphate ion in acidic medium.
The results of these calculations showed that the alkyl substituent at the amino group has pronounce effects on the charge value at the atoms of the molecules at or near the reaction center. On the other hand, the total strain energy of the molecules increases, as the bulkiness of the N-substituents and chain length of alkyl amine increase which prove that these theoretical calculations are in accordance with the well known chemical principle. A simple correlation of the above parameters with experimental values of the rate constant for the substitution reactions at pH equal to 4.9 showed a good relation with charge at the carbon atom attached to the leaving group as well as the charge at the leaving group. While the C-Br bond length and total strain energy showed no significant effects on the reaction.
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