A theoretical study for the effect of polarity, steric energy and energy levels for HOMO and LUMO orbitals on the rate of addition of methyl radical to some monosubstituted Alkenes.

Mostafa R.Aeyd

doi:10.32894/kujss.2008.42054

Abstract

In this work The factors affecting the rate of addition of methyl radical to a number of monosubstitued Alkenes have been studied using quantum mechanical calculations. These factors include physical properties such as: electron densities (Total ED)on the carbon atoms involved in the addition reaction (carbon bearing the single electron in methyl radical and the carbon on the unsubstituted methylene carbon in Alkene),electron density on the substituted carbon, electron density on HOMO orbitals, energy level of HOMO orbital (EHOMO) and energy level of LUMO orbital (ELUMO). In addition, the steric energy in the transition state of the addition reaction also has been considered. The correlation between the logarithm of the addition rate constant (logK) and the physical properties has been established. The results of the regression analysis gave the best correlation coefficient (R=0.97) in the case of using (log K) as a dependent variable with three an independent variables which were: polarity, EHOMO, ELUMO. The most influencing factor on the rate of addition was the polarity followed by ELUMO while EHOMO and steric energy showed an insignificant effect.

Highlights

The importance of polar effects in the addition of methyl radical to Alkenes has been emphasized for many years (Heberger & Fischer,1992).This trend in the studies was encouraged by mechanistic studies and Quantum methods in which the effect of the substituents on the addition rate of free radicals was studied (Minisci & Citterio,1980).This type of studies are included in a branch of chemistry which is called computational Chemistry
The importance of polar effects has arisen from the fact that the differences in the electron densities on the carbon atoms which are involved in the addition reaction affect the formation of a complex in which the transition state between the attacking free radical and the Alkenes has an asymmetric structure
Using PCA free radicals can be classified according to their behavior in the addition process to: strong nucleophile, medium nucleophile, weak nucleophile, strong electrophile, medium electrophile and weak electrophile (Heberger & Lopata,1995), (Heberger, K and Lopata, A 1998) .Both older experimental studies(Arnaud, R et al,1986) and recent theoretical studies(Denisov,1999) have led to the conclusion that the methyl radical is nucleophilic in character

Summary

Introduction

The importance of polar effects in the addition of methyl radical to Alkenes has been emphasized for many years (Heberger & Fischer,1992) This trend in the studies was encouraged by mechanistic studies and Quantum methods in which the effect of the substituents on the addition rate of free radicals was studied (Minisci & Citterio,1980).This type of studies are included in a branch of chemistry which is called computational Chemistry. -SPSS: The statistical analysis of the data has been achieved using SPSS program to find the correlation coefficient ( R ) between the logarithm of the addition rate constant (log K) for methyl radical and the physical properties of the Alkenes in order to determine the most influencing factor of the addition reaction. The square of the correlation coefficient ( R2 ) and the coefficients a1,a2,...etc associated with the independent parameters have been calculated

Results and discussion

Total ED

Total Ed

Steric energy

Total Ed Steric