The quadrapolar character of the Markovnikov reaction transition state

•Calculation of the Markovnikov transition state for H2O and HX (X = F, Cl, Br).•Validation of transition state using intrinsic reaction coordinate and vibrational frequency.•Quantitative agreement of transition state energy based on partition function.•Dipolar nature of transition state validated using Hirshfeld charge and Fukui function.•Explanation of Markovnikov regioselectivity for H2O based on dipolar transition state.

The properties of the transition states for the electrophilic addition reactions of four molecules of the type HX (X = Br, Cl, F and OH) to 1-propene form 2-X propane by the Markovnikov mechanism have been calculated using density functional theory (DFT). A comparative study of the transition state along the reaction path for both M and AM addition was studied in order to understand the origins of regioselectivity of H2O. The quadrapolar nature of the transition state is arises because of a sequential mechanism, in which the addition across the π-bond occurs in two steps, first H+ and then X−. The M reaction mechanism is consistent with a H+X− dipole which induces an oppositely polarized C1−C2+ dipole in the transition state, resulting in a quadrupole. The C1−C2+ dipole in the M mechanism is consistently larger than the C1+C2− dipole of the AM transition state for all species studied.

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