Under the catalysis of HF,the alkylation process of 16 polycyclic aromatic hydrocarbons (PAHs) in coal tar pitch (CTP) with ethylene was studied by using the first principle.The alkylation reaction mechanism of 16 PAHs with ethylene was discussed according to the adsorption enthalpy and activation energy.The reactants could go through three processes to form the six-membered-ring transition state.In any reaction process,ethylene molecules are activated by HF and the fluorine atom help to remove hydrogen from the aromatic rings.Acenaphthylene,chrysene,benzo [a] pyrene and benzo[g,h,i] perylene were selected from 16 PAHs as objects of study,and the activation energy of alkylation reaction of the four substances were calculated.The calculated activation energy locates in two discontinuous ranges.The lower energies are between 35 and 41 kJ/mol and higher energies are between 58 and 59 kJ/mol.An obvious feature of the inactive sites with higher activation energies is that the carbon is linked by two carbon atoms which have only one C-H bond.The activation energies of active sites of the rest 12 PAHs were calculated in the following calculations.The difficulty level of reactions of 16 PAHs with ethylene was achieved on the basis of the calculated activation energies.Among the 16 PAHs,the substance with lowest activation energy is benzo [a] anthracene and the substance with highest activation energy is fluorine.This paper provides guidance to select model compounds for the modification of CTP.