The pyrolysis mechanism of alkyl side chain of n-butyl anthracene as model compound of coal was studied by using the DFT/B3LYP/6-311G method in the Gaussian program package.The thermodynamic parameters of all reaction processes and the frontier orbital energy difference of the product free radical were calculated.The results indicated that the β C—C bond dissociation in the alkyl side chain of n-butyl anthracene was the easiest to occur.In the three isomeric hydrocarbons of n-butyl anthracene the alkyl side chain connected to the γ C atom had the highest reactivity.The size of aromatic rings had positive effect on the β C—C bond dissociation of the alkyl side chain

but had only a little effect on the other C—C bonds.