The present disclosure relates to a process for preparing linear alkyl benzne (LAB). The process comprises alkylation of benzene with an alkylating agent in the presence of an ionic liquid to obtain a first product mixture comprising a first organic phase and a first aqueous phase comprising first partially spent ionic liquid. The first organic phase is deacidified and fractionally distilled to obtain a fraction comprising LAB and a fraction comprising HAB. The fraction comprising HAB is transalkylated with benzene in the presence of the ionic liquid to obtain a second product mixture comprising a second organic phase comprising LAB and a second aqueous phase comprising second partially spent ionic liquid. The partially spent ionic liquids are regenerated, and reused in the steps of alkylation or transalkylation for at least 6 cycles.