AbstractThe dehydrohalogenation mechanism of 2‐haloacetic acids (XCH2CO2H, X=F, Cl and Br) has been studied theoretically by HF/3–21G and AM1 methods. The results indicate that these reactions are most probably proceeded in terms of a polar five‐membered cyclic transition state in the gas phase. Their microscopic processes are beleived to be a stepwise reaction and the rate‐determining step is the first one. By comparing the energy barriers of different 2‐haloacetic acids, it can be realized that 2‐fluoroacetic acid is easier to react than 2‐chloroacetic and 2‐br