The effect of substituents on the properties of acyl derivatives has been examined. Geometry optimizations were carried out at the MP2/6-31G * theoretical level, followed by calculation of energies at the MP3/6-311++G ** level. The energies were studied via isodamic reactions with ethane leading to acetone and a methyl derivative. The calculated energy changes were in good agreement with the available experimental data and showed that groups more electronegative than carbon stabilize a carbonyl group more than methyl, whereas the opposite is true with the more electropositive groups