Theoretical Study Of Carbocation Stability&the Pyrolysis Of β-hydroxyl Carbonyl Compounds

A series of carbocations have been calculated at MP2/AUG-CC-PVDZ. The p-pconjugation effect between the empty p orbital on the cation and the filled p orbital onthe α-heteratom (N, O, P, S) is more effective than the p-π conjugation effect and thehyperconjugation effect. The stability of cycloalkyl cations enhances with the anglestrain decreasing. The calculated result shows that cyclopentadienyl cation is annon-aromatic structure and the carbocation with aromaticity is more significantlystable than the carbocation with a non-aromatic structure. The stability ofhalo-substituted alkyl cations enhances with the increase in the number of methylsubstituent. And the stability order of the halo-bridged cabocations (BXE and BXP) isX=I> Br> Cl>> F. Trifluoromethyl group shows the largest destabilizing effect onthe carbocation, which is mainly caused by the strongest electron-withdrawinginductive effect of the trifluoromethyl group in the studied groups. A relative stabilitydiagram of carbocations was pictured, which is concise and convenient for reader tosearch and compare the relative stability of different carbocations quickly.The pyrolysis of various β-hydroxyl carbonyl compounds has been investigated bytheoretical calculation method. The result shows that the pyrolysis of β-hydroxylcarbonyl compound is a concerted and unsynchronized mechanism with asix-membered cyclic transition state. The corresponding activation energy andreaction energy reduce when an electron withdrawing substituent was attached to thecarbonyl carbon; the corresponding activation energy and reaction enenrgy increasewhile an electron donating substituent connected to the carbonyl carbon. Theconjugation effect of substituent on the α-carbon atom effectively promotes thepyrolysis of β-hydroxyl carbonyl compounds. The pyrolysis of β-hydroxyl carbonylcompounds is more easier when an electron donating substituent or fluorine atom onthe β-C (bonding directly with hydroxyl group). The effect of substituent on β-C ismore obvious than that of substituent on α-C. The activation energy and chargedifference between oxygen and carbon atoms of carbonyl group have a good linearrelationship when carbonyl carbon atom has different substituents (R2=0.897).