COMPUTATIONAL MODELLING OF THE KINETIC AND THERMODYNAMIC STUDIES OF DIELS-ALDER REACTION OF ACROLEIN AND 1, 3-BUTADIENE

Abstract

The kinetics and thermodynamics of the Diels-Alder reaction of acrolein and 1, 3-butadiene to produce 3-cyclohexenecarboxaldehyde (1,2,3,6-Tetrahydrobenzaldehyde), a fragrance and masking ingredient used in many skin care products was modelled in the gas-phase using Density Functional Theory (DFT) at B3LYP level with 6-311++G (2df, 2P) basis set. Geometric parameters such as bond length, bond angle and dihedral angle, atomic charge distribution for the reactant (GS), transition state (TS) and the product (PRD) were obtained, kinetic and thermodynamic parameters were calculated. The reaction proceeded through a concerted asynchronous six-membered cyclic transition state. Bond breaking as shown by the values obtained for the change in the bond length between the transition state and the reactants ( 􀀀) for [C1 –C5 = + 0.03Å, C3-C8 = +0.054 Å and C4-C11= +0.058 Å] occurred first while the bond making process is lagging behind in a single step, with values [C1 –C3 = - 0.06 Å, C3-C11 = -2.301 Å and C5-C14= -1.514 Å] lower, compared with the values for bond breaking. This showed the concertedness and asynchronicity of the reaction. Electrostatic potential maps, LUMO maps, local ionization potential maps and HOMO-LUMO maps perfectly explain the molecular charge distribution. The kinetic [∆H*= 73.290 kJ/mol, ∆G*=80.766kJ/mol, Ea =78.472kJ/mol, Rate, k =8.41 x 105] and thermodynamic [∆Hreaction = -145.945 kJ/mol, ∆Sreaction= -29.867J/mol.K, ∆Greaction = -127.255 kJ/mol] values with the Arrhenius plots which resulted in a negatively sloped line with a steeper slope, showed that the activation energy is higher, this means that the reaction changes more rapidly and the rate constant is inversely proportional to the temperature. Negative values obtained for the enthalpy of reaction and Gibb's free energy showed that the reaction is exothermic and spontaneous respectively, energy of formation obtained confirms the exothermicity of the reaction as stated by Hammond’s postulate.  The plot of rate constant, k versus temperature showed that increase in temperature increases the rate constant and the plot of (1/T versus ln k) showed that as the temperature increases the rate constant decreases. Keywords: Diels Alder reaction, Acrolein, 1, 3-butadiene, modelling