DiscussionThe reaction of (-)-α-phellandrene, 1, and maleic anhydride, 2, gave a Diels-Alder adduct, 4, 7-ethanoisobenzofuran-1, 3-dione, 3a, 4, 7, 7a-tetrahydro-5-methyl-8-(1-methylethyl), 3, this reaction gave white crystals in a yield of 2.64 g (37.56%). Hydrogen and carbon NMR as well as NOESY, COZY and HSQC spectra were used to prove that 3 had formed. These spectroscopic techniques also helped identify whether the process was being attacked via the top of the underside, as well as whether this reaction was occurring via the endo or exo process. These possible attacks give rise to four possible products, however, in reality, due to steric interactions and electronics, only one product is formed. Scheme 1. Formation of the Diels-Alder adduct with maleic anhydride and (-)-α-phellandrene. The IR spectrum obtained from the white crystals showed several functional groups present in the molecule. The spectrum shows a weak, sharp peak between 2865 and 2964 cm-1, which is often associated with CH, sp3 hybridized, stretching into the molecule, peaks in this region often represent a methyl group or CH2 groups. There are also peaks at 1369 cm-1, associated with CH3 stretching. There is also a C=O stretching at 1767 cm-1, which is a large peak due to the large dipole created by the large difference in electronegativity of the carbon and the oxygen atom. A CO anhydride resonates between 1000 and 1300 cm-1, which is at least two bands. The peak is present in 13C NMR at 1269 and 1299 cm-1, it is of medium intensity. Figure 1. Atoms labeled 3 used in NMR assignment. The 1H NMR spectrum shows that there are 18 protons in 11 different proton environments. This corresponds to the Diels-Alder reaction taking place in the middle of paper......teraction of the HOMO of the diene and the LUMO of the dienophile. This reaction was carried out at relatively low temperatures because dry ether has a boiling point of 34.6 °C. At low temperatures, endo preference predominates, unless there is extreme steric hindrance, which is not the case in this case. The endo product forms almost exclusively because the activation barrier for endo is much lower than for exo. This means that the endo form forms more quickly. When reactions proceed via endo, the reaction is under kinetic control. Under kinetic control, the adduct is more sterically hindered, and therefore thermodynamically less stable. The endo form has a lower activation energy, however, the EXO form has a more stable product. As this is a symmetric Diels-Alder reaction, there are no two possible isomers of the product. Figure 5. Mechanism of formation of 3.