Naphthalene, hydrogenation methyl substituent effect

The thermochemical database for substituted naphthalenes is far less extensive than for substituted benzenes and is reproduced in Table 5. For a given R, there are two isomeric monosubstituted naphthalenes, the 1-R and the 2-R. The latter might be expected to be more stable because it lacks the 1.8-or peri-repulsion found in the former between the substituent and a small, but unavoidably nearby, hydrogen. For a methyl substituent, the difference between the enthalpies of formation of 1- and 2-me-thylnaphthalene, 113.5 1.7 and 110.7 1.7 kJ/mol, respectively, reveals a very small 2.8 2.4 kJ/mol peri effect. For identical substituents, there are the following naphthalene isomers 1.2-. 1.3-. 1.4-. 1,5-, 1.6-. 1.7-. 1.8-, 2,3-, 2.6-. and 2.7-isomers. There is no substituent for which all 10 isomers have been thermochemically characterized. Furthermore, there is only one substituent for which there are enthalpy of formation data for the isomer of greatest interest— 1,8-dimethylnaphthalene. Equation 20. analogous to eq 18, is now recast for mono- and disubstituted naphthalenes ... 

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