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"Monoalkylbenzenes substituted by functional groups in the side chain. The effect of alkyl groups spatial structure on the chemical shift of aromatic ortho-protons in NMR1H spectra"
Published: 14 November 2011 by MDPI in The 15th International Electronic Conference on Synthetic Organic Chemistry session Computational Chemistry
Abstract: In our previous work (see ECSOC-12, 2008) we found some interesting dependencies of ortho-protons chemical shift (δoH) in monoalkylbenzenes NMR 1H spectra upon the place and degree of branching of alkyl chain. If the branching takes place at the α-carbon atom, the downfield shift of basic spectral parameter - δoH - (α-effect) is observed and entitled as "α-effect"; the branching at β-carbon atom leads to the shift toward upfield ("β-effect"). α-Effect has a positive value because δoH value increases with the increase of amount of methyl groups in α-position, contrary, β-effect has a negative value. The natural question emerges: which other classes of aromatic compounds (in addition to monoalkylbenzenes) the founded regularities apply to? We obtain the answer while studying NMR 1H spectra of 17 especially selected rows of monoalkylbenzenes with functional groups in alkyl substituent. Extended interpretation of the term "functional groups" allows to examine alkyl groups with multiple bonds C=C, CºC and C=O as a functionalized fragment. Depending upon the place of alkyl chain branching all compounds are divided into two types. In the compounds of A type the branching takes place at α-carbon atom and in the compounds of B type – at β-carbon atom. The variable fragments may be only hydrogen atom or methyl group. The use of differential spectral parameters (ΔδНо) instead of basic ones (δоН) is more advisable because of the great scattering of δоН,N(n) values. The differential spectral parameter is a difference between the value of ortho-protons chemical shift δоН,N(n) of the investigated compound Nn and the same value δоН,N(0) of the standard compound, when all variable substituents R1 = R2 = R3 = Н. The δНо and ΔδНо values of all A and B type compounds are demonstrated in 2 tables and 2 diagrams. The new regular "structure-property" dependencies which were determined for nonfunctionalized monoalkylbenzenes are observed for all values of basic spectral parameters δ0H, N(n) in the investigated rows of monoalkylbenzenes substituted by functional substituents in the side chain. α- and β-Effects (change of chemical shift of ortho-protons δ0H in NMR1H spectra) are explained by changes of saturation of definite spatial areas near phenyl ring by methyl groups. It should be stressed that for the investigated compounds 1-17 there are no facts contradicting with the existing of "α-effect" at the branching of alkyl chain near α-carbon atom and "β-effect" observed at the branching near β-carbon atom. The similar regular "structure-property" dependencies are reported for monosubstituted para-alkylaromatic compounds in two our accompanied communications, which we also intend to sent to ECSOC-15.