Oxidation of Alkyl Substituents on the Aromatic Ring

With side chains longer than methyl, oxidation with air (or oxygen) apparently involves a hydroperoxide radical and the reaction appears to occur, at least initially, at the benzylic position (i.e., a to the aromatic ring) as shown in Equation 6.82. Interestingly, as shown in Equation 6.83, careful adjustment of oxidizing reagent, solvent, and reaction conditions will even allow the conversion of methyl groups to aldehydes.  

The lability of the aldehyde function under oxidizing conditions can be ameliorated by carrying out the oxidation under conditions where the aldehyde group is protected vide supra) (Chapters 8 and 9) as shown in Equation 6.84. Here the acylal (an ester) produced can be hydrolyzed with aqueous acid or base to the corresponding aldehyde (Chapter 9). 

This is particularly interesting because aldehydes (Chapter 9), lying (in a redox sense) between primary alcohols and carboxylic acids, are generally too labile to be isolated in the presence of oxidizing agents. Indeed, as will be encountered later (Chapter 9), many aldehydes spontaneously oxidize (in the presence of air) to the corresponding carboxylic acids. 

Arenes are reduced with more difficulty than alkenes and alkynes so that if double and/or triple bonds are present in side chains attached to the ring, hydrogenation in the presence of a catalyst will cause them to be reduced first. Further, the conditions for catalytic reduction of the aromatic ring are vigorous enough to insure that intermediates are not isolated. That is, reduction of the first double bond is the 

Finally, in this regard, if the reduction of the aromatic ring is carried out with calcium (Ca) rather than sodium (Na) in the presence of an amine rather than ammonia, it is possible to isolate cyclohexene rather than cyclohexadiene. Thus, all three double bonds can be reduced, two of them, or, only one (albeit to the unconjugated diene).  

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