Allylic carbon definition

A considerable difference has been observed between the spectrum of cyclohexyl and that of the cyclopentyl radical, the former exhibiting a pronounced shoulder at 250 nm with e = 920 m -1 cm-1. Cyclohexenyl and cyclopentenyl radicals show a much stronger absorption with definite maxima at 240 nm. These are allyl type radicals and like the allyl radical itself they show extinction coefficients of 7000-9000 M -1 cm-1. The optical spectrum of the allyl radical is greatly affected by unsaturated substituents which conjugate with the allylic 1 and 3 positions. These positions bear all the spin density and their interaction with carboxyl groups, for example, shifts max to 270 nm with extinction coefficients of 20,000-40,000 M 1 cm 1 (Neta and Schuler, 1975). A carboxyl group attached to the central carbon of allyl has only a minimal effect on the absorption. 

Unsymmetrically substituted allylsilanes have different substitutents on the a- and y-carbon atoms of the allylic systems (the silane moiety is by definition on the a-carbon). 

The products in the deuteration of 2-methyl-l,3-butadiene over different catalysts are summarized in Table 4.32. Over ThOz and ZrOz 89% ( ) form and 11% (2) form are formed, while over MgO, 60% ( ) form and 40% (Z) form are formed. Although it is not definite yet which is formed first, anion 1 or 3, the carbon skeleton structure is retained during the reaction over ThOz, and the interconversion within the allylic anions extends over MgO. The dynamic nature of the allylic anions on LazOs is of an intermediate nature between that for MgO and that for ThOz. 

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