CH and CH2 groups have higher chemical shift than CH3 groups
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص274-275
2025-05-19
568
Electronegative substituents have a similar effect on the protons of CH2 groups and CH groups, but with the added complication that CH2 groups have two other substituents and CH groups three. A simple CH2 (methylene) group resonates at 1.3 ppm, about 0.4 ppm further downfi eld than a comparable CH3 group (0.9 ppm), and a simple CH group resonates at 1.7 ppm, another 0.4 ppm downfi eld. Replacing each hydrogen atom in the CH3 group by a carbon atom causes a small downfi eld shift as carbon is slightly more electronegative (C 2.5; H 2.2) than hydrogen and therefore shields less effectively.
●Chemical shifts of protons in CH, CH2, and CH3 groups with no nearby electron withdrawing groups.
The benzyl group (PhCH2–) is very important in organic chemistry. It occurs naturally in the amino acid phenylalanine, which you met in Chapter 2. Phenylalanine has its CH2 signal at 3.0 ppm and is moved downfi eld from 1.3 ppm mostly by the benzene ring. Amino acids are often ‘protected’ as the Cbz (carboxybenzyl) derivatives by reaction with an acid chloride (we’ll discuss this more in Chapter 23). Here is a simple example together with the NMR spectrum of the product. Now the CH2 group has gone further downfi eld to 5.1 ppm as it is next to both oxygen and phenyl.
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