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.

مواضيع ذات صلة

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres

Diastereoisomers can arise when structures have more than one stereogenic centre

Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

R and S can be used to describe the configuration of a chiral centre

Many chiral molecules are present in nature as single enantiomers