Many substituents are more complicated than just a single atom and electronegativity is only part of the story. We need to look at all the common substituents and see what shifts they cause relative to the CH skeleton of the molecule. Our zero really ought to be at about 0.9 ppm for protons and at 8.4 ppm for carbon, that is, where ethane (CH₃–CH₃) resonates, and not at the arbitrary zero allocated to Me4Si. In the table below we give such a list. The reason for this is that the shifts (from Me₄Si) themselves are not additive but the shift differences (from 0.9 or 8.4 ppm) are.

The effects of groups based on carbon (the methyl group is joined directly to another carbon atom) appear in entries 2 to 11. All the electron-withdrawing groups based on carbonyl and cyanide have about the same effect (1.1–1.3 ppm downfi eld shift from 0.9 ppm). Groups based on nitrogen (Me–N bond) show a similar progression through amine, ammonium salt, amide, and nitro compound (entries 15–18). Finally, all the oxygen-based groups (Me–O bond) show large shifts (entries 19–22).

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific