Electronegative groups, deshielding

The chemical shift is determined by the extent to which a proton is deshielded by the groups to which it is attached. The more a proton is shielded by the electron density around it, the lower its 3 value. If a proton is attached to a system that withdraws electrons from its environment such as an electronegative group or to a group which affects its environment by creating a field opposing the applied... 

Alternating single and double bonds (conjugation) relax the restriction on the quick drop-off of the effects of electronegative groups in molecules. That is, an EWG many bonds distant may exert a profound deshielding effect on a given or if resonance structures can be drawn to link the two. 

The two CHj groups in isopropyl benzene (Fig. 8) are identical and their resonance is split into a doublet by the adjacent CH, which is deshielded by the ring current. The methine resonance is split into a septet by the six equivalent methyl protons. The aromatic protons are almost equivalent as there is no electronegative group present to affect their chemical shifts. The integral ratios are 5 1 6. 

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded Normally the carbon of C=0 is the least shielded one m... 

Electronegative substituents, such as halogens, alcohol, and ether functions, deshield the fluorine nuclei of CF2 groups when they are attached directly to the carbon bearing the two fluorine substituents, whereas electronegative substituents at the (3-position always have a shielding influence. 

A chlorine substituent at the 2-position of a 1,1-difluoroalkene will deshield the fluorines modestly, but as seen in Scheme 4.46, a vicinal alkoxy group shields both fluorines, the trans fluorine more greatly. With the additional electronegative substituent, three-bond F—H coupling constants for such compounds become even smaller. 

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