CHEM 2444 Lecture Notes - Lecture 5: Covalent Bond, Chemical Shift, Electronegativity
Document Summary
Electron-releasing & electron-withdrawing groups: electron withdrawing atom or group leads to less electron density around the alpha- hydrogen atom, which requires a lesser magnetic field to come into resonance. Electron withdrawing deshielded lower magnetic field, downfield. Greater the oxygen, further downfield: most deshielding = most electron-withdrawing, electronegativity. More electronegative should be more polarized and should be more electron- withdrawing: electron density surrounding h. More electron density surrounding h = more shielded, higher magnetic field, upfield. Less electron density surrounding h = more deshielded, lower magnetic field, downfield. Chemical shift patterns: inductive effects stop at about 4-4. 5 tms, pi electrons are able to set up their whole magnetic field (anisotropy) Alkyl protons show a range of chemical shifts, depending on the substitution. Disubstituted carbon leads to a weaker ch bond relative to methyl, so it is more downfield: trisubstituted carbon leads to an oven weaker ch bond. Furthest downfield: carbonyl induces a downfield shift of about 1. 2 ppm.
