CHE 211 Study Guide - Midterm Guide: Spectroscopy, Aldehyde, Ring Current
Organic Chemistry 2: NMR Spectroscopy
Effects of Molecular Structure on 1H Chemical Shifts
➢ To see how the electron density of a carbon-hydrogen bond is affected by
electronegativity of other atoms bonded to the carbon atom
➢ Lets consider the spectrum for fluromethane:
o The C-F bond is polar, therefore the fluorine atom bears a
partial (-) charge and the carbon atom bears a partial (+)
charge
o Because the carbon atom has a partial (+) charge, the
electrons in each C-H sigma bond are drawn towards the
carbon atom and away from the hydrogen atom as a way to
minimize the partial (+) charge on carbon (this is called the inductive effect)
o Inductive effect: the electron-attracting or electron-withdrawing effect transmitted
through sigma bonds; electronegative elements have an electron-withdrawing
inductive effects
o The inductive effect occurs through the sigma bonds; the electron withdrawing
effect of the fluorine atom results in a greater electron density around the
fluorine atom and less electron density around each hydrogen atom therefore,
the protons of fluoromethane are deshielded and absorbed downfield compared to
the protons of methane
• Lets look at a series of methyl halides and methane (both above) to try to
cement this very important concept in proton NMR; the series of methyl halides
show increased shielding of the hydrogen nuclei with decreasing
electronegativity of the atom attached to the carbon of the methyl group
notice that the hydrogen atom of methane are completely shielded due to the
circulation of electrons of the sigma bond; when you replace one of the
hydrogen atoms with an electronegative atom, the hydrogen atoms are de-
shielded and appear downfield (away from TMS)
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Document Summary
Effects of molecular structure on 1h chemical shifts. To see how the electron density of a carbon-hydrogen bond is affected by electronegativity of other atoms bonded to the carbon atom. Most elements in organic compounds are more electronegative than carbon; these inductive effects of these electronegative atoms is electron withdrawing and the protons affected by it are. The deshielding effects of the electronegative atoms deshielded attached to carbon are cumulative the protons will appear. It is more important to get a feel for the regions and types of protons than to memorize a bunch of numbers. This correlation table shows the regions of chemical shifts of where most of the common types of protons appear in proton nmr. We will see this time of deshielding with alkenes, aromatic rings and aldehydes carbon of an aldehyde appears further. The proton attached to the carbonyl downfield than aromatic ring protons do carbon=carbon double bond are observed.
