1. Show which carbon atoms correspond with which peaks in the ¹³CNMR spectrum of butan-2-one (Figure 13-45).
2. Draw the proton NMR spectrum you would expect for butan-2-one. How well do the proton chemical shifts predict the carbon chemical shifts using the “15 to 20 times as large” rule of thumbs?

The chemical shift value of a carbon will depend on the environment in which it is in. If a carbon is attached to an electronegative atom, then the value of chemical shift of that carbon atom will increase as electronegative atom tend to withdraw all the electron density from the atom to which it is connected because of which, the electron density on carbon decreases and chemical shift value increases and it will lie in deshielded region. In butan-2-one, the carbonyl carbon has maximum value of chemical shift that is, 209, this is due to attachment to an oxygen atom. Methylene carbon has chemical shift value 37 as it is attached to carbonyl group. Methylene carbon has more chemical shift value than methyl carbons.

Chemical shift values of carbon atoms in carbon-NMR spectrum of butan-2-one

The proton-NMR spectrum for butan-2-one have been drawn and peaks along with splitting pattern have been depicted. Proton chemical shifts predict the carbon chemical shifts using the “15 to 20 times as large” rule of thumb satisfactory as the chemical shift values of carbon and hydrogen are compared and it is found out that, for methyl carbon attached to carbonyl group, the chemical shift value of hydrogen is 2.0 whereas for carbon it is 30, thus it is 15 times more than proton-NMR value. Similarly, for methylene carbon, value in carbon-NMR is 37 and in proton-NMR for methylene hydrogen is 2.7. Thus, 14 times large. For methyl carbon not attached to carbonyl, the value of chemical shift for carbon is 8 and in proton NMR, value of hydrogen is 1, thus in this case it does not predict well, as it is 8 times more.

Proton NMR spectrum for Butan-2-one