13C NMR shifts Flashcards
What factor increases the chemical shift on the Carbon 13 NMR spectrum
The higher the electron deficiency of the Carbon is, the higher the chemical shift
Which functional group has the highest chemical shift in 13C NMR and why?
Aldehyde & Ketons (~200)
The electrons have the ability to delocalise from the double bond onto the oxygen, increasing the electron deficiency of the carbon
What is the chemical shift on 13C NMR and why?
Out of ester and acids OR amides
Both ~165
There is delocalisation but OR and NR group can have an electron donating effect, so less electron deficient than ketones etc
What is the ppm value of aromatics and why in a 13C spectra?
~125
Carbons on Benzene have lost their electron density due to negative ring above and below, creating a degree of positivity
What is the chemical shift for alkenes and why?
~115
There is electron density above and below due to the double bond, hence making the carbons electron deficient
Is the chemical shift higher or lower in alkynes compared to alkenes
lower
Chlorine is highly electronegative, hence why would a chlorinated carbon not always show a higher chemical shift
Chlorine has a lone pair which it can push electron density towards the carbon BUT due to chlorine being relatively big compared to carbon, it cannot cause overlap between orbitals
What is the chemical shift on 13C NMR for ethers
~70
The oxygen will pull electron density towards itself, decreasing the electron density of carbon
What is the chemical shift of an acyl group on 13C NMR and why?
~30 ppm
Electrons can delocalise onto the oxygen, withdrawing electron density from the carbon
What is the chemical shift using 13C NRM of a carbon substituent off an aromatic group
~25 ppm
The Benzene ring will withdraw electron density away from the carbon, increasing chemical shift
What is the chemical shift on 13C NMR for a internal methylene on an alphatic carbon chain
~25
There are subtle inductive effects from other carbons
What is the chemical shift on a 13C NMR for a terminal methyl group
~12
The carbon will pull electron density towards itself, decreasing the chemical shift