13C NMR shifts

Question 1

What factor increases the chemical shift on the Carbon 13 NMR spectrum

Answer 1

The higher the electron deficiency of the Carbon is, the higher the chemical shift

Question 2

Which functional group has the highest chemical shift in 13C NMR and why?

Answer 2

Aldehyde & Ketons (~200)
The electrons have the ability to delocalise from the double bond onto the oxygen, increasing the electron deficiency of the carbon

Question 3

What is the chemical shift on 13C NMR and why?
Out of ester and acids OR amides

Answer 3

Both ~165
There is delocalisation but OR and NR group can have an electron donating effect, so less electron deficient than ketones etc

Question 4

What is the ppm value of aromatics and why in a 13C spectra?

Answer 4

~125
Carbons on Benzene have lost their electron density due to negative ring above and below, creating a degree of positivity

Question 5

What is the chemical shift for alkenes and why?

Answer 5

~115
There is electron density above and below due to the double bond, hence making the carbons electron deficient

Question 6

Is the chemical shift higher or lower in alkynes compared to alkenes

Answer 6

lower

Question 7

Chlorine is highly electronegative, hence why would a chlorinated carbon not always show a higher chemical shift

Answer 7

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

Question 8

What is the chemical shift on 13C NMR for ethers

Answer 8

~70
The oxygen will pull electron density towards itself, decreasing the electron density of carbon

Question 9

What is the chemical shift of an acyl group on 13C NMR and why?

Answer 9

~30 ppm
Electrons can delocalise onto the oxygen, withdrawing electron density from the carbon

Question 10

What is the chemical shift using 13C NRM of a carbon substituent off an aromatic group

Answer 10

~25 ppm
The Benzene ring will withdraw electron density away from the carbon, increasing chemical shift

Question 11

What is the chemical shift on 13C NMR for a internal methylene on an alphatic carbon chain

Answer 11

~25
There are subtle inductive effects from other carbons

Question 12

What is the chemical shift on a 13C NMR for a terminal methyl group

Answer 12

~12
The carbon will pull electron density towards itself, decreasing the chemical shift