13C NMR shifts Flashcards

1
Q

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

A

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

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2
Q

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

A

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

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

What is the chemical shift for alkenes and why?

A

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

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6
Q

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

A

lower

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7
Q

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

A

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

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8
Q

What is the chemical shift on 13C NMR for ethers

A

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

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9
Q

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

A

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

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10
Q

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

A

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

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11
Q

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

A

~25
There are subtle inductive effects from other carbons

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12
Q

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

A

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

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