Carbon 13 NMR Spectroscopy

Question 1

What is the C NMR spectra range for alkanes?

Answer 1

0-50 ppm

Question 2

How does branching affect the chemical shift of alkanes?

Answer 2

Branching increases the chemical shift of alkanes.

Question 3

What does the number of signals in a C-NMR spectrum indicate?

Answer 3

The number of chemically unique carbons in a molecule.

Question 4

What does the location (chemical shift) of a peak in a C-NMR spectrum indicate?

Answer 4

The type of functional group.

Question 5

What is the chemical shift range of a substituted alkyl (a carbon bonded to a heteroatom)?

Answer 5

40-80 ppm

Question 6

What is the chemical shift range of alkenes?

Answer 6

90-140 ppm

Question 7

Which has a lower chemical shift, a terminal alkene or an internal alkene carbon? What are the ranges of each?

Answer 7

Terminal < 120 ppm

Internal = 120-140 ppm

Question 8

Is it possible to determine cis/trans configuration of an alkene from a C-NMR spectrum?

Answer 8

Perhaps from alkyl shifts, but it is difficult (basically, no).

Question 9

What is the C-NMR spectrum range for an aromatic system?

Answer 9

100-150 ppm

Question 10

How many unique carbons will a mono-substituted benzene ring have?

Answer 10

4

Question 11

How many unique carbons will a 1,2-disubstituted benzene ring have?

Answer 11

6

Question 12

How many unique carbons will a 1,3-disubstituted benzene ring have?

Answer 12

6

Question 13

How many unique carbons will a 1,4-disubstituted benzene ring have?

Answer 13

4

Question 14

What type of substitution on an aromatic ring results in shielding (and, therefore, lower chemical shift) in the ortho and para positions?

Answer 14

An EDG

Question 15

What type of substitution on an aromatic ring results in deshielding (and, therefore, higher chemical shift) in the ortho and para positions?

Answer 15

An EWG

Question 16

What is the chemical shift range for carbonyl carbons?

Answer 16

150-250 ppm

Question 17

Where do benzylic carbons typically appear on a C-NMR spectrum?

Answer 17

129 ppm

Question 18

Why do carbonyls have a large chemical shift?

Answer 18

Their strong dipole results in significant deshielding.

Question 19

How does conjugation affect the chemical shift of a carbonyl carbon?

Answer 19

Conjugation lowers the chemical shift, because there is less of a positive charge on the carbonyl carbon (the charge is distributed).

Question 20

What is the chemical shift range for aldehydes and ketones?

Answer 20

200-250 ppm

Question 21

What is the chemical shift range for acids, esters, and amides?

Answer 21

150-200 ppm

Question 22

What is the affect on the chemical shift of an amide carbonyl carbon of increasing the number of alkyl groups attached to the nitrogen?

Answer 22

Chemical shift decreases (because of electron donating of alkyl groups)

Question 23

What is the chemical shift of an amide carbon bonded to a primary amine?

Answer 23

178 ppm

Question 24

What is the chemical shift of an amide carbon bonded to a secondary amine?

Answer 24

172 ppm

Question 25

What is the chemical shift of an amide carbon bonded to a tertiary amine?

Answer 25

170 ppm