Carbon 13 NMR Spectroscopy Flashcards
What is the C NMR spectra range for alkanes?
0-50 ppm
How does branching affect the chemical shift of alkanes?
Branching increases the chemical shift of alkanes.
What does the number of signals in a C-NMR spectrum indicate?
The number of chemically unique carbons in a molecule.
What does the location (chemical shift) of a peak in a C-NMR spectrum indicate?
The type of functional group.
What is the chemical shift range of a substituted alkyl (a carbon bonded to a heteroatom)?
40-80 ppm
What is the chemical shift range of alkenes?
90-140 ppm
Which has a lower chemical shift, a terminal alkene or an internal alkene carbon? What are the ranges of each?
Terminal < 120 ppm
Internal = 120-140 ppm
Is it possible to determine cis/trans configuration of an alkene from a C-NMR spectrum?
Perhaps from alkyl shifts, but it is difficult (basically, no).
What is the C-NMR spectrum range for an aromatic system?
100-150 ppm
How many unique carbons will a mono-substituted benzene ring have?
4
How many unique carbons will a 1,2-disubstituted benzene ring have?
6
How many unique carbons will a 1,3-disubstituted benzene ring have?
6
How many unique carbons will a 1,4-disubstituted benzene ring have?
4
What type of substitution on an aromatic ring results in shielding (and, therefore, lower chemical shift) in the ortho and para positions?
An EDG
What type of substitution on an aromatic ring results in deshielding (and, therefore, higher chemical shift) in the ortho and para positions?
An EWG
What is the chemical shift range for carbonyl carbons?
150-250 ppm
Where do benzylic carbons typically appear on a C-NMR spectrum?
129 ppm
Why do carbonyls have a large chemical shift?
Their strong dipole results in significant deshielding.
How does conjugation affect the chemical shift of a carbonyl carbon?
Conjugation lowers the chemical shift, because there is less of a positive charge on the carbonyl carbon (the charge is distributed).
What is the chemical shift range for aldehydes and ketones?
200-250 ppm
What is the chemical shift range for acids, esters, and amides?
150-200 ppm
What is the affect on the chemical shift of an amide carbonyl carbon of increasing the number of alkyl groups attached to the nitrogen?
Chemical shift decreases (because of electron donating of alkyl groups)
What is the chemical shift of an amide carbon bonded to a primary amine?
178 ppm
What is the chemical shift of an amide carbon bonded to a secondary amine?
172 ppm
