Question 1

Which of the following statements best describes the chemical shift range (in ppm) typically observed for carbonyl carbons in a $$^{13}\mathrm{C}$$ NMR spectrum?

Accepted Answer

$$160 to$$ $$220$$\ \mathrm{ppm}$

Question 2

A compound shows $$^{13}\mathrm{C}$$ NMR signals at $$20$$, $$40$$, $$120$$, and $$180$$ ppm. Which functional group is most likely present in this compound?

Accepted Answer

Aldehyde or ketone

Question 3

Which of the following $$^{13}\mathrm{C}$$ NMR spectra would best represent a molecule containing both an aromatic ring and a carboxylic acid group?

Accepted Answer

Signals between $$100$$–$$160$$ ppm and a strong signal between $$160$$–$$180$$ ppm

Question 4

A student analyzes a $$^{13}\mathrm{C}$$ NMR spectrum and observes a peak at $$210$$ ppm. Which type of carbon is most likely responsible for this signal?

Accepted Answer

Ketone carbonyl carbon

Question 5

Which of the following molecules would you expect to have the greatest number of distinct $$^{13}\mathrm{C}$$ NMR signals?

Accepted Answer

1,3-Butadiene

Question 6

A compound with the formula $$\mathrm{C}_3\mathrm{H}_6\mathrm{O}$$ shows $$^{13}\mathrm{C}$$ NMR signals at $$30$$ ppm, $$45$$ ppm, and $$210$$ ppm. Which structure is most consistent with this data?

Accepted Answer

Acetone

Question 7

Which of the following best explains why $$^{13}\mathrm{C}$$ NMR spectra typically show fewer peaks than the number of carbon atoms in a molecule?

Accepted Answer

Chemically equivalent carbons produce a single signal

Question 8

A $$^{13}\mathrm{C}$$ NMR spectrum of an unknown compound shows peaks at $$20$$ ppm, $$40$$ ppm, $$130$$ ppm, and $$175$$ ppm. Which of the following compounds is most likely?

Accepted Answer

Ethyl acetate

Question 9

Which of the following statements about $$^{13}\mathrm{C}$$ NMR is correct?

Accepted Answer

The chemical shift depends on the electronic environment of the carbon atom

Question 10

A chemist is analyzing a molecule and observes $$^{13}\mathrm{C}$$ NMR signals at $$15$$ ppm, $$25$$ ppm, $$35$$ ppm, and $$180$$ ppm. Which functional group is most likely present?

Accepted Answer

Carboxylic acid or ester

Question 11

Which of the following best describes the effect of conjugation on the $$^{13}\mathrm{C}$$ NMR chemical shift of a carbonyl carbon?

Accepted Answer

Conjugation decreases the chemical shift, moving it to lower ppm values

Question 12

A $$^{13}\mathrm{C}$$ NMR spectrum shows a single peak at $$77$$ ppm. Which compound is most likely responsible for this signal?

Accepted Answer

Deuterated chloroform ($$\mathrm{CDCl}_3)$$

Question 13

Which of the following is a reason why $$^{13}\mathrm{C}$$ NMR is less sensitive than $$^1\mathrm{H}$$ NMR?

Accepted Answer

$$^{13}\mathrm{C}$$ has a lower natural abundance

Question 14

A molecule with the formula $$\mathrm{C}_6\mathrm{H}_6$$ shows a single $$^{13}\mathrm{C}$$ NMR signal. What does this indicate about the molecule's structure?

Accepted Answer

All carbons are chemically equivalent

Question 15

Which of the following best explains why quaternary carbons (carbons with no attached hydrogens) often appear as weak signals in $$^{13}\mathrm{C}$$ NMR spectra?

Accepted Answer

They relax more slowly and have lower signal intensity

Question 16

A student is given a $$^{13}\mathrm{C}$$ NMR spectrum with peaks at $$20$$ ppm, $$30$$ ppm, $$130$$ ppm, and $$170$$ ppm. Which compound is most consistent with this data?

Accepted Answer

Ethyl benzoate

Question 17

Which of the following would you expect to have a $$^{13}\mathrm{C}$$ NMR signal at the lowest chemical shift (ppm)?

Accepted Answer

A methyl carbon in methane

Question 18

A chemist observes $$^{13}\mathrm{C}$$ NMR signals at $$30$$ ppm, $$70$$ ppm, and $$175$$ ppm. Which functional group is most likely present?

Accepted Answer

Ester

Question 19

Which of the following best describes the trend in $$^{13}\mathrm{C}$$ NMR chemical shifts as the electronegativity of atoms bonded to the carbon increases?

Accepted Answer

Chemical shift increases

Question 20

A $$^{13}\mathrm{C}$$ NMR spectrum of a compound shows four signals at $$15$$ ppm, $$25$$ ppm, $$35$$ ppm, and $$55$$ ppm. Which of the following compounds is most likely?

Accepted Answer

Butanol

Step-by-Step Guidance for 13C NMR Spectrum Interpretation

Guida di studio - Domande di pratica

Study guide - Flashcard