Question 1

Which of the following functional groups is most likely to show a strong, broad absorption between $$3200$$ and $$3600 cm$^{-1} in an IR $$spectrum?

Accepted Answer

Alcohol

Question 2

A compound shows a strong absorption at $$1715 cm$^{-1}$$ and a pair of peaks at $$2700$$ and $$2800 cm$^{-1} in $$its IR spectrum. Which functional group is most likely present?

Accepted Answer

Aldehyde

Question 3

Which IR absorption is characteristic of a terminal alkyne but not an internal alkyne?

Accepted Answer

Csp-H stretch at $$3300 cm$^{-1}$$

Question 4

An unknown compound with formula $$C_8H_{16}$$ shows a strong absorption just above $$3000 cm$^{-1}$$ and a peak at $$1600$$–$$1680 cm$^{-1}. $$What is the most likely structure?

Accepted Answer

Alkene

Question 5

Which of the following statements about IR spectroscopy is TRUE?

Accepted Answer

Only bonds with a change in dipole moment during vibration absorb IR radiation.

Question 6

A sample shows a very broad absorption from $$2500 to$$ $$3600 cm$^{-1}$$ and a strong peak at $$1700$$–$$1725 cm$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Carboxylic acid

Question 7

Which equation correctly relates the frequency $$
u of $$electromagnetic radiation to its wavelength $$\lambda$$ and the speed of light $$c$$?

Accepted Answer

$$
u = \dfrac{c}{\lambda}$$

Question 8

A compound shows two absorptions above $$3000 cm$^{-1} in $$its IR spectrum. Which type of amine is most likely present?

Accepted Answer

Primary amine ($$1^\circ)$$

Question 9

Which IR region is most diagnostic for the C≡N stretch in nitriles?

Accepted Answer

$$2100$$–$$2200 cm$^{-1}$$

Question 10

A student analyzes an IR spectrum and observes a strong absorption at $$1738 cm$^{-1}$$ and no broad O-H stretch. Which functional group is most likely present?

Accepted Answer

Ester

Question 11

Which of the following compounds would show both saturated C-H stretches below $$3000 cm$^{-1}$$ and unsaturated C-H stretches above $$3000 cm$^{-1} in $$its IR spectrum?

Accepted Answer

1-Heptene

Question 12

A molecule shows a strong absorption at $$2100$$–$$2200 cm$^{-1}$$ but no absorption at $$3300 cm$^{-1}. $$What does this indicate?

Accepted Answer

Internal alkyne

Question 13

Which functional group is indicated by a strong absorption at $$1710$$–$$1750 cm$^{-1}$$ and no additional peaks at $$2700$$–$$2800 cm$^{-1}$$?

Accepted Answer

Ketone

Question 14

Which of the following is NOT a typical IR absorption for aromatic compounds?

Accepted Answer

C=O stretch at $$1710$$–$$1750 cm$^{-1}$$

Question 15

A sample suspected to be an ether shows a strong absorption at $$1470$$–$$1350 cm$^{-1}$$ and saturated C-H stretches below $$3000 cm$^{-1}. $$What additional feature should you check to rule out alcohol contamination?

Accepted Answer

Broad absorption above $$3000 cm$^{-1}$$

Question 16

Which of the following best explains why IR spectroscopy can distinguish between cis- and trans-alkenes?

Accepted Answer

Trans-alkenes have a symmetric C=C stretch that is IR inactive.

Question 17

A molecule with formula $$C_8H_{14}$$ shows a strong absorption at $$2100$$–$$2200 cm$^{-1}$$ and no absorption at $$3300 cm$^{-1}. $$Which structure is most likely?

Accepted Answer

Internal alkyne

Question 18

Which of the following is the correct formula for the energy of a photon in terms of frequency $$
u$$ and Planck's constant $$h$$?

Accepted Answer

$$E = h 
u$$

Question 19

A compound shows a strong absorption at $$1470$$–$$1350 cm$^{-1}$$ and two absorptions above $$3000 cm$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Primary amine

Infrared (IR) Spectroscopy: Principles and Applications in Organic Chemistry

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