Organic Chemistry: Spectroscopy

Question 1

Spectroscopy

Answer 1

The process of measuring energy differences by determining the frequencies absorbed by the molecules.

Question 2

What does IR spectroscopy measure?

Answer 2

The stretching, bending, and rotation of the bonds that changes the dipole moment (so symmettrical molecules and molecules of the same electronegativity will not exhibit absorption). Percent transmittance plotted versus frequency. The wave number is equal to 1/λ. %T = 1/A

Question 3

What are some important IR functional groups to know?

Answer 3

Alcohols (broad peaks): 3100 - 3500 1/cm
Ketones (sharp peaks): 1700 - 1750 1/cm
Amines (sharp peaks): 3100 - 3500 1/cm

Question 4

Nuclear Magnetic Resonance Spectroscopy

Answer 4

Measures the absorption related to the excitation of the aligned nuclei due to electromagnetic radiation. Plot frequency versus absorption of energy during resonance, where frequency decreases towards the right and the plotted magnetic field increases towards the right.

Question 5

1H-NMR

Answer 5

Proton NMR determines the relative number of protons and their relative chemical environments while showing how many adjacent protons there are and certain functional groups. Deshielding pushes the plot towards a higher ppm, while shielding pushes the plot to a lower ppm. Each peak represents a group of equivalent protons where the relative area of each peak is proportional to the ratio of protons producing that peak.

Question 6

13C-NMR

Answer 6

This requires a larger sample than for 1H-NMR and carbon atom coupling is not observed (although, coupling between carbon atoms and the protons attached to them can be seen).

Question 7

What types of information does NMR provide?

Answer 7

NMR can provide the number of nonequivalent nuclei (peak number), magnetic environment of nucleus (chemical shift), relative numbers of nuclei (peak areas), and the number of neighboring nuclei (splitting pattern).

Question 8

Ultraviolet Spectroscopy

Answer 8

Absorbance versus wavelength after passing ultraviolet light through the sample, useful in studying double bonds. A = εbc: Beer’s Law

Question 9

Mass Spectrometry

Answer 9

A destructive technique that uses a high-speed beam of electrons to ionize a sample, a particle accelerator, a magnetic field for deflection, and a detector that measures the number of particles of each mass. m/e ratio (mass to charge ratio). Only cations appear on the spectrum.