Organic Chemistry Chapter 11: Spectroscopy

Question 1

Infared spectroscopy measures

Answer 1

absorption of infared light, which causes molecular vibration (stretching, bending, twisting and folding)

Question 2

IR spectra are generally plotted as:

Answer 2

percent transmittance vs. wavenumber (1/wavelength)

Question 3

The normal range of spectrum is:

Answer 3

4000 to 400 cm-1

Question 4

What is the fingerprint region?

Answer 4

between 1500 and 400 cm-1. It contains a number of peaks that can be used by experts to identify a compound.

Question 5

What needs to happen to appear on an IR spectrum?

Answer 5

Vibration of a bond must change the bond dipole moment. Certain bonds have characteristic absorption frequencies, which allow us to infer the presence or absence of particular functional groups.

Question 6

O-H peak

Answer 6

A broad peak around 3300 cm-1

The caboxylic acid O-H peak will be around 3000 cm-1

Question 7

N-H peak

Answer 7

a sharp peak around 3300 cm-1

Question 8

C=0 peak

Answer 8

sharp peak around 1750 cm-1

Question 9

UV spectroscopy

Answer 9

measures absorption of ultraviolet light, which causes movement of electrons between molecular orbitals.

Question 10

UV spectra are generally plotted as:

Answer 10

percent transmittance or absorbance vs. wavelength.

Question 11

To appear on a UV spectrum, a molecule must have a small enough energy difference between:

Answer 11

highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) to permit an electron to move from one orbital to another.

Question 12

The smaller the difference between HOMO and LUMO –

Answer 12

The longer the wavelengths a molecule can absorb.

Question 13

Conjugation occurs in:

Answer 13

molecules with unhybridized p-orbitals. Conjugation shifts the absorption spectrum to higher maximum wavelengths (lower frequencies)

Question 14

Nuclear magnetic resonance spectroscopy

Answer 14

measures alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself. It is useful for identifying the structure of a compound.

Question 15

What are the two states of nuclei?

Answer 15

lower energy alpha-state or higher energy beta-state. radiofrequency pylses push the nucleus from the alpha-state to the beta-state and these frequencies can be measured.

Question 16

NMR is generally plotted as

Answer 16

frequency versus absorption of energy. Standardized using chemical shift, measured in parts per million of spectrophotometer frequency.

Question 17

NMR spectra are calibrated using

Answer 17

tetramethylsilane (TMS), which has a chemical shift of 0 ppm

Question 18

Integration area under curve

Answer 18

proportional to the number of protons contained under the peak.

Question 19

Deshielding of protons occurs when:

Answer 19

electron-withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by the magnetic field. Deshielding moves a peak further downfield.

Question 20

Spin-spin coupling (splitting)

Answer 20

when hydrogens are on adjacent atoms, they interfere with each other’s magnetic environment - peak is split into n+ 1 subpeaks, where n is the number of protons that are three bonds away from the proton of interest.

Question 21

Protons of on sp3 hybridized carbons appear

Answer 21

usually in the 0 to 3 ppm range

Question 22

sp2 hybridized protons appear

Answer 22

4.6 to 6.0 ppm range

Question 23

sp hybridized protons appear

Answer 23

2.0 to 3.0 ppm range

Question 24

aldehydic hydrogens appear

Answer 24

9 to 10 ppm range

Question 25

CA hydrogens appear

Answer 25

10.5 and 12 ppm range

Question 26

Aromatic hydrogen appear

Answer 26

6.0 and 8.5 ppm range