Organic Chemistry Ch 11. Spectroscopy

Question 1

IR spectroscopy

Answer 1

Measures absorption of infrared light, which causes molecular vibration (stretching, bending, twisting, and folding), the vibration of a bond must change the bond dipole moment to appear on IR

Question 2

IR spectra plot

Answer 2

Percent transmittance vs wavenumber (1/lambda), normal range 4000 to 400 cm^-1, finger print region is between 1500 and 400 cm^-1 that can be used by experts to identify a compound

Question 3

O-H peak IR

Answer 3

Broad peak around 3300 cm^-1, molecules with O-H include alcohols, water, and carboxylic acids, the carboxylic acid O-H peak will be shifted around 3000 cm^-1

Question 4

N-H peak IR

Answer 4

Sharp peak around 3300 cm^-1, molecules with N-H include some amines, amines, and amides

Question 5

C=O peak IR

Answer 5

Sharp peak around 1750 cm^-1, molecules with C=O includes aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides

Question 6

UV spectroscopy

Answer 6

Measures absorption of ultraviolet light which causes movement of electrons between molecular orbitals

Question 7

UV spectra

Answer 7

Plotted as percent transmittance or absorbance vs wavelength, a molecule must have a small enough energy difference between its highest occupied molecules orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) to permit an electron to move from one orbital to the other, the smaller the difference between HOMO and LUMO the longer the wavelengths a molecule can absorb, conjugation shifts the spectrum to higher maximum wavelengths

Question 8

HOMO

Answer 8

Highest occupied molecular orbital

Question 9

LUMO

Answer 9

Lowest unoccupied molecular orbital

Question 10

Conjugation

Answer 10

Occurs in molecules with unhybridized p-orbitals, shifts the absorption spectrum to higher maximum wavelengths (lower frequencies)

Question 11

Nuclear magnetic resonance spectroscopy

Answer 11

NMR - measures alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself, useful for determining the structure of a compound, including functional groups, nuclei may be in the lower energy (alpha state) or higher energy (beta state), radio frequency pulses push the nucleus from the alpha-state to the beta-state and these frequencies can be measured

Question 12

Magnetic resonance imaging

Answer 12

Medical application of NMR spectroscopy

Question 13

NMR plots

Answer 13

Generally plotted as frequency vs absorption of energy, standardized by using chemical shift(delta), measured in parts per million of spectrophotometer frequency, calibrated using tetramethylsilane (TMs) at a delta = 0, higher chemical shifts are downfield (to the left) and lower chemical shifts are upfield (to the right)

Question 14

Proton NMR

Answer 14

Each unique group of protons has its own peak

Question 15

Integration

Answer 15

Area under the curve, proportional to the number of protons contained under the peak in 1H NMR

Question 16

Deshielding

Answer 16

Deshielding of protons occurs when 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 17

Spin-spin coupling

Answer 17

aka splitting, when hydrogen are on adjacent atoms, they interfere with each others magnetic environment, a protons (or group of protons) peak is split into n+1 sub peaks where n is the number of protons that are three bonds away from the proton of interest, splitter patterns include doublets, triplets, and multiplets

Question 18

Protons on sp3 hybridized carbons NMR

Answer 18

Usually in the 0 to 3 ppm range (but higher if EWG are present)

Question 19

Protons on sp2 hybridized carbons NMR

Answer 19

Usually in the 4.6 to 6 ppm range

Question 20

Protons on sp hybridized carbons NMR

Answer 20

Usually between 2 and 3 ppm

Question 21

Aldehyde hydrogens NMR

Answer 21

Tends to appear between 9 and 10 ppm

Question 22

Carboxylic acid hydrogens NMR

Answer 22

Tends to appear between 10.5 and 12 ppm

Question 23

Aromatic hydrogens NMR

Answer 23

Tends to appear between 6 and 8.5 ppm