UV Spectroscopy

Question 1

Unsaturation Index (U)

Answer 1

Count of Hydrogen molecules needed. U=(2c+2-h+n)/2

Question 2

Molecular Spectroscopy

Answer 2

Technique used to determine a compound’s structure. Measure radiation absorbed and emitted by wavelength.

Question 3

UV Spectroscopy

Answer 3

Gives information on the nature of conjugated pi-electron system. (200-400nm)

Question 4

UV Spectrum

Answer 4

Wavelength maxima vs. absorptivity.

Question 5

Chromophores

Answer 5

Moiety in molecule that has electrons responsible for radiation absorption (Highest occupied orbital to LOMO).

Question 6

Alkanes

Answer 6

Single bonds, no atoms without unshared electron pair, VERY HIGH ENERGY OUTSIDE UV RANGE

Question 7

Alcohols, Ether, Amines, Sulfur

Answer 7

175-200nm (Alcohol & Amine) 200-220nm (Thiols & Sulfides), much overlap in UV absorption.

Question 8

Alkenes & Alkynes

Answer 8

Unsaturated molecules, 175nm Alkenes, 170nm Alkynes. Sensitive to substituents.

Question 9

Carbonyl

Answer 9

N-pi* (280nm), pi-pi* (188nm), sensitive to substituents.

Question 10

Effect of Conjugation

Answer 10

Increasing conjugation of double bonds in a molecule increases the wavelength and the intensity of light.

Question 11

Hyperconjugation

Answer 11

Overlap of C-H sigma orbital with pi system.

Question 12

Molar absorptivity

Answer 12

Measure of strength the sample absorbs light at a particular wavelength.

Question 13

Ketones and Aldehydes

Answer 13

Conjugated double bonds show pi-pi star absorption at 210nm base peak. Conjugation increases this by 30nm, substituent increases by 10nm

N-pi star is very weak and often overlooked. Base peak 280-300nm, conjugation increases this by 30nm.