11.2 Ultraviolet Spectroscopy

Question 1

Note: you will not have to INTERPRET UV spectroscopy data on the MCAT, just understand how it works and when it is used

Question 2

how are UV spectra obtained?

Answer 2

UV light is passed through a sample (usually dissolved in an inert, non absorbing solvent), and recording the absorbance

the absorbance is then plotted vs wavelength

Question 3

intert meaning

Answer 3

non chemically reactive

Question 4

the absorbance is caused by…

Answer 4

electronic transitions between orbitals

Question 5

wavelength of maximum absorbance

Answer 5

tells is the extent of conjugation within conjugated systems

the more conjugated the compound = the lower the energy of the transition = greater wavelength of maximum absorbance

Question 6

UV spectroscopy is most useful for…

Answer 6

studying compounds containing double bonds or heteroatom with lone pairs that create conjugated systems

Question 7

heteroatom

Answer 7

any atom that is no carbon or hydrogen

Question 8

how does UV spectroscopy work (electron transitions)?

Answer 8

molecule with pi electrons or nonbonding electrons can be edited by UV light to higher-energy anti bonding orbitals

molecules with a lower energy gap between HOMO and LUMO are more easily excited and can absorb longer wavelengths with lower energy

Question 9

HOMO

Answer 9

higher occupied molecular orbitals

Question 10

LUMO

Answer 10

lowest unoccupied molecular orbital

Question 11

conjugated molecules

Answer 11

molecules with unhybridized p orbitals

a system of connected p orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability

It is conventionally represented as having alternating single and multiple bonds

Question 12

conjugation results in…

Answer 12

higher maximum wavelengths

ex. benzene has 3 broad absorbances at 180, 200, and 255nm