11. Spec

Question 1

bend vs stretch

Answer 1

going inwards vs stretching outwards

Question 2

wavelength for alc vs CAs vs carbonyl vs N-H bonds vs sp3 C-H bonds

Answer 2

broad peak at 3300 cm-1 vs broad peak at 3000 cm-1 vs 1700 cm-1 vs same as alc (3300 cm-1) but sharp instead of broad vs 3000 cm-1

Question 3

Range of wavelengths for fingerprint region. What’s the fingerprint region?

Answer 3

400-1500 cm-1. Region where specific vibrations = unique to specific molec –> determines identity of complex molec

Question 4

What does IR spec measure?

Answer 4

molec vibrations such as bending and stretching, if there are vibrations —> dipole moments; most useful to detect diff functional groups, double bonds and triple bonds

Question 5

How is IR spectra plotted?

Answer 5

% transmittance vs wavenumber; T = amount of light passing thru molec

Question 6

How are absorbance (A) and transmittance (T) related?

Answer 6

low transmittance => high absorption –> max absorption = at bottom of valleys

Question 7

What’s UV spec?

Answer 7

UV light passing thru a sample that’s been dissolved in inert, nonabsorbing solvent and measuring absorbance; when light passes thru sample in UV spec, e- = excited –> transition to higher energy lvl and then back down to orig lvl –> causes variable absorptions –> variable extinction coeff

Question 8

UV spec = most useful for identifying what type of cmpds?

Answer 8

double bonds or heteroatoms (non-C or H atoms) w/ lone pairs => conjugated systems

Conjugated systems can also be: Alternating single bonds and multiple bonds; this allows pi e- to delocalized throughout unhybridized p orbitals

Question 9

HOMO vs LUMO

Answer 9

highest occupied molecular orbital vs lowest
unoccupied molecular orbital. The smaller the difference in energy between the two, the longer the wavelengths that can be absorbed by the molecule

When e- get excited and absorbs a photon, it moves from HOMO to LUMO

Question 10

What does position of peak (upfield and downfield) mean in H+ NMR?

Answer 10

de/shielding effects and reflects chemical environment of p+ (DESHIELD IS DOWNFIELD but deshielding = further down ppm)

Question 11

What does area of peak mean in H+ NMR?

Answer 11

ratio of H+

Question 12

What do # of peaks mean? Coupling constant?

Answer 12

neighboring H’s (N+1 rule: # of peaks = neighboring H + 1); magnitude of splitting is called the coupling constant, J (units in Hz)

Question 13

How to find Hydrogen Deficiency Index (HDI)?

Answer 13

1. find max # of H can be paired w/ C’s and subtract that # with actual # of H in formula
2. Put the missing # of H into a pair value
3. You can use the formula: 1/2(2C + 2 + N - H - X)

Question 14

How to know if peaks represent CH2 or CH3?

Answer 14

Divide integration value by the lowest, the quotient must be a whole number (multiply by a factor if necessary)

Question 15

Memorize shifts for H NMR

Answer 15

CH3: 0-3ppm, alkynes: 2-3, alkenes: 4.6-6; aromatics: 6-8.5, ald: 9-10, CA: 10-12, -OH: 2-5

Question 16

Which HDI values tell you what?

Answer 16

HDI = 1 --> double bond
HDI = 0 --> no double bond or ring
HDI = 4 --> benzene ring
HDI = 5 --> benzene + 2x bond

Question 17

What kinds of isotopes do you need to see a magnetic moment in NMR?

Answer 17

Even numbered isotopes with odd atomic numbers or odd numbered isotopes
No magnetic moment —> no resonance detected

Question 18

Higher order bonds absorb higher frequencies on IR spec

Answer 18

Yep

Question 19

Wave number is directly proportional to? vs Inversely proportional to?

Answer 19

Frequency vs wavelength

Question 20

What do HNMR resonances mean?

Answer 20

basically means how many H’s will show up on HNMR. if H’s = equivalent –> count as a single resonance b/c there’s free rotation and it’ll show as one on NMR; if H’s = nonequivalent (ie. they have pi character) –> count as multiple resonances b/c each H has its own peak on NMR (pg 353 on princeton)