Analytical

Question 1

How do you calculate no. atoms

Answer 1

no. atoms = (decimal percent x Mr) / atomic mass

Question 2

How do you work the no. DBE

Answer 2

Fill out no. H available

No. addition H needed for saturated compound
divided by 2

Question 3

What does a mass spec show

Answer 3

Fragmentation

M+ = Mr molecule

Question 4

What is the equation of reduced mass

Answer 4

μ = (Ma x Mb) / (Ma + Mb)

Question 5

What is the Beer-Lambert law

Answer 5

A = εlc

Question 6

What are the types of transition

Answer 6

π –> π* & n –> π*
ΔE occurs inside UV/vis range

σ –> σ* & n –> σ*
ΔE usually large
often occurs outside UV/vis range

Question 7

What do UV range transitions show

Answer 7

Due to HOMO/LUMO energy differences

π –> π* generally more intense

Question 8

What does n –> σ* transitions show

Answer 8

No many molecules with transitions in UV

Still rather high in energy
some compounds show peaks in 𝜆 = 150 - 250nm

Question 9

What is the effect of wavenumber

Answer 9

Stronger bond = higher ṽ

Heavier atom = smaller ṽ

Question 10

How is the type of molecule determined

Answer 10

Non-linear molecules = 3N - 6 vibrations

Linear molecule = 3N - 5 vibrations

Question 11

What are the typical peaks of O-H & N-H

Answer 11

~3300 cm-1

Alcohol peak broad w/ rounded tip

2° amine peak broad w/ 1 sharp spike
1° amine peak broad w/ 2 sharp spikes
3° amine have no signal

Question 12

What are the typical peaks of ketones, aldehydes & CA

Answer 12

~1710 cm-1

CA has O-H peak
Aldehyde has C-H peak ~2700-2800 cm-1

Question 13

What are the typical peaks of C=O

Answer 13

C=O conjugated w/ C=C ~1680 cm-1
lowers stretching frequency

Amide C=O ~1640-1680 cm-1

Ester C=O ~1730-1740 cm-1

Question 14

What are the typical peaks of CN

Answer 14

C-N ~1200 cm-1

C=N ~1660 cm-1

C≡N just above 2200 cm-1
C≡C just below 2200 cm-1

Question 15

What is the affect of shielding

Answer 15

Protons shielded different amounts
depending on chemical environment

More shielded => absorbs at higher frequency
Less shielded => absorbs lower frequency

Question 16

What is the role of the environment in NMR

Answer 16

H bonded to electronegative atom
H deshielded = larger shift value

H bonded to neutral/electropositive
H shielded = lower shift value

Question 17

What does the NMR signal show

Answer 17

No. signals
how many kinds of proton

Location
how shielded/deshielded

Intensity
no. protons of that type

Splitting
no. protons on adjacent C

Question 18

What is the internal reference

Answer 18

TMS
tetramethylsilane

Question 19

What are the NMR signals of O-H & N-H

Answer 19

N-H ~δ3.5
O-H ~δ4.5

Question 20

What is the role of D2O

Answer 20

Verify if peak O-H/N-H

D2O exchanges protons
2nd NMR peaks absent/less intense

Question 21

What is spin-spin splitting

Answer 21

Non-equivalent proton on adjacent C
have magnetic field
may align with/expose external field

Question 22

What is the stereochemical non-equivalence

Answer 22

If either H in CH2
replaced w/ ‘z’
gives stereoisomers
proton non-equivalent
& will split each other

Question 23

What are the typical C NMR shifts

Answer 23

sp2 = 100-220
sp = ~75-95
sp3 = 0-100

electronegativity
increases going left

Question 24

What si the proton spin decoupling

Answer 24

To simplify spectrum

Protons continually irradiated w/ ‘noise’
so rapidly flipping

C nuclei see average
of all possible spin states
∴ each different C gives single unspoilt peak

Question 25

What is off-resonance decoupling

Answer 25

C split only by protons attached directly

C w/ no.protons gives signal w/ n+1 peaks

Question 26

What is a DEPT spectra

Answer 26

Uses differences in relaxation
to identify types of C

CH3 & CH peak up
CH2 peak down