1. IB Chemistry Option 1 Analytical Chemistry
Determination of Structure

2. How Does One Determine the Structure of an Organic Compound?
We must use several Analytical Techniques to determine structure of any Organic Compound.
Why do we need Analytical Techniques?
To identify the structure of a compound
To check the purity of compound
It will help to avoid undesirable side effects of medicines

3. Why more than one technique?
Ans:
- One technique is usually insufficient to provide all the necessary information.
- It may provide limited information, but it may not be possible to accurately predict the structure of the compound.

4. Spectroscopic Methods
Which are the main spectroscopic methods addressed? (In the SL/HL chemistry exam)
1. Infrared Spectroscopy (IR)
2. Mass Spectrometry (Mass Spec)
3. Nuclear Magnetic Resonance Spectroscopy (NMR)

5. IR Spectroscopy (IBO Reference Table 17)

6. The Electromagnetic Spectrum
high
Frequency (n)
low
high
Energy
low
MICRO-
WAVE
X-RAY
ULTRAVIOLET
INFRARED
RADIO
FREQUENCY
Nuclear
magnetic
resonance
Vibrational
infrared
Ultraviolet
Visible
2.5 mm
15 mm
1 m
5 m
200 nm
400 nm
800 nm
BLUE
RED
short
Wavelength (l)
long

7. IR Absorption
IR radiation is of too low an energy to excite electronic transitions
IR spectroscopy measures the absorption of light due to bond stretching or bending
Absorption is limited to vibrational and rotational levels
For liquids and solids, molecular rotation is often limited so the major type of interaction is vibrational
Different types of bonds absorb at different energies (frequencies)

8. Analyzing IR Spectra (1 of 3)
Look for C=O peak ( cm-1)
If C=O check for OH ( cm-1)
indicates carboxylic acid
If C=O check for NH (3500 cm-1)
indicates amide
If C=O check for C-O ( cm-1)
indicates ester
If no OH, NH or C-O then ketone

9. Analyzing IR Spectra (2 of 3)
If no C=O check for OH ( cm-1)
indicates alcohol
If no C=O check for NH (3500 cm-1)
indicates amine
If no C=O & no OH check C-O (1300 cm-1)
indicates ether
Look for C=C ( cm-1) then aromatic

10. Analyzing IR Spectra (3 of 3)
Hydrogen stretching region (3700 to 2700 cm-1)
Triple bond region (2700 to 1850 cm-1)
Double bond region (1950 to 1550 cm-1)
Finger-print region (single bonds) (1500 to 700 cm-1)

11. Interpreting IR Spectra
It is easiest to read an IR spectrum from left to right. Look for OH’s, Carbonyls, and alkene (aromatic ring) or alkanes.
Functional Group
Wavenumber (cm-1)
Appearance
-OH (alcohols)
Strong, broad
-OH (acids)
Moderate, more narrow
-NH
Medium (NH2 shows two peak)
-C=C-H (aromatic, alkene)
Medium to strong
-C-C-H (alkane)
Medium-strong
C=O (all carbonyls)
Strong

12. Qualitative Analysis C B A O C CH3
Acetophenone C B A
A) C=O (1730) B) C=C aromatic (1590) C) C-H aromatic (3050)

13. Identify IR stretches corresponding to the functional Groups in given compound

14. Identify IR stretches corresponding to the functional Groups in given compound

15. Identify IR stretches corresponding to the functional Groups in given compound

16. Identify IR stretches corresponding to the functional Groups in given compound

17. A - CO-OH stretch (3000) B - CH stretch (2800)
C - C=O ester (1757) D - C=O carboxy (1690)
E - C=C aromatic (1608) F - C=C aromatic (1460)

18. Mass Spectrometry

19. MS Principles
Different elements can be uniquely identified by their mass

20. The Mass Spectrum
Plot the mass of ions (m/z) (x-axis) versus the intensity of the signal (roughly corresponding to the number of ions) (y-axis)
Tallest peak is base peak (100%)
Other peaks listed as the % of that peak
Peak that corresponds to the unfragmented radical cation is parent peak or molecular ion (M+)

21. Masses are graphed or tabulated according to their relative abundance.
The Mass Spectrum
Masses are graphed or tabulated according to their relative abundance.

22. Fragmentation of CH3OH CH3OH CH3OH+ CH3OH CH2O=H+ + H CH3OH + CH3 + OH

23. Electron Impact MS of CH3OH
Molecular ion
EI Breaks up Molecules in Predictable Ways

24. FRAGMENTATION PATTERNS
ALKANES & ALKENES
The mass spectra of simple hydrocarbons have peaks at m/z values corresponding to the ions produced by breaking C-C bonds. Peaks can occur at ...
m/z etc CH3+ C2H5+ C3H C4H C5H C6H13+
• the stability of the carbocation formed affects its abundance
• the more stable the cation the higher the peak
• the more alkyl groups attached to the carbocation the more stable it is
most stable tertiary 3° > secondary 2° > primary 1° least stable
alkyl groups are electron releasing and stabilise the cation

25. Mass Spectra of Alkanes
More stable carbocations will be more abundant.

26. Mass Spectra of Alkenes
Resonance-stabilized cations favored.

27. FRAGMENTATION PATTERNS
HALOGENOALKANES
Multiple peaks occur in the molecular ion region due to different halogen isotopes.
There are two peaks for the molecular ion of C2H5Br, one for the molecule containing the isotope 79Br and the other for the one with the 81Br isotope. Because the two isotopes are of similar abundance, the peaks are of similar height.
m/z
Abundance %
molecular ion contains...79Br Br

28. Another MS Example with Bromine

29. Mass Spectrum with Chlorine

30. FRAGMENTATION PATTERNS
ALDEHYDES AND KETONES
Cleavage of bonds next to the carbonyl group (C=O) is a characteristic fragmentation of aldehydes and ketones. A common fragment is carbon monoxide (CO) but as it is a molecule and thus uncharged it will not produce a peak of its own. However, it will produce an m/z drop of 28 somewhere in the spectrum.
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group
the more stable the acylium ion RCO+, the more abundant it will be and
the more abundant the species the taller its peak in the mass spectrum

31. FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group. O
CH3 C C4H9
MOLECULAR ION
has m/z = 100
• +

32. FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group. O
CH3 C C4H9
MOLECULAR ION
has m/z = 100
• + O
C4H9 C+
CH3•
Breaking the bond between the methyl group and the carbonyl group produces two possible ions, depending on how the bond breaks.
Two peaks at m/z values 15 and 85 will appear in the mass spectrum.
m/z = 85 O
C4H C•
CH3+
m/z = 15

33. FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group. O
CH3 C C4H9
MOLECULAR ION
has m/z = 100
• + O
CH3 C+
Breaking the bond between the butyl group and the carbonyl group produces two further ions, depending on how the bond breaks.
Two peaks at m/z values 43 and 57 will appear in the mass spectrum.
C4H9•
m/z = 43 O
CH3 C•
C4H9+
m/z = 57

34. FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group. O
CH3 C C4H9
Example;
MOLECULAR ION
has m/z = 100
• + O
C4H9 C+ O
CH3 C+
CH3•
C4H9•
m/z = 85
m/z = 43 O
C4H C• O
CH3 C•
CH3+
C4H9+
m/z = 15
m/z = 57
A further peak occurs at m/z = 72 (100-28) due to loss of CO

35. Mass Spectra of Alcohols
Alcohols usually lose a water molecule.
M+ may not be visible.

36. NMR Spectroscopy (IBO Reference Table 19)

37. Principles of NMR
Measures nuclear magnetism or changes in nuclear magnetism in a molecule
NMR spectroscopy measures the absorption of radio waves due to changes in nuclear spin orientation
NMR only occurs when a sample is in a strong magnetic field
Different nuclei absorb at different energies (frequencies)

38. Chemical Shifts Key to the utility of NMR in chemistry
Different 1H in different molecules exhibit different absorption frequencies
Arise from the electron cloud effects of nearby atoms or bonds, which act as little magnets to shift absorption n up or down
Mostly affected by electronegativity of neighbouring atoms or groups

39. Spin-Spin Coupling
Many 1H NMR spectra exhibit peak splitting (doublets, triplets, quartets)
This splitting arises from adjacent hydrogens (protons) which cause the absorption frequencies of the observed 1H to jump to different levels
NOT NEEDED FOR SL CHEM EXAM!

40. Typical 1H NMR Spectrum
Absorbance

41. NMR Spectrum of Phenylacetone
NOTICE THAT EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT PLACE ………
YOU CAN TELL HOW MANY DIFFERENT TYPES OF HYDROGEN THERE ARE!

42. Benzyl Acetate 55 : 22 : 33 = 5 : 2 : 3 integral line
The integral line rises an amount proportional to the number of H in each peak
integral
line
55 : 22 : = : 2 : 3
simplest ratio
of the heights

43. PEAKS ARE MEASURED RELATIVE TO TMS
Rather than measure the exact resonance position of a
peak, we measure how far downfield it is shifted from TMS.
reference compound
tetramethylsilane
“TMS”
Highly shielded
protons appear
way upfield.
TMS
Chemists originally
thought no other
compound would
come at a higher
field than TMS.
shift in Hz
downfield n

44. THE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field
instruments (300 MHz, 500 MHz) than they are in the
lower field instruments (60 MHz, 100 MHz).
We can adjust the shift to a field-independent value,
the “chemical shift” in the following way:
parts per
million
shift in Hz
chemical
shift
= d =
= ppm
spectrometer frequency in MHz
This division gives a number independent
of the instrument used.
A particular proton in a given molecule will always come
at the same chemical shift (constant value).

45. NMR Correlation Chart d (ppm)
-OH
-NH
DOWNFIELD
UPFIELD
DESHIELDED
SHIELDED
CHCl3 ,
TMS
d (ppm) 12 11 10 9 8 7 6 5 4 3 2 1 H
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2-C-
C-CH-C
RCOOH
RCHO
C=C C
C-CH2-C
C-CH3 O
Ranges can be defined for different general types of protons.
This chart is general, the next slide is more definite.

46. R-CH3 0.7 - 1.3 R-N-C-H 2.2 - 2.9 R-C=C-H R-CH2-R 1.2 - 1.4 4.5 - 6.5
APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR SELECTED TYPES OF PROTONS
R-CH
R-N-C-H
R-C=C-H
R-CH2-R
R-S-C-H
R3CH
I-C-H H
R-C=C-C-H
Br-C-H O
Cl-C-H
R-C-C-H O O
RO-C-H
R-C-N-H
RO-C-C-H
HO-C-H O O O
R-C-H
HO-C-C-H
R-C-O-C-H O
N C-C-H
O2N-C-H
R-C-O-H
R-C C-C-H
F-C-H
C-H
R-N-H Ar-N-H R-S-H
R-O-H Ar-O-H
R-C C-H

47. Characteristic Chemical Shifts

48. ALSO – You have IBO Reference Table 19!
YOU DO NOT NEED TO MEMORIZE THE PREVIOUS CHART!
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES OF HYDROGENS COME IN SELECTED AREAS OF THE NMR CHART
ALSO – You have IBO Reference Table 19!
C-H where C is attached to an
electronega-tive atom
CH on C
next to
pi bonds
acid
COOH
aldehyde
CHO
benzene CH
alkene
=C-H
aliphatic
C-H
X=C-C-H
X-C-H 12 10 9 7 6 4 3 2
MOST SPECTRA CAN BE INTERPRETED WITH
A KNOWLEDGE OF WHAT IS SHOWN HERE

49. Some Basic Comparisons of Spectroscopic Techniques

50. NMR vs. IR NMR has narrower peaks relative to IR
NMR yields far more information than IR
NMR allows you to collect data on solids & liquids but NOT gases
NMR is more quantitative than IR or UV
NMR samples are easier to prepare
NMR is much less sensitive than IR or UV
NMR spectrometers are very expensive

51. NMR vs. IR
Absorbance

52. MS vs. NMR MS peaks are narrower than NMR peaks
MS is much more (104 x) more sensitive than NMR (among most sensitive tools)
MS generally allows one to analyze much larger molecules (>50 kD) than NMR
MS samples are more difficult to prepare
MS is not particularly quantitative
MS instruments cost a little less than NMR

53. MS vs NMR
Absorbance
aspirin
MS NMR

54. Mass Spec vs. IR Mass Spec is very expensive relative to IR
Mass Spec can find molecular weights
Both methods reveal which functional groups are present
Mass Spec samples are harder to prepare
IR is “quick & dirty”, and found in almost every organic lab in the world.
Mass Spec is highly involved and specialized