Mattingly "AI and Prompt Design: LLMs with Text Classification and Open Source"
YR12 IB CHEM Lesson 1 Spectroscopic identification of organic compounds.pptx
1. Spectroscopic identification of organic
compounds
• Name the following
compounds.
• What homologous
series are they in?
• What are the
functional groups?
What are the
classes?
Get out your measurement
exam questions booklet HW.
3. Index of hydrogen deficiency
• The degree of unsaturation or index of hydrogen
deficiency (IHD) can be used to determine from a
molecular formula the number of rings or multiple
bonds in a molecule.
4. IHD from molecular formula
• The degree of unsaturation or index of hydrogen deficiency (IHD) provides a useful
clue as to the molecular structure of an organic compound.
• It gives the number of molecules of hydrogen (H2) needed to convert the organic
compound to a saturated molecule.
• For a hydrocarbon, CxHy, with x number of carbon atoms and y number of
hydrogen atoms, the IHD is given by the following equation:
IHD = 2x + 2 - y
2
where each double bond or ring counts as an IHD of 1, and each triple bond
counts as an IHD of 2.
5. IHD from molecular formula
Rules:
• Oxygen and sulfur atoms in a compound do not affect the IHD.
• Halogen atoms are treated like hydrogen atoms, so one hydrogen atom is
added to the molecular formula for each halogen atom.
• For each nitrogen atom in the compound, add one carbon atom and one
hydrogen atom to the molecular formula.
8. Mass spectrometry (MS)
• A mass spectrometer can be used to determine the Ar of an element.
• Mass spectrometry can also be used to determine the structure of an
organic compound. One of the stages involved in mass spectroscopy is
ionisation, where the sample in question is bombarded by high-energy
electrons (which knocks an e- off the molecule). This process results in
the formation of a molecular ion (M+). The equation for the formation of
a molecular ion can be represented as:
M(g) + e− → M+
(g) + 2e−
• In ionisation, an electron is removed from the molecule, leaving it with a
1+ charge.
9. MS - Fragmentation
• Excess energy from ionisation can be
transferred to molecular ion making it
vibrate
• This can cause bond to weaken and can
split molecular ion into fragments -
Fragmentation
C2H5OH+ → CH3 + CH2OH+
• Molecular ion fragments are detected in MS
13. Proton nuclear magnetic resonance
spectroscopy (1H NMR)
• Nuclear magnetic resonance (NMR) is a powerful tool in
determining the structures of molecules in analytical organic
chemistry. Atoms with an odd number of protons in their nuclei are
the key species in this type of spectroscopy.
• 1H NMR is a particularly powerful tool as it enables information to be
gained on the precise environment of all the protons in the
molecule.
14. Magnetic resonance imaging (MRI)
• In medicine, the use of NMR is immensely useful as the energy of the
radio waves involved is completely harmless and there are no known
side-effects. 31P NMR is very useful in studying the extent of damage to
the body caused by heart attacks and also in monitoring the control of
diabetes. 1H NMR is used in body scanning. The whole body of the
patient can be placed inside the magnet of a large NMR machine. The
protons present in water, lipids, and carbohydrates give different signals
so an image of the whole body can be obtained.
20. The n+1 rule
n n+1 Multiplicity
0 0+1 = 1 Singlet
1 1+1 = 2 Doublet
2 2+1 = 3 Triplet
3 3+1 = 4 Quartet
The relative intensities of the lines can be
determined by Pascal’s triangle.
i.e. a doublet has a relative intensity of 1:1
a triplet has an intensity of 1:2:1
a quartet has an intensity of 1:3:3:1
The gap between the peaks also stays the
same and is determined by the adjacent
proton environment, this is called the
coupling constant.
23. Infrared spectroscopy (IR)
• Infrared spectroscopy is used to identify the bonds present within a molecule.
• When chemical bonds absorb certain wavelengths of infrared radiation, they can
undergo stretching or bending.
• In order for a gas to absorb IR radiation, there must be a change in the dipole
moment of the molecule as the bonds undergo asymmetrical stretching and
bending. Gases that absorb IR radiation are known as being IR active (e.g.
greenhouse gases, such as CO2).
• Diatomic molecules such as N2 and O2 do not act as greenhouse gases as there is no
change in the dipole moment of the molecule. These gases are known as being IR
inactive.
• Non-polar bonds such as C-C, for example, usually do not absorb IR radiation and do
not show up on an IR spectrum.
• IR spectroscopy is used in heat sensors and remote sensing in physics.
24. • All bonds vibrate at a characteristic frequency.
• There are different types of vibration.
Symmetric stretch Assymmetric stretch Bending
• The frequency depends on the mass of the atoms in the
bond, the bond strength, and the type of vibration.
• The frequencies at which they vibrate are in the infra-red
region of the electromagnetic spectrum.
IR energy and molecular vibrations
25. Interpreting infrared spectroscopy (IR)
• An IR spectrum can be divided into two sections. The fingerprint region (below 1500
cm–1) is unique for each compound but is difficult to interpret.
• However, it is possible to identify an unknown compound by comparing this region
with that of known compounds.
• The functional group region is used to determine the type of bonds and hence the
functional groups in the compound.
26. Interpreting infrared spectroscopy (IR)
• Some characteristic frequencies are
given in Table 1. Note that the unit
given in the table is the
wavenumber. The wavenumber is
equal to the number of wavelengths
per centimetre (cm–1).
Wavenumbers are used because
infrared frequencies are very large
and inconvenient to use. The
information in the table below can
be found in section 26 of the IB
Chemistry data booklet.
30. O-H STRETCH
C=O STRETCH
O-H STRETCH
C=O STRETCH
AND
ALCOHOL
ALDEHYDE
CARBOXYLIC
ACID
You can tell the difference between alcohols, aldehydes and carboxylic acids by
comparison of their spectra.
Quick test - What is it?
