Download Physical and Chemical Tests

10-1 Physical and Chemical Tests
Purification:
Chromatography
Distillation
Recrystallization
Comparison to known compounds:
Melting point
Boiling point
… many other properties
When the properties of an unknown purified substance match those in the
literature for a known compound, the identity and structure of the substance are
still not known with certainty.
Many new substances are newly synthesized for the first time and their properties
are not in the literature.
Elemental analysis reveals the gross composition of the sample.
Chemical tests identify the functional groups present.
For larger molecules, knowledge of the composition and functional groups
present in a substance are not enough to determine the chemical structure of the
substance. For instance, the alcohol C7H16O:
10-2 Defining Spectroscopy
Spectroscopy is a technique for analyzing the structure of molecules, usually
based on how they absorb electromagnetic radiation. Four types are most often
used in organic chemistry:
Nuclear Magnetic Resonance spectroscopy (NMR)
Infrared spectroscopy (IR)
Ultraviolet spectroscopy (UV)
Mass spectroscopy (MS)
NMR spectroscopy of C and H provides the most detailed information regarding
the atomic connectivity of a molecule.
10-2 Defining Spectroscopy
Molecules undergo distinctive excitations.
Electromagnetic radiation can be described as a wave having a wavelength (λ), a
frequency (ν) and a velocity (c).
The speed of light in a vacuum is 3 x 1010 cm s-1 or 3 x 108 m s-1.
The units of wavelength must match those used for the speed of light.
The units of frequency are cycles s-1 (or just s-1) or Hertz (Hz).
Molecules absorb energy in discrete packets called “quanta.” A quanta of
electromagnetic radiation is referred to as a photon.
The energy of a photon is determined by the frequency of the incident radiation:
E = hν
When a photon of energy is absorbed by a molecule, it causes electronic
excitation or mechanical motion to occur.
The electronic excitations and motions of a particular molecule are also quantized
so only certain frequencies of radiation are able to be absorbed.
An analysis of the frequencies of electromagnetic radiation absorbed by a
molecule provides information about the arrangement of the atoms in the
molecule.
The lowest energy state of a molecule is called the ground state.
Absorption of electromagnetic radiation causes the molecule to move to an
excited state.
The difference in energy between the excited state and the ground state must be
exactly equal to the energy of the photon absorbed.
Absorption of X-rays results in the promotion of electrons from inner atomic shells
to outer ones (electronic transitions). This requires X-ray energies greater than
300 kcal mol-1.
UV and visible absorption excites valence shell electrons, typically from a filled
bonding to an unfilled antibonding orbital. This involves energies between 40 and
300 kcal mol-1.
IR absorption causes bond vibration excitation: 2 to 10 kcal mol-1.
Microwave radiation excites bond rotations: ~10-4 kcal mol-1.
Radiowaves, in the presence of a magnetic field, produces alignment of nuclear
magnetism: ~10-6 kcal mol-1. This is the basis of NMR.
In this diagram, frequency is specified in units of wavenumbers, defined as 1/λ,
which is the number of waves per centimeter.
Wavenumbers are used to specify energy in infrared spectroscopy.
A spectrometer records the absorption of radiation.
Continuous Wave Spectrometry (CW)
Radiation of a specific wavelength (UV, IR, NMR, etc.) is generated and passes
through a sample.
The frequency of the radiation is continuously changed and the intensity of
the transmitted beam is detected and recorded.
Frequencies that are absorbed by the sample appear as peaks deviating from
a baseline value.
Fourier Transform Spectroscopy (FT)
A much faster technique.
A pulse of electromagnetic radiation covering the entire spectrum under
scrutiny (NMR, UV, IR) is used to obtain the whole spectrum instantly.
The pulse may be applied multiple times and the results accumulated and
averaged, which provides for very high sensitivity.
The signal measured is actually the decay, with time, of the absorption event.
This signal is then mathematically transformed using a Fourier transform,
producing the more familiar frequency versus absorption plot.
10-3 Proton Nuclear Magnetic Resonance
Nuclear spins can be excited by the absorption of radio waves.
Many nuclei can be thought of as spinning on their axes, either clockwise or
counterclockwise.
One such nucleus is the hydrogen nucleus: 1H.
A 1H nucleus is positively charged and its spinning motion generates a magnetic
field.
In the presence of an external magnetic field, H0, the magnetic field of the
hydrogen nucleus can be oriented either with H0 (lower energy) or against H0
(higher energy). These two states are called  and  spin states, respectively.
The difference in energy between the  and  states depends directly on the
external magnetic field strength, H0.
21,150 G
90 MHz
42,300 G
180 MHz
70,500 G
300 MHz
The actual energy difference is small. At 300 MHz, the energy difference for a
proton is about 3 x 10-5 kcal mol-1.
Because the energy difference is so small and the equilibrium between the two
states is so fast, the numbers of nuclei in the two states are nearly equal, however,
a slight excess will be in the  state because of the external magnetic field.
When electromagnetic radiation having the same energy as energy difference
strikes the nucleus, the electromagnetic radiation is absorbed and the slight
excess of nuclei in the  state is reduced.
Many nuclei undergo magnetic resonance.
In general, nuclei composed of an odd number of protons (1H and its isotopes, 14N 19F, and
31P) or an odd number of neutrons (13 C) show magnetic behavior.
If both the proton and neutron counts are even (12C or 16O) the nuclei are non-magnetic.
In a hypothetical scan of CH2ClF in a 70,500-G magnet, the following spectrum
would be observed:
High-resolution NMR spectroscopy can differentiate nuclei of the same
element.
In the NMR spectrum of ClCH2OCH3 at 70,500-G from 0 to 300 MHz, one peak
would be observed for each element present.
Using high-resolution NMR spectroscopy, the region around each of these peaks
can be expanded and additional spectral details can be observed.