Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Physical Characterisation of Organic Compounds

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There are several spectroscopic techniques which can be used to identify organic molecules: infrared (IR), mass spectroscopy (MS) UV/visible spectroscopy (UV/Vis) and nuclear magnetic resonance (NMR).

IR, NMR and UV/vis spectroscopy are based on observing the frequencies of electromagnetic radiation absorbed and emitted by molecules. MS is based on measuring the mass of the molecule and any fragments of the molecule which may be produced in the MS instrument.

UV/Visible SpectroscopyEdit

UV/Vis Spectroscopy uses ultraviolet and/or visible light to examine the electronic properties of molecules. Irradiating a molecule with UV or Visible light of a specific wavelength can cause the electrons in a molecule to transition to an excited state. This technique is most useful for analyzing molecules with conjugated systems or carbonyl bonds.

NMR SpectroscopyEdit

Nuclear Magnetic Resonance (NMR) Spectroscopy is one of the most useful analytical techniques for determining the structure of an organic compound. There are two main types of NMR, 1H-NMR (Proton NMR) and 13C-NMR (Carbon NMR). NMR is based on the fact that the nuclei of atoms have a quantized property called spin. When a magnetic field is applied to a 1H or 13C nucleus, the nucleus can align either with (spin +1/2) or against (spin -1/2) the applied magnetic field.

These two states have different potential energies and the energy difference depends on the strength of the magnetic field. The strength of the magnetic field about a nucleus, however, depends on the chemical environment around the nucleus. For example, the negatively charged electrons around and near the nucleus can shield the nucleus from the magnetic field, lowering the strength of the effective magnetic field felt by the nucleus. This, in turn, will lower the energy needed to transition between the +1/2 and -1/2 states. Therefore, the transition energy will be lower for nuclei attached to electron donating groups (such as alkyl groups) and higher for nuclei attached to electron withdrawing groups (such as a hydroxyl group).

In an NMR machine, the compound being analyzed is placed in a strong magnetic field and irradiated with radio waves to cause all the 1H and 13C nuclei to occupy the higher energy -1/2 state. As the nuclei relax back to the +1/2 state, they release radio waves corresponding to the energy of the difference between the two spin states. The radio waves are recorded and analyzed by computer to give an intensity versus frequency plot of the sample. This information can then be used to determine the structure of the compound.



Aromatics in H-NMR Electron Donating Groups vs. Electron Withdrawing Groups

On monosubstituted rings, electron donating groups resonate at high chemical shifts. Electron donating groups increase the electron density by releasing electrons into a reaction center, thus stabilizing the carbocation. An example of an electron donating group is methyl (-CH3).

Accordingly, electron withdrawing groups are represented at low chemical shifts. Electron withdrawing groups pull electrons away from a reacting center. This can stabilize an electron rich carbanion. Some examples of electron withdrawing groups are halogens (-Cl, -F) and carboxylic acid (-COOH).

Looking at the H NMR spectrum of ethyl benzene, we see that the methyl group is the most electron withdrawing, so it appears at the lowest chemical shift. The aromatic phenyl group is the most electron donating, so it has the highest chemical shift.

Ethyl.gif

Phenol.gif


Disubstituted Rings

The sum of integrated intensity values for the entire aromatic region shows how many substituents are attached to the ring, so a total value of 4 indicates that the ring has 2 substituents. When a benzene ring has two substituent groups, each exerts an influence on following substitution reactions. The site at which a new substituent is introduced depends on the orientation of the existing groups and their individual directing effects. For a disubstituted benzene ring, there are three possible NMR patterns. Omp.gif

Note that para-substituted rings usually show two symmetric sets of peaks that look like doublets.

The order of these peaks is dependent on the nature of the two substituents. For example, the three NMR spectra of chloronitrobenzene isomers are below:

Ortho.gif

Meta.gif Para.gif

Mass SpectroscopyEdit

A mass spectroscope measures the exact mass of ions, relative to the charge. Many times, some form of separation is done beforehand, enabling a spectrum to be collected on a relatively pure sample. An organic sample can be introduced into a mass spectroscope and ionised. This also breaks some molecules into smaller fragments.

The resulting mass spectrum shows:

1) The heaviest ion is simply the ionised molecule itself. We can simply record its mass.

2) Other ions are fragments of the molecule and give information about its structure. Common fragments are:

species formula mass
methyl CH3+ 15
ethyl C2H5+ 29
phenyl C6H5+ 77

Infrared spectroscopy.Edit

Absorbing infrared radiation makes covalent bonds vibrate. Different types of bond absorb different wavelengths of infrared:

Instead of wavelength, infrared spectroscopists record the wavenumber; the number of waves that fit into 1 cm. (This is easily converted to the energy of the wave.)

For some reason the spectra are recorded backwards (from 4000 to 500 cm-1 is typical), often with a different scale below 1000 cm-1 (to see the fingerprint region more clearly) and upside-down (% radiation transmitted is recorded instead of the absorbance of radiation).

The wavenumbers of the absorbed IR radiation are characteristic of many bonds, so IR spectroscopy can determine which functional groups are contained in the sample. For example, the carbonyl (C=O) bond will absorb at 1650-1760cm-1.

Summary of absorptions of bonds in organic moleculesEdit

Infrared Spectroscopy Correlation Table

Bond Minimum wavenumber (cm-1) Maximum wavenumber (cm-1) Functional group (and other notes)
C-O 1000 1300 Alcohols and esters
N-H 1580 1650 Amine or amide
C=C 1610 1680 Alkenes
C=O 1650 1760 Aldehydes, ketones, acids, esters, amides
O-H 2500 3300 Carboxylic acids (very broad band)
C-H 2850 3000 Alkane
C-H 3050 3150 Alkene (Compare intensity to alkane for rough idea of relative number of H atoms involved.)
O-H 3230 3550 H-bonded in alcohols
N-H 3300 3500 Amine or amide
O-H 3580 3670 Free –OH in alcohols (only in samples diluted with non-polar solvent)

Absorptions listed in cm-1.


IR summary version 2.gif

Typical methodEdit

[1]

Typical apparatus

A beam of infra-red light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained.

A reference is used for two reasons:

  • This prevents fluctuations in the output of the source affecting the data
  • This allows the effects of the solvent to be cancelled out (the reference is usually a pure form of the solvent the sample is in).

References & notesEdit

  1. Wikipedia article.

SDBS is a free online database of Spectral analysis including many IR, NMR and MS graphs.

Last modified on 7 April 2011, at 09:19