Organic and analytical chemistry - Essay Example

Organic and Analytical Chemistry Infra-red spectroscopy (IR), Mass Spectrometry (MS), and Nuclear Magnetic Resonance spectroscopy (NMR). are three basic methods that are commonly used for the identification of organic compounds. Each of these techniques measures specific properties of molecules that are unique to the particular compound and can be used to determine the structure of its molecules. Together, the information from MS, IR and NMR can be used to determine the chemical identity and three-dimensional structure of the molecules of an unknown compound.

MS provides information about the different component pieces of a molecule according to their masses. IR provides information about the functional groups that are present in the molecules. NMR provides information about the structural relationship of the nuclei in a molecule.Mass Spectrometry: A mass spectrometer measures the deflection of charged particles as they travel across a magnetic field. This depends on their mass and velocity. A small amount of a sample material injected into the device becomes fragmented into various electrically-charged bits of molecules.

These ions are then accelerated into the magnetic field section where they travel along a curved trajectory determined by their mass-to-charge ratio, m/e. Adjusting their velocity as they enter this section allows particles of each different mass to strike a detector. The rate at which different particles are detected provides the relative number of pieces of different mass in the molecule. The most important fragment is the M+ fragment, corresponding to the whole molecule. This gives a direct measurement of the molecular weight of the compound.

Other fragments can be assigned to certain structural units. But MS cannot provide information about the relative positions of those parts in the molecule.Infra-red Spectroscopy: IR spectroscopy measures the frequency of infra-red light that is absorbed as it passes through a thin film sample of a compound. The different bonds in a molecule undergo various vibrations and rotations. The frequencies of those motions depends on the masses of the atoms involved and the particular type of motion, and correspond to frequencies in the IR spectrum of light.

As IR light is passed through the sample, each bond absorbs energy at its characteristic frequency. The IR spectrograph records the amount of light absorbed at each frequency across the IR spectrum. Some absorbances are characteristic of specific functional groups and appear in very certain ranges of wavelengths. The overall pattern of an IR spectrograph is unique to a compound, like a fingerprint. The absorption bands are given in units called “wave numbers” (or cm-1), which are the reciprocal values of their corresponding wavelengths.

IR spectroscopy provides information about the various component parts of a molecule and its unique spectrum, but it cannot provide detailed information about the overall structure of the molecule.Nuclear Magnetic Resonance: The most powerful analytical tool for determining molecular structure is NMR. This technique is based on the principle that certain elements have a magnetic state in their nuclei, particularly the proton and carbon-13, which can be reversed in a magnetic field. When this happens the nucleus absorbs energy in the radio frequency range (typically 100 – 200 MHz), and these absorbances can be measured.

The exact value and pattern of this energy absorption depends directly on the location of the particular nucleus in the molecule and its geometric relation to the surrounding nuclei. In a proton NMR (1H-NMR) spectrum the positions of the peaks are given relative to an internal standard, usually tetramethylsilane (TMS). Because the deuterium nucleus does not have a magnetic component, the solvents used in 1H-NMR are typically deuteriated versions of normal solvents, like deuteriochloroform, (CDCl3).

Each proton in a molecule is in a slightly different magnetic environment, so each one has its own specific “resonance” frequency that is “split” by echoes from adjacent protons. This splitting depends on how many neighbouring protons there are. By the relative sizes of the absorption signals, the relative number of each type of proton can also be measured. This produces characteristic patterns of signals that can absolutely identify the structural features of a molecule.An Illustrative Example: A sample of an unknown colourless liquid tested by MS, IR and 1H-NMR showed these characteristics.

The M+ peak in the MS spectrum appeared at m/e = 88, with other significant peaks at m/e = 15, 29, 44 and 59. These are consistent with -CH3, -C2H5, CO2, C2H3O2 and C4H8O2 as the M+ peak. The IR spectrum showed strong absorbances at 1720 cm-1, consistent with the C=O bond, at 1240 and 1380 cm-1, consistent with the C-O bond, and at 3000 cm-1 and 1440 cm-1, consistent with alkyl C-H bonds. The 1H-NMR spectrum showed a single sharp peak (a “singlet”) at 2.0 ppm corresponding to 3 protons, four small peaks (a “quartet”) corresponding to 2 protons at 4.

1 ppm, and three peaks (a “triplet”) due to 3 protons at 1.25 ppm relative to the TMS signal. The distance between the peaks in the quartet and the singlet was the same. This pattern and signal range is characteristic of an ethyl group in an ester. Based on this information the unknown compound is positively identified as ethyl acetate, CH3-COO-C2H5.1H-NMR Spectrum of ethyl acetate, from http://www.muhlenberg.edu/depts/chemistry/chem201woh/1Hethylacetate.htmlIR Spectrum of ethyl acetate from http://online.cit.act.edu.au/toolboxes/labtech/Laboratory/StudyNotes/snInterInfraSpec.

htmMass Spectrum of Ethyl Acetate (top) from http://pharmgrads.blogspot.com/2008/04/lecture3.htmlReferences:Morrison, Robert T., Robert N. Boyd, Organic Chemistry, 6th Edition, London: Prentice-Hall International (1992)“1H-NMR Spectrum of Ethyl Acetate”, Muhlenberg College, Allentown, Pa, USA. Retrieved from http://www.muhlenberg.edu/depts/chemistry/chem201woh/1Hethylacetate.html, May 21, 2009“Infra-red Spectrum of Ethyl Acetate”, Canberra institute of Technology, Canberra, Australia. Retrieved from http://online.cit.act.edu.au/toolboxes/labtech/Laboratory/StudyNotes/snInterInfraSpec.

htm, May 21, 2009Hanaa Hashem, “Principles of Mass Spectrum Analysis”, April 28, 2008, Retrieved from http://pharmgrads.blogspot.com/2008/04/lecture3.html, May 21, 2009