Chemistry

How are Chemicals Identified



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In the modern chemistry laboratory, chemicals are identified based on their structure of atoms. Namely, if they are organic or inorganic. One basic method to identify the formula of a compound is the elemental analysis method. In this method the molecular formula of the compound is determined by identifying all the elements in the compound. This means the number of each atom in the compound.

This is an important step because without the mlecular formula it would be extremely difficult if not impossible to identify the compound.

Another method to determine the molecular formula of a compound uses a spectroscopic method which is called mass spectrometry and in particular one of its diverse applications which is called chemical ionization. By this way the molecular formula of the compound is determined.

This step of deciding the molecular formula of a compoud would be possible only if the compound is in a pure state. This is not the case in most chemical reactions in which usually a mixture of two or three compounds exist.

If one has a mixture then there are possible ways to purify the desired compound. This can be accomplished by either a technique that is called chromatography. This technique relies on the difference in polarity of the compounds in order to separate them. Another technique that can be used for the separation of the mixture is by distillation. This technique relies on the difference in boiling points between the compounds in the mixture.

After the step of purification has happened and assuming that the compound that we have is pure then we can know from mass spectrometry or elemental analysis if the compound is organic or inorganic by knowing its elements in the compound.

If the compound is inorganic then X-ray crystallography is a usuall technique for the elucidation of the structure of the compound. If the compoud is organic then we first decide if it is saturated or unsaturated. Namely we must know if it has double bonds between its carbons or not. We know this by checking the double bond equivalence using a formula from organic chemistry. After we decide on this point then There ara many techniques in organic chemistry that can help us identify the compound.

An old technique that is used in students laboratories but not in research labs is to prepare a derivative of the compoud that we have and then measure its melting point and compare it to a literature reference value. This technique is good only if the compound that we wish to know its structure is known in the literature. If this compound is new then this technique will not work.

Another spectroscopic technique that is used in the elucidation of structure of organic compounds is IR spectroscopy. This technique is good mainly for the identification of functional groups that might exist in the compound such as a ketone group or an alcohol. This technique thus contributes to the knowing of the structure but does not elucidate its complete structure.

A powerful method for identifying organic compounds is a spectroscopic technique that is called NMR or nuclear magnetic resonance. This technique has many variations such as two dimensional NMR and NOE and COSY techniques in addition to carbon 13 NMR. All these techniques together can give a definite structure of the compound that we wish to know its structure.

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