STRUCTURE DETERMINATION OF A SAMPLE

Prior to spectroscopic techniques were introduced, the structure determination of organic compounds based on a comparison with compounds whose structures are known. Structure determination is now mainly carried out by spectroscopic and diffraction methods.
a. Mixed Melting Point Test
Before the mid-there are 20, the main procedure in determining the structure of organic compounds is to prove that the compound is identical with the known compounds. This evidence is mainly achieved by a mixture melting point test (test mixture). This method is based on the principle that the highest melting point of the solid when the solid is pure. When the two samples A and B have the same melting point, then determined the melting point of pure A, B pure and mixed a number of equal A and B. When the results of all three together, it is evident that A and B are identical. In practice, there are some complications. The melting point is not always sharp, and the materials tend to melt in the temperature range specified. So it's not easy to say whether the two melting points at or not.
b. The Use Of Derivative Solids
When the sample is liquid or gas tangible, mixed melting point method can not be used. When the sample gas or liquid has a reactive functional group, this sample can be converted into a solid that may result in a beautiful crystal. Aldehydes and ketones, which are very important in organic chemistry, tend to be liquid when low molecular m assa. In such cases these compounds are usually converted into derivatives lewbih easily handled solid for structure determination. Reagent which can react with aldehydes and ketones, such as hydroxylamine, hydrazine and fenilhidrazin. Some reactions to obtain crystalline derivatives are given below.

CH3CHO	+	NH2OH	–>	CH3CH=NOH	+	H2O	(13.1)
Asetaldehida		hidroksilamin		asetaldoksim

(CH3)2C=O	+	C6H5NHNH2	–>	(CH3)2C=NNH C6H5	+	H2O	(13.2)
aseton		fenilhydrazin		asetonfenilhidrazon

Derivatives are crystalline compounds can be used for structure determination of unknown compounds.
c. Comparison Of Physical Properties
Other physical properties such as boiling point, refractive index, dipole moment, and the specific rotation for the optically active compounds can provide useful information. Such data can provide information on the nature of the whole molecule. Sometimes, the nature of the whole molecule can be a number of different contribution parts of the compound. In the case just as it is, information on specific parts of the compound can be obtained. For example, the use of the dipole moment μ will be given below.Dipole moment for the results perconaan nitrobenzen (3.98 D) and khlorobenzen (1.58 D), the moment dipolnya determined by the electronic properties of functional groups (eg electronegativity). In discussing the dipole moments of organic compounds, the moment of CC and CH bonds are assumed zero. So the moment of the compounds were determined mainly by the moment of bonding function groups. Two isomers khloronitrobenzen dipole moment is 2.50 D and 3.40 D. Because the moment of bonding has been identified as the and meta isomer as shown in Figure 13.1 (b).

Figure 13.1 The dipole moment derivatives of benzene substituted. Comparison between the observed and calculated clearly show the relative orientation substituennya.
d. Qualitative Reaction
Structure determination of organic compounds usually involves two approaches. In contrast, structural information obtained by the determination of roughly molecular mass, elemental analysis, etc.. Likewise, information type and amount of functional groups must also be obtained. Thus, information about the molecule as a whole and as substituennya obtained. Prior to the development of spectroscopy, the identification of functional groups depends mainly on its reactivity. Typical examples are the detection of carbonyl groups (aldehyde and ketone-CHO-C = O) by using the silver mirror reaction and Fehling test.