Classification Test for Organic Halides

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  • Topic: Halogen, Sodium chloride, Nucleophilic substitution
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James Anand L. Regala, Sabrina Nicolle G. Sarte, Ann Michelle Siao, Michael Sibulo, Victoria Tan
Group 8 2C Pharmacy Organic Chemistry Laboratory

This experiment is done to classify organic halides. Most organic halides are synthetic and are not flammable. One way to classify organic halides is by classifying its -carbon atom as primary, secondary or tertiary. If the -carbon is attached to one R group, it is then primary. If the -carbon is attached to R groups, it is then secondary, and if attached to 3 R groups, it is then said to be tertiary. But this is only applicable if the -carbon is tetragonal or sp3 hybridized. Another way of classifying organic halides is by differentiating its SN1 and SN2 mechanisms. SN1 is a substitution reaction and the rate is determined by the organic halide and the nucleophile. SN1 is also a two-step reaction. First, the halide moves out. Then, the nucleophile approaches and binds with the alkyl group. SN2 is also a substitution reaction. But, instead of a two-step reaction, SN2 is only a one-step reaction. Its rate is determined by both the organic halide and the nucleophile. Several organic halide were used to complete this experiment. INTRODUCTION

An alkyl halide is another name for a halogen-substituted alkane. The carbon atom, which is bonded to the halogen atom, has sp3 hybridized bonding orbitals and exhibits a tetrahedral shape. Due to electronegativity differences between the carbon and halogen atoms, the σ covalent bond between these atoms is polarized, with the carbon atom becoming slightly positive and the halogen atom partially negative. Halogen atoms increase in size and decrease in electronegativity going down the family in the periodic table. Therefore, the bond length between carbon and halogen becomes longer and less polar as the halogen atom changes from fluorine to iodine. [1] The Beilstein test is a simple chemical test used in chemistry as a qualitative test for halides. It was developed by Friedrich Konrad Beilstein. A copper wire is cleaned and heated in a Bunsen burner flame to form a coating of copper(II) oxide. It is then dipped in the sample to be tested and once again heated in a flame caused by formation of copper halide. [2] The next classification test done is the SN1 reactivity: Reaction with alcoholic AgNO3. If a compound is known to contain a halogen (bromine, chlorine, or iodine), information concerning its environment may be obtained from observation of its reaction with alcoholic silver nitrate.  The overall reaction is shown in the following equation:

  RX + AgNO3 ---------------> AgX + RONO2

Such a reaction will be of the SN1 type. Tertiary halides are more reactive in an SN1 reaction than secondary halides, which are in turn more reactive than primary halides. Differing rates of silver halide precipitation would be expected from halogen in each of these environments, namely, primary < secondary < tertiary. These differ

ences are best determined by testing in separate test tubes authentic samples of primary, secondary, and tertiary halides with silver nitrate and observing the results.  Alkyl bromides and
iodides react more rapidly than chlorides, and the latter may require warming to produce a reaction in a reasonable period.  Aryl halides are unreactive toward the test reagent, as are any vinyl or alkynyl halides generally.  Allylic and benzylic halides, even when primary, show reactivities as great as or greater than tertiary halides because of resonance stabilization of the resulting allyl or benzyl carbocations.

Another method for distinguishing between primary secondary, and tertiary halides makes use of sodium iodide dissolved in acetone. This test complements the alcoholic silver nitrate test, and when these two tests are used together, is possible to determine fairly accurately the gross structure of the attached alkyl...
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