Alkyl and Aryl Halides - Practical Organic Chemistry III - PDF

Alkyl and aryl halides practical organic chemistry III 2nd grade/pharmacy college ‫ﺑﺮاء ﻏﺴﺎن ﻋﺒﺪاﻟﻐﻔﻮر‬.‫م‬.‫م‬ Physical properties All alkyl halides and chlorobenzene are colourless liquids when pure except iodoform, CHI3, which is a yellow crystalline...

Alkyl and aryl halides practical organic chemistry III 2nd grade/pharmacy college ‫ﺑﺮاء ﻏﺴﺎن ﻋﺒﺪاﻟﻐﻔﻮر‬.‫م‬.‫م‬ Physical properties All alkyl halides and chlorobenzene are colourless liquids when pure except iodoform, CHI3, which is a yellow crystalline solid with a characteristic odour. Methyl iodide, ethyl iodide and bromide, chloroform, and carbon tetrachloride have sweetish odours. Benzyl chloride has a sharp irritating odour and is lachrymatory. Chlorobenzene possesses aromatic odour. Alkyl and aryl halides (R‫ـــ‬X, Ar‫ـــ‬X) have boiling points higher than the parent hydrocarbon because of the heavier molecular weight. Accordingly, for a given compound, iodides have the higher boiling point than bromides and chlorides. In spite of their polarity alkyl halides are insoluble in water due to their inability to form hydrogen bonds. They are soluble in most organic solvents. Iodo-, bromo-, and polychloro- compounds are denser than water. Chemical reactions 1. Reaction with alcoholic silver nitrate. CCl4 carbon tetrachloride Cl chlorobenzene Alcoholic silver nitrate reagent is useful in classifying halogen compounds. Many halogen containing compounds react with silver nitrate to give an insoluble silver halide (AgX), and the rate of this reaction indicates the degree of reactivity of the halogen atom in the compound. Besides, the identity of the halogen can sometimes be determined from the colour of the silver halide produced; silver chloride is white (turns to purple on exposure to light), silver bromide is pale yellow, and silver iodide is yellow. It is obvious that this reaction follows SN1 mechanism. Generally the reactivity of alkyl halides towards this reagent is: R3CX > R2CHX >RCH2X tert-chlorides, methyl and ethyl-iodides,allylic chlorides,and ethyl bromides give fastr esult at room temperature Pri- and sec- chlorides, benzyl chloride, and 1-chloro-2,4- dinitrobenzene give result only on warming Chlorobenzene, chloroform, iodoform, carbon tetrachloride, and vinylic chlorides don't give any positive result even on heating: Cyclohexyl halides exhibit a decreased reactivity when compared with the corresponding open-chain secondary halides. Cyclohexyl chloride is inactive, and cyclohexyl bromide is less reactive than 2 bromohexane, although it will give a precipitate with' alcoholic silver nitrate. 2. Sodium iodide in acetone test. This test, complementing the alcoholic silver nitrate test, is used to classify aliphatic chlorides and bromides as primary, secondary, or tertiary. This test depends on the fact that sodium chloride and sodium bromide are only very slightly soluble in acetone. The mechanism follows direct displacement (SN2) process; therefore, the order of reactivity of simple halides is: primary> secondary> tertiary With sodium iodide, primary bromides give a precipitate of sodium bromide within 3 min at 25°C, whereas the chlorides give no precipitate and must be heated to 50°C in order to effect a reaction. Secondary and tertiary bromides react at 50°C, but the tertiary chlorides fail to react within the time specified. Tertiary chlorides will react if the test solutions are allowed to stand for a day or two 3. Differentiationbetween alkyl and aryl halides (Formaldehyde- sulfuric acid test) With this test aryl halides (chlorobenzene) produce pink, red, or bluish red colour whereas alkyl halides produce yellow, amber, or brown colour. 4. Special tests for chloroform a) Riemer- Tiemann reaction For procedure and chemical equations refer to "Identification of Phenols" experiment. Resorcinol results in a red colour with slight fluorescence in the aqueous layer while α- or β- naphthol results in a deep blue aqueous layer fading to green. b) Reduction of Fehling's reagent For preparation of Fehling's reagent and chemical equations refer to "Identification of Aldehydes and Ketones" experiment. Boil 1 mL of chloroform gently (water bath) with 3 mL of Fehling's reagent with constant shaking for 3- 4 minutes. Reduction occurs and reddish cuprous oxide slightly separates.