Organic Chemistry III: Identification of Alkyl and Aryl Halides

Physical Properties of Alkyl and Aryl Halides

• Alkyl and aryl halides are colourless liquids when pure, except iodoform (CHI3), which is a yellow crystalline solid with a characteristic odor.
• They have special odors, e.g., methyl iodide, ethyl iodide, and bromide have pleasant odors, while benzyl chloride has a sharp irritating odor.
• Alkyl and aryl halides have higher boiling points than their parent hydrocarbons due to their heavier molecular weight.

Chemical Reactions of Alkyl and Aryl Halides

• Reaction with alcoholic silver nitrate: useful for classifying halogen compounds, forming an insoluble silver halide (AgX).
• The rate of reaction indicates the degree of reactivity of the halogen atom in the compound, following the S1N mechanism.
• Reactivity of alkyl halides: R3CX > R2CHX > RCH2X.

Procedure for Alcoholic Silver Nitrate Test

• Add one drop or a couple of crystals of the unknown to 2 ml of 2% ethanolic silver nitrate solution.
• Observe the result; if no reaction, stand for 5 minutes at room temperature or warm the mixture in a water bath for 30 seconds.
• If a precipitate forms, add several drops of 1 M nitric acid solution to it; silver halides (AgX) are insoluble in this acid.

Reactivity of Specific Halides

• Allylic and tert-chlorides, methyl and ethyl iodides, and ethylbromides give fast results at room temperature.
• Pri- and sec-chlorides, benzyl chloride, and 1-chloro-2,4-dinitrobenzene give results only on warming.
• Chlorobenzene, chloroform, iodoform, carbon tetrachloride, and vinylic chlorides do not give positive results even on heating.
• Cyclohexyl halides have lower reactivity than the corresponding open-chain secondary halides.

2-Sodium Iodide in Acetone Test

• Used to classify aliphatic chlorides and bromides as primary, secondary, or tertiary.
• Sodium chloride and sodium bromide are only very slightly soluble in acetone.
• The mechanism follows a direct displacement (SN2) process.
• Reactivity order: Primary > secondary > tertiary; primary bromides give a precipitate of sodium bromide within 3 min at 25°C, whereas chlorides give no precipitate and must be heated to 50°C to effect a reaction.

This quiz covers the identification of alkyl and aryl halides in the laboratory setting. Topics include physical properties, characteristics, and classification based on the presence of halogen atoms. Test your knowledge of organic compounds with halogen attachments!