Organic Chemistry Lab - Identification of Alcohols - Bilad Alrafidain University

Bilad Alrafidain University College Department of Pharmacy second stage organic chemistry Lab. By M.SC. Asmaa Bahjat...

Bilad Alrafidain University College Department of Pharmacy second stage organic chemistry Lab. By M.SC. Asmaa Bahjat Identification of Alcohols Alcohols are organic compound that may be considered as derivatives of water in which one of the hydrogen atoms of water molecule (H-O-H) has been replaced by an alkyl or substituted alkyl group. Therefore, properties of alcohols may be related to properties of both water and hydrocarbons. The alkyl group could be primary, secondary, or tertiary, and may be open chain or cyclic. Accordingly, alcohols may be defined as organic compounds that contain hydroxyl groups attached to alkyl, substituted alkyl, or cyclic alkyl group.  Physical properties Alcohols are colourless liquids with a special faint odour. Benzyl alcohol and Cyclohexanol have characteristic odours. Aliphatic alcohols burn with blue flame (without smoke) while aromatic alcohols burn with yellow smoky flame. Boiling points of alcohols are considerably high (being associated liquids); they increase as the molecular weight (number of carbons) increases. Alcohols are miscible with water except benzyl alcohol, cyclohexanol, and sec- butanol (which is very slightly soluble in water ). solubility classification Alcohols are polar compounds because of the of presence of the hydroxyl group Which is also responsible for their ability to form hydrogen bonding. The degree of the polarity depends on the size of the alkyl side chain; the polarity decreases as the size of the alkyl side chain increases, or in another word, as the hydroxyl group/ hydrocarbon ratio of alcohols increases, their water solubility increases and vice versa. Besides, low molecular weight alcohols are soluble in water due to hydrogen bonding ability with water molecules. Therefore, alcohols that are soluble in water and ether are classified under class S1 such as ethanol and methanol. Alcohols that are insoluble in water are related to class N such as benzyl alcohol, esc-butanol, and cyclohexanol. Chemical properties Alcohols are neutral compounds that don’t change the colour of litmus paper. All reactions of alcohols are related to its active hydroxyl group and are of two types: Removal of the hydroxyl itself as in the reaction with hydrogen halides to form alkyl halides or in the dehydration reaction to form a double bond. removal of the proton only from the active hydroxyl as in the formation of esters or in the reaction with active metals such as sodium.  1- general test (ceric ammonium nitrate reagent) ceric ammonium nitrate (yellow solution ) is an oxidizing agent that reacts with alcohols to give a brown to greenish brown precipitate. The red coloured complex is an intermediate for the oxidation of alcohols by the Ce(IV) solution. This red colour disapperars after a reasonable time due to completing the oxidation of this intermediate and the reduction to the colourless Ce(III)solution producing the corresponding aldehyde or ketone. Procedure Water soluble ( miscible) alcohols; Mix two drops of the alcohol with one drop of ceric ammonium nitrate solution.a red complex indicates a positive test. Water insoluble ( immiscible) alcohols; mix two drops of the alcohol with 0.5 ml dioxane, shake well, and add one drop of the reagent to get a positive red complex. This test gives positive results with primary, secondary, and tertiary alcohols (up to 10 carbons), poly hydroxylated compound such as carbohydrate carboxylic acids, aldehydes and ketones.