Organic Chemistry 2 Lab 2: Identification of Alcohols Fall 2024 PDF

Organic Chemistry 2 Lab 2 Identification of Alcohols Fall 2024 1  ALCOHOLS Alcohols have a hydroxyl (–OH) group bonded to a saturated carbon atom. The alcohol carbon may be part of simple alkyl group as in methanol or it may be in...

Organic Chemistry 2 Lab 2 Identification of Alcohols Fall 2024 1  ALCOHOLS Alcohols have a hydroxyl (–OH) group bonded to a saturated carbon atom. The alcohol carbon may be part of simple alkyl group as in methanol or it may be in more complex molecule, such as cholesterol. Alcohols are classified as 1°, 2°, 3°, depending on the number of carbons bonded to the alcohol carbon. Physical properties Alcohol molecules can associate with each other through hydrogen bonding, whereas ethers and hydrocarbons cannot. The presence of polar OH group, alcohols are more water-soluble than hydrocarbons. Methanol, ethanol, propanol alcohols and tert-butyl alcohol are completely miscible with water, while butanol moderately soluble in water. The solubility of alcohols in water gradually decreases as the hydrocarbon portion of the molecule lengthens; long chain alcohols are more alkane-like, and the hydrogen bonds formed between OH group molecules cannot counteract the non-polar alkane part, and are therefore, less like water, Because of hydrogen bonding, alcohols have higher boiling points than comparable hydrocarbons (alkanes, alkenes, and alkynes) or ethers. Alcohols (R-O-H) have weak acidic property, the strong electronegative oxygen atom pulls electron density away from hydrogen, making it more like an H+ ion. Sterically hindered alcohols such as tert-butyl alcohol are less acidic, and hence their conjugate base more basic, than unhindered alcohols such as methanol or ethanol. One reason for this difference is the effect of salvation. Alkoxide ion of tert-butyl alcohol is less stabilized by salvation and destabilized by inductive effect of alkyl groups. The unhindered alcohols are easily solvated and the alkoxide anion is stabilized, and the hydrogen is easily taken up by the base. The order of acidity 1°> 2° > 3°, 2 Reactions of Alcohols 1. Sodium Metal Alcohols are slightly weaker acids than water, they react with sodium or potassium metal or with metal hydrides such as sodium hydride to form sodium or potassium alkoxide. Because most alcohols are weaker acids than water, most alkoxide ions are stronger bases than the hydroxide ion. - Take 1 ml of alcohol in dry test tube, and add small piece of sodium metal. An evolution of gas will be observed. This test gives positive result with all types of alcohols (1°, 2°, 3°) 2. Oxidation reaction Primary alcohols (R-CH2OH) can be oxidized to aldehydes (R-CHO) or to carboxylic acids (R-CO2H). Oxidation of secondary alcohols (R1R2CH-OH) produces 1 2 1 2 3 ketone (R R C=O), while tertiary alcohols (R R R C-OH) are resistant to oxidation. Alcohols can be oxidized to aldehydes by using pyrinium chlorochromate (PCC) or by Swern oxidation 3 Chromic acid (H2CrO4) oxidation Jones reagent is one well known source of chromic acid (K2Cr2O7 (or CrO3) + Conc. H2SO4) K2Cr2O7 + H2O + 2 H2SO4 → 2 H2CrO4 + 2 KHSO4 -In dry test tube, take 5 drops of alcohol, then add 2 drops of chromic acid reagent. -If the color of reagent (orange) turns blue or green, the result indicates the alcohol is 1°or 2°. -If there is no change of reagent color, the alcohol is 3°,. Lucas' test Lucas' reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The reaction is substitution in which the chloride replaces a hydroxyl group. A positive test is indicated by a change from clear and colorless to turbid solution, which indicates to formation of a chloroalkane. The best results are observed with tertiary alcohols, the alkyl halides form very fast fastest due to higher stability of the intermediate tertiary carbocation. Lucas test in alcohols is a test to differentiate between primary, secondary, and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an SN1 reaction mechanism: 4 -Take 0.5 ml of alcohol (10 drops) in dry test tube. -Add 2.0 ml Lucas reagent, and shake well. The result is indicated by formation of alkyl halide and makes the solution turbid. -If the cloudiness appears immediately, the alcohol is 3° -If the cloudiness appear within 2-10 min in 2° alcohol -10 alcohols don't give change in solution. Borax test (boric acid test) -Take 1 ml of borax solution in a test tube and add 1 drop of phenolphthalein (ph.ph), the solution turns pink in color. -Add 1 drop of glycerol (polyhydric alcohol), the pink color disappears, but reappears on warming and vanishes again on cooling. This reaction is due to the combination of two cis- hydroxyl groups of the compound with boric acid to form reversibly a much stronger monobasic acid. Esterification:- Alcohols react with carboxylic acids to form esters in presence of strong acid as a catalyst such as concentrated sulfuric acid R-OH + R'-CO2H → R'-CO2R + H2O Mechanism of acid catalyzed esterification 5 -Take 0.1 g of salicylic acid in dry test tube, add 2 ml methanol, add 2 drops Conc. H2SO4 -heat the mixture until formation of ester (odor) Pharmaceutical Applications of Alcohols:  Methanol and ethanol are very important organic solvents in organic synthesis reaction and re-crystilsation  Ethanol used as antiseptic and also in perfumes  Benzyl alcohol is used as a preservative in pharmaceutical preparations  Glycerol used as softener for the ears and sweetening agent 6