Transcript Alcohols
Chemistry 2412L Properties of Alcohols pre-lab lecture Alcohol Structure • Alcohols are characterized by a hydroxyl bonded to a saturated (sp3) carbon • Alcohols can be classified as primary (1°), secondary (2°), or tertiary (3°) depending on the type of carbon to which the hydroxyl group is attached • The classification of an alcohol as primary, secondary or tertiary affects the chemical properties of the alcohols, which can be used as a means of identification. Examples of alcohol classification CH3 CH3 CH3 CH3 CH2 OH CH3 CH C OH OH CH3 ethanol (primary) 2-propanol (secondary) 2-methyl-2-propanol (tertiary) Properties of Alcohols • In this experiment you will be investigating the chemical and physical properties of alcohols. • You will observe results of 4 tests on known compounds, and use your observations to identify an unknown compound. Unknowns are either primary, secondary, or tertiary alcohols. Procedure Notes • The four tests are: 1. 2. 3. 4. Solubility Lucas test Oxidation Ester formation Procedure Notes 1. Solubility – – – – The hydroxyl group present in all alcohols is a polar functional group which allows alcohols to form hydrogen bonds. Small chain alcohols are miscible with water because of the hydrogen bonding interactions that will occur between the hydroxyl functional group and the water. If the R group of the alcohol becomes too large, however, the solubility of the alcohol will decrease. Note how many drops must be added until the alcohol is no longer soluble. Classify each compound you test as very soluble, slightly soluble or insoluble. Procedure Notes 2. Lucas test – – The hydroxyl group is a poor leaving group and is not readily displaced by nucleophiles. In strongly acidic solutions, however, protonation of the -OH group can occur, and water (a good leaving group) may be displaced. The Lucas reagent (anhydrous zinc chloride dissolved in concentrated hydrochloric acid) will cause the conversion of alcohols to insoluble alkyl chlorides. R-OH + HCl ZnCl2 R-Cl + H2O (insoluble) – – – The alcohol being tested must dissolve initially in the Lucas reagent. The test is positive if the mixture then becomes cloudy or a second layer of droplets forms. Note how quickly the reaction occurs for each type of alcohol. Tertiary alcohols should react immediately, secondary alcohols in 3 to 10 minutes, and primary alcohols may take an hour or longer, or may never react. Procedure Notes 3. Oxidation using chromic acid – – – – Primary and secondary alcohols are oxidized rapidly to carboxylic acids and ketones, respectively, by chromic acid reagent (see next slide). Chromic acid reagent (H2CrO4, also called the Jones reagent) is prepared by dissolving CrO3 in water and adding concentrated sulfuric acid. A positive test is indicated by a change in color from orange-red to a turbid blue-green. The bluegreen color arises from the formation of Cr(III) sulfate salts as the Cr(VI) is reduced. Tertiary alcohols are not oxidized by this reagent. However, they can dehydrate and the dehydration products can oxidize after 2-3 minutes. You will need to make the decision about whether or not the test was positive quickly after adding the reagent. Procedure Notes 3. Oxidation using chromic acid Procedure Notes 4. Ester formation: – Alcohols react with carboxylic acids to yield esters in a condensation reaction in which water is eliminated. O O + + C R OH R'OH H + C R H2O OR' – Many esters have fragrant odors and are the materials primarily responsible for the odor of flowers and ripe fruits. – A positive test (presence of the ester) is indicated by the odor of the product. Safety Notes • Concentrated hydrochloric acid and sulfuric acid, glacial acetic acid, and acetic anhydride are corrosive materials and should be used with care. • Chromic acid is a suspected carcinogen. • If any of the above chemicals are spilled on the skin rinse thoroughly with water, then wash with soap and water. • Waste from the Lucas test must go in the halogenated waste container. All other waste may go in the non-halogenated waste container.