Answer Happy

Identification of Alcohols and Phenols Ref: Bettelheim, F. A.; Landesberg, J. A. "Experiments for Introduction to Organic Chemistry," Saunders College Publishing, Fort Worth, TX, 1997, pp. 85-94. Background Specific groups of atoms in an organic molecule can determine its physical and chemical properties. These groups are referred to as functional groups. Organic compounds which contain the functional group -OH, the hydroxyl group, are called alcohols. Alcohols are important commercially and include use as solvents, drugs, and disinfectants. The most widely used alcohols are methanol or methyl alcohol, CH3OH, ethanol or ethyl alcohol, CH₂CH₂OH, and 2-propanol or isopropyl alcohol, (CH3)2CHOH. Methyl alcohol is found in automotive products such as antifreeze and "dry gas." Ethyl alcohol is used as a solvent for drugs and chemicals but is more popularly known for its effects as an alcoholic beverage. Isopropyl alcohol (isopropanol or 2-propanol) also known as "rubbing alcohol," is an antiseptic. Alcohols may be classified as primary, secondary, or tertiary: 2 Phenols bear a structural resemblance to alcohols in that the hydroxyl group is present. However, since the -OH group is bonded directly to a carbon that is part of an aromatic ring, the chemistry is quite different from that of alcohols. Phenols are more acidic than alcohols; concentrated solutions of the compound phenol are quite toxic and can cause severe skin burns. Phenol derivatives are found in medicines; for example, thymol is used to kill fungi and hookworms. (Also see table 9.1 attached) X In this laboratory, you will examine physical and chemical properties of representative alcohols and phenols. You will be able to compare the differences in chemical behavior between these compounds and use this information to identify an unknown. Physical Properties Since the hydroxyl group is present in both alcohols and phenols, these compounds are polar. The polarity of the hydroxyl group, coupled with its ability to form hydrogen bonds, enables alcohols and phenols to mix with water. The cutoff for water solubility of simple alcohols is around four carbons. As the alcohol becomes larger it becomes more alkane-like and the solubility decreases. If more than one -OH group is present on the molecule the solubility
around four carbons. As the alcohol becomes larger it becomes more alkane-like and the solubility decreases. If more than one -OH group is present on the molecule the solubility increases because it becomes more water-like. Since these compounds also contain non-polar portions, they show additional solubility in organic solvents, such as methylene chloride (CH2Cl2) and diethyl ether (CH3CH2)2O. 2 Chemical Properties The chemical behaviors of the different classes of alcohols and of phenols can be used as a means of identification. Quick, simple tests that can be carried out in test tubes will be performed. 1. Lucas Test. This test is used to distinguish between primary, secondary and tertiary alcohols. Lucas reagent is a mixture of zinc chloride, ZnCl2, in concentrated HCl. Upon addition of this reagent a tertiary alcohol reacts rapidly and immediately gives an insoluble white layer. A secondary alcohol reacts slowly and after heating slightly gives the white layer within 10 min. A primary alcohol does not react. Any formation of a heterogeneous phase or appearance of an emulsion is a positive test. The test is also time and heat dependent. CH3CH2-OH + HCl + ZnCl2 ---> no reaction 1º alcohol (CH3)2CH-OH + HCl + ZnCl₂ ---> (CH3)2CH-Cl + H₂O (10 min., heat) 2º alcohol insoluble (CH3)C-OH + HCl + ZnCl₂ ---> (CH3)3C-Cl + H₂O (immediate) 3⁰ alcohol insoluble 2. Chromic acid test. This test is able to distinguish primary and secondary alcohols from tertiary alcohols. Using chromic acid, also known as Bordwell-Wellman reagent, primary alcohols are oxidized to aldehydes and subsequently to carboxylic acids; secondary alcohols are oxidized to ketones; tertiary alcohols are not oxidized. In the oxidation, the orange color of the chromic acid changes to a blue-green solution. Phenols are oxidized to nondescript brown tarry masses. 13 H₂O 3 CH3CH2OH + 4H2CrO4 + 6 H2SO4 -> 3 CH3CO2H+ 2 Cr2(SO4)3 + 1º alcohol orange carboxylic acid blue-green 3 (CH3)2CHOH + 2 H2CrO4 + 3 H2SO4 -> 3 (CH3)2C-O + Cr2(SO4)3 + 8 H₂O 2º alcohol ketone orange blue-green No Reaction (CH3)3COH + 3° alcohol H₂CRO4 + H₂SO4 orange remains orange
3. Iodoform test. This test is more specific than the previous two tests. Only ethyl alcohol and 2° alcohols with a methyl group directly attached to the carbon bearing the hydroxyl group react. These alcohols react with iodine in aqueous sodium hydroxide to give the yellow precipitate iodoform. Phenols also react under these conditions. With phenol, the yellow precipitate triiodophenol forms. 4. Acidity of phenols. Phenol is also called carbolic acid. Phenol is a weak acid and will react with a base; thus phenols readily dissolve in base solutions. In general, organic compounds that bear a charge are soluble in water. In contrast, alcohols are not acidic. 5. Ferric chloride test. Addition of aqueous ferric chloride to a phenol gives a colored solution. Depending on the structure of the phenol, the color can vary from green to purple. This color change can be used to differentiate alcohols from phenols.
Pre-lab Questions 1. Complete the following table. Structure 1º,2º, or 3⁰ Mol. Wt. Bp. Hazards 1-butanol 2-propanol 2-methyl-2- propanol Resorcinol 2. What is the basis for the solubility of alcohols in H₂O? What is the "cutoff" for alcohol solubility? 3. Write an equation for the reaction of 2-butanol with the Lucas reagent. 4. A modification of the Bordwell-Wellman reagent is sometimes used for a breathalizer test for ethanol. Show the reaction that occurs when someone is tested for intoxication and what would identify the alcohol present.