Experiment 6 – Alcohols and Phenols, High school final essays of Chemistry

Download Experiment 6 – Alcohols and Phenols and more High school final essays Chemistry in PDF only on Docsity! Experiment 6 — Alcohols and Phenols Alcohols are organic molecules that contain a hydroxyl (-OH) group. Phenols are molecules that contain an —OH group that is directly attached to a benzene ring. Alcohols can be classified as primary, secondary, or tertiary. This classification is based on whether the alcohol carbon is attached to one, two, or three alkyl groups. This classification is important, because the different classes of alcohols react differently. Examples of the classes of alcohols are shown below. Because alcohols contain an —OH group, they are able to form hydrogen bonds to H CH CH HRC C—OH H3C-——C——0H H3C-———C——OH H H CH3 A primary alcohol A secondary alcohol A tertiary alcohol one another. They therefore have high boiling points. Alcohols can also form hydrogen bonds with water, so small alcohols are water-soluble. The smallest alcohols, methanol (CH,OH) and ethanol (CH,CH,OH), are completely soluble in water in any proportions. As the hydrocarbon part of an alcohol gets larger, the alcohol becomes less water soluble and more soluble in nonpolar solvents. Phenol is somewhat soluble in water. It acts as a weak acid in water, so a solution of phenol will be slightly acidic. Oxidation Alcohols can be oxidized by oxidizing agents such as chromate or dichromate ions (these contain chromium in the +6 oxidation state). A primary alcohol is oxidized to an aldehyde and then oxidized further to a carboxylic acid. HH oO: | [0] | [0] I HAC H > H3C oH H3C > —OH | A primary alcohol An aldehyde A carboxylic acid Secondary alcohols are oxidized to ketones. OH 0 | 101 | HRC | CH3 KC cH, A secondary alcohol A ketone Tertiary alcohols cannot be oxidized. 27 CH; [0] ERC: ‘C——OH ———> No reaction CH3 A tertiary alcohol In the process of oxidation, the orange chromate solution is reduced to a green solution containing chromium in the +3 oxidation state. During this oxidation, hydrogen atoms are removed from the alcohol (one hydrogen is removed from the -OH group and the other is removed from the carbon atom that is attached to the -OH group). Tertiary alcohols cannot be oxidized because there is no hydrogen atom attached to the alcohol carbon. In this test, if the orange solution of chromate turns green, it is taken as evidence that oxidation has occurred. If the substance tested is an unknown alcohol or phenol and you see a positive reaction, it means that it cannot be a tertiary alcohol. Phenols can also be oxidized, but they are not oxidized to aldehydes or ketones. Lucas Test Some alcohols react with ZnCl, in an acidic aqueous solution to give an alkyl choride. The Lucas test involves a substitution reaction, where the -OH group of the alcohol is replaced by a Cl atom. The Lucas reagent consists of a mixture of HCl and ZnCl, that are dissolved in water. R R | ZnCl | R OH R cl | Hcl | R R A tertiary alcohol A tertiary alkyl chloride The mechanism of this reaction involves a carbocation intermediate: first an OH ion is removed from the alcohol, leaving the carbon atom in the molecule positively charged. Then a Cl ion adds to the positively charged carbon atom. The result is the alcohol is converted to a chlorinated alkane, with the Cl occupying the spot that the -OH was in. Since tertiary carbocations are much more stable than primary or secondary carbocations, tertiary alcohols will react readily with the Lucas reagent. Secondary alcohols will react slowly (usually in 5-10 minutes). Primary alcohols will not react much at all, since a primary carbocation is so unstable. The chlorinated product is usually insoluble in water (the solvent in this reaction), whereas the initial alcohol is usually soluble in water because of its OH group. Cloudiness appearing in the reaction mixture is taken as evidence of a positive reaction. You will need to note the time at which the cloudiness appears. Jodoform Test The iodoform test is used to identify secondary alcohols that have a methyl group on the alcohol carbon. This type of alcohol will react with I, in NaOH to give a yellow precipitate of iodoform, CHI;. The reaction is shown below. 28 Part 5: Iodoform Test 7. In this part, you will test ethanol, 2-propanol, phenol solution, and your unknown. Place about | mL of water (20 drops) in each of 4 test tubes. Add 5 drops of the liquid to be tested. Add 10 drops of 10% NaOH and mix by shaking each tube side to side. Add 10 drops of KI/iodine solution and shake to mix. A yellow precipitate indicates a positive reaction. Part 6: FeCl, Test [ Caution: avoid skin contact with phenols! | 8. You will test ethanol, solid phenol, solid salicylic acid, and your unknown. To test a solid, put a pea-sized amount of solid in a test tube and add 1 mL of water. Shake the tube to dissolve the solid as well as possible (don’t worry if the solid doesn’t completely dissolve). Add 1 drop of FeCl, solution and watch for a color change. A purple color indicates a positive reaction. To test a liquid, put 5 drops in a test tube and add 1 drop of FeCl, solution. Part 7: Phenol Solubility in Base 9. Obtain 3 test tubes. Place 2 mL of deionized water in one test tube, 2 mL of 5% NaHCO, (q) in another, and 2 mL of 5% NaOH ,,) in the third. Add 0.25 g of solid phenol to ‘Sach tube. Shake each one thoroughly to try to dissolve the solid. Record the relative solubility of phenol in each of the solutions. Part 8: Identification of the Unknown Substance 10. Based on the results from each of the experiments, discuss what you know about the structure of your unknown. Questions 1. Label each of the following alcohols as primary, secondary, tertiary, or phenolic and name each one. a. b. CH, HC OH HC ——0oH CH c. d. OH OH e f. 31 OH OH An unknown has a chemical formula of C,H,,O. When chromic acid was added to the unknown, it turned green. When the Lucas reagent was added to a separate portion of the unknown, it tured cloudy in 5 minutes. When NaOH and KI/iodine was added to the unknown, a yellow precipitate formed. What is the structure of the unknown compound? Write the structure of the product(s) of each of the following reactions. ZnCly a. CH,CH,CH,OH —> Ha CH3 ZnCl, b. CH CH, —cCH,;CH, =» ————» HCL OH OH OH dC C——CH,CH; OH NaOH 32 Would you expect 1-propanol to be soluble in water? Would you expect 1- heptanol to be soluble in water? Would either of them be more soluble in hexane than in water? Explain. An unknown alcohol or phenol was tested in the lab. When chromic acid was added, it turned from orange to green. When FeCl, was added to a separate portion of the unknown, it tured purple. What can you say about the structure of this unknown? 33