Oxidation of Alcohols Use of an Environmentally Friendly (Green) Oxidant

Transcription

1 xidation of Alcohols Use of an Environmentally Friendly (Green) xidant xidation of Alcohols xidation of alcohols to carbonyl compounds is one of the most important reactions of alcohols. [] In the oxidation of an alcohol, two hydrogens are removed by the oxidizing agent one from the carbinol carbon (blue ) and one from the hydroxyl oxygen (red ). 1

2 xidation of Secondary Alcohols Secondary alcohols are oxidized cleanly to ketones by a variety of oxidizing agents. R [] R R' R' Tertiary alcohols are not oxidized under mild conditions because there is no hydrogen on the carbinol carbon. R R [] X no reaction R' xidation of Primary Alcohols Primary alcohols are oxidized to aldehydes, but many reagents can oxidize the aldehyde further to a carboxylic acid. R 2 [] R [] R? A key to preventing oxidation beyond the aldehyde is the use of anhydrous oxidation conditions. 2

3 xidation of Alcohols Laboratory scale oxidations of alcohols have frequently been carried out using chromium salts as the oxidizing agents. owever, chromium (VI) salts are toxic and potentially carcinogenic and require careful handling and appropriate disposal. The true story and the movie Erin Brockovich involved inappropriate disposal of chromium salts. xidation of Alcohols In today s lab, we will use an environmentally friendly procedure in which household bleach (6% Nal) is the oxidizing agent and the reduced by-product is sodium chloride, a chemical easily disposed of in small amounts! The reaction is conducted in acetic acid, so the active oxidizing agent is hypochlorous acid, l. 3

4 ypochlorite xidation of Alcohols - Mechanism The initial steps of the mechanism involve nucleophilic substitution at chlorine by the alcohol oxygen. a) l l b) c) + l l 2 l l ypochlorite xidation of Alcohols - Mechanism The final and key step of the mechanism involves the elimination of the elements of l from the chlorine-substituted alcohol. hlorine is transformed from oxidation state of +1 to -1 (a 2-electron change) in this reaction. 2 l l 4

5 xidation of a Unknown Alcohol In today s experiment, the objective is to oxidize one of four (4) secondary alcohols to the corresponding ketone using bleach as the oxidizing reagent. Your TA will assign each pair of students a specific starting material bottle. 3-heptanol cyclopentanol cyclohexanol 2-heptanol xidation of a Unknown Alcohol Today, you will purify the oxidation product by distillation to obtain a boiling point range for your product. You will also need to obtain an IR spectrum of both your distilled product and of the alcohol which was your starting material for this oxidation. 5

6 Identity of Alcohol and Ketone! You should be able to determine the identity of your starting alcohol and product ketone. You will use the observed boiling point of your product, the infrared spectrum of your product, and the infrared spectrum of your starting material to determine their structures. Reference spectra of the starting alcohols and of the product ketones will be posted for comparison with the IR spectra that you obtain on your products. xidation of a 2 o Alcohol Exptl. You will use an Erlenmeyer flask with a stirbar as your reaction flask. You will insert a thermocouple into the reaction mixture to monitor the temperature. Do not allow temperature to stay above 45 o. Make sure the temperature does go above 30 o. An excess of oxidant is necessary. You will use iodide-starch indicator paper to determine if you have excess oxidant at the end of the reaction. 6

7 xidation of a 2 o Alcohol Exptl. To test the solution for presence of excess bleach, remove the thermocouple from the flask and touch the end of the thermocouple to a piece of starch iodide paper and observe the color. Replace the thermocouple into the flask. Place used test strips on watch glass until time to clean up. xidation of a 2 o Alcohol Exptl. Question - What color does starch-iodide paper turn if excess oxidant is present? (Write down an answer) heck your answer with your neighbor The starch-iodide paper turns blue-black when it is treated with an oxidant. 7

8 xidation of a 2 o Alcohol LEANUP PREDURES The aqueous phases from the extractions may be washed down the sink. Place the contaminated test strips and other paper products in the container for solid waste. Dispose of the dichloromethane and the product in the organic waste jar. SAFETY Glacial acetic acid is a dehydrating agent, an irritant, and causes burns. It is much stronger than the dilute solutions found in vinegar. Dispense it in a hood and avoid contact with skin, eyes, and clothing. Sodium hypochlorite solution can emit chlorine gas, which is a respiratory and eye irritant. Dispense it in a hood and keep your graduated cylinder and reaction flask in your mini-hood. 8

9 SAFETY Set up your reaction DIRETLY in front of your mini-hood intake above your stirrer. ave an ice bath ready in case the reaction temperature exceeds 45 o. 9