Lab #10: Oxidation of an Aromatic Side Chain: Preparation of Benzoic Acid from

Ethyibenzene.

Chem 233

Tuesday 04/03/06

8:00-12:00

TA: David Dickson

Purpose:

The purpose of this lab was to prepare benzoic acid from ethylbenzene. This was done by oxidizing ketone and hydrolyzing of nitriles, esters, and amides.First, ethylbenzene reacted slowly with permanganate ion to remove benzylic hydrogen atom and form a carbocation. Then it went under another fast reaction of oxidizing of the side chain and formed 1-phenylethanol. Next, it went under another oxidation and formed acetophenone. The oxidant used during the oxidation was chromic acid. Finally the formation of benzoic acid was another fast reaction. The final product formed after the purification came out to be 0,75grams. The percent yield was about 75%. For this reaction to occur bezyllic hydrogen atom is removed in a free radical reaction started by the permanganate ion. Subsequent reactions lead to the formation of a ketone, which is further oxidized to a carboxylic acid. In the side chain reaction of this experiment ethyl benzene is converted to benzoic acid by reacting with potassium permanganate. The intermediate acetopeone loses a methyl group when it is oxidized to give out carbon dioxide. Further sodium carbonate is produced in the reaction by addition of carbon dioxide with aqueous sodium hydroxide.

Procedure:

First, a 250-mi round bottom flask was set with a reflux condenser. Then,5.6 grams of potassium permanganate with 60 ml of water, 0.8 ml of 3M aqueous NaOH, and 0.9grams of ethyl benzene was added to the round-bottom flask. After the boiling chips were added the solution was heated for one hour. Then the solution was tested on the filter paper to see if the permanganate still exist or not. Next, when the test came out positive, more of sodium bisulfate was added to the solution to destroy the excess permanganate ions. Then the solution was vacuum filtered with two portions of 5-mi of hot water.

Next, the filtrate was distilled until there was 15-ml left in the round-bottom flask. Then the solution was acidif’ with concentrated hydrochloric acid until the benzoic acid separated. Then the mixture was cooled off in an ice-bath for few minutes, and the solution went under vacuum filtration again. Next, the crude was purified again by recrystallization with hot water. Next, the crude was dried off, and the percent yield and the melting point of benzoic acid was determined.

Reactions:

M.formula / Name / Structure / M.W / BP / MP
KMnO4 / Potassium permanganate / 158.0 / 240
C8H10 / Ethyl Benzene / 106.16 / 136.2 / -94.9
NaOH / Sodium hydroxide / 39.99 / 1390 / 318
HCl / Hydro chloric acid / 36.4609 / -85.06 / -114.24
HNaO4S / Sodium bisulfate / 120.05

Calculations:

Mass of Sodium Sulfate / 2.4 grams
Molar mass of Benzoic acid / 123.26 grams
Volume of collected filtatrate / 47 ml
Volume of Distillate / 32 ml
Mass of Benzoic acid (before) / 1.1 grams
Mass of Benzoic acid (after) / 0.75 grams
Theoretical moles / 0.0081 mol
Actual moles / 0.00608 mol
Melting point / 122

Actual Moles = (0.7sgraxns)/(123.26grams /mol) = 0.00608 moles

Percent Yield = (actual yield)! (Theoretical yield) * 100%

= (O.OO6O8moles) / (0.OO8lSmoles) * 100% = 74.6% ~ 75%

]Discussion:

In this lab experiment an aromatic hydrocarbon (ethylbenzene) was reacted with potassium permanganate to form an aromatic carboxylic acid, which in this case was benzoic acid. First, ethylbenzene reacted slowly with permanganate ion to remove benzylic hydrogen atom and form a carbocation. Then it went under another fast reaction of oxidizing of the side chain and formed 1-phenylethanol. Next, it went under another oxidation and formed acetophenone. The oxidant used during the oxidation was chromic acid. Finally the formation of benzoic acid was another fast reaction. The final product formed after the purification came out to be 0,75grams. The percent yield was about 75%. Some of the human error factors that might have contributed towards the results can be; inaccurate measurements of chemicals, spillage of solutions either before or during the experiment, lack of time, scrapping off the filter paper from the filters, not using the correct equipment for certain step such as using of large graduate cylinder instead of small one, or weighed the filter paper along with the product, or letting the product cool off before filtering it out first and then cooling it in the ice bath, etc. Lot of equipment in the lab does not function properly which makes it really difficult to perform the experiment and get the most closest and accurate results.