Video Transcript
Fill in the blanks. Complete oxidation of ethanol using acidified potassium dichromate solution produces blank that has a blank odor. (A) Acetone, fruity. (B) Ethanoic acid, vinegary. (C) Acetaldehyde, fruity. (D) Methanoic acid, pungent. Or (E) ethanoic acid, fruity.
To answer this question, we need to determine the product of the oxidation of ethanol using acidified potassium dichromate solution. Then, we need to determine what that product smells like. Ethanol is a two-carbon alcohol where the carbon atoms are joined together by single bonds. We can draw this chemical structure for ethanol. As the hydroxy group of ethanol is bonded to a carbon atom that has one alkyl substituent, we can classify ethanol as a primary alcohol. So let’s take a look at the oxidation of a generic primary alcohol.
In the question, we are told that acidified potassium dichromate solution is used. This solution serves as an oxidizing agent. For simplicity, we can represent the oxygen of this oxidizing agent by writing a capital O in brackets. When a primary alcohol is reacted with an oxidizing agent, the hydrogen of the hydroxy group and a hydrogen atom bonded to a carbon atom adjacent to the hydroxy group will be removed. To make up for this loss of bonds, a new carbon-oxygen double bond is formed. This produces an aldehyde and water.
To determine the product of the oxidation of ethanol, let’s redraw its structure so that it looks similar to the one given in the generic equation. When ethanol is reacted with an oxidizing agent, the hydrogen of the hydroxy group along with a hydrogen atom bonded to a carbon atom adjacent to the hydroxy group will be removed. And a new carbon-oxygen double bond will be formed. This produces an aldehyde and water. This two-carbon aldehyde is called ethanal, eth- meaning two carbon atoms, A-N indicating that the carbon atoms are single bonded together, and A-L for aldehyde. This compound also has a common name, acetaldehyde. Acetaldehyde is one of the answer choices, so it seems like answer choice (C) is the correct answer to this question. However, the question asks for the product of the complete oxidation of ethanol.
The reaction we have shown is the incomplete oxidation of ethanol. This oxidation is incomplete as aldehydes are easily oxidized. If there is still water and oxidizing agent present in the reaction vessel along with the newly formed aldehyde, the aldehyde will be oxidized in a series of complex steps to produce a carboxylic acid. So, for ethanol to be completely oxidized, the acetaldehyde produced via the first oxidation will need to undergo a further oxidation. We can see from the generic equation that the only difference between the reactant aldehyde structure and the product carboxylic acid structure is the group bonded to the carbonyl carbon. So the product of the oxidation of acetaldehyde will have the same basic structure as acetaldehyde, except that the hydrogen atom bonded to the carbonyl carbon will be replaced by a hydroxy group.
This carboxylic acid is called ethanoic acid, eth- meaning two carbon atoms, A-N indicating that the carbon atoms are single bonded together, and -oic acid for carboxylic acid. This is the product of the complete oxidation of ethanol. So we now know that the correct answer to this question is either answer choice (B) or answer choice (E). We just need to determine if ethanoic acid has a vinegary odor or a fruity odor. As it turns out, ethanoic acid, also called acetic acid, is the main component of vinegar apart from water. So we should expect ethanoic acid to have a vinegary odor, not a fruity odor.
Therefore, to answer the question, we should fill in the first blank with ethanoic acid and the second blank with vinegary, or answer choice (B). So the complete oxidation of ethanol using acidified potassium dichromate solution produces ethanoic acid that has a vinegary odor.