Question 79: [Organic > Hydroxyl Compounds]

When the apparatus below was used with compound Z, a brick-red precipitate formed in the right-hand tube.

Which compound could be Z?

1 CH₃CH(OH)CH₃

2 CH₃CH₂CH₂OH

3 CH₃OH

Reference: Past Exam Paper –June 2003 Paper 1 Q39

Solution 119:

Answer: C.

1, 2 and 3 are all alcohols. When an alcohol is presented to heated acidified potassium dichromate they will oxidise. In 1, the alcohol has 1 H atom bonded to the alcohol-bearing C atom and is secondary which will form a ketone. In 2 and 3 the alcohols have 2 or more H atoms bonded to the alcohol-bearing C atom and are primary which will form aldehydes. Aldehydes turn into a brick red precipitate when presented to Fehling’s reagent. Thus 2 and 3 are the only possible compounds that Z can be. Thus the correct answer is option C

Question 69: [Organic > Hydroxyl Compounds]

In a preparation of ethene, ethanol is added a drop at a time to a heated reagent Y. To purify the ethane it is bubbled through a solution Z and then collected.

What could reagent reagent Y and solution Z be?

Reference: Past Exam Paper –June 2003 Paper 1 Q29

Solution 69:

Answer: C.

Alcohol undergoes dehydration to form an alkene. Concentrated H₂SO₄ is among the possible conditions for this reaction. This eliminates options A and D as possible answers. After dehydration occurs there may be remaining H₂SO₄ mixed with the alkene. This remainder must be neutralised. It is acidic and in option B, the addition of more H₂SO₄ will contaminate the alkene more, thus is an incorrect answer. A basic solution will neutralise the acid, such as the dilute NaOH in option C, which is the correct answer.