9701 MCQ Solution 20

Question 20: [Organic > Introductory Organic]:

The Russian composer Borodin was also a research chemist who discovered a reaction in which two ethanal molecules combine to form a compound commonly known as aldol (reaction I). Aldol forms another compound on heating (reaction II).

I 2CH₃CHO àCH₃CH(OH)CH₂CHO

II CH₃CH(OH)CH₂CHO à CH₃CH=CHCHO + H₂O

Which of the following describes reactions I and II?

	I	II
A	addition	elimination
B	addition	reduction
C	elimination	reduction
D	substitution	elimination

Reference: Past Exam Paper – November 2002 Paper 1 Q20

Solution 20:

Answer: A.

In the first reaction 2 molecules, regardless of the fact that it is the same molecule, combine to form 1 product, which can only be defined as an addition reaction, eliminating options C and D. 

The second reaction is a form of dehydration which in turn categorises under elimination reactions (it eliminates water molecules).