Past paper questions (paper 2)

Question 1

• Butanoic acid
• 2-methylpropanal
•2 - methylpropanoic acid
•2-methylpropan-1-ol

step 1 : two of these compounds can be identified by simple chemical tests
step 2: the other two compounds that contain the same functional group as each other can then be distinguished using a spectroscopic technique .

Describe how these two steps could be used to identity which compound is which (6)

Answer 1

Stage 1
• 2-methylpropanal can be distinguished using Fehlings or Tollens.
• silver mirror or orange - red ppt

Stage 2:
• 2-methylpropan-1-ol can be distinguished using acidified potassium dichromate
• orange to green
• if aldehyde test was done first , it would give the same result

Stage 3:
Distinguishing the acids
• Using IR spectroscopy
• use finger - print region of iR spectrum
• Look for exact match to spectra of known compounds ( what the difference is )

Question 2

In europe , some of the glucose form crops is fermented to produce ethanol.
Use of carbon - neutral fuel leads to no net emissions of carbon dioxide to the atmosphere .
The ethanol produced by fermentation of glucose may be regarded as a carbon - neutral fuel.
Justify this statement . Include relevant chemical equations in your answer (4)

Answer 2

6CO2 + H2O => C6H12O6 + 6O2
Photosynthesis absorbs 6 CO2 molecules.
C6H12O6 => 2C2H5OH + 2CO2
Fermentation releases 2CO2 molecules.
2C2H5OH + 6O2 => 4CO2 + 6H2O
Combustion of ethanol releases 4 CO2 molecules

Question 3

Explain how an infrared spectrum would confirm that the cyclohexane obtained from chromatography column did not contain any cyclohexanol.(1)

Answer 3

• Would be no peak at 3250-3550 due to O-H (alcohol)