AQA A level Chem 3.6 Organic Analysis

A student has samples of these four compounds but does not know which is which:

• 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 identify which compound is which. (6)

Level 3 5-6 marks All stages are covered and each stage is generally correct and virtually complete

Stage 1

To identify 2-methylpropanal

Add Tollen’s reagant

(and the positive result for 2-methylpropanal) is silver mirror

Stage 2

To identify 2-methylpropan-1-ol

add acidified potassium dichromate

(and the positive result willl be) the colour change of the solution from orange to green

The Tollen’s reagant test should be done first to distinguish aldehyde from alcohol

Stage 3

to distinguish the acids,

use IR spectroscopy to distinguish the two acids

use the finger-print region of IR spectrum to look for exact match to spectra of known compounds

This question is about the structural isomers shown

Separate samples of each isomer are warmed with ethanoic acid and a few drops of concentrated sulfuric acid.

In each case the mixture is then poured into a solution of sodium hydrogencarbonate.

Identify the isomer(s) that would react with ethanoic acid.

Suggest a simple way to detect if the ethanoic acid reacts with each isomer. Give a reason why the mixture is poured into sodium hydrogencarbonate solution. (3)

P, Q, R, S (1)

Sweet smelling liquid (1)

to react with (excess acid) (1)

esterification occurs which when a carboxylic acid reacts with an alochol to form an ester and water

The oxidation of propan-1-ol can form propanal and propanoic acid

In a preparation of propanal, propan-1-ol is added dropwise to the oxidising agent and the aldehyde is separated from the reaction mixture by distillation.

Give two ways of maximising the yield of propanal obtained by distillation of the reaction mixture. (2)

Keep the temperature of the reaction mixture below the boiling point of propan-1-ol (1)

Cool the distillate / collecting vessel (1)

Compounds A, B, C and D are isomers with the molecular formula C₄H₁₀O

Compounds A and B are both straight-chain compounds. A can be oxidised to form P. B can be oxidised to form Q.

P and Q are isomers with molecular formula C₄H₈O

Tollens’ reagent and Fehling’s solution can be used to distinguish between isomers P and Q. The results shown in the table are obtained.

Use the information about compounds P and Q to identify compounds A and B. Explain your answer with reference to the functional groups in P and Q. (3)

A = butan-2-ol (1)

B= butan-1-ol (1)

Product from A / P is a ketone AND product from B/Q is an aldehyde (1)

Complete the empty boxes in the table. (3)

Row 1 = no (visible) change (1)

Row 2 = sodium hydrogencarbonate (1)

Row 3 = no (visible) change/reaction (1)