3.5 Carbonyl Compounds

This question is about the reactions of an organic compound R.

Analysis of 15.0 g of an organic compound, R, showed it to contain 69.8% carbon, 2.79 g of oxygen and the remaining mass was hydrogen.

Compound R is a straight chain organic compound. One isomer of compound R produces optical isomers of compound S when reacted with hydrogen cyanide followed by dilute acid.

Compounds R and S were tested with acidified potassium dichromate(VI) and Tollens reagent. The results are shown in Table 2.1.

Table 2.1

Using this information and your answer from part (a), identify compounds R and S.
Justify your answer.

Draw 3D representations of the two optical isomers of compound S.

There are some isomers of compound S that do not display optical isomerism.

Draw the skeletal formula of these isomers.

LiAlH₄ is a reducing agent.

LiAlH₄ cannot be used in aqueous solution because it reacts with water to produce LiOH(aq), H₂(g) and a white precipitate which is soluble in excess sodium hydroxide.

Identify the white precipitate.

Two students try to prepare 2‑hydroxybutanoic acid in the laboratory as shown in Fig. 3.1

Both students oxidise butane‑1,2‑diol to form P in reaction 1.
One student then reduces P using LiAlH₄. Q is formed.
The other student reduces P using NaBH₄. R is formed.

A third student prepares 2‑hydroxybutanoic acid using propanal as the starting material as shown in Fig. 3.2. In step 1 the student reacts propanal with a mixture of NaCN and HCN. 

Draw the mechanism for the reaction of propanal with the mixture of NaCN and HCN to form S.

● Identify the ion that reacts with propanal.
● Draw the structure of the intermediate of the reaction.
● Include all charges, partial charges, lone pairs and curly arrows.

Complete the equation for the reaction in step 2, when S is heated under reflux with HCl (aq).

C₂H₅CH(OH)CN + ..................... → C₂H₅CH(OH)COOH + ......................

Two isomeric compounds are shown below in Fig. 2.1. 

Fig. 2.1

[2]

The same reaction in part (b) can be used to oxidise ethanol into ethanal or ethanoic acid, depending on the reaction conditions.

Outline how the reaction conditions can be changed to produce ethanal or ethanoic acid.

Under the right conditions, the two molecules of compound A can react together to produce a dimer.

Compound W can be converted into three different organic compounds as shown by the reaction scheme in Fig. 3.1.

Fig. 3.1

Write an equation for the formation of X. Use [H] to represent the reagent in the equation.

Identify the specific type of isomerism shown by the product Y, in Fig. 3.1. 

Draw the mechanism to show the formation Y and explain how this leads to the isomers of Y. 

When 5.00 cm³ of propanal (Mr = 58.0) were reacted with an excess of acidified potassium dichromate (VI) solution, 4.25 g of propanoic acid (Mr  = 74.0) were obtained.

The density of propanal is 0.810 g cm⁻³. 

Calculate the percentage yield for this reaction.

A different carbonyl compound, Q, (M_r = 72) reacts with 2,4-dinitrophenylhydrazine but not with Tollens’ reagent.

Q can be reduced to compound R. For this reaction

2-hydroxybutanoic acid can be made from propanal by via a reaction with hydrogen cyanide and the subsequent hydrolysis of 2-hyrdoxybutanenitrile. A possible pathway is shown in Fig. 4.1.The incomplete mechanism for reaction 2 is shown in Fig. 4.2. 

Fig. 4.1

Fig. 4.2

2-hydroxybutanenitrile contains a chiral centre. Draw the three-dimensional structures of the two optical isomers.

State the reagent(s) for reaction 3.

Compound A forms propanal when reacted with acidified potassium dichromate. Give the structural formula of compound A. 

Compound A and propanal were tested using a series of reagents outlined in Table 4.1. For each of the tests listed: 

• State the positive result
• Identify which compound(s) would give a positive result

Table 4.1