Optical Isomerism (AQA A Level Chemistry)

Two isomers of C₃H₆O are shown below in Figure 1

Figure 1

i) Give the IUPAC names of Compounds A and B.

ii) Name and outline the mechanism for the reaction of compound A with HCN.

iii) Name the product formed from this reaction. 

Name the product of the reaction between compound B and HCN and explain why this compound is optically inactive.

Explain why the reaction of compound A and HCN can produce a racemic mixture.

Explain why the racemic mixture produced from the reaction of propanal and HCN has no effect on plane polarised light.

The structure of pentanal is shown below in Figure 1. 

Figure 1

Draw the structure of a branched isomer of pentanal that exhibits optical isomerism.

Draw the structure of a branched isomer of pentanal that does not exhibit optical isomerism.

In terms of optical isomer products, compare the reactions of unbranched aldehydes and unbranched ketones with HCN.

Some examples of unbranched aldehydes and unbranched ketones are shown in Figure 2.

Figure 2

One of the ketones, shown in Figure 2, will form an optical isomer when it reacts with HCN.

Figure 2

i) Identify the ketone that will form an optical isomer with HCN and name the product.

ii) Draw 3D representations of the two optical isomers produced.

Lactic acid can be synthesised from ethanal. The first step of this synthesis is the reaction with HCN. The mechanism for this reaction is shown in Figure 1.

Figure 1

State the role of the CN^- in this reaction.

Lactic acid is an optically active organic acid with the molecular formula CH₃CH(OH)COOH. 

Draw the displayed formula of lactic acid. 

Lactic acid exists as stereoisomers. 

i) Explain the meaning of the term stereoisomerism.

ii) Explain how you can distinguish between the isomers. 

Lactic acid can undergo condensation polymerisation to form polylactic acid, shown in Figure 2.

Figure 2

Label the chiral carbons in the polylactic acid molecule with an ‘x’.

Erythrulose is a carbohydrate, which is used in combination with dihydroxyacetone (DHA) in many self-tanning products because of its natural ability to dye the skin. The sugar reacts with the amino acids of the keratin found in the dead skin cells of the upper layer of the skin.

The structure of Erythrulose is shown in Figure 1.

Figure 1

Identify which carbon in this molecule is chiral. Justify your answer. 

Dihydroxyacetone has the molecular formula C₃H₆O₃.

i) Draw the two isomers of dihydroxyacetone which contain the carbonyl group.

ii) Name the type of isomerism exhibited by these isomers.

A student incorrectly states that 1,1-dihydroxypropanone exhibits optical isomerism because the central carbon atom has different groups attached.

Explain the student’s error.

The  keratin found in the dead skin cells of the upper layer of the skin are rich in the amino acid cysteine.

Using your data booklet, draw 3D representations of the two optical isomers of cysteine.

The full displayed formula of threonine is shown in Figure 1.

Figure 1

State the systematic name of threonine.

Threonine contains two chiral centres.

Draw 3D representations of the pair of enantiomers from one of the chiral centres, showing how the two structures are related to each other.

State the bond angle around the chiral carbon drawn in part (b).

State the relationship between two chiral compounds with the same structural formula.

Cholesterol, shown in Figure 1, is a fatty chemical used by the body to build healthy cells.

Figure 1

State two chemical tests that could be used to identify functional groups within the cholesterol structure.

Your answer should include any expected observations and diagrams of the product of each test.

State the number of chiral carbons in the cholesterol structure.

A student suggested that cholesterol could be tested with plane polarised light to show that it contains chiral centres. 

Justify whether the student is correct.

There are a large number of naturally occurring biological molecules that contain chiral carbons. These biological molecules are often produced as enantiopure compounds in living organisms. All amino acids, except glycine, are a good example of this.

Suggest why amino acids are able to be produced as enantiopure compounds in living organisms.

Limonene, shown in Figure 1, is a naturally occurring hydrocarbon with the molecular formula C₁₀H₁₆ commonly found in the rinds of citrus fruits such as grapefruit, lemon, lime and oranges.

Figure 1

Limonene exists as a pair of enantiomers; one enantiomer is responsible for a strong orange smell while the other is thought to smell like lemons.

Draw 3D representations of the two enantiomers of limonene. 

Suggest why receptors in the human nose can distinguish between the orange and lemon enantiomers of limonene.

Limonene can undergo full hydrogenation to form menthane as shown in Figure 2.

Figure 2

For the complete hydrogenation of an impure sample of 3.00 g of limonene at room temperature and pressure,  870 cm³ of hydrogen was required. 

Calculate the percentage purity of limonene.

(1 mol of hydrogen occupies 24 dm³ at room temperature and pressure)

Limonene can be used in the multi-step synthesis of menthol. The final step of this reaction is shown in Figure 3.

Figure 3

State the reagent for this reaction and the type of reaction.

Lactic acid has the molecular formula C₃H₆O₃. The oxidation product of lactic acid does not react with Fehling’s solution.

Draw the skeletal formula of lactic acid.

With the aid of diagrams, describe the types of isomerism shown by lactic acid. Your answer should not consider ethers.

The general structure of polylactic acid is shown in Figure 1.

Figure 1

Draw two possible structures formed from two repeating units. 

Your answer should keep the main polymer chain in the same plane but show the 3D representation of the chiral carbons.

State why the polymer formed from the uncontrolled condensation polymerisation of lactic acid is not a racemate.

Draw the displayed formula and 3D representations of the smallest aldehyde to form optical isomers.

i) Draw the displayed formula of the oxidation product of the aldehyde identified in part (a).

ii) State a suitable reagent, including observations, for the oxidation of the aldehyde identified in part (a).

i) Write a balanced symbol equation, using structural formulae, to show the reduction of the aldehyde identified in part (a). You should use [H] to show the reducing agent.

ii) State a suitable reducing agent.

Name the organic compound formed by the reaction of the oxidation product in part b) and the reduction product in part c)of the aldehyde identified in part (a).

Evaluate the statement that "aldehydes and ketones always react with HCN to form optical isomers".

Your answer should use the general formulae RCHO and R'COR'', and include equations where appropriate.

A β-hydroxyaldehyde, shown in Figure 1, reacts with acidified potassium dichromate(VI)  solution under reflux.

Figure 1

Explain the change in chirality, if any, between the β-hydroxyaldehyde and its organic product.

Explain the change in chirality, if any, when pent-1-en-3-ol is hydrogenated.

Other isomers of pent-1-en-3-ol, C₅H₁₀O, exist.

i) Draw the displayed formula of the secondary alcohol isomer of pent-1-en-3-ol that exhibits both stereoisomerism and optical isomerism and identify the chiral centre.

ii) Give the IUPAC name for this isomer.