Optical Isomerism (OCR A Level Chemistry)

Two isomers of C₃H₆O are shown below.

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.

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.

Some skeletal examples of unbranched aldehydes and unbranched ketones are shown below.

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

One of the ketones, shown in part (c), will form an optical isomer when it reacts with HCN.

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 below.

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. 

Lactic acid can undergo condensation polymerisation to form polylactic acid as shown below.

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

Erythrulose is a carbohydrate, which is shown below. It 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.

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

Dihydroxyacetone has the molecular formula C₃H₆O₃.

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, shown below.

Draw 3D representations of the two optical isomers of cysteine.

The full displayed formula of threonine is shown.

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 a chiral carbon.

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

This question is about cholesterol, which is a fatty chemical that is used by the body to build healthy cells.

State two chemical tests that could be used to identify the functional groups of cholesterol. Your answer should include observations and diagrams of the product for each test.

State the number of chiral carbons in the cholesterol structure.

There are a large number of naturally occurring biological molecules that contain chiral carbons. These biological molecules are often produced as enantiomerically pure 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 enantiomerically pure compounds in living organisms.

This question is about the chemistry of limonene.

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

Limonene exists as a pair of enantiomers:

• One enantiomer is responsible for a strong orange smell
• The other enantiomer is responsible for a lemon smell.

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.

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 24.0 dm³ at room temperature and pressure)

The final step of the multi-step synthetic conversion of limonene to menthol is shown.

 

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

Lactic acid is an organic acid with the molecular formula C₃H₆O₃.

The oxidation product of lactic acid reacts with 2,4-dinitrophenylhydrazine but does not react with Tollens' reagent.

 Draw the displayed formula of lactic acid.

Illustrate the different types of isomerism shown by lactic acid. Your answer should not consider ethers.

The general structure of polylactic acid is shown below.

Draw two possible structures that can be 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.

This question is about aldehydes.

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

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

This question is about chemical reactions and their effects on chirality.

Aldehydes and ketones always react with HCN to form optical isomers. 

Discuss this statement, using the general formulae RCHO and R’COR’’. 

 Your answer should include equations where appropriate.

Acidified potassium dichromate(VI) solution was heated under reflux with the following β-hydroxyaldehyde.

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.