Ch 4 · Isomerism: structural and stereoisomerism · AS & A Level Chemistry 9701

Topic 4 of 4 in An introduction to AS Level organic chemistry — AS & A Level Chemistry 9701 — 58 lessons.

Introduction

Isomers are compounds with the same molecular formula but different arrangements of atoms. Cambridge examiners expect isomers to be classified precisely into their correct category, and expect chiral centres and cis/trans arrangements to be identified directly from a given structural or skeletal formula, so the definitions in this lesson must be applied, not just recited.

Key ideas — Structural isomerism

Structural isomers have the same molecular formula but different structural formulae, meaning their atoms are connected in a different order. There are three types:

🧑‍🏫 Teacher's explanation

Imagine you have a set of LEGO bricks. If you build a tall tower and then take those same bricks and build a wide house, you haven't changed the number of bricks you used, but you have changed the shape and function of the object. In chemistry, molecules can do the exact same thing.

📖 Definitions

🔍 Worked example — Easy

Identify the type of isomerism between butane (C_4H_10) and 2-methylpropane (C_4H_10).

🔍 Worked example — Standard

A compound has the molecular formula C_2H_4O_2. It can exist as ethanoic acid (CH_3COOH) or methyl methanoate (HCOOCH_3). What type of isomerism is this?

🔍 Worked example — Exam-style

Draw and name the positional isomers of pentan-1-ol (C_5H_12O) where the hydroxyl (-OH) group is on the second carbon.

⚠️ Common errors

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🎯 Exam question

(a) Name the type of isomerism exhibited by these two compounds. [1] (b) Draw the structural formula for 2-chloropropane. [1] [2 marks]

Key ideas — Stereoisomerism: an overview

Stereoisomers have the same molecular formula and the same structural formula (atoms bonded in the same order), but a different arrangement of atoms in space. Stereoisomerism is divided into geometrical (cis/trans) isomerism and optical isomerism.

🧑‍🏫 Teacher's explanation

Imagine you have a pair of hands. Your left and right hands have the same components (four fingers, one thumb) and the same connectivity (the thumb is next to the index finger), but you cannot perfectly overlap them by just sliding them around; you have to flip one over. This is the essence of stereoisomerism: the 'parts list' and the 'blueprint' are identical, but the 3D orientation makes them different molecules.

📖 Definitions

🔍 Worked example — Easy

Identify the type of isomerism between 1-chloropropene and 2-chloropropene.

🔍 Worked example — Standard

Explain why But-2-ene exhibits cis-trans isomerism, whereas Propene does not.

🔍 Worked example — Exam-style

A molecule has the formula CH_3CH(OH)Cl. How many optical isomers does it have, and identify the chiral centre.

⚠️ Common errors

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🎯 Exam question

Key ideas — Geometrical (cis/trans) isomerism

Geometrical isomerism arises in alkenes because rotation about the C=C bond is restricted: free rotation would require breaking the sideways overlap of the p orbitals that forms the pi bond. If each carbon of the double bond carries two different groups, two distinct arrangements are possible: the cis (or Z) isomer, with the higher-priority groups on the same side of the double bond, and the trans (or E) isomer, with them on opposite sides. For example, but-2-ene, CH_3CH=CHCH_3, exists as cis-but-2-ene and trans-but-2-ene, which have different physical properties such as melting point.

🧑‍🏫 Teacher's explanation

Imagine a playground merry-go-round that is bolted to the ground. Unlike a spinning wheel that can turn freely, this one is stuck in one position. In chemistry, the double bond (C=C) acts like that bolt. The π bond is formed by the sideways overlap of p orbitals; to rotate the molecule, you would have to physically break that overlap, which requires a lot of energy. Because the atoms are 'locked' in place, they cannot spin around to change their position.

📖 Definitions

🔍 Worked example — Easy

Identify whether the following molecule can exhibit cis-trans isomerism: CH_2=CHCH_3.

🔍 Worked example — Standard

Draw/describe the two geometrical isomers of pent-2-ene, CH_3CH=CHCH_2CH_3.

🔍 Worked example — Exam-style

A molecule has the formula C_4H_8. It shows geometrical isomerism. How many such stereoisomers exist?

⚠️ Common errors

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🎯 Exam question

Key ideas — Optical isomerism and chiral centres

A chiral centre is a carbon atom bonded to four different atoms or groups. A molecule containing a single chiral centre can exist as two optical isomers, called enantiomers, which are non-superimposable mirror images of each other, in the same way that a left hand and a right hand are mirror images that cannot be perfectly overlaid. Enantiomers have identical physical properties, such as melting point and solubility, except that each rotates the plane of plane-polarised light by an equal angle but in opposite directions: one enantiomer rotates it clockwise, the other anticlockwise. A 50:50 mixture of both enantiomers is a racemic mixture (racemate), which shows no overall optical activity because the rotations cancel exactly.

🧑‍🏫 Teacher's explanation

Imagine you are holding your hands up to your face. Your left hand is a mirror image of your right hand. No matter how you twist, turn, or flip your left hand, you can never make it look exactly like your right hand (the thumb will always be on the wrong side). This is exactly what happens with certain molecules.

📖 Definitions

🔍 Worked example — Easy

Identify the number of chiral centres in 2-chlorobutane (CH_3CHClCH_2CH_3).

🔍 Worked example — Standard

A sample contains a 50:50 mixture of two enantiomers of lactic acid. What is the observed rotation of plane-polarised light if one enantiomer rotates light by +15^°?

🔍 Worked example — Exam-style

How many stereoisomers (enantiomers) are possible for 2,3-dichlorobutane if the molecule has two chiral centres and no internal plane of symmetry?

⚠️ Common errors

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🎯 Exam question

(a) Draw the structure of the two enantiomers of 2-bromobutane. [2] (b) Explain why these two molecules are considered stereoisomers rather than structural isomers. [1] [3 marks]

Key ideas — Identifying isomerism from a structural formula

To identify a chiral centre, examine each carbon atom in turn and list the four groups attached to it; if all four are different, that carbon is chiral. To identify possible geometrical isomerism, examine each carbon of a C=C bond; if both carbons carry two different substituents, cis and trans forms are both possible. To deduce all the possible isomers of a molecule of known molecular formula, systematically vary the position of the functional group and the branching of the carbon skeleton, checking each proposed structure gives the same molecular formula and is not a repeat of a structure already drawn.

🧑‍🏫 Teacher's explanation

Imagine you have a set of LEGO bricks. If you build a car, then take it apart and use the exact same bricks to build a plane, you have created an 'isomer'. The 'ingredients' (atoms) are identical, but the 'blueprint' (structure) is different. In chemistry, we call these isomers.

Ch 4 · Isomerism: structural and stereoisomerism · AS & A Level Chemistry 9701

All chapters in An introduction to AS Level organic chemistry