Functional groups and isomerism

CIE A-Level Chemistry· Unit 12: Introduction to Organic Chemistry, Topic 2· 20 min read

1. Functional Groups: Identification and Classification★★☆☆☆⏱ 5 min

📘 Definition

Functional Group

Varies by group

A group of atoms responsible for the characteristic chemical reactions of an organic compound, shared by all members of the same homologous series.

Example:

-OH (hydroxyl) in alcohols, -COOH (carboxyl) in carboxylic acids

CIE exams require you to recognize all common functional groups from displayed, structural, and skeletal formulas. Functional groups are the foundation for predicting reactivity and classifying organic compounds.

Hydroxyl (-OH): Alcohols
Carboxyl (-COOH): Carboxylic acids
Carbonyl (C=O): Aldehydes/ketones
Alkene (C=C): Alkenes
Halo (-X): Haloalkanes (X=Cl, Br, I)
Ester (-COO-): Esters
Amino (-NH2): Amines

📐 Worked Example

Identify all functional groups present in the compound: $H O C H_{2} C H (N H_{2}) C O O C H_{3}$

1
1. Scan the structure for distinct reactive groups: The terminal $H O C H_{2} -$ group contains a hydroxyl group.
2
1. The central $- C H (N H_{2}) -$ group contains an amino group.
3
1. The $- C O O C H_{3}$ end group is an ester functional group.
4
Final answer: hydroxyl, amino, and ester groups.

2. Structural Isomerism★★★☆☆⏱ 7 min

📘 Definition

Structural Isomerism

Isomerism where compounds share the same molecular formula but have different covalent bond connectivity (different arrangements of atoms).

CIE tests three common sub-types of structural isomerism:

Chain isomerism: Different arrangements of the carbon skeleton (e.g. straight vs branched chain alkanes)
Position isomerism: Same functional group in different positions on the same carbon skeleton
Functional group isomerism: Completely different functional groups (e.g. alcohols vs ethers)

📐 Worked Example

Name one isomer of $C_{4} H_{10} O$ from each sub-type of structural isomerism

1
1. Chain isomer (alcohol): 2-methylpropan-1-ol (branched carbon skeleton, compared to straight chain 1-butanol)
2
1. Position isomer (alcohol): 2-butanol, hydroxyl group on carbon 2 (vs 1-butanol with hydroxyl on carbon 1)
3
1. Functional group isomer: methoxypropane, an ether with a different functional group to alcohols
4
All three isomers have the same molecular formula $C_{4} H_{10} O$ .

3. Stereoisomerism★★★★☆⏱ 7 min

📘 Definition

Stereoisomerism

Isomerism where compounds have the same molecular formula and same bond connectivity, but different spatial arrangement of atoms.

CIE tests two main sub-types of stereoisomerism at A-Level: cis-trans/E-Z isomerism (across alkenes double bonds) and optical isomerism (around chiral centers).

📐 Worked Example

Draw and label the E and Z isomers of but-2-ene ( $C H_{3} C H = C H C H_{3}$ )

1
1. Confirm E-Z is possible: each carbon in the double bond has two different groups ( $C H_{3}$ and $H$ ), so isomerism exists.
2
1. Z isomer (zusammen = together): The two higher priority $C H_{3}$ groups are on the same side of the double bond.
3
$H_3C - \underset{|}{C} = \underset{|}{C} - CH_3 \\ \ \ \ \ \ \ H \ \ \ \ \ \ \ H$
4
1. E isomer (entgegen = opposite): The two higher priority $C H_{3}$ groups are on opposite sides of the double bond.
5
$H_3C - \underset{|}{C} = \underset{|}{C} - CH_3 \\ \ \ \ \ \ \ H \ \ \ \ \ H_3C$

Exam tip:

E-Z priority is determined by atomic number, not the size of the carbon chain. A higher atomic number substituent always gets higher priority.

4. Drawing Isomers for CIE Exams★★★☆☆⏱ 5 min

CIE will specify the format required for your drawing: displayed (all bonds and atoms shown), structural (groups written out), or skeletal (only bonds, carbons at vertices). Always follow the required format to get full marks.

📐 Worked Example

Draw an optical isomer of 2-chlorobutane, showing the chiral center.

1
1. Identify the chiral center: Carbon 2 in 2-chlorobutane is bonded to four different groups: $- C H_{3}$ , $- C H_{2} C H_{3}$ , $- C l$ , $- H$ .
2
1. Draw the chiral carbon at the center of a tetrahedron: Use a solid wedge for the group coming out of the page, and a dashed wedge for the group going into the page to show 3D arrangement.

5. Common Pitfalls

Wrong move:

Claiming an alkene with two identical groups on one double-bond carbon has cis-trans isomerism.

Why:

E-Z/cis-trans isomerism only exists when both carbons of the double bond have two distinct groups attached.

Correct move:

Check each carbon of the double bond for two different groups before confirming isomerism.

Wrong move:

Confusing structural isomerism with stereoisomerism.

Why:

Structural isomerism has different bond connectivity, while stereoisomerism has the same connectivity.

Correct move:

First check if the connectivity is different: if yes, it is structural; if no, it is stereoisomerism.

Wrong move:

Counting a carbon with two identical groups as a chiral center.

Why:

A chiral center requires four distinct groups attached to the carbon atom.

Correct move:

Check all four groups attached to the carbon to confirm all are different before marking it as chiral.

Wrong move:

Not matching the molecular formula after drawing an isomer.

Why:

It is easy to accidentally add or remove a hydrogen or carbon when drawing structures.

Correct move:

Count the number of each atom in your drawn isomer to confirm it matches the given molecular formula.

Wrong move:

Calling functional group the same as homologous series.

Why:

A homologous series is a family of compounds with the same functional group, differing by $C H_{2}$ . They are not interchangeable terms.

Correct move:

Remember: the functional group is the reactive atom group, homologous series is the family the compound belongs to.

6. Quick Reference Cheatsheet

Isomerism Class	Sub-Type	Key Feature
Structural	Chain	Different carbon skeleton
Structural	Position	Functional group in different location
Structural	Functional Group	Different functional group
Stereoisomerism	E-Z/cis-trans	Different arrangement across double bond
Stereoisomerism	Optical	Chiral center with four different groups

7. Frequently Asked

What is the difference between structural and stereoisomerism?

Structural isomers have different covalent bond connectivity (different atoms bonded to each other), while stereoisomers have the same connectivity but different spatial arrangement of atoms.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Functional group identification
2023 · 2
Drawing five isomers of C5H10O
2024 · 1
Classify types of isomerism

Going deeper

syllabusCIE 9701 Official Syllabus

What's Next

This sub-topic is the foundation for all organic chemistry topics in CIE A-Level Chemistry. Understanding functional groups allows you to predict the chemical reactivity of any organic compound, while recognizing isomerism is critical for drawing reaction products and explaining stereochemistry in reaction mechanisms. Mastery of this topic is required for all organic exam questions, from multiple choice to extended open response questions. You will apply these concepts to every subsequent organic chemistry unit.