Introduction to organic chemistry: functional groups

IB Chemistry HL· IB Chemistry HL Topic 10.1· 15 min read

1. What is a Functional Group?★☆☆☆☆⏱ 5 min

📘 Definition

Functional Group

A specific arrangement of atoms or covalent bonds within an organic molecule that determines the molecule's characteristic chemical reactivity. All members of a homologous series share the same functional group.

Example:

The hydroxyl group ( $- O H$ ) is the functional group common to all alcohols.

Functional groups allow chemists to organize millions of different organic compounds into logical families. Compounds with the same functional group undergo similar reactions, regardless of the length of their carbon backbone, making it easy to predict reaction outcomes.

📐 Worked Example

Identify the functional group present in $C H_{3} C H_{2} C O C H_{3}$ .

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Step 1: Expand the condensed formula to show the full arrangement of atoms:
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$CH_3-CH_2-\underset{\underset{O}{||}}{C}-CH_3$
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Step 2: Identify the reactive group: the double-bonded oxygen (carbonyl group) is bonded to two carbon atoms, one on each side.
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Step 3: Match to the functional group classification: a carbonyl bonded to two alkyl groups is a ketone.

2. Common Oxygen-Containing Functional Groups★★☆☆☆⏱ 5 min

Most organic reactions you will study for IB HL involve oxygen-containing functional groups. These are the most commonly tested functional groups in exams, so it is important to memorize their structures.

Name	General Structure	Example
Hydroxyl (Alcohol)	$R - O H$	Ethanol: $C H_{3} C H_{2} O H$
Aldehyde	$R - C H O$	Ethanal: $C H_{3} C H O$
Ketone	$R - C O R^{'}$	Propanone: $C H_{3} C O C H_{3}$
Carboxylic Acid	$R - C O O H$	Ethanoic acid: $C H_{3} C O O H$
Ester	$R - C O O R^{'}$	Methyl ethanoate: $C H_{3} C O O C H_{3}$

📐 Worked Example

Classify $H O C H_{2} C H_{2} C H_{2} C O O H$ by its primary functional group.

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Step 1: Identify all functional groups present: a hydroxyl group ( $- O H$ ) on one end, and a carboxyl group ( $- C O O H$ ) on the other.
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Step 2: Recall that functional group priority determines the primary classification: carboxylic acids have higher priority than hydroxyl groups.
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Step 3: Confirm that the entire $- C O O H$ unit is a single functional group, with very different reactivity from a standalone hydroxyl group.
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Conclusion: This molecule is 4-hydroxybutanoic acid, classified primarily as a carboxylic acid.

3. Nitrogen and Halogen-Containing Functional Groups★★☆☆☆⏱ 4 min

📘 Definition

Amide

$R - C O N H_{2}$ , $R - C O N H R^{'}$

A functional group consisting of a carbonyl group bonded directly to a nitrogen atom. It is structurally distinct from amines, with very different reactivity.

Example:

Ethanamide: $C H_{3} C O N H_{2}$

Halogenoalkanes (alkyl halides) have the general formula $R - X$ , where $X = F, C l, B r, I$ . They are common starting materials for organic synthesis reactions tested frequently in mechanism questions. Amines are derivatives of ammonia, classified as 1°, 2°, or 3° based on the number of alkyl groups bonded to nitrogen.

📐 Worked Example

Identify all functional groups in paracetamol, which has a benzene ring bonded to $- O H$ and $- N H C O C H_{3}$ on opposite carbons.

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Step 1: Analyze the first group: $- O H$ bonded to an aromatic ring is a hydroxyl (phenol) functional group.
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Step 2: Analyze the second group: $- N H C O C H_{3}$ has a carbonyl group bonded directly to nitrogen, forming an amide linkage.
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Conclusion: The molecule contains two functional groups: hydroxyl and amide.

4. Functional Group Priority for Nomenclature★★★☆☆⏱ 5 min

When a molecule contains more than one functional group, you must assign the highest priority group as the main functional group for naming. All other groups are treated as substituents, with lower priority. IB requires you to know the standard priority order for common functional groups.

📐 Worked Example

Identify the main functional group in $O H C - C H_{2} - C H (O H) - C H_{3}$ and give its correct name.

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Step 1: List all functional groups: aldehyde ( $- C H O$ ) and hydroxyl ( $- O H$ ).
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Step 2: Compare priority: aldehydes have higher priority than alcohols.
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Step 3: Number the 4-carbon chain starting from the aldehyde carbon, giving the aldehyde C1 and the hydroxyl C3.
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Conclusion: The main functional group is aldehyde, and the name is 3-hydroxybutanal.

5. Common Pitfalls

Wrong move:

Confusing aldehydes and ketones, misclassifying the functional group

Why:

Both contain a carbonyl ( $C = O$ ) group, so students often mix them up

Correct move:

Check what the carbonyl carbon is bonded to: if bonded to a hydrogen ( $R - C H O$ ) it is an aldehyde; if bonded to two carbons ( $R - C O R^{'}$ ) it is a ketone

Wrong move:

Treating the hydroxyl group in a carboxylic acid as an alcohol functional group

Why:

Carboxylic acids contain a hydroxyl group, leading students to incorrectly count it as an alcohol

Correct move:

Recognize the entire $- C O O H$ unit is the carboxylic acid functional group, which has higher priority than alcohol

Wrong move:

Confusing amines and amides

Why:

Both contain nitrogen, so students often misidentify them

Correct move:

Amides have a carbonyl group bonded directly to nitrogen ( $- C O N H -$ ), while amines have only nitrogen bonded to carbon/hydrogen

Wrong move:

Assigning the lowest number to a double bond instead of the higher priority functional group

Why:

Students remember numbering rules for alkenes, but forget functional group priority comes first

Correct move:

Always number the chain to give the highest priority functional group the lowest possible number

Wrong move:

Forgetting to list all functional groups when the question asks for multiple

Why:

Students often only name the highest priority group, even when asked for all present

Correct move:

Carefully scan the entire molecule to find all non-alkane functional groups and list all requested ones

6. Quick Reference Cheatsheet

Functional Group	Structure	Priority (High → Low)
Carboxylic acid	$- C O O H$	1
Ester	$- C O O R^{'}$	2
Amide	$- C O N H_{2}$	3
Aldehyde	$- C H O$	4
Ketone	$- C O R^{'}$	5
Alcohol	$- O H$	6
Amine	$- N H_{2}$	7
Alkene	$C = C$	8
Alkyne	$C \equiv C$	9
Halide	$- X$	10

7. Frequently Asked

Do I need to memorize all functional group structures for the exam?

Yes, all functional groups listed in the IB syllabus (hydroxyl, carbonyl, carboxyl, amide, amine, halide, alkene, alkyne) are required for identification and nomenclature across all papers.

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.

2025 · 1
Identify functional groups in a molecule
2024 · 2
Name compound by functional group priority

Going deeper

syllabusOfficial IB Chemistry HL Topic 10Full syllabus reference

What's Next

Mastering functional groups is the first critical step to succeeding in organic chemistry for IB HL. All subsequent topics, from nomenclature to reaction mechanisms, rely on your ability to quickly identify and classify functional groups. You will next learn to apply what you know about functional groups to name more complex organic molecules, then study the characteristic reactions of each functional group family. Functional group identification is a common first question in organic sections of Paper 2, so solidifying this knowledge will earn you easy marks on every exam.