Introduction to organic chemistry

IB Chemistry SL· Reactivity 3: Equilibrium and Organic Chemistry· 45 min read

1. Core Definitions and Classification★☆☆☆☆⏱ 10 min

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📘 Definition

Organic compound

A covalent compound containing carbon, excluding inorganic carbon compounds like oxides, carbonates, carbides, and cyanides

Example:

Ethanol ( $C_{2} H_{5} O H$ ) is organic; sodium carbonate ( $N a_{2} C O_{3}$ ) is inorganic

Organic compounds are grouped into homologous series, which share a common functional group, have the same general formula, similar chemical properties, and gradual changes in physical properties (e.g. boiling point) as carbon chain length increases.

📐 Worked Example

Classify urea ( $N H_{2} C O N H_{2}$ ) as organic or inorganic, and explain your reasoning.

1
Check the composition and exception rules
2
$NH_2CONH_2$
3
Urea contains covalently bonded carbon, and is not a carbonate, oxide, or carbide.
4
Therefore, urea meets the modern definition of an organic compound.

Exam tip:

Always remember the common exceptions to the organic compound definition, they are frequently tested in multiple choice questions.

2. Functional Group Identification★★☆☆☆⏱ 15 min

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📘 Definition

Functional group

A specific group of atoms or covalent bonds that gives an organic molecule its characteristic chemical reactivity

Example:

The hydroxyl group (-OH) is the functional group for all alcohols

Alkane: Only single C-C and C-H bonds, no reactive functional group
Alkene: Contains a carbon-carbon double bond ( $C = C$ )
Alcohol: Contains a hydroxyl group (-OH)
Carboxylic acid: Contains a carboxyl group (-COOH)
Halogenoalkane: Contains a halogen atom (-X = Cl, Br, F, I) bonded to carbon

📐 Worked Example

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

1
Expand the condensed formula to see all groups
2
$CH_3CH_2-C(=O)-OH$
3
The group at the end of the chain is a carbon bonded to a hydroxyl group and double bonded to an oxygen, which is the carboxyl functional group.
4
This compound belongs to the carboxylic acid homologous series.

3. Representing Organic Molecules★★☆☆☆⏱ 15 min

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IB Chemistry requires you to use multiple standard notations to represent organic molecules, each with a specific purpose. It is important to use the correct notation when asked in an exam question to earn full marks.

Notation	Purpose	Example for propane
Empirical formula	Simplest whole number atom ratio	$C_{3} H_{8} \to C H_{3} \frac{8}{3} so C_{3} H_{8}$
Molecular formula	Actual number of each atom	$C_{3} H_{8}$
Condensed structural formula	Shows atom grouping, omits C-H bonds	$C H_{3} C H_{2} C H_{3}$
Skeletal formula	Compact representation, vertices = carbons	3-carbon straight line chain

📐 Worked Example

Write the molecular formula for the skeletal structure of methylpropane.

1
Count carbon atoms from vertices and endpoints
2
Methylpropane has 4 carbon atoms total
3
Add hydrogen atoms to give each carbon 4 bonds: 4 carbons × 4 bonds = 16 total bonds, minus 12 C-C bonds = 10 C-H bonds
4
$C_4H_{10}$

4. Basic IUPAC Naming for Alkanes★★★☆☆⏱ 20 min

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IUPAC naming for branched alkanes follows 3 core rules: 1) Find the longest continuous carbon chain (the parent chain), 2) Number the chain to give any alkyl branches the lowest possible position number, 3) Combine position, branch name, and parent name.

📐 Worked Example

Name the alkane with condensed formula $C H_{3} C H_{2} C H (C H_{3}) C H_{2} C H_{3}$ .

1
Find the longest continuous carbon chain: 5 carbons, so parent is pentane
2
Number the chain to give the branch the lowest number: the methyl branch is on carbon 3 from either end
3
The branch is a 1-carbon alkyl group, so it is called methyl
4
Combine to get the full IUPAC name: 3-methylpentane

5. Common Pitfalls

Wrong move:

Classifying calcium carbonate as an organic compound

Why:

Students forget carbonates are an exception to the organic definition despite containing carbon

Correct move:

Always classify oxides, carbonates, carbides, and cyanides as inorganic

Wrong move:

Choosing a shorter carbon chain as the parent alkane chain

Why:

Students often pick the obvious straight line instead of the longer chain that includes a branch point

Correct move:

Always trace all possible continuous carbon chains to find the longest before naming

Wrong move:

Numbering the parent chain from the end that gives a higher branch position number

Why:

Students often number from the top or left of the diagram regardless of branch position

Correct move:

Always number from the end that gives the first branch the smallest possible number

Wrong move:

Incorrectly counting hydrogens for skeletal structures

Why:

Students forget that unshown bonds on carbon vertices are to hydrogen, to fill carbon's 4-bond valency

Correct move:

Count all bonds to each carbon, add enough hydrogen atoms to reach 4 total bonds per carbon

6. Quick Reference Cheatsheet

Carbon length	Prefix	Functional group	General formula
1	Meth-	Alkane	$C_{n} H_{2 n + 2}$
2	Eth-	Alkene	$C_{n} H_{2 n}$
3	Prop-	Alcohol (R-OH)	$C_{n} H_{2 n + 1} O H$
4	But-	Carboxylic acid (R-COOH)	$C_{n} H_{2 n} O_{2}$
5	Pent-	Halogenoalkane (R-X)	$C_{n} H_{2 n + 1} X$
6	Hex-	Ketone (RCOR')	$C_{n} H_{2 n} O$

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
Functional group identification
2024 · 2
IUPAC naming of branched alkanes
2023 · 1
Organic/inorganic classification

What's Next

Introduction to organic chemistry is the universal foundation for all organic topics you will study in IB SL Chemistry. Every organic exam question will require you to apply the classification, naming, and structural drawing skills you learned here. Recognizing functional groups lets you predict reactivity, which is critical for studying reaction types and mechanisms. Examiners consistently penalize incorrect IUPAC names or wrong structural notations, so mastering this sub-topic is essential to earn full marks on organic questions. You will now build on this foundation to study more complex nomenclature and the reactivity of different organic compound classes.