(a) Introduction

Learning objectives

4.1HydrocarbonsKnow that a hydrocarbon is a compound of hydrogen and carbon only 4.2Representing organic moleculesUnderstand how to represent organic molecules using empirical formulae, molecular formulae, general formulae, structural formulae and displayed formulae 4.3Homologous series, functional groups and isomersKnow what is meant by the terms homologous series, functional group and isomerism 4.4IUPAC nomenclatureUnderstand how to name compounds relevant to this specification using the rules of International Union of Pure and Applied Chemistry (IUPAC) nomenclature students will be expected to name compounds containing up to six carbon atoms 4.5Structures from molecular formulaeUnderstand how to write the possible structural and displayed formulae of an organic molecule given its molecular formula 4.6Types of organic reactionUnderstand how to classify reactions of organic compounds as substitution, addition and combustion knowledge of reaction mechanisms is not required

A hydrocarbon is a compound that contains hydrogen and carbon only.

Substance	Hydrocarbon?	Reason
methane, $\ce{CH4}$	yes	it is a compound containing only carbon and hydrogen
ethene, $\ce{C2H4}$	yes	it is a compound containing only carbon and hydrogen
ethanol, $\ce{C2H5OH}$	no	it also contains oxygen
carbon dioxide, $\ce{CO2}$	no	it contains no hydrogen and also contains oxygen

Check every element symbol in the formula. Both C and H must be present, and any additional element means the substance is not a hydrocarbon.

A compound is not a hydrocarbon merely because it contains carbon and hydrogen. The word ‘only’ excludes alcohols, carboxylic acids and every other compound containing an additional element.

Different formula types answer different questions about an organic substance: atom ratio, actual atom count, homologous-series pattern, connectivity, or every bond.

Formula type	What it shows	Example for ethene
empirical	simplest whole-number ratio of atoms	$\ce{CH2}$
molecular	actual number of each atom in one molecule	$\ce{C2H4}$
general	algebraic pattern shared by a homologous series	$\ce{C_nH_{2n}}$ for alkenes
structural	how atoms or groups are connected, with bonds condensed	$\ce{CH2=CH2}$
displayed	every atom and every covalent bond	draw both C atoms, four H atoms and all six bonds

To obtain an empirical formula, divide every molecular subscript by their highest common factor. To obtain a molecular formula from a displayed or structural formula, count each atom; a general formula is not the formula of one particular molecule until a value of $n$ is chosen.

A molecular formula does not show connectivity, so different structures can share it. A structural formula may group bonds such as $\ce{CH3CH2OH}$ ; a displayed formula must show the O–H bond and every other bond explicitly.

Homologous series, functional group and isomerism describe three different relationships: membership of a family, the reactive part of a molecule, and alternative structures for one molecular formula.

Term	Meaning	Diagnostic question
homologous series	a family with the same functional group and general formula, similar chemical reactions, a trend in physical properties, and successive members differing by $\ce{CH2}$	do the compounds follow one family pattern?
functional group	the atom or group of atoms responsible for a compound’s characteristic chemical reactions	which part controls the typical reactions?
isomerism	the existence of compounds with the same molecular formula but different structural formulae	is the atom count unchanged but the connectivity different?

\ce{CH3CH2CH2CH3} \qquad \ce{CH3CH(CH3)CH3}

The two formulae above both have molecular formula $\ce{C4H10}$ but different carbon connectivity, so they are structural isomers.

Isomers must have the same molecular formula, not merely the same empirical or general formula. Members of a homologous series have similar chemical properties and a trend in physical properties; they do not all have identical physical properties.

An IUPAC name combines a carbon-chain stem, any substituent prefixes and a suffix that identifies the principal bond or functional group. Number the chain so the important feature receives the lowest possible locant.

Carbon atoms in the longest relevant chain	1	2	3	4	5	6
stem	meth-	eth-	prop-	but-	pent-	hex-

Feature	Naming pattern	Example
alkane	stem + -ane	$\ce{CH3CH2CH3}$ : propane
alkene	lowest C=C locant + -ene	$\ce{CH2=CHCH2CH3}$ : but-1-ene
alcohol	lowest –OH locant + -ol	$\ce{CH3CH2CH2OH}$ : propan-1-ol
carboxylic acid	stem + -anoic acid; acid carbon is C1	$\ce{CH3CH2COOH}$ : propanoic acid
halogen or branch	numbered prefix before the parent name	$\ce{CH3CHClCH3}$ : 2-chloropropane

Choose the longest chain containing the principal functional group or C=C bond, number it from the nearer important feature, identify substituents, then assemble locants, prefixes, stem and suffix. For an ester, name the alkyl group attached to oxygen first and the acid-derived alkanoate second, for example methyl ethanoate.

Count the parent chain, not every carbon in a branch. Do not omit a locant when more than one position is possible, and keep this specification’s naming work to compounds containing no more than six carbon atoms.

A molecular formula fixes the number of each atom but not their connectivity. Generate possible structures systematically while satisfying normal valencies: carbon forms four bonds, oxygen two, hydrogen and halogens one.

Step	Check
1	write the exact inventory of C, H and any other atoms
2	draw distinct carbon skeletons, beginning with the longest chain and then branching
3	place any multiple bond or functional group in each genuinely different position
4	add H atoms so every atom has its normal valency
5	recount atoms and remove drawings that differ only because they are rotated or reversed
6	convert each valid structural formula to a displayed formula by showing every atom and bond

\ce{CH3CH2CH2CH3} \qquad \ce{CH3CH(CH3)CH3}

For $\ce{C4H10}$ , the only carbon skeletons are a four-carbon chain and a three-carbon chain with one methyl branch. These give butane and methylpropane; redrawing either from the opposite end does not create another isomer.

A valid drawing must match the molecular formula exactly and obey every valency. Different orientations of the same connectivity are not different structures; different atom connectivity is required.

Classify an organic reaction from its overall change: replacement is substitution, joining across an unsaturated bond is addition, and reaction with oxygen that releases energy is combustion.

Type	Overall pattern	Example clue
substitution	one atom or group in an organic molecule is replaced by another	alkane + halogen under ultraviolet light gives a haloalkane and hydrogen halide
addition	atoms add across a C=C bond and two reactants form one organic product	alkene + bromine gives a dibromoalkane
combustion	an organic compound burns in oxygen	oxygen is a reactant and energy is released

\ce{CH4 + Cl2 ->[UV] CH3Cl + HCl}

\ce{CH2=CH2 + Br2 -> CH2BrCH2Br}

\ce{CH4 + 2O2 -> CO2 + 2H2O}

Use reactants and products to identify the overall pattern. Addition does not replace an atom and normally gives one organic product; substitution retains the carbon framework while replacing a group. Reaction mechanisms are not required.

Next Topic

—(b) Crude oilRead

(a) Introduction

Learning objectives

Identify hydrocarbons using the word ‘only’

Translate between five organic formula types

Distinguish series, functional groups and isomerism

Build systematic organic names up to six carbons

Generate possible structures from a molecular formula

Classify substitution, addition and combustion

Next Topic