3.2 Functional groups: Organic compounds

Representation	What it preserves or shows
Empirical	Simplest whole-number atom ratio
Molecular	Actual atom counts
Structural/condensed	Connectivity in compact form
Skeletal	Carbon framework and implied hydrogen
Stereochemical/3D	Spatial arrangement

Translate representations without changing atom connectivity. For a skeletal formula, count every vertex and line end as carbon, add enough hydrogens to give carbon four bonds, and write heteroatoms explicitly. Then verify both the molecular formula and the connectivity.

An empirical formula is a ratio, not necessarily the complete molecule: hydrogen peroxide has molecular formula H₂O₂ but empirical formula HO. Reduce all subscripts by their greatest common factor; if no common factor exists, the molecular and empirical formula are identical.

Matching atom totals alone cannot prove two drawings are the same compound: connectivity and, where relevant, stereochemistry must also agree. Do not reduce a molecular formula when the subscripts already have no common factor.

Representative question

[Maximum number: 1]

State the type of structural formula shown.

Family	Complete recognition pattern	Bounded characteristic cue
Halogenoalkane	C–F/Cl/Br/I	polar C–X bond
Alcohol / hydroxyl	C–OH, not the –OH inside –COOH	can donate and accept hydrogen bonds
Aldehyde	terminal –CHO carbonyl	polar C=O; terminal carbonyl
Ketone	–CO– between carbons	polar C=O; internal carbonyl
Carboxylic acid	–COOH	acidic proton and hydrogen bonding
Ether / alkoxy	C–O–C	oxygen accepts hydrogen bonds but has no O–H donor
Amine / amino	C–N without adjacent carbonyl	basic lone-pair chemistry; N–H species may donate H bonds
Amide / amido	–CONH₂/–CONHR/–CONR₂	nitrogen directly attached to carbonyl
Ester	–COO– between carbon groups	carbonyl and single-bond O in one group
Phenyl	C₆H₅– attached as a substituent	aromatic ring pattern

Identify the whole local bonding pattern before naming the group; these cues support classification, not a complete reaction mechanism.

Identify the characteristic atoms and bonding pattern first, then give the functional-group name and relevant property context.

Identify the complete bonding pattern: an aldehyde has a terminal –CHO carbonyl, a ketone has C=O between carbons, and an ester contains –C(=O)–O–. Do not label every O–H as an alcohol or every C–N as an amine without checking the neighbouring carbonyl.

Saturation describes carbon-carbon bonding: a saturated compound has only C-C single bonds, while an unsaturated compound contains at least one C=C or C≡C bond. A carbonyl C=O does not by itself make the carbon skeleton unsaturated. Identify saturation separately from identifying hydroxyl, carbonyl, carboxyl or other functional groups.

Representative question

[Maximum number: 2]

State the structural formula, functional group name and homologous series of the CHO functional group.

Structural formula drawing	Functional group name	Homologous series name

isomers

Accept "same molecular formula".

3.

(e)

(ii)

Full structural

formula

Functional group

name

Homologous series

$-\mathrm{c}_{\mathrm{O}}^{\prime} \mathrm{H}$

carbonyl

aldehyde

\end{tabular}

structure

carbonyl AND aldehyde
\end{tabular}

Accept R/C attached to functional group in the full structural formula.
Central C must have 4 bonds for M1.

Members of a homologous series share a functional-group pattern and general formula. Successive members differ by CH₂.

Recognize the series by its functional group and general formula, including alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, acids, ethers, amines, amides, esters, and halogenoalkanes.

Homologous series	Recognition pattern / common acyclic general formula
Alkane	only C-C single bonds; $\ce{C_nH_{2n+2}}$
Alkene / alkyne	C=C: $\ce{C_nH_{2n}}$ ; C≡C: $\ce{C_nH_{2n-2}}$
Halogenoalkane	C-X; $\ce{C_nH_{2n+1}X}$
Alcohol / ether	C-OH: $\ce{C_nH_{2n+1}OH}$ ; C-O-C: $\ce{C_nH_{2n+2}O}$
Aldehyde / ketone	terminal -CHO or internal C=O; $\ce{C_nH_{2n}O}$
Carboxylic acid / ester	-COOH or -COO-; $\ce{C_nH_{2n}O2}$
Primary amine / amide	-NH2: $\ce{C_nH_{2n+3}N}$ ; -CONH2: $\ce{C_nH_{2n+1}NO}$

Moving from one member to the next adds CH₂, so molar mass and dispersion forces change gradually while the shared functional group gives similar reaction patterns. Use both the functional group and general formula: formula alone can overlap with another structural family.

Representative question

[Maximum number: 1]

State the general formula for the homologous series of alkenes.

$\mathrm{C}_{\mathrm{n}} \mathrm{H}_{2 \mathrm{n}}$

Subscripts of numbers not
essential

Melting and boiling points depend on chain length, branching, polarity, and intermolecular-force strength. Larger molecules often have stronger dispersion forces, while branching changes contact and packing.

Explain a comparison by naming the relevant structural difference and the resulting change in intermolecular attraction or packing.

Straight-chain pentane has a larger contact surface and higher boiling point than more highly branched isomers of the same formula; lengthening a series usually strengthens dispersion forces. For different functional groups, include polarity and hydrogen bonding before attributing the trend to size alone.

Representative question

[Maximum number: 2]

Explain why the boiling point increases from methane to propane.

Step	Naming decision
1	Choose the longest parent chain
2	Number to give the relevant feature the lowest locant
3	Identify unsaturation and functional group
4	Assemble prefixes, locants, and suffix

Apply the systematic sequence to saturated or mono-unsaturated compounds with up to six carbons and one functional-group type.

For CH₃CH(OH)CH(CH₃)CH₃, choose the four-carbon chain containing –OH, number from the end that gives –OH the lower locant, and name 3-methylbutan-2-ol. The principal suffix controls numbering before a substituent does; check locants, punctuation and retained unsaturation at the end.

Representative question

[Maximum number: 1]

Deduce the systematic name of X using IUPAC nomenclature.

Structural isomers have the same molecular formula but different atom connectivities. Types include straight-chain/branched, position, and functional-group isomers.

Classify primary, secondary, and tertiary alcohols, halogenoalkanes, and amines by the carbon or nitrogen environment attached to the functional group.

Family	Primary / secondary / tertiary test
Alcohol	count carbon groups attached to the carbon bearing -OH: 1 / 2 / 3
Halogenoalkane	count carbon groups attached to the carbon bearing X: 1 / 2 / 3
Amine	count carbon groups attached directly to N: 1 / 2 / 3

C₄H₁₀O can represent different carbon skeletons, different –OH positions, or an ether instead of an alcohol. Draw each connectivity once, then compare molecular formulae. Rotating or redrawing one connectivity does not create a new structural isomer.

For alcohols and halogenoalkanes, classify the carbon carrying the functional group; for amines, classify the nitrogen by how many carbon groups are bonded to it. Do not use the position number alone: butan-2-ol is secondary because its OH-bearing carbon is attached to two other carbons.

Representative question

[Maximum number: 1]

Draw a structural isomer of molecule X.

any structural isomer of $\mathrm{CH}_{3} \mathrm{CHBrC}\left(\mathrm{CH}_{3}\right)_{3}$ .

Retrieve the route: translate formulae, identify functional groups and series, name and classify isomers, then use mass, IR, and NMR evidence together to determine structure.

Check connectivity, functional-group evidence, formula/mass constraint, shifts and integration, splitting neighbours, and agreement across every technique.

Objective notes

6 learning objectives

3.2.1Organic formulas• Types: empirical, molecular, structural (full/condensed), stereochemical, skeletal• 3D models• Interconvert molecular, structural, and skeletal formulasView 3.2.2Functional groups• Give characteristic properties• Groups: halogeno, hydroxyl, carbonyl, carboxyl, alkoxy, amino, amido, ester, phenyl• Identify functional groups by name and structureView 3.2.3Homologous series• Successive members differ by CH₂• Series: alkanes, alkenes, alkynes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, ethers, amines, amides, esters• Recognize general formulas and functional group patternsView 3.2.4Physical property trends• Melting and boiling points in homologous series• Explain trends using chain length, branching, polarity, and IMFView 3.2.5IUPAC nomenclature• Systematic naming for organic/inorganic compounds• Up to 6 carbons, one functional group type• Apply nomenclature to saturated or mono-unsaturated compounds with one functional groupView 3.2.6Structural isomers• Same molecular formula, different connectivities• Types: branched, straight-chain, position, functional group isomers• Include primary, secondary, and tertiary alcohols, halogenoalkanes, and aminesView

3.2 Functional groups: Organic compounds

Organic Formula Representations: From Ratio to Structure

Converting Organic Formulae

Assessment in practice

Organic Functional Groups

Identifying Functional Groups

Assessment in practice

Homologous Series

Recognising Homologous Series

Assessment in practice

Organic Physical-Property Trends

Explaining Organic Trends

Assessment in practice

IUPAC Nomenclature

Naming Organic Compounds

Assessment in practice

Structural Isomers

Classifying Structural Isomers

Assessment in practice

Organic Analysis Summary

Objective notes

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