Classification of Organic Compounds
What is Organic Chemistry?
Organic chemistry is the branch of chemistry that deals with the study of carbon-containing compounds. The word "organic" originally referred to substances obtained from living organisms, but today millions of synthetic organic compounds are known.
Carbon is unique because it can form 4 covalent bonds and can bond to itself to form long chains, rings, and branched structures — a property called catenation.
A: Classification by Carbon Skeleton (Chain Structure)
Organic compounds are first grouped by the shape of their carbon backbone:
| Class | Carbon Skeleton | Example | Notes |
|---|---|---|---|
| Acyclic (open-chain) | No ring; carbon atoms form a chain | Butane, CH₃CH₂CH₂CH₃ | Also called aliphatic |
| Cyclic | Contains at least one ring of carbon atoms | Cyclohexane, benzene | Includes aromatic and non-aromatic rings |
| Alicyclic | Cyclic but NOT aromatic | Cyclohexane C₆H₁₂ | Behave like aliphatic compounds |
| Aromatic | Contains a benzene ring (C₆H₆) | Benzene, toluene, phenol | Highly stable due to delocalized π electrons |
| Heterocyclic | Ring contains atoms other than C (e.g. N, O, S) | Pyridine (N in ring), furan (O in ring) | Very common in medicines and DNA bases |
B: Classification by Degree of Unsaturation
Organic compounds are also classified by the type of bonds between carbon atoms:
| Class | Bonds | General Formula | Examples |
|---|---|---|---|
| Saturated | C–C single bonds only | CₙH₂ₙ₊₂ (alkanes) | Methane CH₄, ethane C₂H₆ |
| Unsaturated | C=C double or C≡C triple bonds present | CₙH₂ₙ (alkenes), CₙH₂ₙ₋₂ (alkynes) | Ethene C₂H₄, ethyne C₂H₂ |
Types of Formulas for Organic Compounds
There are four main ways to represent an organic molecule. Each gives different levels of information.
| Type of Formula | What it shows | Example: Butane |
|---|---|---|
| Molecular (Molecular Formula) | The total number of each type of atom in one molecule | C₄H₁₀ |
| Empirical Formula | The simplest whole-number ratio of atoms | C₂H₅ |
| Structural Formula | Shows how atoms are connected (all bonds shown) | CH₃–CH₂–CH₂–CH₃ |
| Displayed (Full Structural) Formula | Shows every atom and every bond explicitly | All H atoms drawn out around each C |
| Skeletal Formula | Zigzag lines; C atoms at each corner/end; H atoms implied | A zigzag with 3 line segments |
| Condensed Structural Formula | Groups H atoms with each carbon, reading left to right | CH₃CH₂CH₂CH₃ |
Molecular: C₄H₁₀
Empirical: C₂H₅
Condensed: CH₃–CH₂–CH₂–CH₃
IUPAC name: butane
Relationship between Molecular and Empirical Formulas
The molecular formula is always a whole-number multiple of the empirical formula.
If empirical formula is CH₂ and the molar mass is 56 g/mol:
Mass of CH₂ unit = 12 + 2 = 14 g/mol → n = 56 ÷ 14 = 4
So molecular formula = (CH₂)₄ = C₄H₈ (this is butene or cyclobutane)
Functional Groups and Homologous Series
| Family | Functional Group | General Formula | Example (n=2) |
|---|---|---|---|
| Alkanes | C–C (single bond, no FG) | CₙH₂ₙ₊₂ | Ethane C₂H₆ |
| Alkenes | C=C | CₙH₂ₙ | Ethene C₂H₄ |
| Alkynes | C≡C | CₙH₂ₙ₋₂ | Ethyne C₂H₂ |
| Alcohols | –OH (hydroxyl) | CₙH₂ₙ₊₁OH | Ethanol C₂H₅OH |
| Halogenoalkanes | –X (X = F,Cl,Br,I) | CₙH₂ₙ₊₁X | Chloroethane C₂H₅Cl |
| Aldehydes | –CHO | CₙH₂ₙO | Ethanal CH₃CHO |
| Ketones | –CO– | CₙH₂ₙO | Propanone CH₃COCH₃ |
| Carboxylic acids | –COOH | CₙH₂ₙO₂ | Ethanoic acid CH₃COOH |
| Esters | –COO– | CₙH₂ₙO₂ | Methyl ethanoate CH₃COOCH₃ |
| Amines | –NH₂ | CₙH₂ₙ₊₁NH₂ | Ethylamine C₂H₅NH₂ |
| Amides | –CONH₂ | — | Ethanamide CH₃CONH₂ |
| Nitriles | –CN | CₙH₂ₙ₊₁CN | Ethanenitrile CH₃CN |
Properties of a Homologous Series — illustrated with Alkanes
As the number of carbon atoms increases by 1 (–CH₂–), the following properties change gradually:
| Alkane | Formula | Mr | Boiling Point (°C) | State at 25°C |
|---|---|---|---|---|
| Methane | CH₄ | 16 | −162 | Gas |
| Ethane | C₂H₆ | 30 | −89 | Gas |
| Propane | C₃H₈ | 44 | −42 | Gas |
| Butane | C₄H₁₀ | 58 | −1 | Gas |
| Pentane | C₅H₁₂ | 72 | +36 | Liquid |
| Hexane | C₆H₁₄ | 86 | +69 | Liquid |
| Decane | C₁₀H₂₂ | 142 | +174 | Liquid |
| Icosane | C₂₀H₄₂ | 282 | +343 | Solid (wax) |
General Rules of IUPAC Nomenclature
What is IUPAC?
IUPAC stands for the International Union of Pure and Applied Chemistry. Their naming system provides a unique, systematic name for every organic compound that conveys its structure.
The Three Building Blocks of an IUPAC Name
Every IUPAC name has up to three parts: PREFIX + ROOT/STEM + SUFFIX
| Part | Tells you… | Example |
|---|---|---|
| Prefix | Names and positions of substituents (branches, halogens) | 2-methyl, 3-chloro |
| Root/Stem | The length of the longest carbon chain | meth(1), eth(2), prop(3), but(4), pent(5), hex(6), hept(7), oct(8), non(9), dec(10) |
| Suffix | The main functional group (class of compound) | -ane, -ene, -yne, -ol, -al, -one, -oic acid |
Step-by-Step IUPAC Naming Rules
Rule 1 — Find the longest chain: Identify the longest continuous chain of carbon atoms that contains the principal functional group. This gives the root name (stem).
Rule 2 — Number the chain: Number the carbon atoms from the end closest to the principal functional group (or the branch, if no functional group). The functional group / substituents must get the lowest possible locants (numbers).
Rule 3 — Name substituents: Identify all substituents attached to the main chain (branches = alkyl groups; halogens, etc.). Use prefixes: methyl (–CH₃), ethyl (–C₂H₅), propyl (–C₃H₇), fluoro (–F), chloro (–Cl), bromo (–Br), iodo (–I).
Rule 4 — Multiple identical substituents: Use di-, tri-, tetra- when the same substituent appears 2, 3, 4 times. List their positions separately: 2,2-dimethyl (not 2-dimethyl).
Rule 5 — Alphabetical order: When there are two or more different substituents, list them alphabetically (ignore di-, tri-, etc. when alphabetising).
Rule 6 — Suffix for class: End the name with the correct suffix: –ane (alkane), –ene (alkene), –yne (alkyne), –ol (alcohol), –al (aldehyde), –one (ketone), –oic acid (carboxylic acid), –oate (ester), –amine (amine).
| Chain length | Prefix (stem) | Example (alkane) | Formula |
|---|---|---|---|
| 1 carbon | meth- | methane | CH₄ |
| 2 carbons | eth- | ethane | C₂H₆ |
| 3 carbons | prop- | propane | C₃H₈ |
| 4 carbons | but- | butane | C₄H₁₀ |
| 5 carbons | pent- | pentane | C₅H₁₂ |
| 6 carbons | hex- | hexane | C₆H₁₄ |
| 7 carbons | hept- | heptane | C₇H₁₆ |
| 8 carbons | oct- | octane | C₈H₁₈ |
| 9 carbons | non- | nonane | C₉H₂₀ |
| 10 carbons | dec- | decane | C₁₀H₂₂ |
Worked Examples
Question: Name the following compound: CH₃–CH(CH₃)–CH₂–CH₃
Question: Draw the condensed structural formula of 3-ethyl-2-methylpentane.
C1–C2–C3–C4–C5
Question: A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its molar mass is 56 g/mol. Determine the empirical and molecular formulas.
Moles C = 85.7 ÷ 12 = 7.14 mol
Moles H = 14.3 ÷ 1 = 14.3 mol
C : H = 7.14/7.14 : 14.3/7.14 = 1 : 2
Empirical formula mass = 12 + 2 = 14 g/mol
Question: For each compound below, identify the functional group and state the homologous series it belongs to:
(a) CH₃CH₂OH (b) CH₃COOH (c) CH₃CHO (d) CH₃CH₂NH₂
Question: Name: CH₃–C(CH₃)₂–CH₂–CH₂–CH₃
Exercises with Answers
Classify the following as saturated or unsaturated, and as aliphatic, aromatic, or alicyclic:
(a) C₆H₆ (b) C₄H₈ (c) C₃H₈ (d) Cyclohexane C₆H₁₂
(b) C₄H₈ → unsaturated, aliphatic (contains C=C double bond; open chain)
(c) C₃H₈ → saturated, aliphatic (propane — only C–C single bonds; open chain)
(d) C₆H₁₂ → saturated, alicyclic (cyclohexane — ring structure but no benzene ring)
Give the IUPAC name for each compound:
(a) CH₃–CH₂–CH₂–CH₂–CH₃
(b) CH₃–CH(Cl)–CH₃
(c) CH₃–CH₂–OH
(d) CH₃–CH₂–COOH
(b) 3-carbon chain, Cl on C2, –ane suffix → 2-chloropropane
(c) 2-carbon chain, –OH group, –ol suffix → ethanol
(d) 3-carbon chain (including the carboxyl C), –oic acid suffix → propanoic acid
A compound has the molecular formula C₄H₈O. It contains the functional group –CHO.
(a) What homologous series does it belong to?
(b) Write its IUPAC name.
(b) 4-carbon chain + aldehyde suffix: butanal (CH₃CH₂CH₂CHO)
A hydrocarbon has the empirical formula CH. Its molar mass is 78 g/mol. Find the molecular formula and identify the compound.
n = 78 ÷ 13 = 6
Molecular formula = (CH)₆ = C₆H₆
This compound is benzene — an aromatic hydrocarbon with a ring structure and delocalized electrons.
Write the condensed structural formula for:
(a) 2-methylpropane (b) 3-methylhexane (c) 2,3-dimethylbutane
(b) CH₃CH₂CH(CH₃)CH₂CH₂CH₃ — hexane chain (6C) + methyl on C3
(c) CH₃CH(CH₃)CH(CH₃)CH₃ — butane chain (4C) + methyl on C2 and C3
State four characteristics of a homologous series and illustrate each with an example from the alcohol series (CₙH₂ₙ₊₁OH).
2. Same functional group: All have the –OH (hydroxyl) group responsible for their chemical behaviour
3. Differ by –CH₂– unit: Methanol → ethanol → propanol each adds one –CH₂– (mass difference of 14 g/mol)
4. Gradual change in physical properties: Boiling points rise steadily: methanol 65°C, ethanol 78°C, propanol 97°C, butanol 118°C
5. Similar chemical properties: All alcohols undergo oxidation, esterification, and dehydration reactions
Quick Quiz — Unit 1
Multiple Choice Quiz
Unit 1 Test
Section A — Short Answer (30 marks)
A1. (2 marks) Define the term organic compound and give two examples of carbon compounds that are NOT classified as organic.
A2. (3 marks) Distinguish between: (a) saturated and unsaturated hydrocarbons; (b) aliphatic and aromatic compounds.
A3. (4 marks) Explain the meaning of the term homologous series. State four characteristics of a homologous series.
A4. (6 marks) Give the IUPAC name for each of the following:
(i) CH₃CH₂CH₂CH₂OH (ii) CH₃CH₂COCH₃ (iii) (CH₃)₃CH
(iv) CH₃CH(Br)CH₂CH₃ (v) CH₃CH₂NH₂ (vi) CH₃COOC₂H₅
A5. (4 marks) Draw the full structural (displayed) formula for: (a) propan-1-ol, (b) propanal, (c) propanone, (d) propanoic acid.
A6. (4 marks) A compound contains 40.0% C, 6.7% H, and 53.3% O by mass. The molar mass is 60 g/mol. Find the empirical and molecular formula. Name the compound.
A7. (7 marks) Complete the table:
| Compound | Molecular Formula | Functional Group | Homologous Series | IUPAC Name |
|---|---|---|---|---|
| CH₃OH | — | — | — | — |
| CH₃CHO | — | — | — | — |
| — | C₃H₇Cl | — | Halogenoalkane | 1-chloropropane |
| CH₃COCH₃ | — | — | — | — |
| — | C₂H₅COOH | — | — | — |
Section B — Structured Questions (20 marks)
B1. (8 marks) Consider the following compounds:
(I) CH₄ (II) C₂H₄ (III) C₂H₂ (IV) C₂H₅OH (V) C₆H₆
(a) Which compound belongs to the alkyne series? Name it. (2)
(b) Which compound is aromatic? Explain why it is classified as aromatic. (2)
(c) Write the general formula for the series to which compound (II) belongs. (1)
(d) Why does compound (I) have a much lower boiling point than compound (IV) despite (IV) having only 2 carbons? (3)
B2. (12 marks) (a) Define the term functional group. (2)
(b) For each functional group listed, name the homologous series, write the general formula, and give one named example: –OH, –COOH, –CHO, –CO–, –NH₂, –COO–. (6 × 1 mark each)
(c) Explain why the boiling points increase steadily within a homologous series. (2)
(d) State and explain TWO differences between the molecular formula and the structural formula of an organic compound. (2)
📋 Model Answers — Unit Test
A4 answers: (i) butan-1-ol | (ii) butan-2-one | (iii) 2-methylpropane | (iv) 2-bromobutane | (v) ethylamine (ethanamine) | (vi) ethyl ethanoate
A6 answer: Moles: C = 40/12 = 3.33; H = 6.7/1 = 6.7; O = 53.3/16 = 3.33 → Ratio 1:2:1 → empirical CH₂O (mass 30). n = 60/30 = 2 → molecular formula C₂H₄O₂ → ethanoic acid (acetic acid) CH₃COOH
B1(d): Compound (IV) ethanol has –OH group capable of forming hydrogen bonds (due to high electronegativity of O and presence of O–H). Methane only has weak van der Waals forces. Hydrogen bonds are much stronger intermolecular forces → much more energy needed to separate ethanol molecules → higher boiling point.