Naming organic compounds

1. Overview

Organic chemistry is the study of carbon-based compounds. Because carbon can form four covalent bonds and create long chains, millions of different molecules exist; therefore, a systematic naming system (IUPAC) is essential to ensure scientists worldwide can communicate clearly. This topic covers how to name and draw the fundamental building blocks of organic chemistry, including alkanes, alkenes, alcohols, and carboxylic acids.

---

Key Definitions

• Hydrocarbon: A compound containing only hydrogen and carbon atoms.
• Homologous Series: A family of similar compounds with the same functional group and general formula, where each member differs from the next by a –CH₂– unit.
• Functional Group: An atom or group of atoms that determine the chemical properties of a homologous series (e.g., –OH in alcohols).
• Displayed Formula: A drawing showing all atoms and all the bonds in a molecule.
• Structural Formula: A formula that shows how atoms are grouped together in a molecule without showing every bond (e.g., CH₃CH₂OH).
• Molecular Formula: The total number of atoms of each element in a compound (e.g., C₂H₆O).

---

Core Content

Naming and Drawing Basics

The name of an organic compound is derived from the number of carbon atoms (the prefix) and the functional group (the suffix).

Prefixes (Number of Carbons):

• 1 Carbon: Meth-
• 2 Carbons: Eth-

Suffixes (Compound Type):

• -ane: Alkane (C–C single bonds)
• -ene: Alkene (C=C double bond)
• -ol: Alcohol (–OH group)
• -oic acid: Carboxylic acid (–COOH group)

Displayed Formulae to Know

Name	Type	Molecular Formula	Displayed Formula
Methane	Alkane	CH₄	📊C atom in center bonded to 4 H atoms
Ethane	Alkane	C₂H₆	📊2 C atoms with single bond, each bonded to 3 H atoms
Ethene	Alkene	C₂H₄	📊2 C atoms with double bond, each bonded to 2 H atoms
Ethanol	Alcohol	C₂H₅OH	📊2 C atoms with single bond, first C with 3 H, second C with 2 H and an O-H group
Ethanoic Acid	Carboxylic Acid	CH₃COOH	📊2 C atoms; first C has 3 H; second C has a double bond to O and a single bond to O-H

Identifying Compounds

You must be able to identify a compound type from its name or formula:

• C₃H₈: Ends in -ane (Propane) because it fits CₙH₂ₙ₊₂.
• C₂H₄: Ends in -ene (Ethene) because it fits CₙH₂ₙ.
• CH₃OH: Ends in -ol (Methanol) because of the –OH group.
• HCOOH: Ends in -oic acid (Methanoic acid) because of the –COOH group.

---

Extended Content (Extended Curriculum Only)

Expanded Alkanes, Alkenes, and Alcohols

For the extended curriculum, you must know prefixes for up to 4 carbons:

• 3 Carbons: Prop-
• 4 Carbons: But-

Alkenes (Isomers): The position of the double bond must be indicated by a number.

• But-1-ene: CH₂=CHCH₂CH₃ (Double bond starts at carbon 1)
• But-2-ene: CH₃CH=CHCH₃ (Double bond starts at carbon 2)

Alcohols (Isomers):

• Propan-1-ol: CH₃CH₂CH₂OH (–OH on the end carbon)
• Propan-2-ol: CH₃CH(OH)CH₃ (–OH on the middle carbon)
• Butan-1-ol: CH₃CH₂CH₂CH₂OH
• Butan-2-ol: CH₃CH₂CH(OH)CH₃

Carboxylic Acids (up to C4)

1. Methanoic acid: HCOOH
2. Ethanoic acid: CH₃COOH
3. Propanoic acid: CH₃CH₂COOH
4. Butanoic acid: CH₃CH₂CH₂COOH

Esters

Esters are formed from an alcohol and a carboxylic acid.

• Naming: [Alcohol part]-yl [Acid part]-oate.
• Example: Ethyl ethanoate (Made from ethanol and ethanoic acid).
• Structural Formula: CH₃COOCH₂CH₃
• Displayed Formula:
  📊CH₃ connected to C=O, then that C connected to O, which is then connected to CH₂CH₃

---

Key Equations

Organic reactions often involve the production of new named compounds.

1. Combustion of Methane (Complete)

• Word: Methane + Oxygen → Carbon dioxide + Water
• Symbol: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

2. Substitution Reaction (Alkanes + Halogens)

• Word: Ethane + Chlorine → Chloroethane + Hydrogen chloride
• Symbol: C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)

3. Addition Reaction (Ethene + Steam to make Ethanol)

• Word: Ethene + Steam → Ethanol
• Symbol: C₂H₄(g) + H₂O(g) → C₂H₅OH(l)

4. Esterification (Ethanoic Acid + Ethanol)

• Word: Ethanoic acid + Ethanol ⇌ Ethyl ethanoate + Water
• Symbol: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)

---

Common Mistakes to Avoid

• ❌ Wrong: Drawing carbon with 3 or 5 bonds.
  • ✓ Right: Carbon always has 4 bonds. Check every carbon in your drawing.
• ❌ Wrong: Forgetting the "H" in the –OH group of an alcohol.
  • ✓ Right: Ensure the bond goes from the Carbon to the Oxygen, then Oxygen to Hydrogen (C–O–H).
• ❌ Wrong: Naming an ester as "Ethyloic ethanyl".
  • ✓ Right: Always name the alcohol part first with "-yl" and the acid part second with "-oate" (Ethyl ethanoate).
• ❌ Wrong: Counting the carbon in the –COOH group separately from the chain.
  • ✓ Right: If there are 3 carbons total including the one in –COOH, it is propanoic acid.

---

Exam Tips

• Command Words: "Draw the displayed formula" means show every single bond. If you write "CH₃", you may lose marks; you must draw the three lines for the C–H bonds.
• Identify the Functional Group: Look for the double bond (alkene), the –OH (alcohol), or the –COOH (acid) first before naming the chain.
• Numbering: When numbering carbon chains (e.g., But-1-ene vs But-2-ene), always start numbering from the end that gives the functional group the lowest possible number.
• Esters: In a displayed formula for an ester, the carbon double-bonded to an oxygen (C=O) always comes from the carboxylic acid. The oxygen in the middle of the chain (–O–) connects the acid part to the alcohol part.

---

Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Draw the displayed formula of ethanol. [2]

(b) Name the type of organic compound that ethanol belongs to. [1]

(c) State one use of ethanol. [1]

(d) Ethanoic acid reacts with ethanol to form an ester. Name the ester formed in this reaction. [1]

Worked Solution:

(a)

1. 📊Ethanol displayed formula. Carbon chain with 2 carbons. First carbon bonded to 3 hydrogens. Second carbon bonded to 2 hydrogens and an -OH group. All bonds must be explicitly drawn.
   *[Drawing the displayed formula, showing all bonds]*

How to earn full marks:

• Correctly draw the carbon chain with two carbon atoms, showing all bonds.
• Correctly show all bonds to hydrogen atoms and the -OH group.

(b)

1. Alcohols [Identifying the functional group]

How to earn full marks:

• Correctly identify the compound as an alcohol. Spelling must be correct.

(c)

1. Fuel / Solvent / Alcoholic beverages / Disinfectant [Stating a valid use]

How to earn full marks:

• State a valid use of ethanol.

(d)

1. Ethyl ethanoate [Stating the correct ester name]

How to earn full marks:

• Correctly name the ester formed. Spelling must be correct.

Common Pitfall: Many students struggle with drawing displayed formulae correctly. Make sure you show every single bond between the atoms. Also, remember that the order of the ester name is derived from the alcohol first, then the acid.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Define the term 'alkene'. [2]

(b) Draw the structural formula of but-2-ene. [2]

(c) Describe a simple chemical test to distinguish between butane and but-2-ene. State the expected result for each compound. [2]

Worked Solution:

(a)

1. A hydrocarbon containing at least one carbon-carbon double bond. [Defining the term alkene]

How to earn full marks:

• State that it is a hydrocarbon.
• State that it contains at least one carbon-carbon double bond.

(b)

1. $CH_3CH=CHCH_3$ [Drawing the structural formula]

How to earn full marks:

• Correctly show the four carbon atoms in a chain.
• Correctly show the double bond between the second and third carbon atoms.

(c)

1. Add bromine water (or aqueous bromine) to each compound. [Stating the reagent]
2. But-2-ene will decolourise the bromine water (or aqueous bromine). [Stating the result for but-2-ene]
3. Butane will not decolourise the bromine water (or aqueous bromine). [Stating the result for butane]

How to earn full marks:

• State the correct reagent (bromine water or aqueous bromine).
• State the correct observation for but-2-ene (decolourises).
• State the correct observation for butane (no change or no decolourisation).

Common Pitfall: When describing the bromine water test, be sure to mention "decolourisation" specifically. Saying "it reacts" is not enough. Also, remember that alkanes do not react with bromine water under normal conditions.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student investigates the properties of organic compounds. They are given two unlabelled bottles, one containing propan-1-ol and the other containing propanoic acid.

(a) Draw the displayed formula of propanoic acid. [2]

(b) Describe a chemical test that the student could use to distinguish between propan-1-ol and propanoic acid. State the expected result for each compound. [3]

(c) Propanoic acid reacts with ethanol to form an ester and water. (i) Name the ester formed in this reaction. [1] (ii) Draw the displayed formula of the ester formed. [2]

Worked Solution:

(a)

1. 📊Propanoic acid displayed formula. Carbon chain with 3 carbons. First carbon bonded to 3 hydrogens. Second carbon bonded to 2 hydrogens. Third carbon bonded to one oxygen with a double bond and to an -OH group. All bonds must be explicitly drawn.
   *[Drawing the displayed formula, showing all bonds]*

How to earn full marks:

• Correctly draw the carbon chain with three carbon atoms, showing all bonds.
• Correctly show the double bond to one oxygen atom and the single bond to the -OH group on the end carbon.

(b)

1. Add a carbonate, such as sodium carbonate or calcium carbonate, to each compound. [Stating the reagent]
2. Propanoic acid will react with the carbonate to produce carbon dioxide gas, which will turn limewater cloudy. [Stating the result for propanoic acid]
3. Propan-1-ol will not react with the carbonate. [Stating the result for propan-1-ol]

How to earn full marks:

• State the correct reagent (a carbonate such as sodium carbonate or calcium carbonate).
• State the correct observation for propanoic acid (effervescence/fizzing and limewater turns cloudy).
• State the correct observation for propan-1-ol (no reaction).

(c) (i)

1. Ethyl propanoate [Stating the correct ester name]

How to earn full marks:

• Correctly name the ester formed. Spelling must be correct.

(ii)

1. 📊Ethyl propanoate displayed formula. Carbon chain with 5 atoms: 3 carbons, then a carbonyl group (C=O) and also to an oxygen atom. This oxygen is bonded to a carbon. This carbon is bonded to two hydrogens. This carbon is bonded to three hydrogens. All bonds must be explicitly drawn.
   *[Drawing the displayed formula, showing all bonds]*

How to earn full marks:

• Correctly show the ester linkage (-COO-) with the correct number of carbon atoms on each side (2 carbons from ethanol, 3 carbons from propanoic acid), showing all bonds.
• Correctly show all bonds to hydrogen and oxygen atoms.

Common Pitfall: For the reaction with carbonates, remember that you need to mention both the observation of effervescence (fizzing) and that the gas produced turns limewater cloudy to get full marks. Also, be careful when drawing esters – the -COO- linkage is often drawn incorrectly.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A chemist investigates the reactions of different hydrocarbons.

(a) Name the following hydrocarbons:

(i) $CH_4$ [1] (ii) $CH_3CH=CHCH_3$ [1]

(b) Ethane reacts with chlorine in the presence of ultraviolet light to form chloroethane and hydrogen chloride.

(i) State the type of reaction that occurs. [1] (ii) Write a balanced chemical equation for this reaction, using molecular formulae. [2]

(c) A student attempts to produce but-1-ene by cracking dodecane ($C_{12}H_{26}$). One possible cracking reaction produces but-1-ene and one other hydrocarbon.

(i) State the general formula for alkanes. [1] (ii) Determine the molecular formula of the other hydrocarbon produced in this reaction. [3]

Worked Solution:

(a) (i)

1. Methane [Stating the correct name]

How to earn full marks:

• Correctly name the hydrocarbon. Spelling must be correct.

(ii)

1. But-2-ene [Stating the correct name]

How to earn full marks:

• Correctly name the hydrocarbon. Spelling must be correct.

(b) (i)

1. Substitution [Stating the type of reaction]

How to earn full marks:

• Correctly state the type of reaction.

(ii)

1. $C_2H_6 + Cl_2 \rightarrow C_2H_5Cl + HCl$ [Writing the balanced equation]

How to earn full marks:

• Correctly state the molecular formula of each reactant and product.
• Ensure the equation is balanced.

(c) (i)

1. $C_nH_{2n+2}$ [Stating the general formula]

How to earn full marks:

• Correctly state the general formula for alkanes.

(ii)

1. Dodecane: $C_{12}H_{26}$ [Identifying the reactants]
2. But-1-ene: $C_4H_8$ [Identifying the products]
3. $C_{12}H_{26} \rightarrow C_4H_8 + X$ [Setting up the equation]
4. $X = C_{12-4}H_{26-8} = C_8H_{18}$ [Calculating the formula of X]

How to earn full marks:

• Show the subtraction of the carbon number correctly (12-4 = 8).
• Show the subtraction of the hydrogen number correctly (26-8 = 18).
• State the correct molecular formula for the other hydrocarbon: $\boxed{C_8H_{18}}$

Common Pitfall: In cracking questions, remember that the number of carbon and hydrogen atoms must be conserved. Double-check your calculations to ensure that the total number of atoms on both sides of the equation are equal. Also, be careful with the alkane general formula; it's easy to mix up the "+2".