Core ConceptRead this first — the foundation of the topic
CARBON COMPOUNDS — CORE CONCEPT Carbon is a special element. It can form bonds with itself and with other elements to make millions of different compounds. This property is called covalent bonding. Carbon compounds are the basis of all living things and many everyday substances like petrol, plastics, and medicines.
Why is Carbon Special? Carbon has 4 electrons in its outer shell. It needs 4 more to complete its shell. So it shares electrons with other atoms. This sharing of electrons is called a covalent bond. Carbon can bond with carbon itself — this is called catenation. This gives carbon the ability to form long chains, branches, and rings.
Key RulesCore rules you must know cold
Covalent Bonds
Carbon always forms 4 covalent bonds. Hydrogen forms 1. Oxygen forms 2. Nitrogen forms 3.
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Homologous Series
A family of compounds with the same general formula and similar properties. Each member differs by CH2 (14 mass units).
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Functional Groups
A group of atoms that gives a compound its characteristic properties.
— OH (Hydroxyl) = Alcohols (e.g., Ethanol)
— COOH (Carboxyl) = Carboxylic Acids (e.g., Acetic Acid)
— CHO (Aldehyde) = Aldehydes
— CO (Ketone) = Ketones
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Saturated vs Unsaturated
— Saturated = only single bonds (Alkanes) — e.g., Methane (CH4)
— Unsaturated = double or triple bonds (Alkenes, Alkynes) — e.g., Ethene (C2H4), Ethyne (C2H2)
Formula BlockMemorise — at least one formula appears in every paper
Exam PatternsWhat examiners ask — read before attempting PYQs
RRB Group D asks
identify functional groups, name of compounds, formula of a given compound, combustion reactions, and properties of ethanol vs ethanoic acid. Expect 1-2 questions from this topic.
SHORTCUT / TRICK
Trick 1 — Formula Memory: Mnemonic for Alkane formula CnH(2n+2) = "Carbon n, Hydrogen Double Plus Two". For Alkene = "Double n". For Alkyne = "Double Minus Two"
Trick 2 — Vinegar Trick
Ethanoic acid = acetic acid = vinegar. 3-4% solution = vinegar. If the question mentions sour taste or vinegar, the answer is ethanoic acid (CH3COOH).
Worked ExampleSolve this step-by-step before moving on
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Step 1
Alkane general formula = CnH(2n+2)
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Step 2
4th member means n = 4
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Step 3
Carbon atoms = 4
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Step 4
Hydrogen atoms = (2×4) + 2 = 8 + 2 = 10
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Step 5
Formula = C4H10 (Butane)
Answer: C4H10
Exam TrapsCommon mistakes students make — avoid these
Students confuse Ethanol (C2H5OH — alcohol, used in sanitisers) with Ethanoic Acid (CH3COOH — vinegar). Remember: Ethanol has OH group. Ethanoic acid has COOH group.
Both have 2 carbons but very different properties. Also, do NOT mix up alkene and alkyne formulas — alkyne has the minus 2 formula, not alkene.
Key Points to Remember
Carbon forms 4 covalent bonds — this is the reason it makes millions of compounds.
Catenation = carbon's ability to bond with itself to form chains, branches, and rings.
Alkane formula: CnH(2n+2) — saturated hydrocarbons with only single bonds.
Alkene formula: CnH(2n) — unsaturated, contain at least one double bond.
Alkyne formula: CnH(2n-2) — unsaturated, contain at least one triple bond.
Ethanol (C2H5OH) is an alcohol; Ethanoic Acid (CH3COOH) is acetic acid (vinegar).
Which of the following is a functional group present in ethanol (C₂H₅OH)?
Practice 2medium
Ethane (C₂H₆) and ethene (C₂H₄) are two carbon compounds. Which of the following statements correctly distinguishes them based on their bonding and chemical properties?
Practice 3medium
Which allotrope of carbon has a layered structure where each carbon atom is bonded to three other carbon atoms, making it a good conductor of electricity?
Practice 4hard
Ethene (C₂H₄) undergoes an addition reaction with bromine water, producing a colourless dibromide product. Which of the following correctly identifies both the type of reaction and the structural reason why this reaction occurs?
60-Second Revision — Carbon Compounds
Remember: Carbon always makes 4 bonds — the foundation of all organic chemistry.