Carbon and its Compounds
Look around the room you are sitting in. The clothes you wear, the plastic pen in your hand, the food on your plate, the fuel that cooked your last meal — nearly all of it is built from one element. Carbon makes up barely 0.02% of the Earth's crust and 0.03% of the atmosphere, yet chemists have catalogued millions of carbon compounds, far outnumbering every compound formed by all the other hundred-odd elements put together. This chapter asks the obvious question: what is so special about carbon? The answer starts with electrons, not atoms bonding the way sodium and chlorine do. Carbon has four electrons in its outer shell — too many to lose easily, too few to gain easily — so instead of forming ions it shares electron pairs with other atoms, forming strong, uncharged covalent bonds. That single choice explains why carbon compounds melt and boil at low temperatures and refuse to conduct electricity, unlike the ionic salts of the previous chapter. Two further properties turn this simple bonding trick into a chemistry of millions of compounds. Carbon can bond to other carbon atoms almost without limit, called catenation, building chains, branches and rings; and because it is tetravalent, each carbon atom can also carry hydrogen, oxygen, nitrogen, sulphur or a halogen as passengers along the way. Small, repeatable groups of these passenger atoms, called functional groups, hand a whole chain its chemical personality: an -OH makes it an alcohol, a -COOH makes it an acid, regardless of how long the carbon chain is. By the end of this chapter you will be able to draw the electron-sharing structure of simple carbon compounds, name them from chain length and functional group, tell a saturated hydrocarbon from an unsaturated one by its reactions, and explain — in terms of hydrophilic heads and hydrophobic tails — exactly how a bar of soap lifts oily dirt into a rinse of plain water.
- Carbon is tetravalent and forms covalent bonds — it shares four electron pairs rather than gaining or losing electrons, since both extremes (C4- or C4+) are energetically unfavourable for a six-proton nucleus.
- Catenation is carbon's near-unique ability to bond with other carbon atoms, building long chains, branches and rings; carbon-carbon bonds are far stronger and more stable than, say, silicon's.
- Tetravalency and catenation together explain the millions of known carbon compounds — no other element combines both properties to this extent.
- Saturated compounds (alkanes, only single bonds) are relatively unreactive; unsaturated compounds (alkenes, one or more double bonds; alkynes, triple bonds) are more reactive and undergo addition reactions.
- Structural isomers share one molecular formula but different carbon skeletons — butane (C4H10) has two, pentane (C5H12) has three.
- Functional groups (-OH alcohol, -CHO aldehyde, >C=O ketone, -COOH carboxylic acid, -X haloalkane) fix a compound's chemical behaviour regardless of chain length, giving a homologous series where successive members differ by a -CH2- unit, about 14 u.
- Naming follows carbon count plus a prefix or suffix for the functional group and for unsaturation (-ane single bonds, -ene double, -yne triple).
- Combustion, oxidation, addition and substitution are the four core reaction types: burning releases heat and light; oxidising agents like alkaline KMnO4 turn alcohols into acids; H2 adds across a multiple bond with a catalyst; sunlight drives chlorine to replace hydrogen atoms one at a time.
- Ethanol and ethanoic acid are the chapter's flagship compounds, reacting with sodium, undergoing dehydration, esterification and saponification, and (the acid) with bases, carbonates and hydrogencarbonates to release CO2.
- Key idea: carbon's four shared covalent bonds, combined with its unmatched ability to bond to itself (catenation), build millions of compounds whose chemical behaviour is fixed by a small functional group riding on a chain of any length, from the fuels we burn to the soap that cleans our clothes.
Because carbon is tetravalent and forms strong covalent bonds by sharing electrons rather than forming ions, and because it uniquely bonds to itself (catenation) to build chains, branches and rings, a small functional group such as -OH or -COOH riding on a carbon skeleton of any length fixes a whole homologous series' chemical behaviour, while combustion, oxidation, addition and substitution reactions convert one carbon compound into another.
What this chapter covers
The chapter builds outward from one fact: carbon has four valence electrons and satisfies its valency by sharing them, not by forming ions. Section 4.1 develops this covalent bond, comparing it against ionic bonding and using hydrogen, oxygen and nitrogen as simple examples before reaching methane. Section 4.2 asks why carbon forms so many compounds, and answers with catenation (bonding to itself) and tetravalency (bonding to four other atoms), which together generate saturated and unsaturated hydrocarbons, chains, branches, rings, structural isomers, functional groups and the homologous series, ending with naming rules. Section 4.3 turns to reactivity: combustion, oxidation, addition and substitution. Section 4.4 studies ethanol and ethanoic acid in depth. Section 4.5 closes with soaps and detergents, explaining cleaning action through micelle formation and why soap fails in hard water. Carry two ideas through every section: carbon shares, it does not transfer; and a functional group's identity survives no matter how long its carbon chain grows.
The core ideas
Covalent bonding and carbon's tetravalency
Ionic compounds like sodium chloride have high melting and boiling points and conduct electricity when molten or dissolved, because charged ions hold the lattice together. Carbon compounds behave the opposite way: acetic acid melts at 290 K and boils at 391 K, ethanol melts at 156 K and boils at 351 K, methane melts at only 90 K, and none conduct electricity — the bonding involves no ions at all. Carbon's atomic number is 6, giving electronic configuration 2, 4: four electrons in the outermost shell. Losing all four to form C4+ would strip a six-proton nucleus down to two electrons, demanding enormous energy; gaining four to form C4- would ask that same nucleus to hold ten electrons, which it cannot do stably. Carbon instead shares its four valence electrons with other atoms, forming covalent bonds, so both atoms complete their outer shell without any net charge appearing.
where: each shared pair of electrons constitutes one single covalent bond, drawn as a short line between two atomic symbols. Hydrogen shares one pair (a single bond, H-H), oxygen shares two pairs with another oxygen atom (a double bond, O=O), and nitrogen shares three pairs (a triple bond, N≡N) — in every case both atoms end with a filled outer shell. Methane is carbon's simplest hydride: carbon contributes one electron to each of four hydrogen atoms, completing its own octet while each hydrogen completes its duplet.
Catenation, saturation and unsaturation
Two features together explain why carbon forms more compounds than every other element combined. First, catenation: carbon-carbon bonds are strong and stable, letting carbon atoms link into chains of essentially any length, branch, or close into rings; silicon forms chains of only seven or eight atoms before its bonds turn too weak and reactive, but carbon shows no such limit. Second, tetravalency: each carbon atom needs four bonds, so it can devote some to other carbons and the rest to hydrogen, oxygen, nitrogen, sulphur or a halogen, letting an enormous range of side elements decorate a chain of any length. Carbon-carbon bonds may be single, double or triple. Compounds linked only by single carbon-carbon bonds are saturated (the alkanes, relatively unreactive); those with one or more double or triple carbon-carbon bonds are unsaturated (alkenes and alkynes, both more reactive, since the extra shared pairs are more easily broken and added across). The same carbon-carbon bonding, arranged differently, also produces carbon's allotropes — diamond, where each carbon bonds rigidly to four others in three dimensions, and graphite, where each carbon bonds to three others in flat hexagonal sheets that slide past one another, which is why diamond is the hardest known substance while graphite is soft, slippery and (unusually for a non-metal) conducts electricity.
Chains, branches, rings and structural isomers
Carbon chains can run straight (as in butane, C4H10) or branch, and the same molecular formula can correspond to more than one carbon skeleton — such compounds, identical in formula but different in structure, are structural isomers. Butane has two: the straight four-carbon chain (n-butane) and a three-carbon chain with one methyl branch (isobutane, or 2-methylpropane). Carbon chains can also close into a ring, as in cyclohexane (C6H12, a saturated ring) or benzene (C6H6, an unsaturated ring); compounds built only of carbon and hydrogen, whatever their shape, are called hydrocarbons — saturated ones are alkanes, unsaturated ones with a double bond are alkenes, and those with a triple bond are alkynes.
Functional groups and the homologous series
A functional group is a specific atom or group of atoms that replaces a hydrogen on a carbon chain and, regardless of chain length, dictates the compound's characteristic chemical behaviour. The chapter's key groups are the halogen (-Cl or -Br, prefix halo-), the alcohol (-OH), the aldehyde (-CHO), the ketone (>C=O) and the carboxylic acid (-COOH). A homologous series is the family obtained by attaching one functional group to carbon chains of increasing length: methanol, ethanol, propanol and butanol are all alcohols with near-identical chemical behaviour, and any two neighbours differ by exactly one -CH2- unit, about 14 u. Chemical behaviour, set by the functional group, stays constant across a series, while physical properties such as melting point, boiling point and solubility rise smoothly as chain length grows.
where: n is the number of carbon atoms in the chain; each formula assumes an open (non-ring) chain, and each successive homologue is reached by adding one -CH2- unit.
Naming carbon compounds
Naming starts from carbon count (meth- for 1, eth- for 2, prop- for 3, but- for 4, and so on), then layers on the functional group and any unsaturation. A functional group is shown as a prefix (halo-) or suffix (-ol alcohol, -al aldehyde, -one ketone, -oic acid carboxylic acid); when the suffix starts with a vowel, the chain name's final "e" is dropped first, so propane minus "e" plus "one" gives propanone. Unsaturation replaces the final "-ane" with "-ene" for one double bond or "-yne" for one triple bond, so a three-carbon chain with a double bond is propene, with a triple bond propyne.
Reactions of carbon compounds
Four reaction types recur throughout. Combustion: carbon compounds burn in oxygen releasing heat and light, producing CO2 (and water, if hydrogen is present); saturated hydrocarbons burn with a clean blue flame, unsaturated ones with a sooty yellow flame, and even a saturated fuel turns sooty if air supply is restricted — why a blocked air-hole blackens a cooking vessel. Oxidation: an oxidising agent such as alkaline KMnO4 or acidified K2Cr2O7 adds oxygen — ethanol is oxidised this way to ethanoic acid. Addition: unsaturated hydrocarbons add hydrogen across a double or triple bond with a nickel or palladium catalyst, becoming saturated — the hydrogenation of vegetable oils into solid fats uses exactly this reaction. Substitution: saturated hydrocarbons, otherwise unreactive, react with chlorine in sunlight, one hydrogen replaced by one chlorine at a time.
Ethanol, ethanoic acid and soaps
Ethanol reacts with sodium metal to liberate hydrogen gas and form sodium ethoxide, and undergoes dehydration when heated to 443 K with excess concentrated sulphuric acid (a dehydrating agent), forming ethene. Ethanoic acid (the 5-8% solution called vinegar) is a weak acid: unlike HCl, it ionises only partially in water, though both redden blue litmus. It reacts with ethanol and an acid catalyst to form a sweet-smelling ester (esterification); treating that ester with sodium hydroxide reverses the reaction, called saponification because it is how soap is made. Ethanoic acid also reacts with a base to give a salt and water, and with carbonates and hydrogencarbonates to release carbon dioxide, water and a salt. Soap molecules are sodium or potassium salts of long-chain carboxylic acids, with a hydrophilic ionic head and a hydrophobic hydrocarbon tail; in water they cluster into micelles, tails inward around trapped oil, heads outward — exactly how soap lifts oily dirt into a rinse. Hard water's dissolved calcium and magnesium salts react with soap to form an insoluble curdy scum instead of lather, which is why detergents, whose charged heads resist this precipitation, remain effective in hard water.
Two worked derivations
The goal here is to show that the general molecular formulas CnH2n+2 for alkanes, CnH2n for alkenes and CnH2n-2 for alkynes are not rules to memorise but direct, countable consequences of carbon's tetravalency once the number of carbon-carbon bonds and the type of bonding between them are fixed — so any alkane, alkene or alkyne formula can be reconstructed from first principles rather than recalled from a table.
Step 1. Build a saturated open chain of n carbons joined only by single bonds. Joining n carbons in a row uses (n-1) carbon-carbon bonds; bonds not used for the chain go to hydrogen. Each end carbon has 3 spare bonds (1 used for the chain); each of the (n-2) middle carbons has 2 spare bonds (2 used, one per side).
Step 2. Count the spare bonds, which become hydrogen atoms. Two end carbons give 2 times 3 = 6; (n-2) middle carbons give (n-2) times 2 = 2n-4. Total: 6 + 2n - 4 = 2n + 2, so the saturated formula is CnH(2n+2) — matching methane, ethane and propane.
Step 3. Introduce one double bond. It uses two of each carbon's four bonds for the link instead of one, removing 2 hydrogens compared with the saturated chain: CnH(2n+2-2) = CnH2n — matching ethene and propene.
Step 4. Introduce one triple bond. It uses three of each carbon's four bonds, removing 4 hydrogens total: CnH(2n+2-4) = CnH(2n-2) — matching ethyne and propyne. Verified: alkane n=4 gives C4H10 (butane, matches Table 4.2); alkene n=4 gives C4H8; alkyne n=4 gives C4H6; each formula reduces correctly to the named chapter examples methane, ethane, ethene and ethyne.
Go deeper · Derivation B · 1 minDeriving why a soap molecule forms a micelle around an oil droplet
The goal here is to explain, from the two-part structure of a soap molecule alone, exactly why soap dissolved in water spontaneously clusters into spherical micelles with oil trapped at the centre — turning the qualitative "soap cleans oily dirt" statement of Activity 4.10 into a step-by-step argument built on nothing more than the idea that a molecule's two ends can prefer two different environments.
Step 1. Describe the soap molecule's two ends: a long hydrocarbon tail (say 15-17 carbons) attached to a short ionic head, -COO- Na+. The tail is non-polar and hydrophobic (avoids water, mixes with oil); the head is charged and hydrophilic (attracted to water, repelled by oil).
Step 2. Place many such molecules in water containing an oil droplet. Each tail faces a choice: sit in water, unfavourable for a non-polar chain surrounded by polar water molecules, or sit inside the oil droplet, favourable since oil is itself non-polar. The ionic head prefers exactly the opposite.
Step 3. Let the system settle to its lowest-energy arrangement. Each molecule orients its tail into the oil droplet and its head into the surrounding water, satisfying both parts at once. As more molecules pack around the droplet, they surround it completely — tails inward, heads outward — and this spherical cluster is the micelle.
Step 4. Read off the consequence for cleaning. Wrapped in a micelle, the droplet's outer surface is entirely ionic, so the whole oil-plus-soap package disperses as a stable colloid rather than separating back into an oil layer. Agitation (beating, scrubbing, a washing machine's tumbling) supplies the energy to detach dirt-laden oil droplets from fabric so the same wrapping happens to them, after which plain water rinses the micelle away. Verified: the two-part soap molecule (hydrophobic tail, hydrophilic head) and the observed lowest-energy geometry, tails inward and heads outward around trapped oil, matches the micelle structure of Fig. 4.12 in the chapter and the emulsification observed in Activity 4.10.
Worked examples
Board level - Balancing the combustion equations for methane and ethanol (NCERT Section 4.3.1, extended)
Methane and ethanol both burn in oxygen to release heat and light, producing carbon dioxide and water. Balance the equations CH4 + O2 to CO2 + H2O and C2H5OH + O2 to CO2 + H2O, and state what changes in the products if the oxygen supply is restricted.
Step 1. Balance methane's combustion by counting atoms. One carbon needs one CO2; four hydrogens need two H2O; that accounts for 2 + 2 = 4 oxygen atoms on the right, so 2 O2 are needed on the left: CH4 + 2O2 to CO2 + 2H2O.
Step 2. Balance ethanol's combustion the same way. C2H5OH already contains one oxygen atom; its two carbons need 2 CO2, its six hydrogens need 3 H2O. The right side has (2 times 2) + 3 = 7 oxygen atoms; ethanol supplies 1, so 6 more, or 3 O2, must come from the left: C2H5OH + 3O2 to 2CO2 + 3H2O.
Step 3. Consider restricted oxygen. Both equations describe complete combustion, every carbon ending as CO2. With limited oxygen, combustion turns incomplete: some carbon only partly oxidises to poisonous CO, some deposits as soot — the sooty flame and blackened vessel-bottoms the chapter describes. Answer: CH4 + 2O2 gives CO2 + 2H2O; C2H5OH + 3O2 gives 2CO2 + 3H2O; restricted oxygen replaces some CO2 with CO and soot.
Board level - Naming and drawing structures from the functional group tables (NCERT Section 4.2.5, extended)
Name the compound CH3-CH2-Br using the carbon count and the halogen prefix rule, then draw the structures of butanone and ethanoic acid from their carbon count and functional-group suffix.
Step 1. Name CH3-CH2-Br. It has a two-carbon chain (eth-) with a bromine atom substituting for one hydrogen; a halogen is always a prefix, bromo-, giving bromoethane.
Step 2. Draw butanone. "Butan-" fixes a four-carbon chain; suffix "-one" fixes a ketone group, >C=O, which (an internal group) sits on an internal carbon: CH3-CO-CH2-CH3.
Step 3. Draw ethanoic acid. "Ethan-" fixes a two-carbon chain; suffix "-oic acid" fixes -COOH, which (like an aldehyde) sits at a chain end since it uses three of that carbon's four bonds: CH3-COOH. Answer: CH3-CH2-Br is bromoethane; butanone is CH3-CO-CH2-CH3; ethanoic acid is CH3-COOH.
Board HOTS / foundation JEE-NEET - Counting the structural isomers of pentane from tetravalency and catenation (NCERT Exercise Q1, extended)
Pentane has the molecular formula C5H12. Using only carbon's tetravalency and its ability to catenate, work out every distinct carbon skeleton consistent with this formula, and state how their physical properties are expected to differ.
Step 1. Check the formula against the alkane rule derived above: CnH2n+2 with n = 5 gives C5H12, confirming pentane is a fully saturated open-chain hydrocarbon, so every carbon-carbon bond is single and every carbon's four bonds are satisfied by carbon and hydrogen neighbours alone.
Step 2. Draw the straight-chain skeleton first: five carbons in an unbranched row, C-C-C-C-C. Filling every remaining valency with hydrogen gives n-pentane.
Step 3. Introduce one branch: a four-carbon chain with the fifth carbon as a methyl (-CH3) branch on the second carbon gives 2-methylbutane (isopentane). Attaching it to the middle carbon instead gives nothing new — by symmetry it is the same skeleton renumbered from the other end.
Step 4. Introduce two branches: a three-carbon chain with two methyl branches on its central (now quaternary) carbon gives 2,2-dimethylpropane (neopentane). No further distinct skeleton exists — any other placement either repeats these three shapes or demands a five-bonded carbon, breaking tetravalency.
Step 5. Read off the physical-property trend. All three isomers share formula C5H12, but as branching increases, molecules pack less efficiently and touch fewer neighbours, so boiling point falls: n-pentane near 36 degrees Celsius, isopentane near 28 degrees Celsius, neopentane near 9.5 degrees Celsius. Answer: exactly three structural isomers exist — n-pentane, 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane) — all C5H12, with boiling point falling as branching increases. Timing note: aim for under four minutes. Fix the longest possible chain first, place the remaining carbon(s) as branches, then check each branch position against every other by symmetry before declaring a "new" isomer — the commonest error is double-counting a skeleton that is identical once renumbered from the opposite end.
Common traps
The single most repeated slip is describing carbon's bonding as ionic — writing C4+ or C4- or talking about carbon "losing" or "gaining" electrons; carbon shares electrons in every ordinary compound. A second trap is muddling saturated and unsaturated: saturated hydrocarbons burn with a clean flame and are relatively unreactive, unsaturated ones burn sooty and are more reactive — students sometimes reverse this. In naming, a frequent error is forgetting to drop the final "e" before a vowel-starting suffix (propanone, not propaneone), or confusing prefix groups (halo-) with suffix groups (-ol, -al, -one, -oic acid). Esterification and saponification are often confused, since they are exact reverses of each other. Students also assume ethanoic acid ionises completely like HCl simply because both redden litmus; it is a weak acid, only partially ionised, a distinction the universal indicator reveals. Finally, many assume more soap "fixes" hard water; it keeps forming scum with dissolved calcium and magnesium regardless, and only a detergent's charged head resists this.
You should be able to…
- Explain why carbon forms covalent bonds rather than ions, using its electronic configuration.
- Draw the electron-dot (Lewis) structure of simple molecules such as H2, O2, N2, CH4 and CH3Cl.
- State catenation and tetravalency and explain how together they account for carbon's millions of compounds.
- Distinguish saturated from unsaturated hydrocarbons by their bonding, reactivity and flame test.
- Draw structural isomers for a given molecular formula, such as butane or pentane.
- Identify a functional group from a structure or formula, and place a compound in its homologous series.
- Name a carbon compound from its chain length, functional group and degree of unsaturation, and vice versa.
- Write and balance combustion equations for hydrocarbons and alcohols, and classify reactions as combustion, oxidation, addition or substitution.
- Describe the key reactions of ethanol and ethanoic acid, including esterification and saponification.
- Explain micelle formation and why soap is less effective than detergent in hard water.
Ask the tutor: Want to work through drawing electron-dot structures or naming a few more carbon compounds together, or practice counting isomers for a formula of your choice?
Why can carbon form millions of compounds when other elements with four valence electrons, like silicon, cannot?
Two properties combine uniquely in carbon. Tetravalency lets each carbon atom bond to four other atoms, so a chain of any length can still carry hydrogen, oxygen, nitrogen or other elements as passengers. More decisively, carbon-carbon bonds are unusually strong and stable, so carbon can catenate into very long chains, branches and rings. Silicon is also tetravalent and can catenate, but silicon-hydrogen chains are limited to about seven or eight atoms before turning highly reactive and unstable, so silicon never builds carbon's structural variety. It is the combination of both properties, not either alone, that gives carbon its millions of compounds.
Why does a candle flame look yellow and sooty near the base but blue higher up, and does this matter for a gas stove?
The yellow, sooty part of a flame is unburnt carbon particles glowing red-hot because the fuel there is not getting enough oxygen — incomplete combustion. Higher up, where air mixes in well, combustion is complete and the flame burns cleaner. This matters directly for a gas or kerosene stove: it is built with air holes so the fuel-air mixture stays oxygen-rich, giving a clean blue flame. If those holes get blocked, the flame turns yellow and sooty, deposits soot on the vessel's underside, and wastes fuel, since some carbon never reaches CO2.
What is actually different between soap and detergent if both are used to clean clothes?
Both have a long hydrocarbon tail and a charged head, and both clean the same way, wrapping oily dirt inside a micelle. The difference is what the charged head is made of. Soap's head is the -COO- of a long-chain carboxylic acid's salt, which reacts with dissolved calcium and magnesium in hard water to form an insoluble curdy precipitate (scum), wasting soap and reducing lather. A detergent's head is typically a sulphonate or ammonium salt, which does not form insoluble precipitates with calcium or magnesium, so detergents keep lathering in hard water where soap struggles.
If both hydrochloric acid and ethanoic acid turn blue litmus red, why is ethanoic acid called a weak acid?
Litmus only tells you a solution is acidic; it does not measure how strongly acidic it is. Hydrochloric acid is a strong acid because, dissolved in water, essentially all of it splits apart into ions. Ethanoic acid is a weak acid because only a small fraction of its molecules ionise at any moment; most stay as intact, uncharged CH3COOH molecules. A universal indicator, reporting a whole pH value rather than just "acid" or "not," shows this clearly: dilute HCl reads a much lower pH than dilute ethanoic acid of the same concentration, even though both redden blue litmus.