⚛️ Carbon and its Compounds
Complete visual notes — every concept, every equation, every question answered in simple language for school students.
1 Bonding in Carbon — The Covalent Bond
Carbon has atomic number 6, so its electron configuration is 2, 4. It has 4 valence electrons in its outermost shell and needs 4 more to complete its octet (noble gas configuration).
Carbon cannot simply gain 4 electrons (would need to hold 10 electrons with only 6 protons — very unstable) or lose 4 electrons (needs too much energy). So carbon solves this by sharing electrons — this is called a covalent bond.
A covalent bond is formed when two atoms share a pair of electrons, so that both atoms achieve a completely filled outermost shell (noble gas configuration).
Single Bond (—)
One pair of electrons shared between two atoms.
Example: H—H (hydrogen), H—Cl (HCl), C—C (ethane)
Double Bond (=)
Two pairs of electrons shared.
Example: O=O (oxygen), C=C (ethene), C=O (CO₂)
Triple Bond (≡)
Three pairs of electrons shared.
Example: N≡N (nitrogen), C≡C (ethyne)
Fig. 1 – Covalent bonding in H₂ (single), O₂ (double) and N₂ (triple bond)
Methane — Simplest Carbon Compound
Methane (CH₄) is the simplest carbon compound. Carbon (valency 4) shares one electron with each of 4 hydrogen atoms (valency 1). All atoms satisfy their octet/duplet.
Fig. 2 – Structure of Methane (CH₄)
Since no charged ions are formed in covalent bonding, covalent compounds are poor conductors of electricity. Intermolecular forces are weak, so they have low melting and boiling points compared to ionic compounds.
2 Allotropes of Carbon
The same element carbon exists in different physical forms called allotropes. These have very different physical properties, but their chemical properties are the same.
💎 Diamond
Each carbon bonded to 4 other carbon atoms in a rigid 3D network. Hardest natural substance known. Does not conduct electricity. Used in cutting tools, jewellery.
✏️ Graphite
Each carbon bonded to 3 others in hexagonal layers. One bond is double. Layers slide — so it is smooth and slippery. Conducts electricity (unique non-metal). Used in pencils, electrodes, lubricants.
⚽ Fullerene (C-60)
Carbon atoms arranged in a football shape. Named after architect Buckminster Fuller (Buckminsterfullerene). First allotrope discovered after diamond and graphite.
Fig. 3 – The three allotropes of carbon
Synthetic diamonds can be made by subjecting pure carbon to very high pressure and temperature. These are small but chemically identical to natural diamonds. Graphite is unique among non-metals — it conducts electricity because one electron per carbon atom is free to move between the layers.
3 Why Carbon is so Versatile
Carbon forms millions of compounds — more than all other elements combined! Two special reasons make this possible:
🔗 Catenation
Carbon's unique ability to form bonds with other carbon atoms, creating long chains, branched chains, or rings. The C–C bond is very strong and stable. No other element shows catenation to this extent.
4️⃣ Tetravalency
Carbon has a valency of 4 — it can bond with four other atoms. It can form bonds with H, O, N, S, Cl and many other elements, creating compounds with a huge variety of properties.
The two reasons for the huge number of carbon compounds are: (1) Catenation — bonding with other carbon atoms, and (2) Tetravalency — forming 4 bonds with different elements. Carbon is also small in size, making C–C and C–X bonds very strong and stable.
4 Saturated & Unsaturated Carbon Compounds
✅ Saturated Compounds
Carbon atoms linked by only single bonds. These are less reactive and generally burn with a clean blue flame.
Called Alkanes — general formula: CnH2n+2
Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈)
⚡ Unsaturated Compounds
Carbon atoms linked by double or triple bonds. These are more reactive and burn with a yellow, sooty flame.
Alkenes (C=C): CnH2n — e.g. Ethene (C₂H₄)
Alkynes (C≡C): CnH2n-2 — e.g. Ethyne (C₂H₂)
Fig. 4 – Comparing single (saturated) vs double/triple (unsaturated) bonds
5 Chains, Branches & Rings
Alkanes — Saturated Hydrocarbons
| No. of C | Name | Formula | General Pattern |
|---|---|---|---|
| 1 | Methane | CH₄ | CnH2n+2 |
| 2 | Ethane | C₂H₆ | → |
| 3 | Propane | C₃H₈ | → |
| 4 | Butane | C₄H₁₀ | → |
| 5 | Pentane | C₅H₁₂ | → |
| 6 | Hexane | C₆H₁₄ | → |
Structural Isomers
Two compounds with the same molecular formula but different structures are called structural isomers.
Example — Butane (C₄H₁₀): Can form a straight chain (n-butane) OR a branched chain (isobutane) — same formula, different arrangement.
Fig. 5 – Structural isomers of Butane — same formula, different structure
Cyclic Compounds
Some carbon compounds have carbon atoms joined in a ring. For example, Cyclohexane (C₆H₁₂) has 6 carbons in a ring, all single bonds (saturated). Benzene (C₆H₆) has 6 carbons in a ring with alternating double bonds (unsaturated).
Fig. 6 – Cyclic compounds: Cyclohexane (saturated) and Benzene (unsaturated)
6 Heteroatoms & Functional Groups
When hydrogen atoms in a hydrocarbon are replaced by other atoms (like O, N, Cl, Br), those replacing atoms are called heteroatoms. The group formed gives the compound its characteristic properties and is called a functional group.
| Heteroatom | Class of Compound | Functional Group | Formula Pattern |
|---|---|---|---|
| Cl/Br | Haloalkane | –Cl or –Br | R–Cl / R–Br |
| Oxygen | Alcohol | –OH (hydroxyl) | R–OH |
| Aldehyde | –CHO | R–CHO | |
| Ketone | –C(=O)– (between carbons) | R–CO–R | |
| Carboxylic Acid | –COOH | R–COOH |
The functional group determines the chemical properties of the compound, regardless of how long or short the carbon chain is. For example, all alcohols (–OH group) react similarly with sodium to release hydrogen gas.
7 Homologous Series
A homologous series is a family of compounds that have:
✦ The same functional group ✦ Each member differs from the next by –CH₂– (molecular mass difference = 14 u) ✦ Similar chemical properties ✦ Gradually changing physical properties (boiling points, melting points increase with chain length)
| Series | General Formula | Example Members | Difference |
|---|---|---|---|
| Alkanes | CnH2n+2 | CH₄, C₂H₆, C₃H₈, C₄H₁₀… | –CH₂– (14 u) |
| Alkenes | CnH2n | C₂H₄, C₃H₆, C₄H₈… | –CH₂– (14 u) |
| Alkynes | CnH2n-2 | C₂H₂, C₃H₄, C₄H₆… | –CH₂– (14 u) |
| Alcohols | CnH2n+1OH | CH₃OH, C₂H₅OH, C₃H₇OH… | –CH₂– (14 u) |
In a homologous series, chemical properties are similar (decided by functional group), but physical properties (melting point, boiling point, solubility) change gradually as molecular mass increases.
8 Nomenclature of Carbon Compounds
The name of a carbon compound = prefix (if any) + root word (carbon chain) + suffix (functional group).
Root word is based on number of carbons: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6)…
| Class | Prefix/Suffix | Example |
|---|---|---|
| Haloalkane (Cl/Br) | Prefix: chloro-, bromo- | Chloropropane, Bromopropane |
| Alcohol | Suffix: -ol | Methanol, Ethanol, Propanol |
| Aldehyde | Suffix: -al | Methanal, Ethanal, Propanal |
| Ketone | Suffix: -one | Propanone, Butanone |
| Carboxylic Acid | Suffix: -oic acid | Methanoic acid, Ethanoic acid, Propanoic acid |
| Alkene (double bond) | Suffix: -ene | Ethene, Propene, Butene |
| Alkyne (triple bond) | Suffix: -yne | Ethyne, Propyne, Butyne |
Step 1: Count carbons → pick root (eth = 2, prop = 3…)
Step 2: Identify functional group → pick suffix (-ol, -al, -one, -oic acid)
Step 3: If suffix starts with a vowel, drop the final 'e' from the root (e.g., propan + one = propanone)
Step 4: If unsaturated, replace '-ane' with '-ene' or '-yne'
9 Chemical Properties of Carbon Compounds
4.3.1 Combustion
Carbon and most carbon compounds burn in oxygen to give CO₂, water, and heat + light. These are oxidation reactions.
(ii) CH₄ + 2O₂ → CO₂ + 2H₂O + heat and light
(iii) C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat and light
🔵 Clean Blue Flame
Saturated hydrocarbons burn with a clean blue flame when air supply is sufficient. This happens in gas stoves with open air holes.
🟡 Yellow Sooty Flame
Unsaturated hydrocarbons burn with a yellow, smoky flame and deposit soot (carbon) on surfaces. Incomplete combustion also causes this.
If the air (oxygen) supply is limited, the carbon in the fuel does not get fully oxidised. Instead of forming CO₂, some carbon comes out as fine particles of soot. This is why cooking vessel bottoms blacken when gas stove air holes are blocked.
4.3.2 Oxidation
Carbon compounds can be oxidised beyond just burning. Alcohols are converted to carboxylic acids using oxidising agents like alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇).
(Ethanol) (Ethanoic acid)
4.3.3 Addition Reaction
Unsaturated compounds add hydrogen across double/triple bonds in the presence of a nickel or palladium catalyst to give saturated compounds. This is called hydrogenation.
Unsaturated (liquid oil) + H₂ → Saturated (solid fat)
Vegetable oils (unsaturated) are hydrogenated using nickel catalyst to form vanaspati ghee (saturated solid fat). This is why some vegetable oils are called "healthy" — they still contain unsaturated fats. Animal fats are saturated and considered less healthy.
4.3.4 Substitution Reaction
Saturated hydrocarbons are generally unreactive, but in the presence of sunlight, chlorine replaces hydrogen atoms one by one. This is called a substitution reaction — one type of atom takes the place of another.
(Methane) (Chlorine) → (Chloromethane) + (Hydrochloric acid)
10 Ethanol (C₂H₅OH)
Ethanol (commonly called alcohol) is a liquid at room temperature. It is the active ingredient in all alcoholic drinks and is also a good solvent used in medicines (tincture iodine, cough syrups). It is completely miscible with water.
🔬 Reaction with Sodium
Ethanol reacts with sodium metal to release hydrogen gas and form sodium ethoxide.
2Na + 2C₂H₅OH → 2C₂H₅O⁻Na⁺ + H₂↑
🌡️ Dehydration (443 K)
Heating with excess conc. H₂SO₄ at 443 K removes water from ethanol to form ethene (unsaturated compound).
C₂H₅OH → (H₂SO₄, 443K) → CH₂=CH₂ + H₂O
Even a small quantity of methanol (not ethanol) can be lethal. Methanol is oxidised to methanal in the liver which destroys cells and causes blindness. Industrial ethanol is denatured (made unfit for drinking) by adding poisonous methanol and blue dye to it.
11 Ethanoic Acid (CH₃COOH)
Ethanoic acid (commonly called acetic acid) belongs to the carboxylic acid group (–COOH). A 5–8% solution of acetic acid in water is called vinegar, used as a preservative in pickles. Pure ethanoic acid melts at 290 K and freezes in winter → hence called glacial acetic acid.
Unlike mineral acids (HCl), carboxylic acids are weak acids — they do not completely ionise in water.
Reactions of Ethanoic Acid
🧪 Esterification
Ethanoic acid + Ethanol → Ester + Water (using acid catalyst). Esters smell sweet and are used in perfumes and flavouring agents.
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
🧼 Saponification
When an ester is treated with NaOH, it converts back to alcohol + sodium salt of carboxylic acid. This is the basis of soap making.
CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa
⚗️ Reaction with Base
Like all acids, ethanoic acid reacts with NaOH to give salt + water.
CH₃COOH + NaOH → CH₃COONa + H₂O
💨 Reaction with Carbonates
Ethanoic acid reacts with carbonates and hydrogencarbonates to give salt, CO₂ (gas) and water.
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑
12 Soaps & Detergents
Soaps are sodium or potassium salts of long-chain carboxylic acids. Each soap molecule has two ends with completely different natures: a hydrophilic (water-loving) ionic end and a hydrophobic (water-hating, oil-loving) hydrocarbon tail.
Fig. 7 – A micelle: hydrophobic tails surround oily dirt; ionic heads face water outward
When soap is added to water with oil, the hydrophobic tails of soap molecules bury themselves into the oil droplet, while the hydrophilic ionic heads stick out into the water. This forms a micelle — a spherical cluster. The micelle traps the oily dirt inside and the whole thing is washed away with water.
Soap vs Detergent
| Property | Soap | Detergent |
|---|---|---|
| Chemical nature | Sodium/potassium salt of long-chain carboxylic acid | Sodium salts of sulphonic acids or ammonium salts |
| Works in hard water? | ❌ No — forms scum (insoluble with Ca²⁺/Mg²⁺) | ✅ Yes — charged ends don't form precipitate with Ca²⁺/Mg²⁺ |
| Foam in hard water | Very little foam, curdy precipitate forms | Good foam even in hard water |
| Use | Bathing, washing (soft water) | Shampoos, laundry products |
Hard water contains Ca²⁺ and Mg²⁺ ions. Soap reacts with these ions to form an insoluble white curdy precipitate called scum. This wastes soap and does not clean properly. Detergents do NOT form scum in hard water — so they are more efficient for washing.
13 All Questions & Answers
O = C = O
In dot structure: Each oxygen shares 2 electrons with carbon (double bond). Carbon satisfies all 4 valencies (2 double bonds). Each oxygen has 4 non-bonding electrons (2 lone pairs). All atoms have complete octets.
In S₈, each sulphur atom forms a single bond with two neighbouring sulphur atoms in a ring structure, satisfying the need for 2 more electrons. Each sulphur also has 2 lone pairs of non-bonding electrons. The ring looks like: –S–S–S–S–S–S–S–S– (connected in a closed ring).
(1) n-Pentane — straight chain: C–C–C–C–C
(2) Isopentane (2-methylbutane) — branch on 2nd carbon:
C–C(–C)–C–C
(3) Neopentane (2,2-dimethylpropane) — 2 branches on middle carbon:
C–C(–C)(–C)–C
All three have the molecular formula C₅H₁₂ but different structural arrangements.
(1) Catenation: Carbon has the unique ability to bond with other carbon atoms, forming long chains, branched chains, and rings. The C–C bond is strong and stable.
(2) Tetravalency: Carbon has a valency of 4, allowing it to bond with four other atoms — whether carbon or other elements like H, O, N, S, Cl. This creates enormous variety in compound structure.
Cyclopentane has 5 carbon atoms in a ring, each connected by single bonds. Using the general formula for cycloalkanes (CnH2n):
Formula: C₅H₁₀
Each C in the ring bonds to 2 other C atoms (ring bonds) and 2 H atoms. All carbons are saturated. The ring structure: –CH₂–CH₂–CH₂–CH₂–CH₂– (closed loop).
H–C–C(=O)–OH with 3 H on first C
Full: CH₃–COOH — 2 carbons, with –COOH group at one end.
(ii) Bromopentane (C₅H₁₁Br):
Br replaces one H on a pentane chain. Example: CH₃–CH₂–CH₂–CH₂–CH₂–Br (1-bromopentane).
Yes — structural isomers are possible. Br can be on C1, C2, or C3 (by symmetry), giving different isomers like 1-bromopentane, 2-bromopentane, 3-bromopentane. Branched isomers also possible.
(iii) Butanone (C₄H₈O):
4 carbons, ketone group (–C=O– between carbons).
CH₃–C(=O)–CH₂–CH₃
(iv) Hexanal (C₆H₁₂O):
6 carbons, aldehyde group (–CHO at end).
CH₃–CH₂–CH₂–CH₂–CH₂–CHO
(ii) H–C(=O)–H (methanal): 1 carbon (meth-) + aldehyde (–al) = Methanal (also called formaldehyde)
(iii) HC≡C–C–C–C–C–H (hex-1-yne): 6 carbons (hex-) + triple bond between C1 and C2 (–yne) = Hex-1-yne or Hexyne
When burnt in pure oxygen, combustion is complete, producing a very hot (about 3000°C) blue flame called the oxy-acetylene flame. This extremely high temperature is needed to melt and join metals in welding.
Test 1 — Litmus/pH paper: Both turn blue litmus red (both are mildly acidic), BUT carboxylic acids are more acidic. Using a universal indicator or pH paper will show carboxylic acid has a significantly lower pH than alcohol.
Test 2 — Sodium carbonate test: Add sodium carbonate (Na₂CO₃) to each.
• Carboxylic acid → produces CO₂ gas (bubbles) and turns lime water milky.
• Alcohol → NO reaction with Na₂CO₃, no bubbles.
Test 3 — Sodium bicarbonate test: Same result — only carboxylic acid reacts with NaHCO₃ to produce CO₂.
The simplest and most reliable test: add NaHCO₃ — brisk effervescence = carboxylic acid; no effervescence = alcohol.
Examples: Alkaline potassium permanganate (KMnO₄) and acidified potassium dichromate (K₂Cr₂O₇) are common oxidising agents. They convert ethanol to ethanoic acid by adding oxygen.
Beating, scrubbing, or machine agitation:
✦ Helps soap molecules reach the dirt more effectively
✦ Breaks up larger dirt clumps
✦ Helps dislodge micelles (with trapped dirt) from cloth fibres
✦ Rinses the dirt-containing micelles away in the water
Without agitation, soap molecules would have less contact with the dirt and cleaning would be inefficient.
Count all covalent bonds in C₂H₆:
Structure: H₃C–CH₃
• 3 C–H bonds on first carbon
• 1 C–C bond between the two carbons
• 3 C–H bonds on second carbon
Total = 3 + 1 + 3 = 7 covalent bonds
The name "butanone" = but- (4 carbons) + -one (ketone suffix). The suffix '-one' specifically indicates a ketone functional group (–C=O– between two carbons). The formula of butanone is CH₃–CO–CH₂–CH₃ (C₄H₈O).
The black deposit on the bottom of cooking vessels is soot (carbon). This is produced during incomplete combustion of the fuel — when insufficient oxygen reaches the flame (air holes of the stove are blocked). In complete combustion, all carbon would oxidise to CO₂ (gas) and no soot would be deposited.
• Carbon (valency 4) shares electrons with 3 hydrogen atoms and 1 chlorine atom.
• Each C–H bond is formed by sharing one electron pair between C and H.
• The C–Cl bond is formed by sharing one electron pair between C and Cl.
• No electrons are transferred (no ions formed) — this is a covalent bond.
• Carbon achieves 8 electrons in outer shell; each H achieves 2; Cl achieves 8.
• Since no ions form, CH₃Cl is a poor conductor of electricity and has low boiling point.
H₃C–C(=O)–O–H: Carbon shares double bond with O (C=O) and single bond with another O–H. First carbon has 3 H atoms. All octets are satisfied.
(b) H₂S: Sulphur (6 valence e⁻) forms 2 single bonds with 2 hydrogen atoms: H–S–H. Sulphur has 2 lone pairs (4 non-bonding electrons).
(c) Propanone (CH₃–CO–CH₃): 3 carbons; middle carbon double-bonded to oxygen (C=O, ketone). Each end carbon has 3 H atoms single-bonded to it. All octets satisfied.
(d) F₂: Fluorine (atomic number 9, 7 valence e⁻) forms a single bond: F–F. Each fluorine has 3 lone pairs. One shared pair between the two F atoms. Both achieve octet.
Example — Alcohols (–OH group):
CH₃OH (Methanol) → C₂H₅OH (Ethanol) → C₃H₇OH (Propanol) → C₄H₉OH (Butanol)
Each differs from the next by –CH₂– (14 u). All have the –OH functional group. All react similarly with sodium to produce H₂. Physical properties (boiling point) increase gradually with chain length.
• Ethanol has a pleasant smell; ethanoic acid has a sharp, vinegar-like smell.
• Ethanoic acid's melting point is 290 K (freezes in winter = glacial acetic acid); ethanol stays liquid (melting point 156 K).
• Ethanoic acid is a weak acid (pH lower); ethanol is neutral in pH.
Chemical Properties:
• Litmus test: Ethanoic acid turns blue litmus red; ethanol does not.
• Na₂CO₃/NaHCO₃ test: Ethanoic acid reacts with Na₂CO₃ to produce CO₂ gas (turns lime water milky); ethanol does not react.
• Reaction with base: Ethanoic acid + NaOH → salt + water; ethanol does not react with NaOH.
In ethanol — No, micelles will NOT form.
Micelle formation happens because of the strong repulsion between the hydrophobic tail and water. Ethanol is an organic solvent and the hydrophobic tails of soap are soluble in ethanol — there is no such repulsion. Since there is no tendency to escape the solvent, soap molecules will simply dissolve uniformly in ethanol without forming micelles.
(1) They burn in oxygen releasing a large amount of heat and light — the oxidation reaction is highly exothermic.
(2) They are widely available — coal, petroleum, natural gas, LPG, CNG are all carbon-based fuels found in abundance.
(3) They have a high calorific value — large amount of energy per unit mass/volume.
(4) Combustion products (CO₂, H₂O) are manageable (though CO₂ contributes to greenhouse effect).
(5) They are easy to store and transport in solid, liquid, or gaseous forms.
2RCOONa + CaCl₂ → (RCOO)₂Ca↓ + 2NaCl
The calcium and magnesium salts of the fatty acid are insoluble in water. They form a white, sticky precipitate called scum. This scum sticks to clothes and does not wash off. Also, soap is wasted because it reacts with the Ca²⁺/Mg²⁺ instead of cleaning the clothes.
• Red litmus paper → turns blue (soap solution is basic)
• Blue litmus paper → stays blue (no change, as it is already blue and soap is basic, not acidic)
Reaction: R–CH=CH–R + H₂ → (Ni catalyst) → R–CH₂–CH₂–R
Industrial Application: Hydrogenation is used to convert vegetable oils (liquid, unsaturated) into vanaspati ghee / margarine (solid, saturated fat). The unsaturated C=C bonds in vegetable oil are converted to saturated C–C bonds. This is done at industrial scale to make solid cooking fats that have a longer shelf life.
• C₂H₆ (Ethane) — saturated (alkane) — ❌ No addition reaction
• C₃H₈ (Propane) — saturated (alkane) — ❌ No addition reaction
• C₃H₆ (Propene) — unsaturated (alkene, has C=C) — ✅ Undergoes addition reaction
• C₂H₂ (Ethyne) — unsaturated (alkyne, has C≡C) — ✅ Undergoes addition reaction
• CH₄ (Methane) — saturated (alkane) — ❌ No addition reaction (undergoes substitution instead)
Test 1 — Bromine water (Br₂ dissolved in water, orange/brown colour):
• Add a few drops of bromine water to the hydrocarbon.
• Unsaturated compound: bromine water is decolourised (turns colourless) — Br₂ adds across the double/triple bond.
• Saturated compound: bromine water remains orange/brown — no addition reaction.
Test 2 — Alkaline KMnO₄ (purple colour):
• Unsaturated compound: purple colour disappears (gets oxidised).
• Saturated compound: purple colour remains.
• Hydrophilic end (ionic, –COO⁻Na⁺) — water-loving, dissolves in water.
• Hydrophobic end (long carbon chain) — water-hating but oil-loving.
Step-by-step cleaning mechanism:
Step 1: Soap is dissolved in water. The hydrophobic tails cannot stay in water, so they orient themselves toward oily dirt on the cloth.
Step 2: The hydrophobic tails of many soap molecules penetrate into the oily dirt, surrounding it. The hydrophilic heads remain pointing outward into the water.
Step 3: This forms a spherical structure called a micelle — oily dirt is trapped in the centre, hydrophilic ends face the water on the outside.
Step 4: Micelles stay suspended as a colloid in water (don't come together due to like-charge repulsion between ionic heads).
Step 5: When clothes are rinsed, the micelles (with trapped dirt inside) are washed away with water, leaving the cloth clean.
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