Carbon and its Compounds
INTRODUCTION:
Ever wondered why diamond is the hardest substance but graphite is soft and slippery, even though both are made only of carbon? Or why LPG burns with a clean blue flame but a candle gives yellow smoke? These observations connect directly to how carbon atoms bond and arrange themselves. By the end of this chapter, naming organic compounds, predicting reactions, and understanding soap action becomes easy—exactly what board exams demand.
Why carbon is special (Bonding basics)
Carbon
cannot form ions easily
Carbon has atomic number 6, so its electronic configuration is 2,4—meaning it has 4 valence electrons. To get a stable octet (8 electrons), carbon would need to either gain 4 electrons (to form C⁴⁻) or lose 4 electrons (to form C⁴⁺), but both are nearly impossible because it would require huge energy.
Solution: Carbon shares its 4 valence electrons with other atoms to form covalent bonds (sharing electrons instead of losing/gaining).
Covalent bonding (electron sharing)
When two atoms share a pair of electrons, both achieve stable configuration; this shared pair forms a covalent bond.
Example: Methane (CH₄) formation: Carbon (4 valence electrons) shares one electron each with four hydrogen atoms (each has 1 valence electron), forming 4 single covalent bonds.
Properties of covalent compounds
|
Property |
Observation |
Reason |
|
Melting/boiling points |
Low |
Weak intermolecular forces |
|
Electrical conductivity |
Poor conductors |
No free ions/charged particles |
|
State |
Often gases/liquids |
Weak forces between molecules |
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Why millions of carbon compounds exist
Catenation
(carbon's superpower)
Catenation is carbon's unique ability to form strong bonds with other carbon atoms, creating long chains, branched chains, and rings. No other element shows catenation to this extent; silicon forms chains of only 7-8 atoms and those compounds are very reactive.
Tetravalency (valency of 4)
Carbon can bond with 4 other atoms (carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine, etc.), which creates enormous variety.
Saturated vs unsaturated compounds
|
Type |
Bond type |
General formula (for hydrocarbons) |
Example |
|
Saturated |
Only single C–C bonds |
CₙH₂ₙ₊₂ (alkanes) |
Methane CH₄, Ethane C₂H₆ |
|
Unsaturated |
Double or triple C–C bonds |
CₙH₂ₙ (alkenes), CₙH₂ₙ₋₂ (alkynes) |
Ethene C₂H₄, Ethyne C₂H₂ |
Unsaturated compounds are more reactive because double/triple bonds can break and add atoms.
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Chains, branches, rings, and isomers
Structural
isomers (same formula, different structure)
In words: Compounds with the same molecular formula but different structures are called structural isomers.
Example: Butane (C₄H₁₀) has two isomers:
· Straight chain: CH₃–CH₂–CH₂–CH₃ (n-butane)
· Branched chain: CH₃–CH(CH₃)–CH₃ (isobutane)
Cyclic compounds
Carbon atoms can also form rings;
example: cyclohexane (C₆H₁₂) has 6 carbon atoms in a ring.
Benzene (C₆H₆) is a cyclic unsaturated compound
with alternating double bonds.
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Functional groups (the game-changer)
In a carbon chain, one or more hydrogen atoms can be replaced by heteroatoms (like O, N, Cl) or groups containing heteroatoms; these are called functional groups. The functional group decides the chemical properties of the compound, not the carbon chain length.
Table: Important functional groups
|
Functional group |
Formula |
Class of compound |
Example |
|
Halo (Cl, Br) |
–Cl, –Br |
Haloalkane |
Chloromethane CH₃Cl |
|
Alcohol |
–OH |
Alcohol |
Ethanol C₂H₅OH |
|
Aldehyde |
–CHO |
Aldehyde |
Ethanal CH₃CHO |
|
Ketone |
>C=O |
Ketone |
Propanone CH₃COCH₃ |
|
Carboxylic acid |
–COOH |
Carboxylic acid |
Ethanoic acid CH₃COOH |
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Homologous series
A homologous series is a family of organic compounds with the same functional group, where each successive member differs by a –CH₂– unit (14 mass units).
Example: Alcohols: CH₃OH, C₂H₅OH, C₃H₇OH… (differ by –CH₂–)
Properties of homologous series
· Same functional group → similar chemical properties
· Increasing molecular mass → gradation in physical properties (melting/boiling points increase)
· General formula can be written (e.g., alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ)
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Nomenclature (IUPAC naming—easy method)
Step-by-step naming:
1. Count carbon atoms: 1C = meth, 2C = eth, 3C = prop, 4C = but, 5C = pent, 6C = hex
2. Identify bond type: single bond = -ane, double bond = -ene, triple bond = -yne
3. Add functional group as prefix or suffix (see table below)
|
Class |
Prefix/Suffix |
Example name |
Structure hint |
|
Haloalkane |
Prefix: chloro/bromo |
Chloropropane |
3C + Cl |
|
Alcohol |
Suffix: -ol |
Propanol |
3C + OH |
|
Aldehyde |
Suffix: -al |
Propanal |
3C + CHO |
|
Ketone |
Suffix: -one |
Propanone |
3C + C=O |
|
Carboxylic acid |
Suffix: -oic acid |
Propanoic acid |
3C + COOH |
|
Alkene |
Suffix: -ene |
Propene |
3C + double bond |
|
Alkyne |
Suffix: -yne |
Propyne |
3C + triple bond |
Chemical properties (key reactions)
Combustion (burning)
Carbon compounds burn in oxygen to give CO₂, H₂O, heat, and light.
Equations:
CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat + light
Saturated vs unsaturated flame:
Saturated hydrocarbons (single bonds) → clean blue flame
Unsaturated hydrocarbons (double/triple bonds) → yellow, sooty flame
Oxidation
Alcohols can be oxidized to carboxylic acids using oxidizing agents like alkaline KMnO₄.
Equation:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (using KMnO₄/K₂Cr₂O₇)
Addition reaction (for unsaturated compounds)
Unsaturated hydrocarbons add hydrogen in the presence of Ni/Pd catalyst to form saturated hydrocarbons.
Equation:
CH₂=CH₂ + H₂ → CH₃–CH₃ (Ni catalyst, hydrogenation)
Real-life use: Vegetable oil (unsaturated) → Vegetable ghee (saturated) using hydrogenation.
Substitution reaction (for saturated compounds)
In the presence of sunlight, chlorine replaces hydrogen atoms one by one in saturated hydrocarbons.
Equation:
CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)
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Ethanol (C₂H₅OH)
Ethanol is the alcohol present in all alcoholic drinks; it's also used in tincture iodine, cough syrups, and as a solvent.
Important reactions of ethanol
(i) Reaction with sodium:
Ethanol reacts with sodium to give sodium ethoxide and hydrogen gas.
Equation: 2Na + 2C₂H₅OH → 2C₂H₅O⁻Na⁺ + H₂↑
(ii) Dehydration (to form ethene):
Heating ethanol with conc. H₂SO₄ at 443 K removes water and gives ethene.
Equation: C₂H₅OH
→ CH₂=CH₂ + H₂O (conc. H₂SO₄, 443 K)
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Ethanoic acid (CH₃COOH)
Ethanoic acid (acetic acid) is present in vinegar (5-8% solution); pure ethanoic acid freezes at 290 K, hence called glacial acetic acid.
Important reactions of ethanoic acid
(i) Esterification:
Ethanoic acid reacts with ethanol in the presence of conc. H₂SO₄ to give ethyl ethanoate (ester, sweet smell).
Equation: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (conc. H₂SO₄)
(ii) Reaction with base:
Ethanoic acid reacts with NaOH to give sodium ethanoate (sodium acetate) and water.
Equation: NaOH + CH₃COOH → CH₃COONa + H₂O
(iii) Reaction with carbonates:
Ethanoic acid reacts with Na₂CO₃/NaHCO₃ to give salt, CO₂, and water.
Equations:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
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Soaps and detergents
Soap = Sodium/potassium salts of long-chain carboxylic acids.
Cleansing action (how soap works)
Soap molecules have two parts:
Hydrophilic (ionic) head: Attracts water
Hydrophobic (carbon chain) tail: Attracts oil/dirt
In water, soap molecules surround oil droplets with tails inside (towards oil) and heads outside (towards water), forming micelles; this pulls dirt into water and cleans clothes.
Why soap doesn't work in hard water
Hard water contains Ca²⁺/Mg²⁺ ions, which react with soap to form insoluble scum (calcium/magnesium salts of fatty acids), wasting soap and preventing lather.
Detergents work even in hard water because their
calcium/magnesium salts are soluble.
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MCQs PYQ
Q1.
Carbon forms compounds mainly by covalent bonding. Why?
(a) Carbon is metal
(b) Carbon can gain/lose 4 electrons easily
(c) Carbon shares electrons to complete octet
(d) Carbon is non-reactive
Answer: (c)
(CBSE 2023)
Q2. The
number of single and double bonds in benzene (C₆H₆) are:
(a) 3 and 3
(b) 9 and 3
(c) 6 and 6
(d) 12 and 0
Answer: (b) 9 single (6 C–H + 3 C–C), 3 double (C=C alternate).
(CBSE 2024)
Q3. Which metal is used in thermite
process? When burnt in air it gives amphoteric oxide:
(a) Fe and Fe₂O₃
(b) Al and Al₂O₃
(c) Fe and Fe₃O₄
(d) Al and Al₃O₄
Answer: (b)
(CBSE 2023)
Q4. The general formula for alkenes
is:
(a) CₙH₂ₙ₊₂
(b) CₙH₂ₙ
(c) CₙH₂ₙ₋₂
(d) CₙHₙ
Answer: (b)
(CBSE 2024)
Q5. Saturated hydrocarbons burn with:
(a) Yellow sooty flame
(b) Clean blue flame
(c) No flame
(d) Green flame
Answer: (b)
(CBSE 2020)
Q6. Ethanol reacts with sodium to
form:
(a) Sodium ethanoate + H₂
(b) Sodium ethoxide + H₂
(c) Sodium ethoxide + O₂
(d) No reaction
Answer: (b)
(CBSE 2017, 2016, 2014, 2011)
Q7. 5% alkaline KMnO₄ acts as:
(a) Reducing agent
(b) Oxidizing agent
(c) Catalyst
(d) Dehydrating agent
Answer: (b)
(CBSE 2020)
Q8. Esterification reaction produces:
(a) Bitter smell
(b) Sweet/fruity smell
(c) No smell
(d) Pungent smell
Answer: (b)
(CBSE 2020)
Q9. Detergents work better than soap
in hard water because:
(a) Detergents form scum
(b) Detergents' Ca/Mg salts are soluble
(c) Detergents are cheaper
(d) Detergents are alkaline
Answer: (b)
(CBSE 2020)
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Short
Answer Questions (PYQ)
Q1.
Define homologous series. Give one example.
Answer: A homologous series is a group of organic compounds with the same functional group, where successive members differ by –CH₂– (14 u). Example: Alcohols CH₃OH, C₂H₅OH, C₃H₇OH. (CBSE 2020, 2019)
Q2. Why
does carbon form covalent compounds only?
Answer: Carbon has 4 valence electrons; gaining/losing 4 electrons requires very high energy, so it shares electrons to form stable covalent bonds. (CBSE 2024, 2021)
Q3. Name
two functional groups and give examples.
Answer: (i) Alcohol (–OH): Ethanol C₂H₅OH (ii) Aldehyde (–CHO): Ethanal CH₃CHO. (CBSE 2024)
Q4. What
happens when ethanol is heated with conc. H₂SO₄ at 443 K?
Answer: Ethanol undergoes dehydration to form ethene; conc. H₂SO₄ acts as dehydrating agent. Equation: C₂H₅OH → C₂H₄ + H₂O. (CBSE 2023, 2021)
Q5.
Distinguish between saturated and unsaturated hydrocarbons by flame test.
Answer: Saturated hydrocarbons burn with
clean blue flame; unsaturated hydrocarbons burn with yellow sooty flame due to
incomplete combustion. (CBSE 2024)
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Long Answer Questions (PYQ)
Q1.
(a) Define catenation and tetravalency. (b) Draw structural isomers of butane.
Answer: (a) Catenation: Carbon's ability to form bonds with other carbon atoms forming long chains/rings. Tetravalency: Carbon has valency 4, can bond with 4 atoms. (b) n-butane: CH₃–CH₂–CH₂–CH₃; isobutane: CH₃–CH(CH₃)–CH₃. (CBSE 2023, 2022)
Q2. A
compound X (C₂H₅OH) is heated with conc. H₂SO₄ at 443 K to give Y. Y on addition of
H₂
(Ni catalyst) gives Z. Identify X, Y, Z and write equations.
Answer: X = Ethanol, Y = Ethene, Z = Ethane.
Equations:
C₂H₅OH → C₂H₄
+ H₂O (443
K, conc. H₂SO₄)
C₂H₄ + H₂ → C₂H₆
(Ni catalyst). (CBSE 2023)
Q3. (a)
Write equations: (i) Combustion of methane (ii) Oxidation of ethanol (iii)
Esterification. (b) Name the product when ethanol burns.
Answer: (a) (i) CH₄ + 2O₂ → CO₂ + 2H₂O
(ii) CH₃CH₂OH + 2[O] →
CH₃COOH + H₂O
(iii) CH₃COOH + C₂H₅OH
→ CH₃COOC₂H₅
+ H₂O
(b) CO₂ and H₂O. (CBSE 2022, 2021)
Q4. Explain cleansing action of soap with diagram. Why doesn't soap work in hard water?
Answer: Soap molecules have hydrophilic head (attracts water) and hydrophobic tail (attracts oil). In water, soap forms micelles with tails inside (towards oil) and heads outside (towards water), pulling dirt into water. In hard water, Ca²⁺/Mg²⁺ ions react with soap to form insoluble scum, wasting soap. (CBSE 2020, 2017)
Q5. (a)
What are hydrocarbons? (b) List two properties that allow carbon to form
millions of compounds. (c) Draw two isomers of butane.
Answer: (a) Hydrocarbons are compounds of
carbon and hydrogen only. (b) (i) Catenation (ii) Tetravalency. (c) n-butane
and isobutane (structures as in Q1). (CBSE 2024)
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Conclusion
Carbon and its Compounds becomes
scoring when you connect three ideas: (1) why carbon forms only covalent bonds
and millions of compounds (catenation + tetravalency), (2) how to name
compounds using IUPAC rules and identify functional groups, and (3) key
reactions (combustion, oxidation, addition, substitution, esterification, soap
action). Revise the tables (functional groups, nomenclature, homologous series)
and practice writing reactions in "words first, then equation"
format.
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