CARBON AND ITS COMPOUNDS

CARBON AND ITS COMPOUNDS  

1. Introduction to Carbon

Carbon (Symbol: C) is a unique element and the basis of all life on Earth. It forms the skeleton of:

• Proteins

• Carbohydrates

• DNA

• Plastics

• Medicines

• Fuels (coal, petrol, LPG, CNG)

Why carbon forms a huge number of compounds?

1. Tetravalency (Valency = 4)
   Carbon has 4 electrons in its outer shell → it needs 4 more to complete octet.
   It cannot lose 4 (needs too much energy), cannot gain 4 (makes ion too unstable).
   Hence, it shares electrons.

2. Catenation
   Carbon can bond with other carbon atoms forming:

   • Straight chains (C–C–C–C…)

   • Branched chains

   • Ring structures
     No other element shows catenation as strongly as carbon.

3. Ability to form Single, Double & Triple Bonds
   Carbon can form:

   • C–C (single)

   • C=C (double)

   • C≡C (triple)
     This creates millions of structural variations.

2. Covalent Bonding in Carbon Compounds

A covalent bond is formed when atoms share electron pairs.

Characteristics of Covalent Compounds

• Usually low melting and boiling points
  (because they have weak intermolecular forces)

• Generally poor conductors of electricity
  (no free ions or electrons except graphite)

• Mostly liquid or gas at room temperature

• Insoluble in water (mostly)

Examples

1. Methane (CH₄)

Carbon shares 4 electrons with 4 hydrogen atoms → single bonds.

2. Ethene (C₂H₄)

A double bond exists between carbons.

3. Ethyne (C₂H₂)

A triple bond exists between carbons.

3. Catenation: The Unique Ability of Carbon

Catenation = ability of an element to form long chains with itself.

Why is carbon the best at catenation?

• C–C bonds are strong and stable

• Small atomic size eliminates repulsion

• Forms single, double, triple bonds

• Forms chains up to thousands of atoms long (like polymers)

Examples:

• Propane: CH₃–CH₂–CH₃

• Cyclohexane: a ring of 6 carbon atoms

• Polyethylene: thousands of carbon atoms linked

4. Allotropes of Carbon (Most Important Topic)

Allotropes = Different forms of the same element with different physical properties.

A. Diamond

Structure:
Each carbon atom is bonded to 4 other carbon atoms → strong 3D network.

Properties:

• Hardest natural substance

• Transparent

• Non-conductor of electricity

• High melting point

• Used in cutting tools, drills, jewellery

B. Graphite

Carbon atoms form layers. Each carbon bonded to 3 atoms, forming hexagonal sheets.

Properties:

• Soft and slippery (layers slide)

• Black, opaque

• Good conductor of electricity (free electrons)

• Used in lubricants, pencil lead, electrodes

C. Fullerenes (C₆₀ and others)

Sphere or cage-like structure (football shape).
First discovered: Buckminsterfullerene (C₆₀)

Properties:

• Hollow cage

• Conductive

• Used in drug delivery, nanotechnology

5. Hydrocarbons

Hydrocarbons = Compounds made of only carbon and hydrogen.

A. Saturated Hydrocarbons (Alkanes)

• Only single covalent bonds

• Least reactive

• General formula: CₙH₂ₙ₊₂

Name	Formula
Methane	CH₄
Ethane	C₂H₆
Propane	C₃H₈
Butane	C₄H₁₀

B. Unsaturated Hydrocarbons

(1) Alkenes — contain double bond

General formula: CₙH₂ₙ
Example: Ethene (C₂H₄)

(2) Alkynes — contain triple bond

General formula: CₙH₂ₙ₋₂
Example: Ethyne (C₂H₂)

Unsaturated hydrocarbons are more reactive because double and triple bonds are easier to break.

6. Functional Groups (Very Important for Board Exams)

Functional groups are atoms or groups that decide the chemical properties of organic compounds.

Functional Group	Symbol	Example
Alcohol	–OH	Ethanol
Aldehyde	–CHO	Ethanal
Ketone	>C=O	Propanone
Carboxylic Acid	–COOH	Ethanoic acid
Halo group	–Cl, –Br	Chloroethane
Amino group	–NH₂	Amines

Functional group + alkyl group = organic compound.

7. Homologous Series

A group of organic compounds with the same functional group and similar properties.

Characteristics:

• Same general formula

• Successive members differ by –CH₂ (14 u)

• Gradual change in physical properties

• Chemical properties almost identical

Example: Alcohol series

• CH₃OH

• C₂H₅OH

• C₃H₇OH
  (difference = 14 u each)

8. Chemical Properties of Carbon Compounds

I. Combustion

Carbon compounds burn in oxygen to produce CO₂ + H₂O + heat.

Example:
CH₄ + O₂ → CO₂ + H₂O + energy

This property makes them useful as fuels.

II. Oxidation

Alcohols can be oxidized to:
alcohol → aldehyde → acid

Example:
Ethanol → Ethanal → Ethanoic acid

Oxidizing agents:

• KMnO₄

• K₂Cr₂O₇

III. Addition Reaction

Unsaturated hydrocarbons (C=C, C≡C) add hydrogen.

Example (Hydrogenation):
C₂H₄ + H₂ → C₂H₆
(Catalyst: Ni, Pt, Pd)

Used in converting vegetable oil into ghee.

IV. Substitution Reaction

Saturated hydrocarbons (alkanes) undergo substitution.

Example:
CH₄ + Cl₂ → CH₃Cl + HCl
(Requires sunlight)

9. Important Carbon Compounds

A. Ethanol (C₂H₅OH)

Properties:

• Colourless, volatile liquid

• Soluble in water

• Burns with blue flame

• Used in medicines, sanitizers, perfumes

Harmful effects:

• Damages liver (cirrhosis)

• Addiction

• Reduces body coordination

B. Ethanoic Acid (CH₃COOH)

• Sour taste

• Vinegar contains 5–8% solution

• Freezes at 16°C (glacial acetic acid)

Esterification Reaction

Alcohol + Acid → Ester + Water
(pleasant fruity smell)

Example:
C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O

Saponification (Reverse reaction)

Ester + Base → Alcohol + Soap

10. Soap and Detergents

Soap

• Made from fats/oils + NaOH or KOH

• Works only in soft water

• In hard water → forms scum

Detergents

• Made from petrochemicals

• Work in hard & soft water

• Used in washing powders and liquids

 Very Short Answer Questions (1 Mark)

1. Why does carbon form covalent bonds?

Answer:
Carbon has 4 electrons in its valence shell. It cannot gain or lose 4 electrons easily, so it completes its octet by sharing electrons, forming covalent bonds.

2. What is catenation?

Answer:
Catenation is the ability of carbon atoms to bond with each other, forming long chains, branched chains, and rings.

3. Give two examples of allotropes of carbon.

Answer:
Diamond and Graphite.

4. What is the general formula of alkanes?

Answer:
The general formula of alkanes is CₙH₂ₙ₊₂.

5. Name the functional group present in alcohols.

Answer:
The functional group in alcohols is –OH (hydroxyl group).

 Short Answer Questions (2–3 Marks)

6. Why is graphite a good conductor of electricity but diamond is not?

Answer:
In graphite, each carbon atom is bonded to only three other carbons, leaving one free electron per atom. These free electrons move freely and conduct electricity.
In diamond, all four valence electrons are bonded, so no free electrons are available for conduction.

7. Differentiate between saturated and unsaturated hydrocarbons.

Answer:

Saturated Hydrocarbons	Unsaturated Hydrocarbons
Contain only single bonds.	Contain double or triple bonds.
Less reactive.	More reactive.
Known as alkanes.	Known as alkenes/alkynes.

8. What is a homologous series? State its characteristics.

Answer:
A homologous series is a group of organic compounds having the same functional group and same general formula, with each successive member differing by –CH₂ (14 u).

Characteristics:

1. Same functional group

2. Same general formula

3. Gradual change in physical properties

4. Similar chemical properties

9. Explain the addition reaction of unsaturated hydrocarbons with an example.

Answer:
Unsaturated hydrocarbons (alkenes/alkynes) undergo addition reactions where atoms like hydrogen are added across the double/triple bond.

Example:
C₂H₄ + H₂ → C₂H₆
(Ethene + Hydrogen → Ethane; catalyst: Ni/Pt/Pd)

10. What is esterification? Give an example.

Answer:
Esterification is the reaction in which a carboxylic acid reacts with an alcohol to form an ester and water.

Example:
C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O
(Ethanol + Ethanoic acid → Ester)

 Long Answer Questions (4–5 Marks)

11. Describe the three allotropes of carbon: Diamond, Graphite, and Fullerenes.

Answer:
(1) Diamond:

• Each carbon atom bonded to four others forming a rigid 3D structure.

• Hardest natural substance.

• Poor conductor of electricity.

• Used in cutting tools and jewellery.

(2) Graphite:

• Each carbon bonded to three others forming hexagonal layers.

• Free electrons make it a good conductor.

• Soft and slippery.

• Used in lubricants and electrodes.

(3) Fullerenes (C₆₀):

• Cage-like spherical molecules (football shape).

• Good conductors.

• Used in nanotechnology and medicines.

12. Explain the chemical properties of carbon compounds with examples.

Answer:
(1) Combustion:
Carbon compounds burn in oxygen to form CO₂, H₂O, and heat.
Example: CH₄ + O₂ → CO₂ + H₂O

(2) Oxidation:
Alcohol → Aldehyde → Acid
Example: Ethanol → Ethanal → Ethanoic acid

(3) Addition Reaction:
Unsaturated hydrocarbons add hydrogen.
Example: C₂H₄ + H₂ → C₂H₆

(4) Substitution Reaction:
Alkanes react with halogens.
Example: CH₄ + Cl₂ → CH₃Cl + HCl

13. Describe the structure and properties of ethanol and ethanoic acid.

Answer:

Ethanol (C₂H₅OH):

• Colourless liquid, soluble in water

• Neutral pH

• Used in sanitizers, medicines

• Excess consumption is harmful (liver damage)

Ethanoic Acid (CH₃COOH):

• Sour taste, strong smell

• Present in vinegar (5–8%)

• Freezes at 16°C (glacial acetic acid)

• Reacts with alcohol to form esters

14. What are soaps and detergents? How do they work?

Answer:
Soaps:

• Made from fats/oils and NaOH/KOH

• Work only in soft water

• Form scum in hard water

Detergents:

• Made from petrochemicals

• Work in both hard and soft water

• More effective in washing clothes

How they work:
Both have two parts:

• Hydrophobic tail (repels water, attaches to dirt/oil)

• Hydrophilic head (attracts water)

They form micelles → trap dirt → wash away.

15. Why does carbon form a large number of compounds?

Answer:
Carbon forms millions of compounds because:

1. Tetravalency – can form 4 covalent bonds

2. Catenation – ability to form long chains and rings

3. Multiple bonds – forms C–C, C=C, C≡C

4. Strong C–C bond – stable and unreactive

5. Bonding with other elements – H, O, N, S, halogens etc.