Chemical Properties of Carbon Compounds

Chemical Reactions of Carbon Compounds

Now we move from learning about structures to how carbon compounds react.

Key reactions covered in Class 10:

1. Combustion — burning in oxygen.
2. Oxidation — adding oxygen / losing hydrogen.
3. Addition reactions — adding atoms across a multiple bond.
4. Substitution reactions — replacing one atom with another.

Why Are These Important?

Carbon compounds undergo specific reactions based on their:

• Functional groups.
• Type of bonds (single, double, triple).
• Conditions (heat, light, catalysts).

Knowing reactions helps us:

• Make new compounds (industry).
• Use them efficiently (fuels, drugs, materials).
• Understand life processes (biochemistry).

1. Combustion

The most important reaction of organic compounds.

Definition

'Combustion' = the rapid reaction of a substance with oxygen, releasing heat and light (flame).

All carbon compounds burn in oxygen.

General Reaction

Hydrocarbon + O₂ → CO₂ + H₂O + heat

Examples

Methane:

$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat (891 kJ/mol)}$$

Ethanol:

$$C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O + \text{heat}$$

Acetic acid:

$$CH_3COOH + 2O_2 \rightarrow 2CO_2 + 2H_2O + \text{heat}$$

Complete vs Incomplete Combustion

Complete Combustion

Plenty of oxygen → CO₂ + H₂O. Clean blue flame. Maximum heat released.

Example: gas stove with proper air supply.

Incomplete Combustion

Insufficient oxygen → CO + H₂O + soot (carbon). Yellow/sooty flame. Less heat released. Produces poisonous CO.

Example: gas stove with restricted air.

Reaction (Incomplete):

$$2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O$$

Why Saturated vs Unsaturated Burn Differently

Saturated (alkanes):

• More H per C.
• Burn cleanly with blue flame.
• Less soot.
• Used as good fuels.

Unsaturated (alkenes, alkynes):

• Less H per C.
• Burn with yellow/sooty flame.
• More soot (incomplete combustion).
• Need more O₂.

Why It Matters

Understanding combustion:

• Choose right fuels (LPG, CNG, natural gas).
• Design efficient engines.
• Avoid CO poisoning.
• Reduce pollution.

2. Oxidation

'Oxidation' = adding oxygen OR losing hydrogen.

Different from combustion — oxidation can be controlled and partial.

Oxidation of Alcohols

Alcohols can be oxidised to acids using oxidising agents.

Mild Oxidation (Step 1)

Alcohol → Aldehyde

Example: $$CH_3CH_2OH \xrightarrow{\text{oxidation}} CH_3CHO + H_2$$

Strong Oxidation (Step 2)

Alcohol → Acid (skipping aldehyde, or going through it)

Example: $$CH_3CH_2OH \xrightarrow{\text{strong oxidation}} CH_3COOH$$

Common Oxidising Agents

Used to oxidise alcohols and aldehydes:

1. Acidified KMnO₄ (potassium permanganate):

• Purple solution.
• Decolourises during oxidation.
• Strong oxidiser.

2. Acidified K₂Cr₂O₇ (potassium dichromate):

• Orange solution.
• Turns green during oxidation.
• Strong oxidiser.

3. Alkaline KMnO₄:

• For mild oxidation.

Reaction with KMnO₄: $$CH_3CH_2OH + [O] \xrightarrow{KMnO_4} CH_3COOH$$

[O] indicates oxygen from oxidising agent.

A Famous Reaction — Wine Souring

Wine = solution of ethanol in water. Ethanol + O₂ (from air) → acetic acid (using bacteria as catalyst).

$$CH_3CH_2OH + O_2 \rightarrow CH_3COOH + H_2O$$

This is why wine 'turns to vinegar' — ethanol is slowly oxidised to acetic acid.

Industrial Significance

Vinegar Production

Industrial process: ethanol + O₂ → acetic acid (vinegar). Uses bacteria as catalyst. Worldwide production: millions of tonnes annually.

Aldehyde to Acid

Aldehydes are intermediates. Easily oxidised to acids. Acetaldehyde → acetic acid (industrial route).

Body Metabolism

Our bodies oxidise alcohol to acid:

• Ethanol (drink) → acetaldehyde → acetic acid → CO₂ + H₂O.
• Liver enzymes do this.
• Hangover symptoms come from acetaldehyde (toxic intermediate).

Oxidation State Changes

During oxidation:

• Alcohol carbon: C with -OH = lower oxidation state.
• Acid carbon: C with -COOH = higher oxidation state.

Each oxidation step adds an oxygen or removes a hydrogen.

Why Functional Groups Matter Here

Different functional groups oxidise differently:

Functional Group	Oxidation Behaviour
Alcohol (-OH)	→ Aldehyde → Acid
Aldehyde (-CHO)	→ Acid
Ketone (>C=O)	Generally not oxidised easily
Acid (-COOH)	Maximum oxidation state (mostly)

Alcohols are 'starting points' for many oxidation reactions.

Practical Significance

1. Industrial chemistry:

• Making acids from alcohols (e.g., vinegar).
• Pharmaceutical manufacturing.

2. Biology:

• Body metabolism of alcohols.
• Many natural processes involve oxidation.

3. Lab tests:

• KMnO₄/K₂Cr₂O₇ tests for alcohols and aldehydes.
• Distinguishing between functional groups.

3. Addition Reactions

'Addition reaction' = atoms from one molecule are added across a multiple bond (double or triple).

Characteristic of unsaturated compounds (alkenes, alkynes).

General Pattern

$C=C + AB \rightarrow A-C-C-B$

The double bond breaks; A and B add to either side.

Hydrogenation — Most Important

'Hydrogenation' = adding hydrogen across a double or triple bond.

Catalyst: Nickel (Ni), Platinum (Pt), or Palladium (Pd).

Example: Ethene + H₂

$$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$$ Ethene → Ethane.

Example: Ethyne + 2H₂

$$HC≡CH + 2H_2 \xrightarrow{Ni} CH_3-CH_3$$ Ethyne → Ethane.

Triple bond can take 2 H₂ molecules → fully saturated.

Industrial Use — Hydrogenation of Vegetable Oils

Vegetable oils have C=C double bonds (unsaturated fats). Hydrogenation converts them to saturated fats (no C=C).

Why?

• Saturated fats are solids at room temperature.
• Easier to store, transport.
• Longer shelf life.

Example: $$\text{Vegetable oil (liquid)} + H_2 \xrightarrow{Ni} \text{Vanaspati (solid)}$$

Vanaspati ghee = hydrogenated vegetable oil. Margarine = similar process.

Health Concerns of Hydrogenation

Hydrogenation also creates trans fats (a type of unhealthy fat).

Trans fats:

• Raise bad cholesterol (LDL).
• Lower good cholesterol (HDL).
• Linked to heart disease.

Many countries are now restricting trans fats in food.

Modern guidance:

• Use natural unsaturated oils (mustard, sunflower, olive).
• Avoid heavily-hydrogenated 'vanaspati' or 'shortening'.

Other Addition Reactions

Addition of Halogens

$$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$ This is the basis of the bromine water test.

Addition of Hydrogen Halides

$$CH_2=CH_2 + HBr \rightarrow CH_3-CH_2Br$$

Addition of Water

$$CH_2=CH_2 + H_2O \xrightarrow{\text{H}_2\text{SO}_4} CH_3-CH_2OH$$ Industrial production of ethanol from ethene.

Why Addition Happens

Multiple bonds (double, triple) have 'extra' electrons (π electrons). These are reactive — easy targets for attack. New atoms add → multiple bond becomes single bond.

Significance

1. Industrial:

• Hydrogenation (vanaspati, margarine).
• Plastic production (polymerisation).
• Drug synthesis.

2. Biological:

• Vegetable oils — unsaturated, healthy.
• Saturated fats — less healthy.
• Unsaturated fatty acids (omega-3, etc.) essential nutrients.

3. Lab:

• Bromine water test for unsaturation.
• Identifying unknown compounds.

4. Substitution Reactions

'Substitution reaction' = an atom or group is replaced by another atom or group.

Characteristic of saturated compounds (alkanes).

General Pattern

R-H + X-Y → R-X + H-Y

Here, H is replaced by X.

Substitution in Alkanes

Alkanes are unreactive — they don't add (no double bonds to break). But they undergo substitution under specific conditions.

Reaction with Halogens (Photochemical)

Methane + chlorine → chloromethane + HCl (in sunlight).

$$CH_4 + Cl_2 \xrightarrow{\text{UV light}} CH_3Cl + HCl$$

Note: 'sunlight' or UV light is the catalyst — initiates the reaction.

Why UV needed?

• Cl-Cl bond breaks under UV.
• Free Cl• atoms attack methane.
• One H replaced by Cl.

Multiple Substitution

If excess Cl₂ is used, more H atoms can be replaced:

$$CH_3Cl + Cl_2 \rightarrow CH_2Cl_2 + HCl$$ (dichloromethane)

$$CH_2Cl_2 + Cl_2 \rightarrow CHCl_3 + HCl$$ (trichloromethane / chloroform)

$$CHCl_3 + Cl_2 \rightarrow CCl_4 + HCl$$ (tetrachloromethane / carbon tetrachloride)

Substitution Products and Their Uses

Compound	Common Name	Use
CH₃Cl	Methyl chloride	Refrigerant, solvent
CH₂Cl₂	Dichloromethane (DCM)	Solvent, paint stripper
CHCl₃	Chloroform	Anaesthetic (historic), solvent
CCl₄	Carbon tetrachloride	Solvent, fire extinguisher (banned)

Famous Substitution — CFCs

Carbon-Fluorine-Chlorine compounds. Made by substituting H with F and Cl in alkanes.

Examples: CFC-11 (CCl₃F), CFC-12 (CCl₂F₂).

Used as:

• Refrigerants (in fridges, AC).
• Aerosol propellants (spray cans).
• Foam blowing agents.

But CFCs damaged the ozone layer — banned in 1987 (Montreal Protocol). Replaced by HFCs (less harmful).

Reactivity Compared

Alkanes — substitution only:

• Need UV light or high temperature.
• Slow reaction.
• Multiple products possible.

Alkenes/Alkynes — addition (preferred):

• Fast, at room temperature.
• Single product.
• More controllable.

This is why alkanes are 'unreactive' fuels and alkenes/alkynes are 'reactive' raw materials.

Substitution vs Addition

Property	Substitution	Addition
Bond type	Single bonds	Multiple bonds
Compound	Alkane	Alkene, Alkyne
Conditions	UV light, heat	Catalyst
Speed	Slow	Fast
Product	One H replaced	Atoms added across bond

Practical Significance

1. Industrial:

• Production of CFCs, halocarbons.
• Plastic precursors.
• Drug synthesis.

2. Daily Life:

• Chloroform — used as anaesthetic historically.
• Solvents in paints, glues.
• Refrigerants.

3. Environment:

• CFCs harmful to ozone — banned.
• Replacing with safer compounds.
• Lesson: chemistry has consequences for environment.

Summary of Reactions — Key Patterns

Reactions Based on Compound Type

Alkanes (Saturated)

Less reactive. Few reaction types:

• Combustion (burns easily).
• Substitution (with halogens, under UV).

Alkenes (Double Bond)

Reactive — many reactions:

• Combustion (burns).
• Addition — H₂, X₂, HX, H₂O.
• Polymerisation (makes plastics).

Alkynes (Triple Bond)

Most reactive:

• Combustion (very hot flame).
• Addition — twice (2 molecules add).
• Acidic hydrogen (terminal alkynes).

Alcohols (-OH)

Several reactions:

• Combustion (burn cleanly).
• With Na → release H₂.
• Oxidation → aldehyde → acid.
• Esterification → ester (with acid).
• Dehydration → alkene.

Aldehydes (-CHO)

Reactive:

• Oxidation → acid (easy).
• Reduction → alcohol.
• Addition (less common).

Ketones (>C=O)

Less reactive than aldehydes:

• Reduction → alcohol.
• Generally don't oxidise easily.

Carboxylic Acids (-COOH)

Several reactions:

• With base → salt + water.
• With alcohol → ester (esterification).
• With metal → salt + H₂.

Key Reaction Examples — Summary

Combustion (general):

$$\text{Carbon compound} + O_2 \rightarrow CO_2 + H_2O + \text{heat}$$

Hydrogenation (alkene/alkyne):

$$C=C + H_2 \xrightarrow{Ni} C-C$$

Bromination test (alkene):

$$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$

Substitution (alkane):

$$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$

Oxidation of alcohol:

$$CH_3CH_2OH \xrightarrow{KMnO_4} CH_3COOH$$

Esterification:

$$CH_3COOH + CH_3OH \rightleftharpoons CH_3COOCH_3 + H_2O$$

Each is a 'tool' in the organic chemist's toolkit.

Why These Reactions Matter

In Industry:

• Polymerisation: makes plastics from alkenes.
• Hydrogenation: makes vanaspati from oils.
• Substitution: makes drugs, refrigerants.
• Oxidation: makes vinegar.

In Biology:

• Combustion: respiration in cells.
• Oxidation: metabolism.
• Reduction: photosynthesis.
• Addition/substitution: enzyme reactions.

In Daily Life:

• Burning fuels: heat, cooking.
• Vinegar production: food.
• Plastic manufacture: materials.
• Refrigeration: household appliances.

Mastering these reactions = understanding most of organic chemistry.

🧠 Memory Capsule

A quick glance just before the board exam.

1. Main Reaction Types

Type	Description	Compound
Combustion	Burning in O₂	All
Oxidation	Adding O / losing H	Alcohols, aldehydes
Addition	Across multiple bond	Alkenes, alkynes
Substitution	Replace one atom	Alkanes (with halogens)

2. Combustion

Hydrocarbon + O₂ → CO₂ + H₂O + heat.

Examples:

• $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
• $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$

3. Oxidation of Alcohol

Alcohol → Aldehyde → Carboxylic acid

Catalyst: KMnO₄ or K₂Cr₂O₇.

$$CH_3CH_2OH \xrightarrow{KMnO_4} CH_3COOH$$

4. Addition

$C=C + H_2 \xrightarrow{Ni} C-C$

Industrial example: hydrogenation of vegetable oils → vanaspati.

5. Substitution

$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$

Multiple substitution possible: CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄

6. Combustion: Complete vs Incomplete

Complete (excess O₂): CO₂ + H₂O, blue flame, max heat. Incomplete (limited O₂): CO + soot, yellow flame, less heat.

CO is poisonous!

7. Important Industrial Reactions

1. Hydrogenation: Oils + H₂ → vanaspati ghee

2. Vinegar production: Ethanol + O₂ (bacteria) → acetic acid

3. Soap making: Fatty acid + NaOH → soap + glycerol

8. Bromine Water Test

Distinguishes saturated from unsaturated:

• Saturated: no change.
• Unsaturated: decolourises.

Reaction (alkene): $$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$

9. Board's 'Golden' Questions

1. Combustion of methane — equation.
2. Oxidation of ethanol to ethanoic acid.
3. Hydrogenation reaction with example.
4. Why CFCs are banned?
5. Substitution vs addition — difference.

Final Formula: Reactions show what compounds DO. Combustion = burn; Oxidation = add O; Addition = across multiple bonds; Substitution = replace one atom.

Solved Examples

Example 1: NCERT — Combustion

Write balanced equations for combustion of: (a) methane (b) ethanol

Solution:

(a) Combustion of Methane

Methane (CH₄) burns in excess oxygen.

Reactants: methane + oxygen Products: CO₂ + H₂O + heat

Balancing:

Carbon: 1 = 1 ✓ Hydrogen: 4 = 2 ⇒ 2 H₂O. Oxygen: 4 = 4 ⇒ 2 O₂.

Balanced Equation: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$$

Heat released: 891 kJ/mol.

(b) Combustion of Ethanol

Ethanol (C₂H₅OH) burns in oxygen.

Reactants: ethanol + oxygen Products: CO₂ + H₂O + heat

Balancing:

Carbon: 2 = 2 ⇒ 2 CO₂. Hydrogen: 6 = 2 ⇒ 3 H₂O. Oxygen: 7 (in products: 4 from CO₂ + 3 from H₂O = 7). Oxygen on left: from C₂H₅OH = 1, plus from O₂. Need: 7 - 1 = 6 from O₂ ⇒ 3 O₂.

Balanced Equation: $$C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O + \text{heat}$$

Heat released: ~1370 kJ/mol.

Comparison

Compound	Equation	Heat (kJ/mol)
Methane	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$	891
Ethanol	$C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$	1370

Why Both Burn Cleanly?

Both have:

• Sufficient H atoms (more H per C).
• Burn with blue flame.
• Complete combustion in plenty of O₂.

Practical Significance

Methane:

• Main component of natural gas, LPG.
• Cooking fuel.
• Highly efficient burner.

Ethanol:

• Used as fuel additive (E10, E20 petrol).
• Burns cleanly.
• Renewable (made from sugarcane, corn).

Both are good fuels — but methane is cheaper.

What If Insufficient O₂?

Incomplete combustion produces CO (carbon monoxide):

$$2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O$$

CO is poisonous — can be deadly. This is why gas stoves should have proper air supply.

[NCERT — important]

Example 2: NCERT — Oxidation of Alcohol to Acid

How can ethanol be converted to ethanoic acid? Write the equations and conditions.

Solution:

Conversion: Ethanol → Ethanoic Acid

This is an oxidation reaction.

Method 1: Using Strong Oxidising Agent

Common oxidising agents:

• Acidified KMnO₄ (potassium permanganate) — purple, decolourises.
• Acidified K₂Cr₂O₇ (potassium dichromate) — orange, turns green.

Reaction:

$$CH_3CH_2OH \xrightarrow{\text{Alkaline KMnO}_4 \text{ + heat}} CH_3COOH$$

Conditions:

• Acidified or alkaline KMnO₄.
• Heated.

Or:

$$CH_3CH_2OH \xrightarrow{\text{Acidified K}_2Cr_2O_7 \text{ + heat}} CH_3COOH$$

Both methods work.

Method 2: Through Aldehyde Intermediate

Sometimes oxidation happens in two steps:

Step 1: Mild oxidation of alcohol to aldehyde: $$CH_3CH_2OH \xrightarrow{\text{mild oxidation}} CH_3CHO + H_2$$

Step 2: Further oxidation of aldehyde to acid: $$CH_3CHO + [O] \rightarrow CH_3COOH$$

Combined: ethanol → ethanal → acetic acid.

Method 3: Industrial — Acetobacter Bacteria

Bacteria can oxidise ethanol to acetic acid.

This is how vinegar is made commercially:

$$CH_3CH_2OH + O_2 \xrightarrow{\text{Acetobacter}} CH_3COOH + H_2O$$

Bacteria use enzymes as catalysts. Slow process, but produces vinegar at low cost.

Visual Indicator

During the reaction with KMnO₄:

• Initial: purple solution.
• During: purple decolourises.
• End: clear (Mn²⁺ ions are colourless).

During reaction with K₂Cr₂O₇:

• Initial: orange solution.
• End: green (Cr³⁺ ions are green).

These colour changes are diagnostic for alcohol oxidation.

Why This Reaction Matters

1. Industrial production of vinegar. Acetic acid (vinegar) is a major commercial product.

2. Wine to vinegar. If wine is exposed to air, ethanol gets oxidised to acetic acid → wine sours.

3. Pharmaceutical synthesis. Acetic acid is precursor for many drugs.

4. Body metabolism. Body converts alcohol to acetaldehyde to acetic acid (via enzymes).

Detailed Step Mechanism (Optional)

Step 1 (oxidation): -OH group: H-O-CH₂- → CH₂=O + H+ (oxidised; loses H) Then aldehyde forms.

Step 2: -CHO + [O] → -COOH

Each oxidation = +1 oxidation state on carbon.

Summary

Ethanol can be converted to ethanoic acid by:

1. Strong oxidising agents (KMnO₄ or K₂Cr₂O₇) + heat.
2. Bacterial fermentation (Acetobacter) — used in industry.

Reaction: $$CH_3CH_2OH \xrightarrow{[O]} CH_3COOH$$

[NCERT — important, every year]

Example 3: NCERT — Hydrogenation

What is hydrogenation? Why is it important industrially?

Solution:

Hydrogenation — Definition

'Hydrogenation' = a reaction where hydrogen is added across a multiple bond.

Used to convert:

• Alkenes (with C=C) to alkanes (saturated).
• Alkynes (with C≡C) to alkanes.

Conditions Required

Hydrogen + alkene/alkyne + catalyst → alkane.

Catalysts: Nickel (Ni), Platinum (Pt), or Palladium (Pd). Conditions: heat, pressure.

Reactions

Hydrogenation of Ethene (Alkene)

$$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$$ Ethene → Ethane.

Hydrogenation of Ethyne (Alkyne)

Triple bond can take up to 2 H₂:

$$HC≡CH + H_2 \xrightarrow{Ni} CH_2=CH_2$$ (intermediate) $$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$$

Or in one step: $$HC≡CH + 2H_2 \xrightarrow{Ni} CH_3-CH_3$$

Industrial Applications

1. Vanaspati Ghee Production

Vegetable oils have C=C double bonds (unsaturated fats).

Hydrogenation: $$\text{Vegetable oil} + H_2 \xrightarrow{Ni, \text{ heat}} \text{Vanaspati (saturated)}$$

Why:

• Vegetable oil = liquid (unsaturated).
• Vanaspati = solid at room temperature (saturated).
• Easier to store, transport, use.
• Longer shelf life.

This was a revolutionary discovery — replaced expensive cow ghee with cheap vegetable substitute.

2. Margarine

Similar to vanaspati — hydrogenated vegetable oil + colouring + flavouring. Used as butter substitute in cooking.

3. Industrial Production of Saturated Compounds

Many industrial chemicals start from unsaturated compounds (cheaper). Hydrogenation converts to saturated forms.

4. Hydrogen Storage

Hydrogenation is reversible. Used as a way to store hydrogen for fuel cells.

The Health Story — Trans Fats

Hydrogenation isn't always perfect. Sometimes creates trans fats (unhealthy form of saturated fats).

Trans fats:

• Raise LDL ('bad') cholesterol.
• Lower HDL ('good') cholesterol.
• Linked to heart disease.

Modern guidance:

• Limit hydrogenated fats (vanaspati, margarine).
• Prefer natural unsaturated fats (mustard oil, olive oil, sunflower).
• Many countries have banned trans fats.

Indian health organizations (FSSAI) regulate trans fats in food.

Benefits vs Concerns

Benefits of Hydrogenation:

• Industrial efficiency: solid fats easier to handle.
• Longer shelf life.
• Cheaper than animal fats.
• Various industrial uses.

Concerns:

• Trans fats are unhealthy.
• Less nutritious than natural oils.
• Need careful regulation.

Why It's Important to Understand

Hydrogenation is one of the most important industrial reactions in chemistry. It changes the physical and chemical properties of compounds dramatically. Affects health and food industry globally.

Understanding hydrogenation:

• Helps make informed food choices.
• Useful in industrial chemistry careers.
• Connects chemistry to daily life.

[NCERT — important]

Example 4: NCERT — Substitution

What is a substitution reaction? Give an example with methane and chlorine.

Solution:

Substitution Reaction — Definition

'Substitution reaction' = a chemical reaction where an atom or group is replaced by another atom or group.

Characteristic of saturated compounds (alkanes).

Why Alkanes Undergo Substitution?

Alkanes have only single bonds (saturated). No double/triple bonds for addition. Cannot 'add' atoms across a bond. Instead, they 'swap' atoms.

Example: Methane + Chlorine (Photochemical Reaction)

Reaction:

$$CH_4 + Cl_2 \xrightarrow{\text{Sunlight (UV)}} CH_3Cl + HCl$$

Methane + chlorine → chloromethane + hydrogen chloride.

What Happens:

One H atom of methane is replaced by Cl. Cl atom of Cl₂ goes to make HCl. UV light starts the reaction (breaks Cl-Cl bond).

Detailed Mechanism (Optional)

Step 1: UV light breaks Cl-Cl: Cl₂ → Cl• + Cl• (free radicals). Step 2: Cl• attacks methane: Cl• + CH₄ → HCl + CH₃• (methyl radical). Step 3: Methyl radical attacks Cl₂: CH₃• + Cl₂ → CH₃Cl + Cl•.

Chain reaction — once started, continues until reactants run out.

Multiple Substitution

If excess chlorine is used, more H atoms are replaced:

First substitution:

$$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$ Chloromethane formed.

Second substitution:

$$CH_3Cl + Cl_2 \xrightarrow{UV} CH_2Cl_2 + HCl$$ Dichloromethane (DCM) formed.

Third substitution:

$$CH_2Cl_2 + Cl_2 \xrightarrow{UV} CHCl_3 + HCl$$ Trichloromethane (chloroform) formed.

Fourth substitution:

$$CHCl_3 + Cl_2 \xrightarrow{UV} CCl_4 + HCl$$ Tetrachloromethane (carbon tetrachloride) formed.

Industrial Importance

All four products are useful:

Product	Use
CH₃Cl	Refrigerant, methyl group source
CH₂Cl₂ (DCM)	Solvent, paint stripper
CHCl₃ (chloroform)	Anaesthetic (historical), solvent
CCl₄	Solvent, fire extinguisher (banned now)

Fluorination — A Special Case

F substitution is even more important industrially.

Example: CFC-12 manufacture

Methane → fluorinated/chlorinated → CCl₂F₂ (CFC-12).

CFCs were used in:

• Refrigerators.
• Aerosol sprays.
• Foam production.

But CFCs damage the ozone layer in the atmosphere. Banned by the Montreal Protocol (1987). Replaced by HFCs (less harmful).

Substitution vs Addition

Property	Substitution	Addition
Compounds	Alkanes	Alkenes, alkynes
Bond type	Single bonds only	Multiple bonds
Conditions	UV light, heat	Catalyst
Product	One H replaced	Atoms added
Example	CH₄ → CH₃Cl	CH₂=CH₂ → CH₃-CH₃

Practical Importance

1. Industrial chemistry:

• Production of solvents (chloroform, DCM).
• Plastic precursors.
• Drugs.

2. Environment:

• CFCs damaged ozone — important lesson.
• New 'safer' chemicals being developed.

3. Daily life:

• Used in many products around us.

A Reflection

Substitution reactions show how alkanes — though 'unreactive' — can be turned into useful compounds.

The challenge: control the conditions to get the right product. Modern chemistry can selectively make any of CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄ as desired.

This is the art of organic chemistry — turning simple compounds into complex useful materials.

[NCERT — important]

Example 5: A Concluding Question

(a) Define the four main types of reactions of carbon compounds. (b) Give one example each. (c) Why does ethanol react with sodium? (d) What is the bromine water test?

Solution:

(a) Four Main Reaction Types

1. Combustion

'Combustion' = burning in oxygen, producing CO₂, H₂O, and heat.

Examples: cooking gas burning, petrol burning.

2. Oxidation

'Oxidation' = adding oxygen or losing hydrogen.

Different from full combustion — controlled, partial.

Examples: alcohol → aldehyde → acid.

3. Addition

'Addition reaction' = atoms add across a multiple bond (C=C or C≡C).

Characteristic of unsaturated compounds.

Examples: hydrogenation, halogenation.

4. Substitution

'Substitution reaction' = atom/group replaced by another atom/group.

Characteristic of saturated compounds.

Examples: methane + chlorine.

(b) One Example of Each

Combustion:

$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$$

Methane (cooking gas) burning in air.

Oxidation:

$$CH_3CH_2OH \xrightarrow{KMnO_4} CH_3COOH$$

Ethanol oxidised to acetic acid.

Addition:

$$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$$

Hydrogenation of ethene.

Substitution:

$$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$

Methane + chlorine in sunlight.

(c) Why Does Ethanol React with Sodium?

Ethanol has -OH group. The H of -OH is slightly acidic (loosely held). Sodium is reactive and can replace this H.

Reaction:

$$2CH_3CH_2OH + 2Na \rightarrow 2CH_3CH_2O^-Na^+ + H_2\uparrow$$

Ethanol + sodium → sodium ethoxide + hydrogen gas.

This is similar to acid + metal: HCl + Na → NaCl + H₂

But ethanol is much weaker than HCl. Hence reaction is slow.

Why This Matters

This reaction is a test for alcohols:

• Add small piece of Na to alcohol.
• If it reacts (bubbling, gas): alcohol present.
• Use to identify -OH groups.

Also used in dehydration of alcohol: Ethanol → ethene (with concentrated H₂SO₄, hot).

(d) Bromine Water Test

Setup

Bromine dissolved in water = orange-brown 'bromine water'.

Procedure

Add bromine water to the unknown organic compound.

Observations

If saturated (alkane, alcohol with no C=C): Bromine water remains orange-brown. No reaction.

If unsaturated (alkene, alkyne): Bromine water becomes colourless. Br₂ adds across the double/triple bond.

Reaction (alkene example)

$$CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$$

1,2-dibromoethane (colourless).

Why This Test Works

Br₂ is reddish-brown due to its electronic structure. When Br₂ adds across a multiple bond, both Br atoms get attached to the molecule. New molecule is colourless.

Practical Use

Quality Control

Industrial test for unsaturated oils. Margarine producers monitor bromine water reaction to track hydrogenation progress.

Identifying Compounds

Distinguish saturated from unsaturated easily. Quick lab test.

Distinguishing Examples

Cyclohexane vs hex-1-ene (both C₆H₁₂):

• Cyclohexane: no change.
• Hex-1-ene: decolourises bromine water.

Summary

The bromine water test is:

• Simple.
• Visual (colour change).
• Quick.
• Reliable for identifying unsaturation.

It's one of the most useful tests in organic chemistry.

Even in advanced research, scientists still use this principle.

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