ORGANIC CHEMISTRY

Answer: (b) Mg

Detailed Explanation:

Alkyl halides can be reduced using:

1. Zinc (Zn) with HCl: Most common method

Zn + 2HCl → ZnCl₂ + 2[H] (nascent hydrogen)

R-X + 2[H] → R-H + H-X

2. Magnesium (Mg): Forms Grignard reagent intermediate

R-X + Mg → R-Mg-X (Grignard reagent)

R-Mg-X + H₂O → R-H + Mg(OH)X

Other options:

(a) Al – Not typically used for this reduction

(c) Ni – Catalyst for hydrogenation, not direct reduction of alkyl halides

(d) Co – Not standard for this reaction

Answer: (c) CO₂ and H₂O

Detailed Explanation:

Complete combustion of hydrocarbons:

Naphtha composition: Mixture of hydrocarbons (C₅-C₁₀)

Complete combustion: Hydrocarbon + excess O₂ → CO₂ + H₂O + heat

Incomplete combustion: If oxygen limited → CO + C (soot) + H₂O

Why not other options:

(a) Alkanes – Starting material, not product

(b) Alkenes – Intermediate in cracking, not combustion product

(d) Both – Incorrect, combustion gives oxides

Answer: (c) n-Butane

Detailed Explanation: Heat of combustion increases with carbon number:

Trend: More carbons = more bonds = more heat released

n-Butane vs iso-Butane: n-Butane releases slightly more heat due to better packing/more efficient combustion

Among options: n-Butane (C₄) has most carbons → highest heat

Answer: (c) Addition

Detailed Explanation:

Alkane reactions:

1. Substitution: ✓ Characteristic reaction (e.g., chlorination)

CH₄ + Cl₂ → CH₃Cl + HCl (with light)

2. Combustion: ✓ Burning with oxygen

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

3. Cracking: ✓ Breaking larger alkanes into smaller ones

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

4. Addition: ✗ NOT given by alkanes

• Alkanes are saturated (all single bonds)

• No double/triple bonds to add to

• Addition characteristic of alkenes/alkynes

Key point: Saturated = no addition reactions

Answer: (c) Methane

Detailed Explanation:

Coal mine gas (firedamp): Mainly methane (CH₄)

Why methane causes explosions:

1. Forms explosive mixture with air (5-15% CH₄ in air)

2. Released from coal seams during mining

3. Highly flammable, ignites easily

4. Complete combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + heat

5. Rapid heat release causes pressure wave → explosion

Other hydrocarbons in mines: Ethane, propane (less common)

Safety measures: Ventilation, methane detectors, flame safety lamps

Historical: “Firedamp” explosions common in 19th/early 20th century mines

Answer: (b) 5 moles

Detailed Explanation:

Propane combustion equation:

Balancing steps:

1. Count C atoms: 3C → 3CO₂ (needs 3O₂ for CO₂)

2. Count H atoms: 8H → 4H₂O (needs 2O₂ for H₂O)

3. Total O₂: 3 (for CO₂) + 2 (for H₂O) = 5 O₂

Stoichiometry: 1 mol C₃H₈ : 5 mol O₂

Alternative method: General formula for alkane combustion:

CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O

For propane (n=3): (3×3+1)/2 = (9+1)/2 = 10/2 = 5 O₂

Answer: Based on carbon content, bonding, properties, and sources.

Exceptions: Carbonates, cyanides, carbides, cyanates, CO, CO₂ classified as inorganic

Answer: Due to carbon’s unique properties: catenation, tetravalency, multiple bonding.

Detailed reasons:

1. Catenation: Carbon forms strong C-C bonds

• Chains of any length: C-C-C-C-C…

• Branched chains: C-C-C, C-C-C

• Rings: C-C-C (cyclopropane)

2. Tetravalency: Forms 4 covalent bonds

• With H, O, N, halogens, other C

• Single, double, triple bonds

3. Isomerism: Same formula, different structures

• C₄H₁₀: n-butane & iso-butane

4. Multiple functional groups: -OH, -COOH, -NH₂, etc.

Result: More carbon compounds than all other elements combined

Answer: By cracking process: heating at high temperature with catalyst.

Detailed process:

Chemical changes:

• Large hydrocarbons (C₁₀-C₁₈) → smaller (C₅-C₁₀)

• Some alkanes → alkenes + smaller alkanes

• C-C bonds break, new bonds form

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Purpose: Convert less useful heavy fractions to more valuable fuels (petrol, diesel)

Answer: Three different formula representations

Explanation:

Molecular formula: Shows atom types and numbers (C₄H₁₀)

Structural formula: Shows atom connections and bonds

Condensed formula: Shows groups without drawing all bonds (CH₃)₃CH

Note: Same molecular formula as n-butane but different structure

Answer: Essential for life, industry, medicine, and daily needs.

Answer: From plants, animals, and natural sources

Note: All biomolecules are organic compounds found naturally

Answer: Due to strong sigma bonds, non-polar nature, and saturation.

Detailed reasons:

1. Strong C-C and C-H sigma bonds:

• Bond energies: C-C (347 kJ/mol), C-H (413 kJ/mol)

• High energy required to break bonds

2. Non-polar molecules:

• C and H have similar electronegativity (C=2.5, H=2.1)

• Bonds almost non-polar → no charged sites for attack

3. Saturation:

• All single bonds (sigma bonds)

• No pi bonds (weaker, more reactive)

• No sites for addition reactions

4. Lack of functional groups:

• Only C-C and C-H bonds

• No -OH, -COOH, -NH₂ etc. to react

5. Inert towards common reagents:

• Don’t react with acids, bases, oxidizing agents at room temp

• Only react under extreme conditions (high temp, light, catalysts)

Exception: Combustion (with O₂) and substitution (with halogens + light)

Answer: Methane forms explosive mixture with air (5-15% CH₄), rapid combustion causes pressure wave.

Detailed explanation:

Chemical reaction:

Why explosion occurs:

1. Rapid combustion releases heat quickly

2. Gases (CO₂, H₂O vapor) expand rapidly

3. Creates pressure wave (shock wave)

4. Damages surroundings

Explosive limits:

• Lower Explosive Limit (LEL): 5% CH₄ in air

• Upper Explosive Limit (UEL): 15% CH₄ in air

Below 5%: Too little fuel, won’t ignite

Above 15%: Too rich, insufficient oxygen

Answer: Organic compounds generally have lower MP/BP than inorganic compounds.

Reasons for difference:

Organic: Weak van der Waals forces between molecules

Inorganic: Strong ionic bonds or covalent networks

Exceptions: Some organic polymers have high MP (nylon: 263°C)

Trend in organic: MP/BP increases with molecular size

Answer: Essential for all aspects of modern life from food to technology.

Without organic compounds: No life (all biomolecules organic), no modern civilization

Answer: Carbon’s unique properties enable vast diversity of compounds forming basis of life.

Detailed reasons:

1. Versatile bonding:

• Forms 4 covalent bonds (tetravalent)

• Single, double, triple bonds possible

• Bonds with itself and many other elements

2. Catenation (self-linking):

• Forms chains of any length (C-C-C-C…)

• Branched chains and rings

• No other element does this to same extent

3. Small size & strong bonds:

• Small atomic radius → short, strong bonds

• C-C bond energy 347 kJ/mol (very strong)

• Forms stable compounds

4. Multiple bonding:

• Forms σ and π bonds

• Allows diverse structures: alkanes, alkenes, alkynes, aromatics

5. Isomerism:

• Same formula, different structures

• Millions of possible compounds

6. Basis of life:

• All biomolecules contain carbon

• DNA, proteins, carbohydrates, lipids

• Life as we know it is carbon-based

Result: More carbon compounds than all other elements combined

Answer: Provides energy for transportation, heating, electricity, and industry.

Chemical basis: Combustion releases large amount of energy

Heat values: Methane: 890 kJ/mol, Propane: 2220 kJ/mol

Advantages: High energy density, relatively clean (compared to coal), easy to transport/store

Disadvantage: Produces CO₂ (greenhouse gas)

Answer: Methane accumulates, forms explosive mixture, prevention through ventilation and safety devices.

Reason for explosion:

Ignition sources: Spark from switch, flame, static electricity

Rapid combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + heat (explosion)

How to avoid:

Why spreads quickly: Methane less dense than air (16 g/mol vs 29 g/mol for air)

Safety first: If gas smell detected, evacuate, don’t use electrical switches, call emergency