Hydrocarbons

 

Hydrocarbons – Detailed Notes with Mechanisms & Electronic Effects
(NCERT Class XI Chemistry)

 

INTRODUCTION

Hydrocarbons are organic compounds containing only carbon and hydrogen. They are the fundamental compounds of organic chemistry and are mainly obtained from petroleum, coal, and natural gas.

CLASSIFICATION OF HYDROCARBONS

1. Saturated hydrocarbons – Alkanes

2. Unsaturated hydrocarbons

   (a) Alkenes

   (b) Alkynes

3. Aromatic hydrocarbons

 

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ALKANES (PARAFFINS)

General formula: CₙH₂ₙ₊₂

All carbon atoms are sp³ hybridised and linked by single covalent (σ) bonds.

 

PREPARATION OF ALKANES

1. Hydrogenation of Alkenes/Alkynes

   CH₂=CH₂ + H₂  →  CH₃–CH₃  (Ni/Pt)

 

2. Wurtz Reaction

   2CH₃–Cl + 2Na  →  CH₃–CH₃ + 2NaCl

 

3. Kolbe’s Electrolysis

   2CH₃COO⁻ → C₂H₆ + 2CO₂ + 2e⁻

 

CHEMICAL REACTIONS OF ALKANES

1. Halogenation (Free Radical Substitution)

   CH₄ + Cl₂  →  CH₃Cl + HCl  (Sunlight)

 

MECHANISM OF HALOGENATION

 

 

 

 

 

 

(i) Initiation:

    Cl₂ → 2Cl·

(ii) Propagation:

    Cl· + CH₄ → CH₃· + HCl

    CH₃· + Cl₂ → CH₃Cl + Cl·

(iii) Termination:

    Cl· + Cl· → Cl₂

 

 

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ALKENES (OLEFINS)

General formula: CₙH₂ₙ

Contain one double bond (one σ and one π bond).

 

PREPARATION OF ALKENES

1. Dehydration of Alcohols

   CH₃–CH₂OH → CH₂=CH₂ + H₂O  (Conc. H₂SO₄)

 

2. Dehydrohalogenation of Alkyl Halides

   CH₃–CH₂Br + KOH → CH₂=CH₂ + KBr + H₂O

 

CHEMICAL REACTIONS OF ALKENES

1. Addition of Hydrogen (Hydrogenation)

   CH₂=CH₂ + H₂ → CH₃–CH₃

 

2. Addition of Halogens

   CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

 

3. Addition of HX (Markovnikov’s Rule)

   CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃

 

MECHANISM OF ELECTROPHILIC ADDITION (HBr)

 

ALKYNES

General formula: CₙH₂ₙ₋₂

Contain one triple bond (one σ and two π bonds).

 

PREPARATION OF ALKYNES

From calcium carbide:

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

 

PROPERTIES

• Terminal alkynes show acidic nature

• Undergo addition reactions

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AROMATIC HYDROCARBONS

Benzene formula: C₆H₆

STRUCTURE OF BENZENE

• Planar hexagonal ring

• All carbon atoms are sp² hybridised

• Resonance stabilised structure

 

RESONANCE STRUCTURES OF BENZENE

Two equivalent Kekulé structures explain the stability of benzene.

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IMPORTANT ELECTRONIC EFFECTS

1. INDUCTIVE EFFECT (–I and +I)

Permanent displacement of σ electrons due to electronegativity difference.

• –I effect: –NO₂, –Cl, –COOH

• +I effect: –CH₃, –C₂H₅

 

2. HYPERCONJUGATION

Delocalisation of σ electrons of C–H bond with adjacent π system.

• Increases stability of alkenes and carbocations

• More substituted alkene → More hyperconjugation → More stable

 

3. RESONANCE EFFECT (+R and –R)

Delocalisation of π electrons across conjugated systems.

• +R groups: –OH, –NH₂

• –R groups: –NO₂, –CN

ENVIRONMENTAL ASPECTS

• Incomplete combustion causes air pollution

• Benzene is carcinogenic

 

IMPORTANT NCERT POINTS

• Alkanes are least reactive

• Reactivity of alkenes and alkynes is due to π bonds

• Aromatic compounds are highly stable due to resonance

 

Biomolecules

BIOMOLECULES –

Biomolecules are organic compounds produced by living organisms. They are essential for life processes such as growth, metabolism, heredity, and reproduction. 

MAIN TYPES OF BIOMOLECULES:

1. Carbohydrates

2. Amino acids

3. Proteins

4. Enzymes

5. Vitamins

6. Nucleic Acids

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1. CARBOHYDRATES

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield them on hydrolysis.

Classification:

• Monosaccharides – e.g., Glucose, Fructose

• Oligosaccharides – Sucrose, Maltose, Lactose

• Polysaccharides – Starch, Glycogen, Cellulose

 

Glucose:

• Aldohexose (C6H12O6)

• Exists in α- and β- forms

• Forms 6-membered cyclic structure (pyranose)

 

Fructose:

• Ketohexose

• Forms 5-membered ring (furanose)

 

Disaccharides:

• Sucrose = Glucose + Fructose (non-reducing)

• Maltose = Glucose + Glucose (reducing)

• Lactose = Glucose + Galactose (reducing)

 

Polysaccharides:

• Starch = Amylose + Amylopectin

• Cellulose = β-D-glucose polymer

• Glycogen = Animal storage polysaccharide

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2. AMINO ACIDS

General structure: NH2—CH(R)—COOH

 

• Essential amino acids – must be taken from diet

• Non-essential amino acids – synthesized by body

 

Zwitter Ion: Amino acids exist as dipolar ions

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3. PROTEINS

Polymers of α-amino acids linked by peptide bonds.

 

Levels of Structure:

• Primary – sequence of amino acids

• Secondary – α-helix, β-sheet

• Tertiary – 3D folding

• Quaternary – multiple polypeptide chains

 

Types:

• Fibrous proteins – keratin, collagen

• Globular proteins – enzymes, hemoglobin

 

Denaturation: Loss of biological activity (e.g., heating egg white)

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4. ENZYMES

Biocatalysts that speed up biochemical reactions.

Characteristics:

• Highly specific

• Efficient

• Work at 37°C optimum 

Examples:

• Amylase – digests starch

• Invertase – sucrose → glucose + fructose

• Zymase – fermentation

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5. VITAMINS

Required in small amounts.

Classification:

• Fat-soluble: A, D, E, K

• Water-soluble: B-complex, C

 Deficiency Diseases:

• Vitamin A – Night blindness

• B1 – Beriberi

• B12 – Pernicious anemia

• C – Scurvy

• D – Rickets

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6. NUCLEIC ACIDS (DNA & RNA)

Composed of:

• Sugar (Ribose/Deoxyribose)

• Base (A, G, C, T, U)

• Phosphate

DNA:

• Double helix

• A–T (2 H-bonds)

• G–C (3 H-bonds) 

RNA:

• Single-stranded

• U replaces T

Types: mRNA, tRNA, rRNA

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7. HORMONES

Chemical messengers.

 Examples:

• Insulin – controls blood sugar

• Adrenaline – emergency hormone

• Thyroxine – metabolism regulation

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8. METABOLISM

• Catabolism: breakdown (respiration)

• Anabolism: synthesis (photosynthesis)

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SUMMARY TABLE:

Carbohydrates → Monosaccharides → Glucose

Proteins → Amino acids → Enzymes

Lipids → Fatty acids → Oils, fats

Nucleic acids → Nucleotides → DNA, RNA

Buffer Solutions: Definition, Types, and Applications

Chemistry plays a vital role in maintaining balance in various natural and industrial processes. One of the most fascinating concepts that ensures chemical stability is the buffer solution. Whether in our blood, biological cells, or industrial formulations, buffer solutions are crucial in maintaining a stable pH. In this blog, we’ll explore what buffer solutions are, how they work, their types, preparation methods, and applications.

What is a Buffer Solution?

A buffer solution is a solution that resists a change in its pH when a small amount of acid or base is added. In simple terms, it helps maintain a nearly constant hydrogen ion concentration even when acids or bases are introduced.

Buffers are usually made from a weak acid and its conjugate base or a weak base and its conjugate acid.

Examples:
- Acidic buffer: A mixture of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa)
- Basic buffer: A mixture of ammonium hydroxide (NH₄OH) and ammonium chloride (NH₄Cl)

How Do Buffer Solutions Work?

The effectiveness of a buffer lies in the common ion effect and equilibrium principle.

Let’s understand this using an acidic buffer example: Mixture of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa).

1. When acid (H⁺) is added:
The acetate ion (CH₃COO⁻) from sodium acetate reacts with the added H⁺ ions to form undissociated acetic acid:
CH₃COO⁻ + H⁺ → CH₃COOH

2. When base (OH⁻) is added:
The weak acid (CH₃COOH) reacts with hydroxide ions to produce water and acetate ions:
CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O

Types of Buffer Solutions

1. Acidic Buffers:
These buffers maintain a pH less than 7 and are made from a weak acid and its salt with a strong base.
Example: Acetic acid and sodium acetate maintain a pH around 4.75.

2. Basic Buffers:
These maintain a pH greater than 7 and are made from a weak base and its salt with a strong acid.
Example: Ammonium hydroxide and ammonium chloride maintain a pH around 9.25.

The Henderson–Hasselbalch Equation

For an acidic buffer:
pH = pKa + log([Salt]/[Acid])

For a basic buffer:
pOH = pKb + log([Salt]/[Base])
and pH = 14 - pOH

Buffer Capacity

Buffer capacity is the measure of a buffer’s ability to resist pH change when acid or base is added. It depends on concentration, ratio of salt to acid/base, and total buffer volume.

Preparation of Buffer Solutions

(a) Acidic Buffer:
To prepare an acetic acid–sodium acetate buffer of pH 4.75:
1. Take acetic acid and sodium acetate in suitable proportions.
2. Mix them in distilled water.
3. Use the Henderson–Hasselbalch equation to adjust the ratio for desired pH.

(b) Basic Buffer:
To prepare an ammonium hydroxide–ammonium chloride buffer of pH 9.25:
1. Mix a known volume of NH₄OH with NH₄Cl.
2. Adjust proportions to obtain the required pH.

Applications of Buffer Solutions

1. Biological Systems:
Blood maintains a constant pH (~7.4) due to the carbonic acid–bicarbonate buffer system.
Enzymatic reactions in cells depend on buffer control for proper function.

2. Pharmaceutical Industry:
Drug formulations use buffers to maintain stability and effectiveness.
Eye drops and injections are buffered to match body pH.

3. Industrial Applications:
Used in fermentation, dyeing, and electroplating processes.
Essential in photographic and cosmetic industries.

4. Analytical Chemistry:
Employed in titrations to maintain constant pH.
Used in preparation of calibration solutions for pH meters.

Examples of Common Buffer Systems

Buffer System	Components	pH Range	Type
Acetic acid–sodium acetate	CH₃COOH + CH₃COONa	4–6	Acidic
Ammonium hydroxide–ammonium chloride	NH₄OH + NH₄Cl	9–10	Basic
Carbonic acid–bicarbonate	H₂CO₃ + NaHCO₃	7.2–7.4	Biological
Phosphate buffer	H₂PO₄⁻ + HPO₄²⁻	6–8	Biological

Conclusion

Buffer solutions are the silent protectors of pH balance in nature and industry. From keeping our blood at the perfect pH to ensuring the accuracy of chemical reactions, they are indispensable in both life and laboratory. A strong understanding of buffers helps chemists, biologists, and engineers design stable systems across multiple applications.

In short — buffers are the guardians of equilibrium.