Unit 1 | Engineering Chemistry Notes | AKTU Notes

UNIT 1: Atomic and Molecular Structure & Chemistry of Advanced Materials

PART A: ATOMIC AND MOLECULAR STRUCTURE

1.1 Molecular Orbitals of Diatomic Molecules

To understand how atoms combine to form molecules, we use the Molecular Orbital (MO) Theory. According to this theory, when two atoms come close together, their atomic orbitals combine to form new orbitals called molecular orbitals.

Key Ideas:

• When two atomic orbitals combine, they form two molecular orbitals.
• Bonding Molecular Orbital (BMO): Lower energy than the original atomic orbitals. Electrons here hold the atoms together. Formed by addition of wave functions (constructive interference).
• Anti-Bonding Molecular Orbital (ABMO): Higher energy than the original atomic orbitals. Electrons here push the atoms apart. Denoted with a star (*), e.g., σ*, π*. Formed by subtraction of wave functions (destructive interference).

Types of Molecular Orbitals:

• Sigma (σ) MO: Formed by head-on (end-to-end) overlap of atomic orbitals. Electron density is along the bond axis.
• Pi (π) MO: Formed by sideways (lateral) overlap of atomic orbitals. Electron density is above and below the bond axis.

Energy Order of MOs for diatomic molecules (like O₂, F₂):

• σ1s < σ*1s < σ2s < σ*2s < σ2pz < π2px = π2py < π*2px = π*2py < σ*2pz

Energy Order for lighter molecules (like N₂, C₂, B₂):

• σ1s < σ*1s < σ2s < σ*2s < π2px = π2py < σ2pz < π*2px = π*2py < σ*2pz

Rules for filling electrons in MOs:

• Aufbau Principle: Fill lowest energy MO first.
• Pauli's Exclusion Principle: Each MO holds maximum 2 electrons with opposite spins.
• Hund's Rule: If two MOs have equal energy, put one electron in each before pairing.

1.2 Bond Order

Bond Order tells us how many bonds exist between two atoms in a molecule.

Formula:

• Bond Order = (Nb − Na) / 2
• Where Nb = number of electrons in bonding MOs, Na = number of electrons in anti-bonding MOs.

Significance of Bond Order:

• Bond Order = 1 → Single bond (like H₂)
• Bond Order = 2 → Double bond (like O₂)
• Bond Order = 3 → Triple bond (like N₂)
• Bond Order = 0 → Molecule does not exist (like He₂)
• Higher bond order → Stronger bond → Shorter bond length.

Examples:

Molecule	Total Electrons	Nb	Na	Bond Order	Magnetic Nature
H₂	2	2	0	1	Diamagnetic
He₂	4	2	2	0 (does not exist)	—
N₂	14	10	4	3	Diamagnetic
O₂	16	10	6	2	Paramagnetic
F₂	18	10	8	1	Diamagnetic

1.3 Magnetic Characters

The magnetic nature of a molecule depends on whether it has unpaired electrons in its molecular orbitals.

• Diamagnetic: All electrons are paired. The molecule is weakly repelled by a magnetic field. Example: N₂, F₂, H₂.
• Paramagnetic: One or more unpaired electrons. The molecule is attracted toward a magnetic field. Example: O₂ (has 2 unpaired electrons in π*2px and π*2py).

Why O₂ is paramagnetic:

• When we fill electrons in O₂'s MOs, the last 2 electrons go into π*2px and π*2py (degenerate orbitals).
• By Hund's rule, one electron goes in each — so both remain unpaired.
• Two unpaired electrons → O₂ is paramagnetic.
• This was a great success of MO theory — it correctly predicted O₂'s paramagnetism, which earlier bond theories failed to explain.

1.4 Numerical Problems on MO Theory

How to solve MO theory numerical problems — Step by step:

• Step 1: Find total number of electrons in the molecule or ion.
• Step 2: Fill electrons into MOs in correct energy order.
• Step 3: Count Nb (electrons in bonding MOs) and Na (electrons in anti-bonding MOs).
• Step 4: Calculate Bond Order = (Nb − Na) / 2.
• Step 5: Check for unpaired electrons → determine magnetic character.

Example — O₂ molecule (16 electrons):

• MO configuration: (σ1s)²(σ*1s)²(σ2s)²(σ*2s)²(σ2pz)²(π2px)²(π2py)²(π*2px)¹(π*2py)¹
• Nb = 10 (electrons in σ1s, σ2s, σ2pz, π2px, π2py)
• Na = 6 (electrons in σ*1s, σ*2s, π*2px, π*2py)
• Bond Order = (10 − 6) / 2 = 2 → Double bond ✓
• 2 unpaired electrons (in π*2px and π*2py) → Paramagnetic ✓

PART B: CHEMISTRY OF ADVANCED MATERIALS

1.5 Liquid Crystals — Introduction

We normally know three states of matter: solid, liquid, and gas. But some special materials exist in a state that is between solid and liquid — this is called the liquid crystal state (also called mesophase).

• Liquid crystals flow like a liquid (they are fluid).
• But their molecules are arranged in an ordered pattern like a solid crystal.
• They have properties of both solids (optical anisotropy) and liquids (fluidity).
• The liquid crystal state exists between the melting point and the clearing point (above which it becomes a true liquid).

Discovery: First discovered by Friedrich Reinitzer in 1888 while studying cholesteryl benzoate.

1.6 Types of Liquid Crystals

Based on molecular arrangement, there are three main types:

1. Nematic Liquid Crystals:

• Molecules are aligned roughly parallel to each other (like a bundle of sticks pointing in the same direction).
• But there is no positional order — molecules are not arranged in layers.
• Most commonly used in LCD displays.
• Example: p-Azoxyanisole (PAA).

2. Smectic Liquid Crystals:

• Molecules are aligned parallel AND arranged in layers.
• More ordered than nematic.
• The layers can slide over each other.
• Used in some display technologies and as lubricants.
• Example: Ethyl p-azoxybenzoate.

3. Cholesteric (Chiral Nematic) Liquid Crystals:

• Similar to nematic but the direction of alignment rotates as you move through layers — forming a helical (twisted) structure.
• They reflect light of specific wavelengths depending on temperature — this is why they appear colorful.
• Used in thermometers and security holograms.
• Example: Cholesteryl esters.

Type	Positional Order	Orientational Order	Structure
Nematic	No	Yes (parallel)	Like parallel sticks, random positions
Smectic	Yes (in layers)	Yes (parallel)	Parallel sticks arranged in layers
Cholesteric	No	Yes (rotating)	Twisted helical layers

1.7 Applications of Liquid Crystals

• LCD Displays (Liquid Crystal Displays): Most common application. Used in TVs, mobile phones, computer monitors, calculators. Nematic liquid crystals control light passage by rotating under electric field.
• Thermometers: Cholesteric LCs change color with temperature — used in medical thermometers (forehead strips) and aquarium thermometers.
• Optical Sensors: Detect changes in temperature, pressure, or chemicals.
• Smart Windows: Windows that turn opaque or transparent under electric field.
• Security Features: Holograms on credit cards and currency notes use cholesteric LCs.
• Medical Imaging: Liquid crystal thermography to detect tumors (tumor areas are warmer).

1.8 Industrially Important Materials Used as Liquid Crystals

• p-Azoxyanisole (PAA): Nematic LC, melting point 118°C, clearing point 135°C. One of the first studied LCs.
• MBBA (N-(4-methoxybenzylidene)-4-butylaniline): Nematic LC at room temperature. First room-temperature nematic LC discovered. Important for early LCD research.
• Cyanobiphenyls (5CB, 7CB, 8CB): Most widely used in modern LCD devices. Chemically stable, good range of liquid crystal phase.
• Cholesteryl esters: Cholesteric type. Used in temperature-sensing applications.

1.9 Graphite — Introduction, Structure and Applications

Introduction:

• Graphite is an allotrope of carbon — it is pure carbon, just arranged differently from diamond.
• It is a soft, black, slippery solid with a layered structure.
• It is the most stable form of carbon under normal conditions.

Structure of Graphite:

• Carbon atoms are arranged in flat, hexagonal layers (like a honeycomb pattern).
• Within each layer: each carbon atom is sp² hybridized and forms 3 strong covalent bonds with neighboring carbons.
• The 4th electron from each carbon forms a delocalized π electron cloud above and below the layers.
• These delocalized electrons can move freely → graphite conducts electricity.
• Layers are held together by weak van der Waals forces → layers can slide over each other easily → graphite is slippery and soft.
• Distance between layers: 3.35 Å (much larger than within-layer C-C distance of 1.42 Å).

Properties:

• Good electrical conductor (due to delocalized electrons).
• Good thermal conductor.
• Soft and slippery (layers slide easily).
• High melting point (3600°C).
• Chemically inert under normal conditions.

Applications of Graphite:

• Pencil leads (mixed with clay).
• Dry lubricant (in machines where oil cannot be used, e.g., high-temperature applications).
• Electrodes in batteries, fuel cells, and electrolytic cells.
• Moderator in nuclear reactors (slows down neutrons).
• Carbon brushes in electric motors.
• Refractory materials for high-temperature furnaces.
• Starting material for making graphene.

1.10 Fullerenes — Introduction, Structure and Applications

Introduction:

• Fullerenes are another allotrope of carbon — discovered in 1985 by Kroto, Curl, and Smalley (Nobel Prize 1996).
• They are cage-like hollow molecules made entirely of carbon.
• The most famous fullerene is C₆₀ (Buckminsterfullerene or Buckyball).

Structure of C₆₀:

• 60 carbon atoms arranged in a spherical cage — like a soccer ball.
• The structure has 20 hexagonal faces and 12 pentagonal faces.
• Each carbon is sp² hybridized and bonded to 3 other carbons.
• Two types of bonds: bonds between hexagon-hexagon junction (shorter, more double bond character) and bonds between hexagon-pentagon junction (longer, more single bond character).
• Diameter of C₆₀ ≈ 0.7 nm.
• Other fullerenes: C₇₀, C₈₄, C₂₄₀, carbon nanotubes (cylindrical fullerenes).

Properties:

• Soluble in organic solvents (unlike graphite and diamond).
• Semiconductor in pure form, becomes superconductor when doped with alkali metals (e.g., K₃C₆₀ is superconductor at 18K).
• Very strong and stable.
• Can trap other atoms inside the cage (called endohedral fullerenes).

Applications of Fullerenes:

• Drug delivery — hollow cage can carry drug molecules to target cells.
• Antioxidants — C₆₀ scavenges free radicals (medical research).
• Solar cells — fullerenes used as electron acceptors in organic photovoltaics.
• Lubricants — act as molecular ball bearings.
• Superconductors — doped fullerenes are superconducting.
• HIV protease inhibitor research — C₆₀ fits into the active site of HIV enzyme.
• Starting material for carbon nanotubes.

1.11 Nanomaterials — Introduction, Preparation and Characteristics

Introduction:

• Nanomaterials are materials with at least one dimension between 1 and 100 nanometers (1 nm = 10⁻⁹ m).
• At this scale, materials show completely different properties from their bulk form due to quantum effects and very high surface area.

Why are nanomaterials special?

• High surface area to volume ratio: As size decreases, more atoms are on the surface (more reactive).
• Quantum confinement: Electrons are confined, leading to discrete energy levels and unique optical, electrical, and magnetic properties.

Preparation of Nanomaterials:

1. Top-Down Approach (Breaking bulk material into nano size):

• Ball milling: Grinding bulk material in a high-energy ball mill.
• Laser ablation: Using laser pulses to vaporize bulk material, vapor condenses into nanoparticles.
• Chemical etching: Selectively removing material to create nanostructures.
• Lithography: Used in semiconductor industry to create nanoscale features.

2. Bottom-Up Approach (Building from atoms/molecules):

• Chemical Vapor Deposition (CVD): Chemical vapor deposits on substrate forming nanostructures.
• Sol-Gel method: Chemical solution converts to gel, then heated to give nanoparticles.
• Co-precipitation: Mixing chemical solutions to precipitate nanoparticles.
• Hydrothermal synthesis: High temperature and pressure in water to grow nanocrystals.

Characteristics of Nanomaterials:

• Optical: Gold nanoparticles appear red/purple (not yellow like bulk gold). Quantum dots emit different colors based on size.
• Electrical: Carbon nanotubes can be metallic or semiconducting.
• Magnetic: Iron nanoparticles show superparamagnetism.
• Mechanical: Harder and stronger than bulk material. Carbon nanotubes are 100× stronger than steel.
• Chemical: Highly reactive catalysts due to large surface area.
• Thermal: Lower melting points than bulk material.

1.12 Carbon Nano Tubes (CNT)

Introduction:

• Carbon Nanotubes are cylindrical tubes made of carbon atoms arranged in a hexagonal lattice — basically a sheet of graphene rolled into a tube.
• Discovered by Sumio Iijima in 1991.
• Diameter: typically 1–50 nm. Length: can be several micrometers to centimeters.

Types of CNT:

• Single-Walled CNT (SWCNT): Single graphene sheet rolled into a tube. Diameter ~1–2 nm.
• Multi-Walled CNT (MWCNT): Multiple concentric graphene cylinders (tube within a tube). Diameter ~5–50 nm.

Properties of CNT:

• Extremely high tensile strength — 100 times stronger than steel at 1/6th the weight.
• Excellent electrical conductors (can carry current density 1000× more than copper).
• Outstanding thermal conductors (better than diamond).
• High aspect ratio (length much greater than diameter).
• Can be metallic or semiconducting depending on how the graphene sheet is rolled (chirality).

Applications of CNT:

• Reinforcement in composite materials (aerospace, sports equipment, bulletproof vests).
• Nano-electronics — transistors, memory devices.
• Field emission displays.
• Drug delivery — CNTs can carry drugs into cells.
• Hydrogen storage for fuel cells.
• Nano-sensors for detecting chemicals and biological molecules.
• Conductive coatings and films.

1.13 Green Chemistry — Introduction and 12 Principles

Introduction:

• Green Chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.
• It aims to make chemistry more environmentally friendly, safe, and sustainable.
• It was developed by Paul Anastas and John Warner in the 1990s.
• Green Chemistry focuses on prevention of pollution at the source rather than cleaning up pollution after it is created.

12 Principles of Green Chemistry:

• 1. Prevention: Prevent waste rather than treating it after formation. Better to not create waste than to clean it up.
• 2. Atom Economy: Design reactions so that the maximum amount of starting materials ends up in the final product. Minimize byproducts. Formula: Atom Economy = (MW of desired product / MW of all products) × 100%.
• 3. Less Hazardous Synthesis: Use and generate substances with little or no toxicity to humans and the environment.
• 4. Safer Chemicals: Design chemicals to be effective but with minimal toxicity.
• 5. Safer Solvents and Auxiliaries: Avoid use of solvents, separation agents, and other auxiliary chemicals wherever possible. If used, make them innocuous.
• 6. Design for Energy Efficiency: Minimize energy requirements. Conduct reactions at room temperature and pressure if possible.
• 7. Use of Renewable Feedstocks: Use raw materials from renewable sources (plants, agricultural waste) rather than depleting petrochemical resources.
• 8. Reduce Derivatives: Avoid unnecessary derivatization (blocking, protection, temporary modification) which generates additional waste.
• 9. Catalysis: Use catalytic reagents (in small amounts) rather than stoichiometric reagents (in large amounts). Catalysts can be reused — less waste.
• 10. Design for Degradation: Chemical products should be designed to break down into harmless substances after use and not persist in the environment.
• 11. Real-Time Pollution Prevention: Monitor and control processes in real time to prevent formation of hazardous substances.
• 12. Inherently Safer Chemistry: Choose substances and processes that minimize accidents — explosions, fires, spills. Inherent safety over add-on safety measures.

1.14 Importance of Green Synthesis and Green Chemicals

• Traditional chemical synthesis uses toxic solvents (benzene, chloroform), generates large amounts of waste, and uses non-renewable raw materials.
• Green synthesis replaces these with safer solvents (water, ethanol), renewable feedstocks, and cleaner reactions.
• Green Chemicals: Chemicals that are non-toxic, biodegradable, and derived from renewable sources.
• Examples of green chemicals: Ethanol (from fermentation of sugarcane), lactic acid (from fermentation), biodegradable plastics from starch.
• Benefits: Reduced environmental pollution, safer working conditions, lower waste disposal costs, sustainable production.

1.15 Synthesis of Adipic Acid and Paracetamol by Green and Conventional Routes

Adipic Acid:

• Used mainly to make Nylon-6,6 (a synthetic fiber).

Conventional Route:

• Benzene → Cyclohexane (hydrogenation) → Cyclohexanol/Cyclohexanone mixture (KA oil) → Adipic Acid (using HNO₃ — generates hazardous N₂O greenhouse gas).
• Problem: Uses benzene (carcinogen), generates N₂O (300× more potent greenhouse gas than CO₂).

Green Route:

• Glucose (renewable) → cis,cis-muconic acid (using genetically engineered bacteria) → Adipic Acid (by catalytic hydrogenation).
• Advantage: No benzene, no N₂O, uses renewable glucose as starting material.

Paracetamol (Acetaminophen):

• Common pain killer and fever reducer.

Conventional Route:

• Nitrobenzene → Acetanilide → p-Nitroacetanilide → p-Aminoacetanilide → Paracetamol.
• Problem: Uses toxic solvents, generates large amounts of waste, poor atom economy.

Green Route:

• p-Aminophenol + Acetic Anhydride → Paracetamol (using water as solvent, with acid catalyst).
• Advantage: Water as solvent, fewer steps, better atom economy, less waste generated.

1.16 Environmental Impact of Green Chemistry on Society

• Reduced pollution: Less toxic chemicals released into air, water, and soil.
• Safer workplaces: Workers are less exposed to hazardous chemicals.
• Conservation of resources: Using renewable feedstocks reduces dependence on petroleum.
• Climate change mitigation: Green processes produce fewer greenhouse gases.
• Economic benefits: Less waste = less disposal cost. Catalytic processes use less raw material.
• Biodegradable products: Green chemicals break down naturally — no persistent pollution.
• Public health: Cleaner environment means fewer health problems caused by chemical pollution.