Unit 2 | Engineering Chemistry Notes | AKTU Notes

UNIT 2: Spectroscopic Techniques and Stereochemistry

PART A: SPECTROSCOPIC TECHNIQUES AND APPLICATIONS

2.1 Introduction to Spectroscopy

Spectroscopy is the study of how matter interacts with electromagnetic radiation. When radiation of specific energy is absorbed or emitted by a molecule, it gives information about the molecule's structure.

Electromagnetic Spectrum (in order of increasing energy):

• Radio waves → Microwaves → Infrared (IR) → Visible → Ultraviolet (UV) → X-rays → Gamma rays

Why spectroscopy is useful:

• Identifies unknown compounds.
• Determines molecular structure.
• Measures concentration of substances.
• Studies chemical reactions.

2.2 UV Spectroscopy (Ultraviolet Spectroscopy)

Principle:

• UV spectroscopy involves absorption of ultraviolet radiation (wavelength range: 200–400 nm) by organic molecules.
• When UV radiation falls on a molecule, electrons are excited from a lower energy molecular orbital to a higher energy orbital.
• This is called an electronic transition.

Types of Electronic Transitions (in increasing energy):

• n → π* transition: Non-bonding electron jumps to anti-bonding π* orbital. Low energy, weak absorption. Shown by compounds with C=O, N=O groups.
• π → π* transition: Bonding π electron jumps to anti-bonding π* orbital. Stronger absorption. Shown by alkenes, aromatic compounds, conjugated systems.
• n → σ* transition: Non-bonding electron to anti-bonding σ* orbital.
• σ → σ* transition: Highest energy transition. Shown by alkanes (below 200 nm — far UV).

Important Terms:

• Chromophore: The part of the molecule that absorbs UV/visible light. Examples: C=C, C=O, N=O, benzene ring.
• Auxochrome: A group that itself does not absorb but when attached to a chromophore, shifts the absorption to longer wavelength. Examples: -OH, -NH₂, -Cl.
• Bathochromic shift (Red shift): Shift of absorption to longer wavelength (lower energy).
• Hypsochromic shift (Blue shift): Shift of absorption to shorter wavelength (higher energy).
• λmax: The wavelength at which maximum absorption occurs.

Beer-Lambert Law:

• A = ε × c × l
• Where A = absorbance (optical density), ε = molar absorptivity (L mol⁻¹ cm⁻¹), c = concentration (mol/L), l = path length (cm).
• This law says: higher concentration → more absorption.

Applications of UV Spectroscopy:

• Identification of conjugated systems and aromatic compounds.
• Determination of concentration of colored solutions.
• Studying the extent of conjugation in organic molecules.
• Pharmaceutical analysis — purity testing of drugs.
• Detection of impurities in organic compounds.

2.3 IR Spectroscopy (Infrared Spectroscopy)

Principle:

• IR spectroscopy involves absorption of infrared radiation (wavelength range: 2.5–25 µm or wavenumber 400–4000 cm⁻¹) by molecules.
• When IR radiation is absorbed, it causes vibrational transitions — bonds in the molecule start vibrating (stretching or bending) more energetically.
• Each type of bond vibrates at a specific frequency — this is like a fingerprint of the bond.

Types of Vibrations:

• Stretching vibrations: Change in bond length. Can be symmetric or asymmetric.
• Bending vibrations: Change in bond angle. Types: scissoring, rocking, wagging, twisting.

Condition for IR absorption:

• The vibration must cause a change in the dipole moment of the molecule.
• Homodiatomic molecules (like N₂, O₂) do NOT absorb IR — they have no dipole moment change.
• Heterodiatomic molecules and polyatomic molecules (like H₂O, CO₂, organic molecules) DO absorb IR.

Important IR Absorption Frequencies:

Bond / Group	Wavenumber (cm⁻¹)	Type of vibration
O-H (alcohol)	3200–3600	Stretching (broad)
N-H (amine)	3300–3500	Stretching
C-H (alkane)	2850–2960	Stretching
C≡N (nitrile)	2200–2260	Stretching
C=O (carbonyl)	1700–1750	Stretching (strong)
C=C (alkene)	1620–1680	Stretching
C-O (ether, alcohol)	1000–1300	Stretching

Fingerprint Region:

• The region 400–1500 cm⁻¹ is called the fingerprint region.
• Every organic compound has a unique pattern of absorption in this region — like a fingerprint.
• Used for compound identification by comparing with known spectra.

Applications of IR Spectroscopy:

• Identification of functional groups in organic molecules.
• Identification of unknown compounds (by matching fingerprint region).
• Studying reaction mechanisms and monitoring reactions.
• Quality control in pharmaceutical and chemical industries.
• Forensic analysis — identifying drugs, explosives.
• Environmental monitoring — detecting atmospheric pollutants (CO₂, CO).

2.4 NMR Spectroscopy (Nuclear Magnetic Resonance Spectroscopy)

Principle:

• NMR spectroscopy is based on the magnetic properties of certain atomic nuclei (mainly ¹H — proton NMR, and ¹³C — carbon NMR).
• Nuclei with odd number of protons or neutrons have a property called nuclear spin.
• When placed in a strong external magnetic field, these spinning nuclei align either with the field (low energy, α state) or against the field (high energy, β state).
• When radio frequency (RF) radiation of the right frequency is applied, nuclei in the α state absorb energy and flip to the β state — this is called resonance.
• The frequency at which resonance occurs depends on the chemical environment of the nucleus.

Chemical Shift (δ):

• Different protons in a molecule resonate at slightly different frequencies depending on their chemical environment (what groups are attached nearby).
• This difference in frequency (relative to a reference compound TMS — tetramethylsilane) is called chemical shift, measured in ppm (parts per million).
• TMS is set as δ = 0 ppm (reference standard).

Typical Chemical Shift Values (¹H NMR):

Type of Proton	Chemical Shift δ (ppm)
TMS reference	0
Alkane (R-CH₃)	0.9–1.5
Allylic (C=C-CH₃)	1.6–2.6
Ketone (R-CO-CH₃)	2.1–2.6
Alkyne (C≡C-H)	2.5
Ether (R-O-CH₃)	3.3–3.5
Vinyl (C=C-H)	4.6–5.9
Aromatic (Ar-H)	6.5–8.0
Aldehyde (-CHO)	9.5–10.5
Carboxylic acid (-COOH)	10–12

Spin-Spin Splitting (Coupling):

• Protons on adjacent carbons influence each other's magnetic environment — this causes splitting of NMR peaks.
• n+1 Rule: A proton with n equivalent neighboring protons splits into (n+1) peaks.
• Doublet (d): 1 neighboring proton → splits into 2 peaks.
• Triplet (t): 2 neighboring protons → splits into 3 peaks.
• Quartet (q): 3 neighboring protons → splits into 4 peaks.

Integration:

• The area under each NMR peak is proportional to the number of protons causing that signal.
• Used to count the number of each type of proton.

Applications of NMR Spectroscopy:

• Determining the structure of organic and biological molecules.
• MRI (Magnetic Resonance Imaging) in medicine — uses NMR principle to image soft tissues of the body.
• Pharmaceutical industry — structure verification of drug molecules.
• Quality control in food and beverage industry.
• Studying protein and DNA structures (biological NMR).

2.5 Numerical Problems on Spectroscopy

UV Spectroscopy Numerical:

• Using Beer-Lambert Law: A = ε × c × l
• Example: If absorbance A = 0.6, path length l = 1 cm, ε = 6000 L mol⁻¹ cm⁻¹, find concentration c.
• Solution: c = A / (ε × l) = 0.6 / (6000 × 1) = 1 × 10⁻⁴ mol/L.

NMR Spectroscopy Numerical:

• Identifying number and type of protons from integration ratio.
• Predicting splitting pattern using n+1 rule.
• Example: In CH₃-CH₂-Br (bromoethane), the CH₃ group has 2 neighboring protons (CH₂), so it appears as a triplet. The CH₂ group has 3 neighboring protons (CH₃), so it appears as a quartet.

PART B: STEREOCHEMISTRY

2.6 Introduction to Stereochemistry

Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects their properties.

• Molecules with the same molecular formula and same connectivity of atoms but different spatial arrangements are called stereoisomers.
• Stereoisomerism is important because different spatial arrangements can lead to completely different biological and chemical properties.

Types of Stereoisomers:

• Geometrical (cis-trans) isomers
• Optical isomers (enantiomers and diastereomers)

2.7 Optical Isomerism in Compounds Without Chiral Carbon

Normally, optical isomerism requires a chiral carbon (a carbon attached to 4 different groups — also called an asymmetric carbon or stereocenter).

But optical isomers can exist even without a chiral carbon — this happens due to restricted rotation or overall molecular asymmetry.

1. Allenes:

• Allenes have the structure R₁R₂C=C=CR₃R₄ (two cumulated double bonds).
• The two ends of the allene are perpendicular to each other.
• If R₁ ≠ R₂ and R₃ ≠ R₄, the molecule is non-superimposable on its mirror image → optical isomers exist.
• Example: 1,3-dimethylallene (CH₃CH=C=CHCH₃).

2. Biphenyls (Atropisomerism):

• In biphenyl compounds (two benzene rings connected), rotation around the central single bond is restricted if there are large groups at ortho positions.
• This restricted rotation leads to non-superimposable mirror image structures → optical isomers without chiral carbon.
• This type of stereoisomerism due to restricted rotation around a single bond is called atropisomerism.

3. Spiranes:

• Molecules with two rings sharing a single common atom (spiro atom).
• If substituents on both rings are appropriately different, the molecule can be chiral without any chiral carbon.

2.8 Geometrical Isomerism

Geometrical isomers (also called cis-trans isomers) arise due to restricted rotation around a double bond or a ring.

Conditions for geometrical isomerism:

• There must be a rigid structure (double bond or ring) preventing free rotation.
• Each carbon of the double bond must have two different groups attached to it.

In Alkenes:

• cis isomer: Same groups on the same side of the double bond.
• trans isomer: Same groups on opposite sides of the double bond.
• Example: 2-butene (CH₃-CH=CH-CH₃): cis-2-butene (both CH₃ on same side) and trans-2-butene (CH₃ on opposite sides).

E/Z Nomenclature (for complex cases):

• When there are four different groups, cis/trans notation is insufficient.
• Priority is assigned using CIP rules (Cahn-Ingold-Prelog) based on atomic number.
• Z (zusammen = together): High priority groups on same side.
• E (entgegen = opposite): High priority groups on opposite sides.

In Cyclic Compounds:

• Ring carbons cannot rotate freely → geometric isomerism exists.
• Example: 1,2-dimethylcyclopentane — cis (both CH₃ on same side of ring) and trans (CH₃ on opposite sides of ring).

Properties of Geometric Isomers:

• Different physical properties: different melting points, boiling points, densities, dipole moments.
• Different chemical reactivity in some reactions.
• Example: cis-butenedioic acid (maleic acid) readily forms anhydride on heating; trans-butenedioic acid (fumaric acid) cannot (groups too far apart).

2.9 Chiral Drugs

Chirality in Drugs:

• Many drug molecules are chiral — they have a chiral center (asymmetric carbon).
• A chiral drug exists as two mirror-image forms called enantiomers (R and S forms, or (+) and (-) forms).
• The mixture of equal amounts of both enantiomers is called a racemate or racemic mixture.

Why chirality matters in drugs:

• Biological receptors and enzymes are also chiral (made of L-amino acids).
• Only one enantiomer usually fits the receptor perfectly → has the desired therapeutic effect.
• The other enantiomer may be: inactive, have different activity, or be harmful/toxic.

Famous Examples of Chiral Drugs:

Drug	Active Enantiomer	Inactive/Harmful Enantiomer
Thalidomide	R-form: safe sedative	S-form: caused severe birth defects (tragedy of 1960s)
Ibuprofen	S-form: anti-inflammatory	R-form: inactive (but converts to S in body)
L-DOPA	L-form: treats Parkinson's	D-form: toxic side effects
Naproxen	S-form: pain reliever	R-form: liver toxin

Chiral Drugs in Pharmaceutical Industry:

• Many older drugs were sold as racemates (mixtures of both forms).
• Modern pharmaceutical companies try to produce and sell only the active enantiomer (called a single enantiomer drug or enantiopure drug).
• This process is called chiral switching.
• Benefits: lower dose needed, fewer side effects, better therapeutic effect.
• Chiral separation methods: chiral HPLC, asymmetric synthesis, enzymatic resolution.