Organic Chemistry: Some Basic Principles and TechniquesNEET Chemistry · Class 11 · NCERT Chapter 8

Tetravalency of Carbon and Classification

Carbon has atomic number 6 and electronic configuration 1s² 2s² 2p². It has four electrons in its outermost shell and needs four more to complete its octet. This is why carbon always forms four bonds — it is tetravalent. This tetravalency, combined with carbon's ability to form stable C-C bonds with itself (catenation), is why millions of organic compounds exist.

Classification of Organic Compounds

Organic compounds are broadly classified based on their carbon skeleton:

• Open chain (acyclic) compounds: Carbon atoms are arranged in straight or branched chains. Example: propane (CH₃CH₂CH₃), 2-methylpropane.
• Cyclic compounds: Carbon atoms form one or more rings.
  • Alicyclic: cyclic but not aromatic. Example: cyclohexane.
  • Aromatic: contain benzene ring or follow Hückel's rule (4n + 2 π electrons). Example: benzene, naphthalene.
  • Heterocyclic: ring contains atoms other than carbon (N, O, S). Example: pyridine, furan, thiophene.

Organic compounds are also classified by their functional groups. A functional group is an atom or group of atoms responsible for the characteristic chemical properties of the compound. Examples: −OH (alcohol), −COOH (carboxylic acid), −NH₂ (amine), −CHO (aldehyde), −C=O (ketone), −CN (nitrile).

IUPAC Nomenclature

CH₃-CH(CH₃)-CH₂-CH₃
(4-carbon chain with a methyl branch at C-2)

IUPAC (International Union of Pure and Applied Chemistry) rules give every organic compound a unique systematic name. Questions on IUPAC naming are asked almost every year in NEET.

Steps for Naming Alkanes

1. Find the longest continuous carbon chain. This is the parent chain. Count the carbons and use the appropriate prefix (meth-1, eth-2, prop-3, but-4, pent-5, hex-6, hept-7, oct-8, non-9, dec-10) followed by -ane.
2. Number the chain from the end nearest the first branch point, giving substituents the lowest set of locants.
3. Name each substituent (alkyl group) as a prefix with its locant. Alphabetical order for multiple substituents (ignore multiplying prefixes: di-, tri- when alphabetising).
4. If two paths give the same locants for the first point of difference, number so as to give the lower number at the first point of difference.

Example: 2-methylpentane (not 4-methylpentane, because we start from the nearer end).

Naming Alkenes and Alkynes

Replace -ane with -ene (one double bond) or -yne (one triple bond). Number the chain to give the double/triple bond the lowest locant. If both double and triple bonds are present, the double bond gets priority in numbering if both get the same lowest number; otherwise give the lower number to the one closer to the end.

Functional Group Priority for Nomenclature

When a compound has multiple functional groups, one is chosen as the principal characteristic group (suffix); others become prefixes. Priority order (highest to lowest):

−COOH > −SO₃H > −COOR (ester) > −COX (acyl halide) > −CONH₂ (amide) > −CHO (aldehyde) > −C=O (ketone) > −OH (alcohol) > −NH₂ (amine) > C=C > C≡C

Hybridisation and Molecular Geometry

Hybridisation is the mixing of atomic orbitals of similar energy to form new hybrid orbitals of equivalent energy for bonding.

• sp³ carbon: 4 σ bonds, no π bonds.
• sp² carbon: 3 σ bonds, 1 π bond (unhybridised p orbital forms the π bond).
• sp carbon: 2 σ bonds, 2 π bonds.

Bond length: C-C (1.54 Å) > C=C (1.34 Å) > C≡C (1.20 Å). Shorter bonds are stronger.

Bond angle increases as hybridisation changes from sp³ to sp² to sp because s-character increases (s orbitals are closer to the nucleus, so the bond angle opens up).

Inductive Effect

The inductive effect is the permanent polarization of sigma bonds due to electronegativity differences. It decreases along the chain.

The inductive effect is the permanent polarisation of a sigma bond due to the electronegativity difference between two atoms. The effect is transmitted through sigma bonds but decreases rapidly with distance (usually negligible beyond 3 bonds).

• −I (electron-withdrawing) groups:More electronegative than carbon. They pull electron density away from carbon, making the carbon slightly positive. Examples: F, Cl, Br, I, OH, NH₂, COOH, NO₂, CN (order of −I: F > OH > Cl > Br > I).
• +I (electron-donating) groups:Less electronegative than carbon, or push electron density toward carbon. Examples: alkyl groups (tert-butyl > iso-propyl > ethyl > methyl). Metal atoms bonded to carbon (organometallics) also show +I.

Effects of the inductive effect:

• −I groups in carboxylic acids increase acid strength (stabilise the carboxylate anion by withdrawing negative charge). HCOOH < CH₃COOH acid strength is reversed: actually F-substituted acetic acids are stronger due to −I of F.
• +I groups destabilise carboxylate anions → weaker acids. Acetic acid (with CH₃, +I) is a weaker acid than formic acid (HCOOH).

Resonance

Resonance (or mesomeric effect) is the delocalisation of electrons (particularly π electrons and lone pairs) over a conjugated system. The molecule is a single species called the resonance hybrid, which is more stable than any single contributing structure.

Conditions for Resonance

• The molecule must have a conjugated system (alternating single and double bonds) or an atom with a lone pair adjacent to a double bond/positive charge.
• All atoms involved must lie in the same plane (for p orbital overlap).
• The more resonance structures, the more stable the molecule (resonance energy).

Rules for Drawing Resonance Structures

• Only electron positions change, never atom positions.
• The number of unpaired electrons must remain the same in all resonance structures.
• No atom should exceed its maximum valency (no more than 8 electrons for second-row elements, except S and P).
• The most stable resonance structure contributes most to the hybrid. Structures with complete octets and no charge separation are more stable.

Mesomeric Effect (+M and −M)

The mesomeric effect is the permanent electron delocalisation via resonance:

• +M effect: Electron density is donated into the ring or conjugated system via resonance. Groups with lone pairs adjacent to the pi system: −NH₂, −OH, −OCH₃, −Cl, −Br (halogens show +M but −I). These groups activate the benzene ring toward electrophilic aromatic substitution and direct ortho/para.
• −M effect: Electron density is withdrawn from the ring via resonance. Groups containing a pi bond toward electronegative atoms: −NO₂, −COOH, −CHO, −CN, −SO₃H. These deactivate the benzene ring and direct meta.

Electromeric Effect

The electromeric effect is a temporary effect that occurs when an attacking reagent approaches a molecule. The π electrons of a multiple bond are completely transferred to one end under the influence of the reagent.

• +E effect: π electrons are transferred toward the attacking group. Occurs when electrophiles attack.
• −E effect: π electrons are transferred away from the attacking group. Occurs when nucleophiles attack.

Unlike the inductive or resonance effect, the electromeric effect exists only in the presence of a reagent. It is temporary.

Hyperconjugation

Hyperconjugation (or Baker-Nathan effect) is the delocalisation of electrons from a C-H sigma bond (or C-C sigma bond) adjacent to a carbocation, radical, or pi system, into the vacant p orbital or pi system.

For a carbocation: the electron density from a C-H bond on the adjacent carbon (alpha carbon) delocalises into the empty p orbital, reducing the positive charge. The more alpha-H atoms (more alkyl groups), the more hyperconjugation, and the more stable the carbocation.

For alkenes: hyperconjugation from alpha-H atoms stabilises the double bond. More substituted alkenes (more alkyl groups, more alpha-H) are more stable. This explains Zaitsev's rule in elimination reactions.

Number of hyperconjugative structures = number of alpha-H atoms.

Reaction Intermediates

Reaction intermediates are short-lived, high-energy species formed during a reaction. They are too reactive to be isolated under normal conditions. NEET tests their structure and stability frequently.

Carbocations (Carbonium Ions)

A carbon with only 6 electrons (a positive charge on carbon): R₃C⁺. Carbon is sp² hybridised with an empty p orbital perpendicular to the plane.

• Stability: 3° > 2° > 1° > CH₃⁺
• Stabilised by: alkyl groups (+I effect), hyperconjugation, adjacent π systems (resonance).
• Allylic and benzylic carbocations are particularly stable due to resonance delocalisation.

Carbanions

A carbon with 8 electrons and a negative charge: R₃C⁻. Carbon is sp³ hybridised.

• Stability: CH₃⁻ > 1° > 2° > 3° (opposite to carbocations).
• Stabilised by electron-withdrawing groups (−I, −M) which help disperse the negative charge.
• Destabilised by electron-donating groups (+I, +M).

Free Radicals

A carbon with 7 electrons (one unpaired electron): R₃C•. Carbon is sp² hybridised.

• Stability: 3° > 2° > 1° > CH₃• (same as carbocations).
• Stabilised by hyperconjugation and resonance.

Carbenes

Carbon with only 2 bonds and no charge: R₂C: (two electrons, either both paired or both unpaired). Example: :CH₂ (methylene). Carbenes are very reactive and short-lived.

Stability of Carbocations and Radicals

The stability order for carbocations:

Special stability: allylic (H₂C=CH-CH₂⁺) and benzylic (Ph-CH₂⁺) carbocations are more stable than simple secondary carbocations due to resonance.

Vinyl (H₂C=CH⁺) and phenyl (Ph⁺) carbocations are very unstable because the positive charge is on sp² or sp carbon with no adjacent hyperconjugation.

Free radical stability follows the same order as carbocations: 3° > 2° > 1° > CH₃•. Allylic and benzylic radicals are also extra stable.

Types of Organic Reactions

1. Substitution Reactions

One atom or group in a molecule is replaced by another. Subtypes:

• Nucleophilic substitution (SN1, SN2): Nucleophile attacks and replaces a leaving group. Common in alkyl halides. SN1 goes via a carbocation intermediate; SN2 is a concerted mechanism (back-side attack).
• Electrophilic substitution (SE): Electrophile replaces H on an aromatic ring. Example: nitration, halogenation of benzene.
• Free radical substitution (SR): Chlorination of alkanes in UV light (chain mechanism: initiation, propagation, termination).

2. Addition Reactions

Two molecules combine to give one product. Happens at multiple bonds (C=C, C=O). Subtypes: electrophilic addition (to alkenes), nucleophilic addition (to C=O), free radical addition.

3. Elimination Reactions

Removal of atoms or groups from adjacent carbons to form a double (or triple) bond. Example: dehydrohalogenation of alkyl halides with KOH/alc. Zaitsev's rule: the more substituted alkene is the major product.

4. Rearrangement Reactions

Atoms rearrange within the molecule to give a more stable product. Example: carbocation rearrangement (1,2-hydride or 1,2-alkyl shift) during SN1 reactions.

Electrophiles and Nucleophiles

Electrophiles: Electron-poor species. Examples: H⁺, Cl⁺, Br⁺, NO₂⁺ (nitronium ion), carbocations, Lewis acids (BF₃, AlCl₃, FeBr₃). They attack electron-rich centres (nucleophiles, pi bonds, aromatic rings).

Nucleophiles: Electron-rich species. Examples: OH⁻, CN⁻, NH₃, H₂O, Cl⁻, Br⁻, I⁻, alkenes (pi system). They attack electron-poor centres (electrophiles).

Isomerism

Isomers are compounds with the same molecular formula but different structural arrangements or spatial arrangements.

Structural Isomerism

• Chain isomerism: Different carbon chains (straight vs branched). Example: n-butane and isobutane (2-methylpropane) are both C₄H₁₀.
• Position isomerism: Same functional group at different positions. Example: 1-propanol and 2-propanol.
• Functional group isomerism: Different functional groups. Example: ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃) are both C₂H₆O.
• Metamerism: Different alkyl groups attached to the same functional group. Example: diethyl ether and methyl propyl ether (both ethers, both C₄H₁₀O).
• Tautomerism: Interconvertible structural isomers that exist in dynamic equilibrium. Most common: keto-enol tautomerism.

Stereoisomerism

• Geometrical (cis-trans) isomerism: Restricted rotation around a double bond. Requires two different groups on each doubly bonded carbon. cis: same groups on same side; trans: same groups on opposite sides. Also possible in cyclic compounds.
• Optical isomerism: Non-superimposable mirror images (enantiomers). Requires a chiral centre (an sp³ carbon with four different substituents). Enantiomers rotate plane-polarised light in opposite directions (+ and −). Diastereomers are stereoisomers that are not mirror images of each other. A meso compound has chiral centres but is achiral overall due to an internal plane of symmetry.

Worked NEET Problems

Summary Cheat Sheet