Organic Chemistry: Basic Principles — NEET notes

Classification of Organic Compounds

Organic compounds are classified based on their carbon skeleton (open-chain vs cyclic) and functional groups. A functional group is the reactive site of a molecule that determines its chemical behavior. The homologous series groups compounds with the same functional group and successive CH₂ differences, and the degree of unsaturation (DBE) formula quickly tells you how many rings + double bonds exist.

DBE = 2C + 2 + N - H - X 2

Open-Chain vs Cyclic

Organic compounds divide into acyclic (open-chain/aliphatic) and cyclic compounds. Cyclic compounds further split into homocyclic (carbocyclic) and heterocyclic based on whether the ring contains only carbon or also heteroatoms.

Carbon's ability to form 4 covalent bonds and link with itself endlessly (catenation) creates two broad skeletal types. Open-chain (acyclic/aliphatic) compounds have straight or branched chains with no ring. Cyclic compounds contain at least one ring of atoms.

• Acyclic (open-chain): straight or branched chains — e.g., butane CH_3CH_2CH_2CH_3 , isobutane
• Carbocyclic (homocyclic): ring contains only C atoms — e.g., cyclohexane C_6H_ 12 , benzene C_6H_6
• Heterocyclic: ring contains at least one non-carbon atom (N, O, S) — e.g., pyridine (N), furan (O), thiophene (S)
• Aromatic: special cyclic compounds obeying Hückel's rule ( 4n+2 electrons) — benzene, naphthalene, pyridine
• Alicyclic: cyclic but NOT aromatic — cyclohexane, cyclopentane

Functional Groups

A functional group is a specific atom or group of atoms within a molecule that determines its chemical reactions. The same functional group gives the same chemical behavior regardless of the size of the carbon chain attached to it.

The carbon skeleton is the body; the functional group is the personality. Methanol ( CH_3OH ) and octanol ( C_8H_ 17 OH ) both undergo the same reactions — because the —OH group dictates reactivity, not the chain length.

• —OH → Alcohol (suffix: -ol)
• —CHO → Aldehyde (suffix: -al)
• >C=O → Ketone (suffix: -one)
• —COOH → Carboxylic acid (suffix: -oic acid)
• —NH_2 → Amine (suffix: -amine)
• —X (F, Cl, Br, I) → Haloalkane (prefix: halo-)
• —NO_2 → Nitro compound (prefix: nitro-)
• —C≡N → Nitrile (suffix: -nitrile)
• —COOR → Ester (suffix: -oate)

Homologous Series

A homologous series is a family of organic compounds with the same general formula, same functional group, and successive members differing by a CH₂ unit (14 u). Physical properties change gradually; chemical properties remain identical.

Each member of a homologous series differs from the next by exactly one —CH_2— unit (molecular mass difference = 14 u). This means they share the same general formula and the same functional group, so their chemistry is predictable.

• Alkanes: C_nH_ 2n+2 — methane, ethane, propane, butane...
• Alkenes: C_nH_ 2n — ethene, propene, butene...
• Alkynes: C_nH_ 2n-2 — ethyne, propyne, butyne...
• Alcohols: C_nH_ 2n+1 OH — methanol, ethanol, propanol...
• Successive members differ by CH_2 = 14 u in molecular mass

Quick recall

Ethanol (C₂H₅OH) and butanol (C₄H₉OH) belong to the same homologous series. Which has a higher boiling point?

Butanol — longer chain → stronger van der Waals forces → higher boiling point. But both undergo the same reactions (esterification, oxidation, dehydration) because both have the —OH group.

Degree of Unsaturation (DBE)

The degree of unsaturation (Double Bond Equivalents, DBE) formula lets you calculate the total number of rings and π bonds in a molecular formula. Each ring or double bond removes 2 H atoms from the saturated formula.

Every ring or double bond in a molecule means 2 fewer hydrogen atoms compared to the fully saturated open-chain compound. A triple bond counts as 2 degrees of unsaturation (2 π bonds). A ring counts as 1.

DBE = 2C + 2 + N - H - X 2

Where C = number of carbons, H = number of hydrogens, N = number of nitrogens, X = number of halogens. Oxygen and sulfur do NOT appear in the formula — they don't change the hydrogen count.

• Benzene C_6H_6 : DBE = 12+2-6 2 = 4 (3 double bonds + 1 ring)
• Cyclohexane C_6H_ 12 : DBE = 12+2-12 2 = 1 (1 ring, 0 double bonds)
• Acetylene C_2H_2 : DBE = 4+2-2 2 = 2 (1 triple bond = 2 π bonds)
• DBE = 0 means the compound is a saturated open-chain molecule
• DBE ≥ 4 with 6 carbons? Think aromatic ring (benzene = 4)

IUPAC Nomenclature

IUPAC nomenclature provides a systematic, unambiguous name for every organic compound. The name is built from four parts: prefix(es) + root word + primary suffix + secondary suffix. The root word comes from the longest continuous carbon chain containing the principal functional group, and numbering gives the lowest locant to the principal group.

Prefix(es) + Root word + Primary suffix + Secondary suffix

Root Words & Word Roots

The root word indicates the number of carbon atoms in the longest continuous chain (parent chain) that includes the principal functional group. The primary suffix indicates saturation (-ane, -ene, -yne).

The root word tells you the chain length. The primary suffix tells you the degree of saturation. Together, they form the skeleton of the IUPAC name.

• C_1 → Meth-, C_2 → Eth-, C_3 → Prop-, C_4 → But-
• C_5 → Pent-, C_6 → Hex-, C_7 → Hept-, C_8 → Oct-
• C_9 → Non-, C_ 10 → Dec-
• Primary suffix: all single bonds → -ane, one C=C → -ene, one C≡C → -yne
• Multiple double bonds: -adiene (2), -atriene (3); multiple triple bonds: -adiyne (2)

Step-by-Step IUPAC Naming

A systematic 6-step procedure to generate the correct IUPAC name for any organic compound: find the principal functional group, select the parent chain, number it, name substituents, arrange alphabetically, and assemble.

Naming an organic compound is like filling out a form — follow the steps in order and you'll never go wrong.

• Step 1: Identify ALL functional groups present in the molecule
• Step 2: Select the principal functional group (highest priority → gets the suffix)
• Step 3: Find the longest continuous chain containing the principal functional group → root word
• Step 4: Number the chain to give the lowest locant to the principal functional group first
• Step 5: Name all substituents (other functional groups → prefixes, alkyl branches → prefixes)
• Step 6: Assemble: prefixes (alphabetical) + root word + primary suffix + secondary suffix

Naming with Functional Groups

Each functional group has a specific suffix (when principal) and prefix (when subordinate). The priority order determines which group gets the suffix and which becomes a prefix.

When multiple functional groups are present, only the highest priority group gets the suffix. All others are expressed as prefixes. This is the most commonly tested aspect of IUPAC nomenclature in NEET.

• Halogens (F, Cl, Br, I) are ALWAYS prefixes (fluoro-, chloro-, bromo-, iodo-) — they never get suffixes
• —NO_2 is ALWAYS a prefix (nitro-) — never a suffix
• Alkyl groups ( —CH_3 , —C_2H_5 ) are always prefixes (methyl-, ethyl-)
• When naming, prefixes go in alphabetical order: bromo- before chloro- before methyl-
• Multiplying prefixes (di-, tri-, tetra-) are NOT considered for alphabetical ordering

Quick recall

Name the compound: CH₃—CH(OH)—CH₂—CHO

The —CHO (aldehyde) has higher priority than —OH (alcohol). Parent chain = 4C with both groups. Name: 3-Hydroxybutanal. —CHO gets the suffix (-al), —OH becomes a prefix (hydroxy-), locant 3.

Common IUPAC Naming Errors

Students lose marks on IUPAC naming by making predictable errors: wrong parent chain, incorrect numbering direction, ignoring priority rules, and misplacing locants. Master these and you'll never drop a naming question.

IUPAC naming is 80% following the algorithm correctly and 20% not falling into traps. These are the errors NEET question-setters exploit repeatedly.

• Lowest set of locants rule: Compare locant sets digit by digit — first point of difference decides (e.g., 2,3,5 beats 2,4,5 )
• Parent chain criteria order: (1) contains principal group → (2) maximum chain length → (3) maximum number of multiple bonds → (4) maximum substituents
• Cyclic compounds: use 'cyclo' prefix before the root word → cyclopentane, cyclohexanone
• When naming substituted benzene: toluene (methylbenzene), phenol (hydroxybenzene), aniline (aminobenzene) are acceptable IUPAC names

Isomerism

Isomers are compounds with the same molecular formula but different structural or spatial arrangements. Structural isomerism includes chain, position, functional group, and metamerism. Stereoisomerism includes geometrical (cis-trans / E-Z) and optical (R/S) isomerism. Recognizing isomer types is one of the highest-yield NEET topics in organic chemistry.

Same molecular formula Same compound

Structural Isomerism

Structural isomers differ in the order in which atoms are bonded to each other. The four types are chain isomerism, position isomerism, functional group isomerism, and metamerism.

In structural isomerism, atoms are connected in a different sequence. Think of it as building different LEGO structures with the exact same set of bricks.

• Chain isomerism: same molecular formula, different carbon skeleton → butane vs isobutane ( C_4H_ 10 )
• Position isomerism: same functional group on the same chain, different position → butan-1-ol vs butan-2-ol
• Functional group isomerism: same formula, DIFFERENT functional group → ethanol ( C_2H_5OH ) vs dimethyl ether ( CH_3OCH_3 )
• Metamerism: same functional group, different alkyl groups on either side of the heteroatom → diethyl ether vs methyl propyl ether ( C_4H_ 10 O )

Geometrical Isomerism (cis-trans / E-Z)

Geometrical isomerism arises due to restricted rotation around a C=C double bond (or in cyclic compounds). When two different groups are attached to each doubly-bonded carbon, cis-trans or E-Z isomers exist.

A C=C double bond cannot rotate freely (breaking the bond would require ~264 kJ/mol). If each carbon of the double bond has two different substituents, the groups can be on the same side (cis/Z) or opposite sides (trans/E).

• Condition: Each doubly-bonded carbon must have two different groups attached to it
• cis (Z): same or similar groups on the same side of the double bond
• trans (E): same or similar groups on opposite sides of the double bond
• E-Z system (CIP rules): Z (zusammen = together) — higher priority groups on same side; E (entgegen = opposite) — higher priority groups on opposite side
• E-Z is more general than cis-trans — it works even when groups are all different

CIP Priority: Higher atomic number → higher priority (Br > Cl > O > N > C > H)

Optical Isomerism

Optical isomers (enantiomers) are non-superimposable mirror images of each other, arising due to the presence of a chiral center (asymmetric carbon with 4 different groups). They rotate plane-polarized light in opposite directions.

Your left and right hands are mirror images but can't be superimposed — this is chirality. A carbon with 4 different substituents is a chiral center (stereocenter), and the molecule exists as two non-superimposable mirror images called enantiomers.

• Chiral center: sp^3 carbon bonded to 4 different groups → denoted by an asterisk (*)
• Enantiomers: non-superimposable mirror images — identical physical properties except optical rotation direction
• Dextrorotatory (d or +): rotates plane-polarized light to the RIGHT (clockwise)
• Laevorotatory (l or -): rotates plane-polarized light to the LEFT (anticlockwise)
• Racemic mixture (dl or ±): 50:50 mix of enantiomers → net rotation = ZERO (optically inactive)
• R/S system (CIP): R (rectus = right) and S (sinister = left) — absolute configuration, independent of optical rotation

Number of optical isomers = 2^n (max), where n = number of chiral centers

Tautomerism

Tautomers are structural isomers that exist in dynamic equilibrium and interconvert by the migration of a hydrogen atom (usually) and a shift of bonding electrons. The most common type is keto-enol tautomerism.

Unlike other isomers, tautomers are NOT separate compounds — they are two forms of the same compound that exist in equilibrium. A proton migrates from one atom to another within the same molecule, accompanied by a switch of a single bond and adjacent double bond.

• Keto-enol tautomerism: —CH_2—C(=O)— leftharpoons —CH=C(—OH)— (most common type)
• In most simple cases, the keto form dominates (more stable due to stronger C=O bond vs C=C bond)
• Exception: in 1,3-diketones and phenols, the enol form is significantly stabilized by conjugation or aromaticity
• Tautomerism requires at least one -hydrogen adjacent to the C=O group
• Not the same as resonance — tautomers are different compounds (different atom positions), resonance structures are different representations of the SAME compound

Electronic Effects

Electronic effects describe how electrons are pushed or pulled within a molecule, altering charge distribution and reactivity. The four main effects are inductive (+I/-I), resonance/mesomeric (+M/-M), hyperconjugation (σ-π delocalization), and electromeric effect (temporary shift under attack). These effects determine acid-base strength, carbocation stability, and reactivity patterns.

+I groups donate electrons through bonds; +M groups donate through bonds

Inductive Effect (+I / -I)

The inductive effect is the permanent displacement of σ-bond electrons toward or away from a substituent due to electronegativity differences. It weakens with distance and operates through σ bonds only.

When an electronegative atom (like Cl) is attached to a carbon chain, it pulls electron density toward itself through the bonds. This electron-withdrawing effect is called -I (negative inductive) effect. Conversely, alkyl groups push electron density toward the chain → +I (positive inductive) effect.

• -I groups (electron-withdrawing): —NO_2 > —CN > —F > —Cl > —Br > —I > —OH > —OR > —C_6H_5
• +I groups (electron-donating): —C(CH_3)_3 > —CH(CH_3)_2 > —CH_2CH_3 > —CH_3 > —H
• Inductive effect decreases rapidly with distance — practically negligible beyond 3-4 bonds
• It operates through bonds ONLY — never through space
• H is the reference: groups with greater electron donation than H → +I; less → -I

Resonance & Mesomeric Effect (+M / -M)

Resonance is the delocalization of π electrons or lone pairs across adjacent p-orbitals. The mesomeric effect is the permanent polarization of a molecule due to this delocalization. Unlike inductive effect, resonance does NOT weaken with distance along the conjugated system.

When a molecule has alternating single and double bonds (conjugation), electrons are not locked between two atoms — they spread across the entire conjugated system. This delocalization stabilizes the molecule and is called resonance.

• +M groups (electron-donating via resonance): —NH_2 > —NHR > —OH > —OR > —NHCOR > —X (halogens)
• -M groups (electron-withdrawing via resonance): —NO_2 , —CN , —CHO , —COR , —COOH , —COOR , —SO_3H
• Resonance does NOT fade with distance along a conjugated system — it operates equally well across the entire chain/ring
• +M groups have a lone pair on the atom directly attached to the conjugated system
• -M groups have a multiple bond to an electronegative atom (C=O, C≡N, N=O)

Hyperconjugation

Hyperconjugation is the delocalization of σ-electrons from C—H bonds adjacent to a π system or positively charged carbon into the empty or partially filled p-orbital. It explains why alkyl groups stabilize carbocations, double bonds, and free radicals.

In hyperconjugation, a C—H bond adjacent to a C=C or C^+ partially overlaps with the empty/partially filled p -orbital. The electrons of the bond delocalize into the system, providing stabilization. It is also called "no-bond resonance" because one contributing structure shows no bond between C and H.

• Only -C—H bonds (C—H on the carbon ADJACENT to C=C or C⁺) participate
• More -H atoms → more hyperconjugation → more stabilization
• Explains carbocation stability: 3° > 2° > 1° > CH_3^+ (9 > 6 > 3 > 0 α-H's)
• Explains alkene stability: more substituted alkene = more stable (Saytzeff's rule)
• Baker-Nathan effect: in some cases, —CH_3 is a better electron donor than —C(CH_3)_3 — because —CH_3 has 3 α-H's for hyperconjugation while —C(CH_3)_3 has 0

Number of hyperconjugative structures = n + 1 , where n = number of -H atoms

Electromeric Effect

The electromeric effect is a temporary and complete transfer of π electrons to one atom of a multiple bond in the presence of an attacking reagent. Unlike inductive and mesomeric effects, it exists ONLY during the attack and vanishes when the reagent is removed.

When an electrophile or nucleophile approaches a multiple bond ( C=C or C=O ), the electrons shift completely to one of the two atoms. This temporary polarization is the electromeric effect. Remove the reagent → the shift disappears.

• +E effect: π electrons shift TOWARD the attacking reagent (e.g., electrophile approaches C=C, electrons shift to the nearer carbon)
• -E effect: π electrons shift AWAY from the attacking reagent (rare, less commonly tested)
• It is a temporary effect — exists only in the presence of the attacking species
• Involves complete transfer of a shared pair, not partial displacement like inductive effect
• Most commonly seen in C=O (carbonyl): nucleophile attacks C, both π electrons shift to O

Bond Fission

When a covalent bond breaks, it can split in two ways: homolytic fission (one electron to each atom → free radicals) or heterolytic fission (both electrons to one atom → cation + anion). The type of fission depends on bond polarity, reaction conditions, and solvent — and it determines the entire reaction mechanism.

A:B homolysis A + B | A:B heterolysis A^+ + :B^-

Homolytic Fission

In homolytic fission, the shared electron pair splits equally — each atom gets one electron, producing two free radicals. This is favored in non-polar bonds, gas phase, UV light, and non-polar solvents.

"Homo" = same. Each atom takes one electron from the shared pair. The result: two electrically neutral species, each with an unpaired electron — these are free radicals. Free radicals are denoted by a dot: Cl , CH_3 .

• Represented by a fishhook arrow (single-barbed, half-headed arrow ⇀) showing movement of ONE electron
• Produces free radicals — species with one unpaired electron
• Favored in: non-polar bonds ( Cl—Cl , C—C , C—H )
• Favored by: UV light, high temperature, gas phase, non-polar solvents, peroxides as initiators
• Bond dissociation energy: energy needed to break the bond homolytically → Cl_2 2Cl needs 242 kJ/mol

Heterolytic Fission

In heterolytic fission, both electrons of the shared pair go to ONE atom — the more electronegative one. This produces a cation (electron-deficient) and an anion (electron-rich). Polar bonds in polar solvents favor heterolysis.

"Hetero" = different. The more electronegative atom grabs both electrons, becoming an anion ( :X^- ). The other atom loses both, becoming a cation ( R^+ ). This is the basis of ionic/polar reaction mechanisms.

• Represented by a regular curved arrow (double-barbed, full-headed arrow →) showing movement of TWO electrons
• Produces a cation (electrophile) and an anion (nucleophile)
• Favored in: polar bonds ( C—Cl , C—O , C—Br )
• Favored by: polar solvents (water, alcohols), high dielectric constant media
• R—X R^+ + :X^- → the carbocation is an electrophile, halide anion is nucleophile

Reaction Intermediates

Reaction intermediates are short-lived, high-energy species formed during multi-step reactions. The four main types are carbocations (C⁺), carbanions (C⁻), free radicals (C·), and carbenes (:C). Their stability order determines which products form and which mechanism dominates — making this one of the most fundamental topics in organic chemistry.

Carbocation stability: 3° > 2° > 1° > CH_3^+

Carbocations (C⁺)

A carbocation is a carbon atom bearing a positive charge with only 6 electrons (electron-deficient). It is sp² hybridized, trigonal planar, and has an empty p-orbital perpendicular to the plane. Stability increases with electron donation (hyperconjugation, resonance, +I groups).

A carbocation has 3 bonds and no lone pair on the charged carbon — it's sp^2 hybridized with a vacant p-orbital. This empty orbital is the reason carbocations are powerful electrophiles — they desperately want electrons.

• Hybridization: sp^2 → trigonal planar geometry, bond angle ≈ 120°
• Has an empty p-orbital perpendicular to the plane of 3 bonds
• Only 6 electrons around carbon (electron-deficient, violates octet rule)
• Stability order: benzyl ≈ allyl > 3° > 2° > 1° > CH₃⁺ > vinyl > phenyl (C₆H₅⁺)
• Stabilized by: +I effect (alkyl groups), hyperconjugation (α-H's), resonance (adjacent π system)
• Destabilized by: -I groups (adjacent electronegative atoms pull electrons away from already deficient C⁺)

Quick recall

Rank in order of carbocation stability: CH₃⁺, (CH₃)₃C⁺, CH₃CH₂⁺, C₆H₅CH₂⁺

C₆H₅CH₂⁺ (benzylic, resonance-stabilized) > (CH₃)₃C⁺ (3°) > CH₃CH₂⁺ (1°) > CH₃⁺ (methyl, no stabilization at all)

Carbanions (C⁻)

A carbanion is a carbon atom bearing a negative charge with 8 electrons (3 bonds + 1 lone pair). It is sp³ hybridized (pyramidal). Stability is the OPPOSITE of carbocations — electron-withdrawing groups stabilize carbanions.

A carbanion has 3 bonds and 1 lone pair → 4 electron pairs → sp^3 hybridization → pyramidal geometry (like ammonia NH_3 ). It has 8 electrons around carbon, satisfying the octet, and carries a formal negative charge.

• Hybridization: sp^3 → pyramidal geometry, bond angle < 109.5°
• Stability order: OPPOSITE of carbocations → CH_3^- > 1° > 2° > 3°
• Stabilized by: -I groups (electronegative atoms pull excess electron density away), -M groups, and s-character (more s-character → electrons held closer to nucleus → more stable)
• s-character stability: sp > sp^2 > sp^3 → acetylide RC≡C^- > vinyl R_2C=CH^- > alkyl R_3C^-
• Carbanions are strong nucleophiles and bases (Grignard reagents RMgX contain carbanion-like carbon)

Free Radicals (C·)

A free radical is a species with one unpaired electron on carbon. It is sp² hybridized (planar) with the unpaired electron in a p-orbital. Free radicals are formed by homolytic fission and are the key intermediates in chain reactions like halogenation.

A free radical has 3 bonds and 1 unpaired electron → sp^2 hybridized → planar (like carbocations!). The unpaired electron sits in the unhybridized p -orbital. Free radicals are electrically neutral but extremely reactive due to the unpaired electron.

• Hybridization: sp^2 → planar, bond angle ≈ 120° (similar to carbocation)
• Stability order: same as carbocations → 3° > 2° > 1° > CH_3
• Stabilized by: hyperconjugation (α-H's), resonance (allyl, benzyl radicals)
• Electrically neutral — no charge
• Formed by homolytic fission (UV light, heat, peroxides)

• Allylic radical ( CH_2=CH—CH_2 ): resonance-stabilized → unusually stable
• Benzylic radical ( C_6H_5CH_2 ): resonance with benzene ring → very stable
• Vinyl radical ( CH_2=CH ): sp^2 C with radical in sp^2 orbital → less stable than alkyl radicals
• Free radical stability: benzylic ≈ allylic > 3° > 2° > 1° > methyl > vinyl

Carbenes (:CR₂)

A carbene is a neutral species in which carbon has only two bonds and one lone pair (or two unpaired electrons). Singlet carbenes (paired electrons, sp²) and triplet carbenes (unpaired electrons, sp) have different reactivities.

Carbenes are the rarest and most exotic of the four intermediates. Carbon has only 2 bonds and either a lone pair (singlet) or 2 unpaired electrons (triplet). With only 6 electrons, carbon is electron-deficient and extremely reactive.

• Singlet carbene: lone pair on C → sp^2 hybridized, bent geometry, paired electrons → reacts as electrophile, stereospecific addition (cis stays cis)
• Triplet carbene: 2 unpaired electrons → sp hybridized, linear geometry → reacts like a diradical, non-stereospecific
• Most common carbene: :CH_2 (methylene) — formed by photolysis of CH_2N_2 (diazomethane) or CHCl_3 + KOH
• Dichlorocarbene ( :CCl_2 ) is generated from CHCl_3 + t-BuOK — key reagent in NEET (Reimer-Tiemann reaction!)
• Carbenes are used in cyclopropane synthesis: :CH_2 + C=C cyclopropane ring

Types of Organic Reactions

Organic reactions are classified into substitution, addition, elimination, rearrangement, oxidation, and reduction. Understanding the type of reaction helps predict products and mechanisms. Substitution replaces one group with another, addition adds across multiple bonds, elimination removes groups to form double bonds, and rearrangements change the carbon skeleton.

Substitution: A—B + C A—C + B | Addition: A=B + C—D A(C)—B(D)

Substitution Reactions

In substitution, an atom or group attached to carbon is replaced by another atom or group. Depending on the attacking species: nucleophilic substitution (SN1, SN2), electrophilic substitution (aromatic), or free radical substitution.

Substitution is the most common reaction type in organic chemistry. One group leaves (leaving group) and another group enters (incoming group/attacking species) at the same carbon atom. The carbon skeleton remains intact.

• Nucleophilic substitution (SN): a nucleophile ( :Nu^- ) replaces the leaving group → R—X + Nu^- R—Nu + X^-
• SN1: 2-step, carbocation intermediate, favored by 3° substrate + polar protic solvent → racemization
• SN2: 1-step, backside attack, favored by 1° substrate + strong nucleophile + polar aprotic solvent → inversion
• Electrophilic substitution: electrophile ( E^+ ) replaces H on aromatic ring → halogenation, nitration, sulfonation, Friedel-Crafts
• Free radical substitution: radical replaces H → alkane halogenation under UV light

Addition Reactions

In addition reactions, two molecules combine to form a single product by adding across a multiple bond (C=C, C≡C, C=O). The π bond breaks and new σ bonds form. Types: electrophilic addition (alkenes), nucleophilic addition (carbonyls).

Addition is the signature reaction of unsaturated compounds. The bond breaks, freeing two bonding sites, and two new groups attach. The product has fewer multiple bonds than the reactant.

• Electrophilic addition: electrophile attacks the π electrons of C=C → CH_2=CH_2 + HBr CH_3CH_2Br
• Markovnikov's rule: H adds to the C with more H's (or: electrophile adds to give the MORE stable carbocation)
• Anti-Markovnikov (peroxide effect): in presence of peroxides, HBr adds via free radical mechanism → Br goes to less substituted C
• Nucleophilic addition: nucleophile attacks the electrophilic C of C=O → aldehydes and ketones react with HCN, Grignard reagents, NaBH₄
• Triple bonds: undergo addition twice (2 mol of reagent) → alkyne → alkene → alkane

Elimination Reactions

Elimination reactions remove two atoms or groups from adjacent carbons (or the same carbon), creating a new π bond. β-elimination (1,2-elimination) is most common. The two mechanisms are E1 (2-step, carbocation) and E2 (1-step, concerted).

Elimination is the reverse of addition. Two groups depart from the molecule, a bond forms, and the degree of unsaturation increases. Most commonly, H leaves from one carbon ( -C) and a leaving group departs from the adjacent carbon ( -C).

• β-elimination (1,2-elimination): most common type → H from β-C and X from α-C leave → C=C forms
• E1: 2-step mechanism — leaving group departs first (forming carbocation), then base removes β-H → favored by 3° substrates, weak base, polar protic solvent
• E2: 1-step concerted — base removes β-H simultaneously as leaving group departs → favored by strong base, high temperature
• Saytzeff (Zaitsev) rule: the MORE substituted alkene is the major product (more stable alkene preferred)
• Hofmann elimination: with bulky base → LESS substituted alkene is major product (steric control overrides stability)

Rearrangement, Oxidation & Reduction

Rearrangement reactions change the carbon skeleton by migration of atoms or groups within the same molecule. Oxidation increases the C—O bond count (or decreases C—H), while reduction does the opposite. These round out the five fundamental reaction types.

In rearrangement, atoms migrate from one position to another within the SAME molecule — the molecular formula stays the same but the structure changes. In oxidation/reduction, the oxidation state of carbon changes: more bonds to oxygen = oxidation, more bonds to hydrogen = reduction.

• 1,2-Hydride shift: H migrates to adjacent C⁺ → 2° carbocation rearranges to 3°
• 1,2-Methyl shift: CH₃ migrates to adjacent C⁺ → ring expansion or degree change
• Pinacol-pinacolone rearrangement: 1,2-diol + acid → ketone (methyl migration)
• Oxidation of organic compounds: R—CH_3 [O] R—CH_2OH [O] R—CHO [O] R—COOH (increasing oxidation state of C)
• Reduction: reverse of above — C=O → C—OH (by NaBH₄, LiAlH₄) or C=C → C—C (by H₂/Pd)

Purification Techniques

Purification of organic compounds exploits differences in physical properties (solubility, boiling point, adsorption affinity, partition coefficient) between the desired compound and impurities. Key methods include crystallization, distillation (simple, fractional, steam, vacuum), chromatography (column, TLC, paper), and differential extraction. Qualitative analysis detects elements (Lassaigne's test) and functional groups.

R_f = Distance moved by solute Distance moved by solvent front

Crystallization & Distillation

Crystallization separates a solid from impurities based on differential solubility at different temperatures. Distillation separates liquids based on boiling point differences. Variants include simple, fractional, steam, and vacuum distillation.

Crystallization works because most solids are MORE soluble in hot solvent than cold. Dissolve the impure solid in hot solvent, filter out insoluble impurities, then cool → pure crystals form while soluble impurities stay in solution (mother liquor).

• Simple crystallization: dissolve in hot solvent → filter hot → cool → crystals of pure compound deposit
• Choice of solvent: must dissolve the compound when hot but NOT when cold; must NOT dissolve impurities (or dissolve them at all temperatures)
• Simple distillation: for liquids with bp difference > 30°C → heat → vapors of lower bp liquid collected first
• Fractional distillation: for liquids with bp difference < 30°C → uses fractionating column (glass beads provide surface for repeated condensation-evaporation)
• Steam distillation: for water-immiscible liquids that decompose at their bp → distill below 100°C with steam (e.g., aniline, essential oils)
• Vacuum distillation (reduced pressure): for liquids that decompose at atmospheric bp → lowering pressure lowers bp → distill safely

Chromatography

Chromatography separates components of a mixture based on their different rates of movement through a stationary phase under the influence of a mobile phase. The technique exploits differences in adsorption, partition, or ion-exchange behavior.

In chromatography, a mixture is applied to a stationary phase (solid or liquid on solid support) and a mobile phase (liquid or gas) moves through it. Components that interact more strongly with the stationary phase move SLOWER; those that interact more with the mobile phase move FASTER.

• Column chromatography: stationary phase = alumina/silica gel packed in glass column; mobile phase = solvent (eluent) poured from top → components separate into bands
• Thin Layer Chromatography (TLC): thin layer of silica gel on glass plate; spot mixture at bottom, dip in solvent → R_f value identifies each compound
• Paper chromatography: filter paper = stationary phase (water trapped in cellulose); organic solvent = mobile phase → partition chromatography
• R_f (Retention factor): R_f = distance moved by substance distance moved by solvent front → unique for each compound in a given solvent system
• Adsorption chromatography: separation based on different adsorption strengths (column, TLC)
• Partition chromatography: separation based on different partition coefficients between two liquid phases (paper chromatography)

R_f = Distance moved by solute from origin Distance moved by solvent front from origin

Differential Extraction

Differential extraction (solvent extraction) separates an organic compound from an aqueous mixture using an immiscible organic solvent. The compound distributes between the two layers based on its partition coefficient. Multiple small extractions are more efficient than one large extraction.

If an organic compound is dissolved in water but is more soluble in an organic solvent (like ether or DCM), you can shake the aqueous solution with the organic solvent. The compound partitions between the two layers according to its partition coefficient K .

K = Concentration in organic layer Concentration in aqueous layer

• Based on the Nernst distribution law — a solute distributes between two immiscible solvents in a fixed ratio at constant temperature
• Multiple small extractions are more efficient than one large extraction with the same total volume of solvent
• Commonly used solvents: diethyl ether, dichloromethane (DCM), chloroform, ethyl acetate
• Separating funnel is the standard lab apparatus for liquid-liquid extraction
• If K 1 → compound prefers organic layer → easily extracted

Detection of Elements (Lassaigne's Test)

Lassaigne's test converts covalently bonded elements (N, S, halogens) in organic compounds into ionic form by fusing with sodium metal. The resulting sodium fusion extract (SFE) is then tested with specific reagents. This is a core NCERT + NEET topic.

Organic compounds contain elements in covalent form — you can't detect them with ionic reagents directly. Lassaigne's solution: fuse the compound with sodium metal at high temperature. Na converts covalent bonds into ionic sodium salts: NaCN (from N), Na_2S (from S), NaX (from halogens).

• Nitrogen test: SFE + FeSO_4 + NaOH → boil → acidify with H_2SO_4 → Prussian blue color confirms N ( Na_4[Fe(CN)_6] + Fe^ 3+ Fe_4[Fe(CN)_6]_3 )
• Sulphur test: SFE + sodium nitroprusside → violet color confirms S ( Na_2S + Na_2[Fe(CN)_5NO] Na_4[Fe(CN)_5NOS] )
• Halogen test: SFE + HNO_3 (boil to remove CN⁻/S²⁻) + AgNO_3 → white ppt (Cl), pale yellow ppt (Br), yellow ppt (I)
• If compound contains both N and S → NaSCN forms → gives blood red color with FeCl_3 (NOT Prussian blue — need to destroy SCN⁻ first for N test)
• Phosphorus test: Heat with Na_2O_2 → Na_3PO_4 → test with ammonium molybdate → yellow ppt