Organic Reagents Cheat Sheet: Reactions, Conditions & Exceptions for Exams

The Complete Organic Chemistry Reagent Guide: Master Every Reaction for Competitive Exams

JEE AdvancedNEETIIT-JAMBITSATGATECSIR-NETTGT/PGT

Organic chemistry reagents are the backbone of every synthesis question in competitive examinations. The moment you see a reagent on your paper, you must instantly recall: What does it do? Which functional group does it attack? What is the stereochemical outcome? This guide presents every major reagent from the classic curriculum — their identity, mechanism, selectivity, and exam-critical applications — in the clearest possible language, supported by precise chemical diagrams and reaction equations.

ℹ️ INFORMATION: This article covers 80+ reagents from a two-semester organic chemistry curriculum. Each section is written to give you deep mechanistic understanding — not just memorisation — because JEE Advanced, GATE, and CSIR-NET questions probe mechanism, not just product.

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1. Lewis Acids — The Electrophilic Activators

Lewis acids are electron-pair acceptors. In organic chemistry they play a pivotal role in activating electrophiles, particularly in Friedel–Crafts reactions, ring halogenation, and the Meerwein–Ponndorf–Verley (MPV) reduction. The four most tested Lewis acids are AlCl3, AlBr3, FeBr3, and FeCl3.

1.1 Aluminium Chloride (AlCl3) and Aluminium Bromide (AlBr3)

AlCl3 is among the strongest Lewis acids in organic chemistry. Its empty p-orbital on aluminium coordinates to halides, carbonyls, and other electron-rich sites, making otherwise sluggish electrophiles dramatically reactive.

⚠️ IMPORTANT: AlCl3 is used in stoichiometric (not catalytic) amounts for Friedel–Crafts acylation because the product ketone coordinates strongly to AlCl3, tying it up. In Friedel–Crafts alkylation it can be used catalytically.

Key Reactions:

• Friedel–Crafts Alkylation: ArH + R–Cl → Ar–R + HCl (carbocation mechanism; prone to rearrangements)
• Friedel–Crafts Acylation: ArH + RCOCl → Ar–CO–R + HCl (proceeds via acylium ion; no rearrangement)
• Electrophilic Chlorination: ArH + Cl2 / AlCl3 → Ar–Cl + HCl
• Meerwein–Ponndorf–Verley Reduction: Ketone + alcohol solvent / AlCl3 → secondary alcohol (transfer hydrogenation)

Friedel–Crafts Acylation of Benzene (Textbook Standard Representation)

🧠 KEY POINT: Acylium ion (RC≡O⁺) is more stable than carbocation — it carries resonance stabilisation. Therefore Friedel–Crafts acylation gives no rearrangement, while alkylation can rearrange.

1.2 Boron Trifluoride (BF3)

BF3 is a classic Lewis acid with an empty p-orbital on boron. It is particularly prized for forming thioacetals from ketones or aldehydes with dithiols. These are important protecting groups for carbonyls.

Ketone + HSCH2CH2SH + BF₃ → Thioacetal + H2O

Mechanistically, BF3 coordinates to the carbonyl oxygen (Lewis acid–base interaction), making the carbonyl carbon highly electrophilic. The thiol sulfur then attacks sequentially to give first a hemithioacetal, then the cyclic thioacetal.

💡 TIP: Thioacetals are resistant to hydrolysis under neutral and basic conditions but cleave easily under acid. They are also removed by Raney Nickel (Ra–Ni) to give the parent C–H, making them useful for overall deoxygenation (Wolff–Kishner alternative for acid-sensitive compounds).

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2. Halogenation Reagents — From Alkenes to Aromatics

Halogenation is one of the most foundational reaction types in organic chemistry. The choice of halogenating agent completely controls whether you get radical substitution, electrophilic addition, aromatic substitution, or alpha-halogenation of carbonyls.

2.1 Bromine (Br2) and Its Surrogates

Bromine is a versatile electrophilic halogenating agent. Its reactions span:

1. Electrophilic addition to alkenes → vicinal dibromides (anti addition via bromonium ion)
2. Halohydrin formation (Br2/H2O) → anti product, Markovnikov OH placement
3. Electrophilic aromatic substitution (Br2/FeBr3) → aryl bromides
4. α-Bromination of ketones (via enol or enolate)
5. Radical halogenation of alkanes (Br2/hν) → selective at 3° > 2° > 1° C–H
6. Hoffmann rearrangement of amides → primary amines
7. Haloform reaction (Br2/NaOH) of methyl ketones → carboxylic acids + CHBr3

Fig 2.1: Bromonium Ion Formation and Anti-Addition Mechanism

⚠️ IMPORTANT: In the bromonium-ion mechanism, nucleophilic attack (by Br⁻ or H2O) always occurs at the back face of the three-membered ring — giving exclusively anti (trans) addition. This is heavily tested in JEE Advanced stereochemistry questions.

2.2 N-Bromosuccinimide (NBS)

NBS is the reagent of choice for allylic bromination under radical conditions (light or AIBN initiation). It provides a steady, low concentration of Br2 — just enough to sustain the radical chain but not enough to add across the double bond.

Alkene–CH2– + NBS ^{hν or AIBN}→ Alkene–CHBr– + succinimide

The mechanism involves:

• Initiation: hν cleaves Br–Br → 2 Br•
• Propagation 1: Br• abstracts allylic H → resonance-stabilised allylic radical
• Propagation 2: Allylic radical + Br2 → allylic bromide + Br•
• NBS continuously regenerates Br2 from HBr formed in propagation 1

NBS also forms bromohydrins from alkenes in aqueous medium — identical outcome to Br2/H2O.

🧠 KEY POINT: NBS, NCS, and NIS (N-halosuccinimides) are interchangeable for halohydrin formation. Their main advantage is being stable crystalline solids — safer to handle than liquid halogens.

2.3 Chlorine (Cl2), NCS, and Iodine (I2)/NIS

These follow completely parallel chemistry to Br2/NBS:

Reagent	Addition to Alkene	Aromatic EAS	Radical	Haloform
Cl2	vicinal dichloride (anti)	with FeCl3/AlCl3	non-selective (3°≈2°≈1°)	yes (NaOH/Cl2)
NCS	chlorohydrin (with H2O)	—	allylic	—
I2	vicinal diiodide; iodohydrin	less reactive	haloform (NaOH/I2)	iodoform
NIS	iodohydrin (with H2O)	—	—	—

Note: Bromine is more selective in radical reactions than chlorine. Br• is less reactive, so it preferentially abstracts the weaker (tertiary or allylic) C–H bonds. Cl• is so reactive it attacks almost randomly.

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3. Hydrohalogenation — HX Reagents

The hydrogen halides (HBr, HCl, HI) add across multiple bonds following predictable regiochemistry. Understanding the difference between ionic (Markovnikov) and radical (anti-Markovnikov) pathways is essential.

3.1 Markovnikov Addition (HX, no peroxide)

In the absence of peroxides or light, HX adds to alkenes by an electrophilic mechanism:

1. Proton (H⁺) adds to the less substituted carbon → most stable carbocation
2. X⁻ attacks the carbocation

CH2=CH–CH3 + HBr → CH3–CHBr–CH3   (Markovnikov: Br on more substituted C)

3.2 Anti-Markovnikov Addition (HBr + Peroxides)

In the presence of peroxides (or light), HBr adds by a free-radical mechanism:

1. RO• (from peroxide) abstracts H from HBr → Br•
2. Br• adds to the less substituted carbon → more stable secondary radical
3. Radical abstracts H from HBr → anti-Markovnikov product + new Br•

⚠️ IMPORTANT: Only HBr shows anti-Markovnikov (peroxide effect). HCl and HI do NOT show this because: HCl has too strong a bond for RO• to abstract H efficiently; HI radical addition is thermodynamically unfavourable. This is a classic one-mark NEET and JEE question.

Fig 3.1: Markovnikov vs Anti-Markovnikov HBr Addition

3.3 HX Addition to Alkynes

• 1 equivalent HX → vinyl halide (Markovnikov)
• 2 equivalents HX → geminal dihalide (both halogens on the same carbon)

HC≡CH + HBr (1 eq) → CH2=CHBr   →   + HBr (2 eq) → CH3–CHBr2 (geminal)

3.4 HX Conversion of Alcohols

HX converts alcohols to alkyl halides by protonation of OH (making H2O a good leaving group) followed by nucleophilic attack of X⁻:

• Primary alcohol + HBr → SN2 (inversion, backside attack)
• Tertiary alcohol + HBr → SN1 (carbocation intermediate, racemisation possible)
• HI cleaves ethers: R–O–R' + HI → ROH + R'I (SN2 or SN1 depending on structure)

💡 TIP for Exams: Reactivity of HX towards alcohols: HI > HBr > HCl (mirrors nucleophilicity and leaving group ability of halide). HF is not used — too weak an acid, F⁻ too poor a nucleophile for carbon.

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4. Oxidising Agents — Systematic Oxidation Level Control

Mastering oxidation requires understanding oxidation state of carbon and which reagent stops at which level. This is one of the most tested conceptual areas in synthesis questions.

4.1 Chromium-Based Oxidants

Fig 4.1: Oxidation Level Control — Which Reagent Stops Where

The critical distinctions:

Reagent	1° Alcohol	Aldehyde	2° Alcohol	Notes
PCC (Py·CrO₃·HCl)	→ Aldehyde (stops)	not oxidised further	→ Ketone	Anhydrous CH₂Cl₂ solvent
DMP (Dess-Martin)	→ Aldehyde (stops)	not oxidised further	→ Ketone	Mild, iodine-based
CrO₃/pyridine	→ Aldehyde (stops)	not oxidised further	→ Ketone	Collins reagent
H₂CrO₄ (Jones)	→ Carboxylic Acid	→ RCOOH	→ Ketone	K₂Cr₂O₇/H₂SO₄ equivalent
KMnO₄ (acidic)	→ Carboxylic Acid	→ RCOOH	→ Ketone	Also cleaves alkenes

⚠️ IMPORTANT: PCC/DMP/CrO₃·pyridine CANNOT oxidise a primary alcohol all the way to the carboxylic acid. They stop at the aldehyde stage. H₂CrO₄ and KMnO₄ go all the way. This distinction appears in almost every synthesis exam question involving alcohols.

4.2 Potassium Permanganate (KMnO4)

KMnO4 is arguably the most versatile oxidant in organic chemistry. Its reactivity is controlled by conditions:

• Cold, dilute, basic KMnO4: dihydroxylation → syn-1,2-diol (like OsO4, but lower yield)
• Hot, acidic KMnO4: oxidative cleavage of alkenes → ketones/carboxylic acids
• 1° alcohol → carboxylic acid; 2° alcohol → ketone; 3° alcohol → no reaction
• Side chain oxidation of alkylaromatics: –CH2– adjacent to benzene ring → –COOH (requires at least one C–H bond)

Fig 4.2: Predicting Products of Oxidative Cleavage

4.3 OsO4 — Syn Dihydroxylation

Osmium tetroxide gives exclusively syn-dihydroxylation of alkenes. Both oxygens are delivered from the same face via a cyclic osmate ester intermediate.

Alkene + OsO4 → [cyclic osmate ester] → syn-diol (NaHSO3 workup)

💡 TIP: OsO4 is expensive and toxic. NMO (N-methylmorpholine N-oxide) is added as a co-oxidant to regenerate OsO4 catalytically (Upjohn dihydroxylation). This is tested in GATE and CSIR-NET.

4.4 Ozonolysis (O3)

Ozone cleaves the C=C double bond completely. The key decision is the workup:

• Reductive workup (Zn/H2O or DMS/Me2S): preserves aldehydes
• Oxidative workup (H2O2/H2O): aldehydes are further oxidised to carboxylic acids

R–CH=CH–R' → [O3] → ozonide → [DMS] → R–CHO + R'–CHO   (reductive workup)

R–CH=CH–R' → [O3] → ozonide → [H2O2] → R–COOH + R'–COOH   (oxidative workup)

🧠 KEY POINT — Ozonolysis Strategy: To determine starting alkene from ozonolysis products, connect the carbonyl carbons of all fragments with a C=C bond. To predict ozonolysis products from an alkene, break each C=C and put =O on each carbon.

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5. Reducing Agents — Selective Reduction Chemistry

Selection of the correct reducing agent is a classic multi-step synthesis skill. The hierarchy from most powerful to most selective must be mastered.

5.1 Lithium Aluminium Hydride (LiAlH4)

LiAlH4 (LAH) is the most powerful common reducing agent. It delivers hydride (H⁻) from the Al–H bond to electrophilic carbons. It reduces virtually every carbonyl-containing functional group:

Substrate	Product	Notes
Carboxylic acid (RCOOH)	Primary alcohol (RCH₂OH)	Goes through aldehyde stage but doesn't stop
Ester (RCOOR')	2 alcohols (RCH₂OH + R'OH)	Two equivalents of hydride consumed
Amide (RCONR₂)	Amine (RCH₂NR₂)	O is better leaving group than N; iminium intermediate
Nitrile (R–C≡N)	Primary amine (RCH₂NH₂)	Via imine intermediate
Ketone	Secondary alcohol
Aldehyde	Primary alcohol
Epoxide	Alcohol (opens at less hindered C)	SN2-like
Azide (R–N₃)	Primary amine

⚠️ IMPORTANT: LiAlH4 cannot be used with protic solvents (water, alcohol) — it reacts violently, releasing H2 gas. Use anhydrous THF or Et2O. Aqueous workup is done AFTER the reaction is complete.

5.2 Sodium Borohydride (NaBH4)

NaBH4 is a milder, more selective reducing agent than LiAlH4. It reduces aldehydes and ketones readily but does NOT normally reduce esters, amides, or carboxylic acids.

R–CO–R' + NaBH4 → R–CHOH–R'   (secondary alcohol)

💡 Exam Tip: NaBH4 can be used in protic solvents (methanol, ethanol) — unlike LAH. The solvent itself provides the proton for workup. This makes NaBH4 operationally much simpler.

5.3 DIBAL — Selective Reduction of Esters to Aldehydes

Di-isobutylaluminium hydride (DIBAL, DIBAL-H) is the key reagent when you need to stop reduction at the aldehyde stage from an ester. The bulky isobutyl groups create steric shielding — at –78 °C, the tetrahedral alkoxide intermediate is stable and does not react with a second equivalent of hydride.

R–COOR' + DIBAL (–78 °C) → [alkoxide intermediate] → H2O workup → R–CHO

• DIBAL also reduces nitriles (R–C≡N) → aldehydes (via imine hydrolysis)
• Reduces acyl halides (RCOCl) → aldehydes
• At room temperature it reduces further — temperature control is critical

5.4 LiAlH(OtBu)3 — Reduction of Acyl Halides to Aldehydes

This bulky aluminium hydride is even more hindered than DIBAL. It cleanly reduces acyl chlorides (RCOCl) to aldehydes at low temperature without over-reduction.

5.5 NaBH(OAc)3 and NaBH3CN — Reductive Amination

Both sodium triacetoxyborohydride and sodium cyanoborohydride are the reducing agents of choice for reductive amination:

1. Ketone/aldehyde + amine → imine (or iminium under acidic pH)
2. NaBH3CN (pH 4–5) selectively reduces iminium ion over free ketone

R–CHO + R'–NH2 → [R–CH=N–R'] + H2O → [NaBH3CN] → R–CH2–NH–R'

🧠 KEY POINT — Reduction Selectivity Summary:
LAH: reduces all C=O, C≡N, epoxides
NaBH₄: only aldehydes and ketones
DIBAL: ester → aldehyde (–78°C); nitrile → aldehyde
NaBH₃CN: iminium > ketone (reductive amination)

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6. Organometallic Reagents — Carbon Nucleophiles

Organometallic reagents are among the most powerful C–C bond forming tools. They behave essentially as carbanions (R⁻) and attack electrophilic carbons.

6.1 Grignard Reagents (R–MgX)

Grignard reagents are formed by treating alkyl/aryl/alkenyl halides (Cl, Br, I — never F) with magnesium in dry ether. They are enormously versatile nucleophiles:

Electrophile	Product (after H₃O⁺)	Key Notes
Aldehyde (RCHO)	2° Alcohol	R'MgX + RCHO → R–CHOH–R'
Ketone (RCOR')	3° Alcohol
Ester (RCOOR')	3° Alcohol (two R groups added)	Grignard adds TWICE via ketone intermediate
Acyl halide (RCOCl)	3° Alcohol (two R groups)	Adds twice
CO₂	Carboxylic acid	R'MgX + CO₂ → R'COO⁻ → R'COOH
Epoxide	Primary alcohol (SN2 at less substituted C)
Acidic H (ROH, RCOOH)	R–H + Mg salt (base reaction)	Destroys Grignard reagent!

⚠️ IMPORTANT: Grignard reagents are destroyed by ANY protic source (water, alcohol, amine, carboxylic acid). The reaction must be conducted in absolutely anhydrous conditions. This is why the reaction vessel must be flame-dried and inert atmosphere (N₂ or Ar) used.

Fig 6.1: Grignard Double Addition to Ester

6.2 Organolithium Reagents (R–Li)

Organolithium reagents are even more reactive than Grignards — the C–Li bond is more ionic, making the carbanion character stronger. All reactions of Grignards apply, plus:

• Can add to carboxylic acids (2 equivalents): RCOOH + 2 R'Li → R–CO–R' (ketone) after workup (first equivalent deprotonates)
• React with phosphonium salts to form ylides (Wittig chemistry)

6.3 Organocuprates — Gilman Reagents (R2CuLi)

Gilman reagents are formed by treating organolithiums with Cu(I) halides (2 R–Li + CuI → R2CuLi). Their key distinction from Grignards and organolithiums is conjugate (1,4) addition to α,β-unsaturated ketones, whereas Grignards/organolithiums give 1,2-addition.

α,β-enone + R2CuLi → 1,4-addition → β-substituted ketone (after workup)

RCOCl + R2CuLi → R–CO–R' (ketone)   [stops at ketone, doesn't add twice]

🧠 KEY POINT — 1,2 vs 1,4 Addition:
Grignard/RLi → 1,2-addition to enone (attack at C=O)
Gilman (R₂CuLi) → 1,4-addition to enone (attack at β-carbon)
This is a guaranteed question in JEE Advanced synthesis.

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7. Reagents for Alcohol → Halide Conversion

Converting an alcohol to a halide is a foundational step in most synthesis routes. The critical factor is often stereochemical retention or inversion.

Reagent	Halide Produced	Stereochemistry	Notes
SOCl₂ (thionyl chloride)	Alkyl chloride	Inversion (SN2)	SO₂ and HCl liberated as gases
PCl₃	Alkyl chloride	Inversion	Also converts RCOOH → RCOCl
PCl₅	Alkyl chloride	Inversion	Same mechanism as PCl₃
PBr₃	Alkyl bromide	Inversion	RCOOH → RCOBr also
SOBr₂ (thionyl bromide)	Alkyl bromide	Inversion	Identical to SOCl₂ mechanism
HBr, HCl, HI	Corresponding halide	1°: inversion; 3°: racemic (SN1)	See Section 3
TsCl/MsCl/BsCl	Alkyl sulfonate	RETENTION at C (new bond is C–O)	Used as leaving group for subsequent reactions

💡 TIP: TsCl, MsCl, and BsCl do NOT directly produce an alkyl halide — they produce a sulfonate ester (OTs, OMs, OBs). These are excellent leaving groups (weaker bases than OH⁻) and can then undergo SN2 with nucleophiles (halide, azide, cyanide etc.) with inversion at carbon. Conversion of OH → OTs → X results in overall inversion (2 inversions = retention? No — TsCl is retention then SN2 is inversion → net inversion).

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8. Epoxidation and Dihydroxylation

8.1 m-CPBA (m-Chloroperoxybenzoic Acid)

m-CPBA epoxidises alkenes in a concerted, stereospecific mechanism. The oxygen is delivered from the same face the peracid approaches — so geometry of the alkene is completely preserved:

• cis-alkene → cis-epoxide (meso or racemic depending on symmetry)
• trans-alkene → trans-epoxide

Alkene + m-CPBA → Epoxide + m-chlorobenzoic acid (byproduct)

m-CPBA also performs the Baeyer–Villiger oxidation: ketones are oxidised to esters. In this reaction, the carbon migrates to oxygen with loss of the peroxyacid leaving group. Migratory aptitude: 3° > 2° ≈ aryl > 1° > methyl.

R–CO–R' + m-CPBA → R–O–CO–R' (ester)   [the group with higher migratory aptitude migrates]

⚠️ IMPORTANT (Baeyer-Villiger): The group that migrates is the one anti-periplanar to the O–O bond. The oxygen is inserted between the carbonyl carbon and the more easily migrating group. E.g., cyclohexanone → caprolactone (7-membered lactone).

8.2 Opening of Epoxides

Epoxides are strained and reactive toward nucleophiles under both acidic and basic conditions — but with opposite regiochemistry:

• Acidic conditions (H3O⁺): SN1-like; nucleophile attacks the more substituted carbon (more carbocation character)
• Basic/neutral (Nu⁻, Grignard, LAH): SN2; nucleophile attacks the less substituted (less hindered) carbon; inversion at that carbon

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9. Diol Cleavage Reagents

Vicinal diols (1,2-diols) can be cleaved oxidatively to give aldehydes and/or ketones. Three reagents accomplish this:

HO–CH(R)–CH(R')–OH + HIO4 → R–CHO + R'–CHO + HIO3 + H2O

Reagent	Conditions	Notes
HIO₄ (periodic acid)	Aqueous	Cyclic iodate ester mechanism; selective for vicinal diol
NaIO₄ (sodium periodate)	Aqueous	Same as HIO₄; I(VII) → I(V)
Pb(OAc)₄ (lead tetraacetate)	Organic solvent	Pb(IV) → Pb(II); cyclic lead ester; used when aqueous conditions unsuitable

💡 TIP: If an α-hydroxy ketone is present, HIO4 will also cleave it (the enol of a ketone is a 1,2-diol). Glycerol (1,2,3-propanetriol) with excess HIO4 gives 2 HCHO + HCOOH.

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10. Elimination and Dehydration Reagents

Elimination reactions convert saturated systems to alkenes. The choice of base and conditions controls both the regiochemistry (Zaitsev vs Hofmann) and the mechanism (E2 vs E1).

10.1 Zaitsev vs Hofmann Selectivity

Base	Alkene Product	Rule
Small, strong base (NaOEt, NaOH)	More substituted (Zaitsev)	Thermodynamic control
Bulky base (KOtBu, LDA, DBU)	Less substituted (Hofmann)	Kinetic/steric control

2-Bromobutane + KOtBu → 1-butene (minor Zaitsev: 2-butene)   [major = Hofmann = terminal alkene]

10.2 LDA — Kinetic Enolate Formation

LDA (lithium diisopropyl amide) is the premier base for forming kinetic (less substituted) enolates of ketones. Its giant isopropyl groups prevent it from attacking as a nucleophile and force deprotonation at the less hindered α-carbon.

2-methylcyclohexanone + LDA (THF, –78°C) → enolate at C-6 (less substituted) → kinetic enolate

10.3 Dehydration with H3PO4, H2SO4, and TsOH

Strong acids protonate the alcohol OH group, converting it to H2O (a good leaving group). E1 elimination of water then gives the alkene. The conjugate bases (HSO4⁻, H2PO4⁻, TsO⁻) are poor nucleophiles, preventing substitution from competing.

💡 TIP: POCl3/pyridine is used when the substrate cannot tolerate acidic conditions. POCl3 converts OH to a good leaving group (the phosphate ester), and pyridine acts as base to drive elimination — this is an E2 mechanism with no rearrangement.

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11. Special Reagents for Named Reactions

11.1 PPh3 — The Wittig Reaction

Triphenylphosphine (PPh3) reacts with alkyl halides (SN2) to form phosphonium salts. Strong base (NaH, n-BuLi) then deprotonates to give the ylide (Ph3P=CR2). The ylide reacts with aldehydes or ketones to give alkenes via an oxaphosphetane intermediate.

R–CHO + Ph3P=CHR' → R–CH=CHR' + Ph3P=O

Driving force: formation of the very strong P=O bond (~540 kJ/mol).

🧠 KEY POINT — Wittig Selectivity:
Stabilised ylides (electron-withdrawing group on carbanion) → mainly E (trans) alkene
Non-stabilised ylides → mainly Z (cis) alkene
This is tested in JEE Advanced and GATE stereochemistry questions.

11.2 DCC — Peptide Bond Formation

N,N'-Dicyclohexylcarbodiimide (DCC) is a dehydration reagent that activates carboxylic acids toward nucleophilic attack by amines to form amide bonds. It is the foundational coupling reagent in peptide synthesis.

R–COOH + DCC → O-acylisourea intermediate → + R'–NH2 → R–CO–NHR' + DCU (byproduct)

11.3 Diazomethane (CH2N2)

Diazomethane is a yellow toxic gas used for three reactions:

1. Methylation of carboxylic acids: RCOOH + CH2N2 → RCOOCH3 + N2↑
2. Cyclopropanation of alkenes (carbene insertion)
3. Wolff rearrangement (Arndt–Eistert synthesis): extends carboxylic acid chain by one carbon

11.4 Hydrazine (NH2NH2) — Wolff–Kishner Reduction

Hydrazine reacts with ketones/aldehydes to give hydrazones. Treatment with strong base (KOH) and heat in ethylene glycol solvent liberates N2 gas and forms the alkane — overall reduction of C=O to CH2.

R–CO–R' + NH2NH2 → R–C(=NNH2)–R' → [KOH, heat] → R–CH2–R' + N2

💡 TIP — Wolff–Kishner vs Clemmensen: Both reduce C=O to CH2, but:
Wolff–Kishner: basic conditions (KOH) — use for acid-sensitive substrates
Clemmensen (Zn/Hg + HCl): acidic conditions — use for base-sensitive substrates

11.5 AIBN and Peroxides — Free Radical Initiators

AIBN [2,2'-azobis(2-methylpropionitrile)] and organic peroxides (RO–OR) initiate free radical chain reactions by thermally fragmenting to give radicals. Their main uses: HBr/peroxide anti-Markovnikov addition, allylic NBS bromination.

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12. Aromatic Chemistry Reagents — Electrophilic Aromatic Substitution (EAS)

EAS is the dominant reactivity of benzene and its derivatives. A comprehensive exam-ready summary:

Fig 12.1: Summary of Electrophilic Aromatic Substitution Reactions

12.1 Nitration (HNO3/H2SO4)

H2SO4 protonates HNO3, which loses water to give the nitronium ion (NO2+) — a very strong electrophile. This attacks the ring to give nitroarenes, which are subsequently reduced to anilines (using Sn/HCl, Fe/HCl, Zn/HCl, or H2/Pd).

12.2 Sulfonation (SO3/H2SO4)

The sulfonic acid group (–SO3H) introduced by SO3 is a useful removable blocking group. It occupies a position on the ring, then can be removed by hot dilute H2SO4, allowing synthesis of regioisomers that would otherwise be inaccessible.

12.3 Diazonium Chemistry — Sandmeyer Reactions

Aromatic amines are converted to diazonium salts (Ar–N2+) by treatment with HONO (NaNO2/HCl, 0°C). Diazonium salts are versatile intermediates:

Reagent	Product
CuBr (Sandmeyer)	Ar–Br
CuCl (Sandmeyer)	Ar–Cl
CuCN (Sandmeyer)	Ar–CN
KI / heat	Ar–I
H₂O / heat	Ar–OH (phenol)
H₃PO₂	Ar–H (deamination)

🧠 KEY POINT: Diazonium chemistry is the only practical route to aryl iodides (Ar–I), aryl fluorides (Balz–Schiemann: Ar–N₂⁺ + BF₄⁻ → heat → Ar–F), and aryl cyanides where direct EAS fails.

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13. Protecting Groups in Synthesis

Protecting groups temporarily mask a functional group so that reactions can be performed selectively elsewhere in the molecule. The key requirement: easy to introduce, stable under reaction conditions, easy to remove.

Group Protected	Protecting Group	Install with	Remove with
Alcohol	Silyl ether (TMS, TBS)	TMSCl / TBSCl + base	TBAF or aq. HF; aq. acid
Alcohol	Acetal	ROH + CH₂(OCH₃)₂ / TsOH	Aq. acid (H₃O⁺)
Ketone/Aldehyde	1,3-Dioxolane (cyclic acetal)	HOCH₂CH₂OH / TsOH	Aq. acid
Ketone	1,3-Dithiolane (thioacetal)	HSCH₂CH₂SH / BF₃	Raney Ni (also removes S → C–H)
Alcohol	Ester (Ac)	Ac₂O / pyridine	NaOH/MeOH (saponification) or aq. acid

💡 Exam Tip: TBAF (tetrabutylammonium fluoride) is the unique reagent for removing silyl protecting groups. Its power comes from the extraordinary strength of the Si–F bond (~565 kJ/mol), which drives F⁻ attack on silicon to displace the oxygen. No other common fluoride source is as effective.

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14. Special Reductions — Birch Reduction and Dissolving Metal

14.1 Birch Reduction (Na or Li in liquid NH3/alcohol)

The Birch reduction converts aromatic rings to 1,4-cyclohexadienes (non-conjugated dienes). The regiochemistry depends on substituents:

• Electron-donating groups (EDG, e.g., –OMe): double bonds remain attached to the carbon bearing the EDG (substituted positions retain sp² character). The unsubstituted positions are reduced.
• Electron-withdrawing groups (EWG, e.g., –COOH): reduction occurs adjacent to the EWG; the EWG carbon remains sp² (in the double bond).

Fig 14.1: Birch Reduction — Effect of EDG vs EWG

14.2 Na/NH3 — Trans Reduction of Alkynes

Sodium (or lithium) in liquid ammonia reduces internal alkynes to trans (E) alkenes — the opposite of Lindlar's catalyst. The trans selectivity arises from the preference of the vinyl radical/anion intermediate to adopt the more stable trans geometry before protonation.

R–C≡C–R' + Na/NH3 → (E)-R–CH=CH–R'   (trans alkene)

R–C≡C–R' + Lindlar + H2 → (Z)-R–CH=CH–R'   (cis alkene)

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15. Reagent Selectivity Summary — Exam-Ready Tables

Oxidising Agents at a Glance

Transformation	Reagent(s)
1° Alcohol → Aldehyde	PCC, DMP, CrO₃/pyridine, Swern (oxalyl chloride/DMSO)
1° or 2° Alcohol → Acid/Ketone	KMnO₄, H₂CrO₄ (Jones), Na₂Cr₂O₇/H₂SO₄
Alkene → Epoxide	m-CPBA (or any peroxyacid)
Alkene → syn-Diol	OsO₄ (±NMO), cold KMnO₄/NaOH
Alkene → Cleavage products	O₃ (then DMS or H₂O₂), hot acidic KMnO₄
Ketone → Ester (BV)	m-CPBA
Vicinal diol → Aldehydes	HIO₄, NaIO₄, Pb(OAc)₄
Aldehyde → Carboxylic acid	KMnO₄, H₂CrO₄, Tollens' (Ag₂O/NH₃)

Reducing Agents at a Glance

Transformation	Reagent(s)
Ester → Primary alcohol	LiAlH₄
Ester → Aldehyde	DIBAL (–78°C)
Acyl halide → Aldehyde	DIBAL, LiAlH(OtBu)₃
Aldehyde/Ketone → Alcohol	NaBH₄, LiAlH₄, DIBAL
Nitrile → Amine	LiAlH₄, H₂/Pd
Nitrile → Aldehyde	DIBAL (then H₂O)
Amide → Amine	LiAlH₄
Carboxylic acid → Alcohol	LiAlH₄
Alkyne → cis-Alkene	Lindlar + H₂, Ni₂B
Alkyne → trans-Alkene	Na/NH₃, Li/NH₃
Nitro → Amine	Sn/HCl, Fe/HCl, Zn/HCl, H₂/Pd
Ketone → Alkane	NH₂NH₂/KOH (Wolff–Kishner), Zn/Hg+HCl (Clemmensen)
Imine → Amine	NaBH₃CN (reductive amination), H₂/Pd

ℹ️ INFORMATION — Synthesis Strategy: When planning a multi-step synthesis in JEE Advanced or GATE, work backwards (retrosynthesis). Identify the target functional group and ask: "What one-step transformation reveals a simpler precursor?" Then identify the reagent for that step using the tables above.

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16. Exam-Critical Tips and Common Traps

⚠️ TOP TRAPS IN EXAMS:

• PCC vs H₂CrO₄: PCC stops at aldehyde; H₂CrO₄ goes to carboxylic acid.
• HBr+peroxide ONLY for anti-Markovnikov — HCl and HI don't work this way.
• Grignard + ester adds TWICE (through ketone intermediate) → tertiary alcohol with two identical groups.
• Organocuprate = 1,4-addition to enone; Grignard/RLi = 1,2-addition.
• DIBAL requires low temperature to stop at aldehyde from ester; at room temp it reduces to alcohol.
• OsO₄ = syn-diol; m-CPBA = epoxide (then opening can give anti-diol).
• Diazonium chemistry requires 0–5°C — above this temperature the diazonium salt decomposes.
• LDA forms kinetic enolate (less substituted); NaOEt/NaOH forms thermodynamic enolate (more substituted).

🧠 MEMORY AID — "PLAN" for Synthesis Questions:
Product — what is the target?
Level — what oxidation state change is needed?
Agent — which reagent achieves it with the right selectivity?
Nucleus — does the stereochemistry demand SN2 (inversion) or SN1 (mixture)?

💡 JEE Advanced Pattern: Paragraph-based questions often test a sequence of 3–4 reagents. Identify the functional group after each step — don't skip ahead. Common multi-step chains: alcohol → tosylate → azide (SN2) → amine (LAH) — this is the Gabriel synthesis shortcut.

💡 CSIR-NET/GATE Pattern: These exams test mechanistic details deeply. Focus on: bromonium/chloronium ion stereochemistry (anti addition); E2 transition state geometry (anti-periplanar H and LG); Baeyer–Villiger migratory aptitude; and oxymercuration/hydroboration regioselectivity (Markovnikov vs anti-Markovnikov).

💡 NEET Pattern: NEET tests: which reagent converts X to Y (functional group interconversion), Markovnikov/anti-Markovnikov addition, haloform reaction conditions, and the Tollens test (Ag mirror) vs Fehling test specificity for aldehydes.

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17. Solvents — Why They Matter

Solvent choice profoundly affects reaction rates and outcomes:

Solvent Type	Examples	Best For
Polar Aprotic	DMSO, DMF, acetone, acetonitrile	SN2 reactions (no H-bonding to nucleophile, enhances reactivity)
Polar Protic	H₂O, MeOH, EtOH, i-PrOH	SN1, E1, reactions requiring proton source
Ethers	THF, Et₂O, DME	Grignard, RLi, LiAlH₄ (coordinate to Mg/Li, stabilise)
Chlorinated	CH₂Cl₂, CHCl₃, CCl₄	mCPBA epoxidation, PCC, halogenation

⚠️ IMPORTANT: Polar aprotic solvents dramatically accelerate SN2 reactions because they solvate the cation (Na⁺, K⁺) but leave the anion (nucleophile) relatively "naked" and highly reactive. This is why NaN₃ in DMSO is far more reactive than NaN₃ in water.

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18. Quick-Reference: pKa Values and Base Selection

A reaction proceeds when the base deprotonates the substrate — this requires the base's conjugate acid to have a higher pKa than the substrate.

Acid / Functional Group	Approximate pKa	Strong enough base needed
HI / HBr / HCl / H₂SO₄	−10 to −3	Any base deprotonates
Carboxylic acid	~4–5	NaHCO₃ suffices
Alcohol	~16–18	NaH, KH, or metal alkyls
Ketone / Aldehyde (α-H)	20–24	LDA, NaH, NaOEt (strong base)
Ester / Nitrile (α-H)	~25	LDA
Alkyne (terminal C–H)	~25	NaNH₂, NaH, n-BuLi
Amine (N–H)	35–38	n-BuLi, LDA (strong)
H–H	42	Only strongest bases (n-BuLi)
Alkene (C–H)	~43	Only metallic Na or n-BuLi

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Conclusion: Building Your Reagent Intuition

The reagent guide is not simply a list to memorise — it is a system of chemical logic. Every reagent in organic chemistry has a defined electronic role (Lewis acid, nucleophile, electrophile, radical source, reductant, oxidant) and a defined selectivity (which functional group, which face, which regiochemical outcome). When you understand the mechanism, the product becomes predictable rather than remembered.

For competitive examinations, the most reliable strategy is:

1. Identify the functional group transformation required
2. Recall which reagents perform that transformation
3. Apply selectivity rules (oxidation level, stereochemistry, regiochemistry) to choose the right one
4. Check conditions (temperature, solvent, anhydrous vs aqueous) for any trap

With this framework, the 80+ reagents in this guide become a navigable landscape rather than an intimidating catalogue. Practice applying this logic to past JEE Advanced, GATE, and CSIR-NET papers — you will find that the same mechanistic principles recur in every question.

🧠 FINAL KEY POINT — The Big Three for Every Synthesis Step:
1. What bond is formed or broken?
2. What reagent creates/breaks that bond with the required selectivity?
3. What are the conditions (temperature, solvent, stoichiometry)?
Answer these three questions for each step and no synthesis problem can defeat you.

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