Aldehydes Ketones

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Aldehydes Ketones — Key Facts for JEE Advanced

Structure & Functional Group:

• Aldehyde: R–CHO (formyl group –CHO, carbonyl C bonded to at least one H)
• Ketone: R–CO–R′ (carbonyl C bonded to two carbon groups; R ≠ H)
• Acetone = propanone = CH₃–CO–CH₃ (most common ketone)
• Formaldehyde = methanal = HCHO (only aldehyde with two H atoms on carbonyl C)
• Bond angle C=O–C in ketones: ~120° (sp² hybridization at carbonyl C)
• C=O bond length: ~123 pm (shorter than C–O single bond)

Nomenclature:

• Aldehydes: suffix –al; carbon bearing –CHO is C-1 (never explicitly numbered)
• CH₃CH₂CHO = propanal (not 1-propanal)
• Ketones: suffix –one; number to give carbonyl lowest number
• CH₃–CO–CH₂–CH₃ = butan-2-one (not 3-butanone)
• Aromatic aldehydes: benzaldehyde (C₆H₅CHO), 4-methylbenzaldehyde = p-tolualdehyde

Key Reactions — Rapid Recall:

Reaction	Aldehyde	Ketone
Tollens’ Test (AgNO₃/NH₃)	Ag mirror (reduces to Ag⁰)	No reaction
Fehling’s Test (CuSO₄/Na-K tartrate)	Red precipitate (Cu₂O)	No reaction
2,4-DNP Test	Yellow/orange precipitate (hydrazone)	Same
Sodium Bisulfite	White crystalline adduct	Ketones > C₄ form adduct; methyl ketones react slowly
Haloform Test (I₂/NaOH)	CH₃CHO (acetaldehyde) → yellow CHI₃	Methyl ketones (CH₃–CO–R) → CHI₃
Oxidation (KMnO₄)	carboxylic acid	No reaction (except strong oxidants like HNO₃)

⚡ Exam Tip: JEE frequently asks: “Which gives positive Tollens’ test?” The answer: only aldehydes. But note that α-hydroxy ketones (Compounds with –OH on the carbon adjacent to carbonyl, e.g., benzoin) ALSO give positive Tollens’ test because they oxidize to α-diketones under the reaction conditions.

⚡ Exam Tip: Ketones do NOT give Fehling’s or Tollens’. However, fructose (a ketose) DOES give positive Fehling’s and Tollens’ because it undergoes tautomerization to an aldehyde under the alkaline test conditions (lobry de Bruyn–van Ekenstein transformation).

⚡ Exam Tip: The haloform reaction is THE distinguishing test for methyl ketones (CH₃–CO– group). Structure: CH₃–CO–R + 3I₂ + 4NaOH → CHI₃ + R–COONa + 3NaI + 3H₂O. Only methyl ketones (R = H, alkyl, aryl) with CH₃–CO– give this. Acetaldehyde also gives it.

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Aldehydes Ketones — Chemistry Study Guide

1. Structure & Bonding:

Carbonyl Group:

• Carbonyl C is sp² hybridized: one σ bond to O, two σ bonds to C (or H in aldehyde)
• π bond formed by sideways overlap of C(sp²) and O(sp²) p-orbitals
• The O atom has two lone pairs: one in sp² orbital (in-plane), one in p orbital (perpendicular)
• The molecule is planar at the carbonyl center (all atoms in one plane)

Resonance structures:

R–C(=O)–R' ⟷ R–C(–O⁻)=R'⁺
Dipole moment: ~2.4 D for acetone (C=O is polar, O is δ–)

Reactivity Factors:

• Aldehydes are generally more reactive than ketones for nucleophilic addition
• Reason: Ketones have two electron-donating alkyl groups that reduce electrophilicity of C=O carbon; aldehydes have one H (less donating)
• Also steric: ketones are more sterically hindered at the carbonyl carbon
• Exception: conjugated aldehydes (e.g., benzaldehyde) can be less reactive than some ketones due to resonance stabilization of the carbonyl

Substituent Effects:

• Electron-withdrawing groups (–NO₂, –CN, –Cl) at α-carbon increase acidity of α-H
• Electron-donating groups (–CH₃, –OCH₃) decrease acidity of α-H
• α-hydrogen acidity: pKa ~17 for acetone (much more acidic than alkanes, pKa ~50)

2. Preparation Methods:

Aldehydes:

1. Oxidation of primary alcohols: R–CH₂OH → R–CHO (PCC, PDC, or Swern oxidation; avoid over-oxidation to acid)
2. Rosenmund reduction: R–COCl + H₂/Pd-BaSO₄ (poisoned) → R–CHO (selective reduction of acid chloride)
3. Gattermann-Koch: Benzene + CO + HCl/AlCl₃ → benzaldehyde (formylation of aromatic rings)
4. Nef reaction: R–CH₂–NO₂ (primary nitroalkane) → R–CHO after hydrolysis with H₂SO₄/NaNO₂
5. From esters: DIBAL-H reduction at low temperature → aldehyde (special reagent; LiAlH₄ over-reduces to alcohol)
6. Oxidation of gem-diols: R–CH(OH)₂ → R–CHO (acid catalyzed)

Ketones:

1. Oxidation of secondary alcohols: R₂CHOH → R₂C=O (PCC, Jones reagent, CrO₃)
2. Friedel-Crafts acylation: R–C(=O)–Cl + ArH/AlCl₃ → aryl ketone (most reliable ketone synthesis)
3. Nef reaction for secondary: R₁R₂CH–NO₂ → R₁R₂C=O (ketone)
4. Hydrolysis of gem-dihalides: Ar–CCl₂–Ar’ → Ar–CO–Ar’ ( hydrolysis)
5. Oxidation of alkenes: Ozonolysis with reductive workup (Zn/H₂O): C=C → two C=O
6. From nitriles: R–CN + RMgX → ketimine → hydrolyze → ketone (Grignard on nitrile)

3. Important Named Reactions:

Nucleophilic Addition Mechanisms:

1. Addition of Grignard Reagents:

R–CHO + RMgX → R–CH(OMgX)R → H₂O/H⁺ → R–CH(OH)R (secondary alcohol)
HCHO + RMgX → R–CH₂OH (primary alcohol)
Ketone + RMgX → tertiary alcohol

⚡ Every Grignard addition to a carbonyl of known MW lets you determine the structure by working backwards. This is a classic JEE problem type: “A compound with MW 116 gives a tertiary alcohol of MW 134 on reaction with MeMgBr. What is the compound?”

2. Addition of HCN:

R–CHO + HCN ⇌ R–CH(OH)–CN (cyanohydrin)
Equilibrium lies left for ketones (steric hindrance); ketones with α-H form cyanohydrins slowly

⚡ Equilibrium constant for acetone cyanohydrin is much lower than for formaldehyde. HCN addition is reversible — important for equilibrium analysis questions.

3. Addition of NaHSO₃:

R–CHO + NaHSO₃ → R–CH(OH)–SO₃Na (bisulfite addition product)
Only works for: (1) aldehydes, (2) methyl ketones, (3) cyclic ketones < C₈
Reason: Steric hindrance blocks addition for bulkier ketones
Bisulfite adduct is water-soluble — used for purification of carbonyl compounds

4. Wittig Reaction:

R–CHO + Ph₃P=CH–R' → R–CH=CH–R' + Ph₃P=O
Phosphonium ylide reacts with carbonyl to give alkenes
This is stereospecific: cis/trans depends on stabilized/non-stabilized ylide

⚡ The Wittig reaction is particularly important for making alkenes that are hard to access by other means. Non-stabilized ylides (R₂C=PPh₃) give Z-alkenes; stabilized ylides (RCH=PPh₃COOEt) give E-alkenes predominantly.

Reduction Reactions:

1. Clemmensen Reduction (Zn-Hg/HCl):

R–CO–R' → R–CH₂–R' (methylene group replacement of carbonyl)
Reduces ketones to alkanes; C=O → CH₂
Cannot reduce esters, amides, nitro groups

2. Wolff-Kishner (N₂H₄/KOH, high T):

Same product as Clemmensen but works for base-sensitive compounds
Hydrazone formation → loss of N₂ → alkane

3. NaBH₄ Reduction:

R–CHO → 1° alcohol (mild, selective for carbonyl)
R₂C=O → 2° alcohol (mild, does not reduce esters, amides, carboxylic acids)

⚡ LiAlH₄ reduces everything (including esters) → over-reduction. NaBH₄ is milder → selective for aldehydes/ketones only.

4. MPV (Meerwein-Ponndorf-Verley):

Ketone + i-PrOH/Al(i-OPr)₃ → secondary alcohol + acetone
Equilibrium reached by distillation of acetone

Oxidation of Aldehydes:

1. Tollens’ Test:

R–CHO + 2[Ag(NH₃)₂]⁺ + OH⁻ → R–COO⁻ + 2Ag + 4NH₃ + H₂O
Silver mirror on test tube walls
Mechanism: Ag⁺ is reduced to Ag⁰; aldehyde is oxidized to carboxylate

⚡ The “silver mirror test” is qualitative — it confirms an aldehyde. Benedict’s/Fehling’s uses Cu²⁺ → Cu₂O (red).

2. Oxidative Cleavage (Ozonolysis):

CH₂=CH–R → O₃ → ozonide → reductive workup (Zn/MeOH or (CH₃)₂S) → formaldehyde + R–CHO
R–CH=CH–R' → H₂O → R–CHO + R'–CHO
Internal alkene cleavage → two carbonyl compounds

α-Haloform Reaction:

CH₃–CO–R + I₂/NaOH → CHI₃ (yellow precipitate) + R–COONa
Mechanism: Halogenation at α-carbon (3 successive halogenations) 
→ α-trihaloketone → hydroxide attacks carbonyl → haloform + carboxylate

⚡ Don’t confuse: only methyl ketones (CH₃–CO–) and acetaldehyde give this test. Diethyl ketone (CH₃CH₂–CO–CH₂CH₃) does NOT give haloform reaction because it has no CH₃–CO– group.

Aldol Condensation:

CH₃–CHO + CH₃–CHO → CH₃–CH(OH)–CH₂–CHO (aldol product)
By base: enolate of one attacks carbonyl of another
Aldol → (heat, acid or base) → α,β-unsaturated carbonyl compound
CH₃–CH(OH)–CH₂–CHO → Δ → CH₃–CH=CH–CHO (crotonaldehyde)

⚡ Crossed aldol: If both components have α-H, mixture of products forms (not useful). Use one component without α-H as electrophile (e.g., benzaldehyde) + one with α-H as nucleophile (acetaldehyde → product is cinnamaldehyde).

⚡ Claisen-Schmidt condensation: crossed aldol between aromatic aldehyde and aliphatic ketone → gives α,β-unsaturated aromatic ketone.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Aldehydes Ketones — Comprehensive Chemistry Notes

1. Detailed Mechanisms:

Nucleophilic Addition — General Mechanism:

Step 1: Nucleophile (Nu:) attacks carbonyl C (electrophilic center)
        → tetrahedral alkoxide intermediate
Step 2: The O⁻ gets protonated (by solvent or acid)
        → alcohol product
Rate: depends on electrophilicity of C=O and nucleophilicity of Nu⁻
Acid catalysis: Protonate O → C=O becomes more electrophilic

⚡ The addition is reversible for most nucleophiles. The equilibrium constant depends on steric and electronic factors. For HCN addition to acetone, Keq ≈ 20 (favors cyanohydrin but not overwhelmingly).

Enolization and α-Substitution:

R–CO–CH₂–R' ⇌ R–C(OH)=CH–R' (enol form)
In acidic conditions: enol gets protonated → iminium-like intermediate
In basic conditions: enolate forms, then reacts with electrophile (E⁺)

⚡ The α-haloform reaction proceeds through three rounds of α-halogenation → base-induced elimination of CX₃⁻ from carbonyl carbon → carboxylate + CHX₃.

Base-Catalyzed Aldol Mechanism:

Step 1: Base abstracts α-H → enolate anion (resonance stabilized: C=C–O⁻)
Step 2: Enolate attacks carbonyl of second aldehyde molecule → alkoxide
Step 3: Protonation → β-hydroxy aldehyde (aldol product)
Step 4 (condensation): Dehydration gives α,β-unsaturated carbonyl

⚡ For acetaldehyde: the aldol of 2 molecules is 3-hydroxybutanal → dehydration gives crotonaldehyde (CH₃–CH=CH–CHO).

Cannizzaro Reaction (Non-Enolizable Aldehydes):

2HCHO + OH⁻ → CH₃OH + HCOO⁻ (formaldehyde + base)
2C₆H₅CHO + OH⁻ → C₆H₅CH₂OH + C₆H₅COO⁻ (benzaldehyde)

⚡ ONLY aldehydes without α-H undergo Cannizzaro. This is because enolizable aldehydes form enolates preferentially (α-H is more acidic). Formaldehyde, benzaldehyde, and p-methoxybenzaldehyde are classic examples.

Mechanism of Cannizzaro:

Step 1: OH⁻ attacks one aldehyde carbonyl → tetrahedral intermediate
Step 2: Hydride transfer from the alkoxide to the second aldehyde molecule
        (hydride is the actual reducing agent — this is intramolecular within the encounter complex)
Step 3: Products: one molecule reduced (CH₂OH), one oxidized (COO⁻)

⚡ In crossed Cannizzaro (formaldehyde + acetaldehyde): formaldehyde is always the reducing agent (gets oxidized to formate), acetaldehyde is reduced to ethanol.

Tischenko Reaction:

2 ArCHO → ArCOOCH₂Ar (using Al(OR)₃ as catalyst)
Ester from two aldehyde molecules without α-H

2. Stereochemistry of Nucleophilic Addition:

Additions to Cyclic Ketones:

• Nucleophile can attack from either face of the planar carbonyl
• If substituent on ring makes one face more accessible → diastereomeric products form
• Examples: attack of MeMgBr on cyclohexanone → predominantly equatorial alcohol (trans product, more stable)

Stereospecificity in Wittig:

• Stabilized ylides (with –COOR, –CN, –COR attached to the ylide carbon) → E-alkenes
• Non-stabilized ylides (alkyl-substituted) → Z-alkenes
• Semi-stabilized ylides (aryl-substituted) → mixture

3. α,β-Unsaturated Carbonyl Compounds:

Michael Addition:

R–CO–CH=CH–R' + Enolate → R–CO–CH₂–CH(R')–Enolate partner (1,4-addition)
The enolate attacks the β-carbon (not the carbonyl carbon)
Conjugate addition (1,4-addition) vs direct addition (1,2-addition)

⚡ Gilman reagents (organocuprates) add 1,4 to enones. Grignards normally add 1,2. However, for α,β-unsaturated ketones with bulky groups at carbonyl, 1,4-addition can dominate. Organocuprates (R₂CuLi) are the classic Michael donor reagents.

Robinson Annulation:

Michael addition followed by intramolecular aldol condensation
Forms bicyclic α,β-unsaturated ketone systems

Example: Methyl vinyl ketone + cyclohexanone enolate → product → dehydration → bicyclic enone.

4. Synthetic Utility — Advanced Transformations:

Convert Acetone to Mesityl Oxide:

CH₃–CO–CH₃ + CH₃–CHO (aldol condensation) → CH₃–C(CH₃)=CH–CHO (mesityl oxide)
Further aldol → phorone → mesitylene oxidation → benzene derivatives

Benzoin Condensation:

2 PhCHO + CN⁻ → Ph–CH(OH)–C(O)–Ph (benzoin)
Mechanism: Cyanide attacks one benzaldehyde → carbanion on the other → coupling
Required: aromatic aldehyde + base + cyanide (no α-H)

⚡ Benzoin condensation is unique to aromatic aldehydes (or other non-enolizable aldehydes). The product benzoin can be oxidized to benzil (α-diketone) which with base gives benzilic acid rearrangement.

Benzilic Acid Rearrangement:

Ph–C(O)–C(O)–Ph (benzil) + OH⁻ → (Ph)₂C(COO⁻) (benzilate)
α-diketone + base → α-hydroxy acid (rearrangement)

⚡ This rearrangement involves migration of one aryl group to the adjacent carbonyl carbon — the group that migrates is typically the one that can better stabilize the developing positive charge in the transition state.

5. Quantitative Analysis:

Identification by Derivatives:

• 2,4-DNP: Aldehydes and ketones → 2,4-dinitrophenylhydrazone (yellow/orange crystals)
• Semicarbazide: Aldehydes and ketones → semicarbazone (crystalline derivatives)
• Hydroxylamine: Aldehydes and ketones → oxime
• All three are condensation reactions: R₂C=O + H₂N–Y → R₂C=NY + H₂O

Molecular Formula Problems (JEE favorite type):

Example: An organic compound (MW = 72) gives CHI₃ but negative Tollens' test.
Analysis:
- Positive CHI₃ → methyl ketone or acetaldehyde
- Negative Tollens' → NOT aldehyde → must be methyl ketone
- MW = 72: possible ketones
  - C₄H₈O (CH₃–CO–CH₂–CH₃ = butan-2-one, MW = 72) ✓
  - C₃H₄O (propenal, no CHI₃) ✗
  - C₄H₈O (cyclobutanone) ✓ (cyclic ketone with CH₃CO– equivalent)
Answer: Butan-2-one

⚡ JEE常常考: A compound with molecular formula C₅H₁₀O gives yellow precipitate with I₂/NaOH but negative Tollens’. This is 2-pentanone or 3-pentanone. Both are methyl ketones (CH₃–CO– is always terminal or internal if CH₃ is attached).

6. Environmental & Biological Significance:

• Formaldehyde: preservative, disinfectant, precursor to resins (Bakelite)
• Acetaldehyde: intermediate in metabolism (ethanol oxidation), toxic, carcinogenic
• Acetone: nail polish remover, solvent, ketone body (diabetic ketoacidosis)
• Benzaldehyde: bitter almond oil, flavoring agent, starting material for dyes
• Cinnamaldehyde: cinnamon flavor, α,β-unsaturated aromatic aldehyde
• Furfural: from agricultural waste (corn cobs), used to make furan derivatives
• Carvone: spearmint/caraway flavor, α,β-unsaturated ketone

Metabolism Connections:

• Glycolysis: glyceraldehyde-3-phosphate is an aldehyde
• β-oxidation of fatty acids produces acetyl-CoA → in liver converts to ketone bodies (acetoacetate, β-hydroxybutyrate, acetone)
• Formaldehyde is produced in significant amounts in the human body (one-carbon metabolism)

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