Class 12 Chemistry · Chapter 8
Short answer:
Ye chapter Class 12 Chemistry Part II ka Chapter 8 hai — organic chemistry ka sabse "reaction-heavy" chapter, isliye board me weight bhi acha milta hai. NCERT textbook me is chapter ke exercise me karib 20+ questions hain — nomenclature, preparation methods (oxidation, ozonolysis, Rosenmund reduction, Stephen reaction, Gattermann-Koch), physical properties (boiling point comparison H-bonding ki wajah se), aur phir chemical reactions ka bada hissa: nucleophilic addition mechanism, aldol condensation, Cannizzaro reaction, haloform (iodoform) test, oxidation-reduction, aur carboxylic acid ki acidity + substituent effect. Neeche sab kuch Hinglish me, exam-ready tareeke se cover kiya hai.
Carbonyl group (C=O) is chapter ka hero hai — aldehyde, ketone aur carboxylic acid teeno isi group se ban'te hain, bas attached groups alag hote hain. Aldehyde me carbonyl carbon ke saath ek H aur ek alkyl/aryl group hota hai (R-CHO), ketone me do alkyl/aryl groups (R-CO-R'), aur carboxylic acid me carbonyl ke saath -OH group hota hai (R-COOH). Ye teeno functional groups bahut reactive hain kyunki carbonyl carbon electron-deficient (electrophilic) hota hai — is wajah se poora chapter nucleophilic addition reactions ke around ghoomta hai. Board exam me is chapter se naming, conversions, distinguishing tests, aur mechanism-based questions bahut common hain.
Chapter 8 Summary — 5 Minute Revision
1. Nomenclature aur Structure
Aldehyde ka IUPAC suffix -al hota hai (e.g., CH3CHO = ethanal), aur ketone ka -one (e.g., CH3COCH3 = propan-2-one). Carboxylic acid ka suffix -oic acid hai (e.g., CH3COOH = ethanoic acid). Numbering hamesha carbonyl carbon (ya -COOH carbon) ko lowest number dene ke liye ki jaati hai — carboxylic acid me COOH carbon hi C1 maana jaata hai.
Carbonyl carbon sp2 hybridised hota hai, planar structure, bond angle ~120°. C=O bond me ek sigma aur ek pi bond hota hai. Oxygen electronegative hone ki wajah se carbonyl carbon par partial positive charge (δ+) aur oxygen par partial negative charge (δ−) hota hai — yehi resonance/polarity carbonyl compounds ki reactivity ka core reason hai.
2. Preparation Methods (Aldehydes aur Ketones)
- Oxidation of alcohols: 1° alcohol → aldehyde (PCC ya mild oxidising agent se, taaki further oxidation na ho carboxylic acid tak); 2° alcohol → ketone (acidified KMnO4/K2Cr2O7).
- Dehydrogenation of alcohols: alcohol vapours ko garam Cu catalyst (573 K) se pass karke.
- Ozonolysis of alkenes: alkene + O3 phir Zn/H2O (reductive workup) se C=C bond todkar do carbonyl compounds bante hain.
- Rosenmund reduction: acid chloride (RCOCl) ko H2 gas se Pd/BaSO4 (poisoned catalyst) ki presence me reduce karke aldehyde banate hain — BaSO4 catalyst ko "poison" karta hai taaki reaction aldehyde tak hi ruke, alcohol tak na jaaye.
- Stephen reaction: nitrile (R-CN) ko SnCl2/HCl se reduce karke imine banaya jaata hai, jo hydrolysis se aldehyde deta hai.
- Gattermann-Koch reaction: benzene + CO + HCl, anhydrous AlCl3/CuCl catalyst se — benzaldehyde banane ka industrial-type method.
- Ketones for benzene ring: Friedel-Crafts acylation (benzene + acid chloride, AlCl3 catalyst).
3. Physical Properties
Aldehydes/ketones me C=O polar hai isliye dipole-dipole attraction hoti hai, par inme N-H ya O-H nahi hota to ye khud aapas me H-bond nahi bana sakte. Isliye boiling point corresponding alcohols se kam hota hai lekin same molecular mass ke hydrocarbons/ethers se zyada hota hai. Lower members (HCHO, CH3CHO, acetone) water me miscible hain kyunki ye water ke saath H-bond bana sakte hain.
4. Chemical Reactions — Nucleophilic Addition Mechanism
Nucleophilic addition me carbonyl carbon electrophilic hota hai kyunki oxygen electronegative hai — isliye nucleophile carbon par attack karta hai. Mechanism: nucleophile (Nu⁻) carbonyl carbon par attack karta hai, C=O ka pi bond tootkar electron pair oxygen par chala jaata hai (alkoxide intermediate banta hai), phir proton (H⁺) us oxygen se attach hokar -OH group deta hai.
Reactivity order: HCHO > other aliphatic aldehydes > ketones (aryl aldehydes < alkyl aldehydes). Iski do wajahein hain — steric factor (bigger groups nucleophile ke attack ko block karte hain) aur electronic factor (alkyl groups +I effect se electron density carbonyl carbon par badhate hain, jisse electrophilicity kam ho jaati hai). Isliye HCHO sabse reactive hai (do H hi hain, koi bulky group nahi) aur ketones sabse kam reactive (do alkyl groups, dono steric aur electronic hindrance).
5. Aldol Condensation
Jin aldehydes/ketones me α-hydrogen hota hai, unme dilute base (NaOH) ya acid catalyst se do molecules aapas me react karke β-hydroxy aldehyde/ketone (aldol) banate hain. Ye aldol garam karne par water lose karke α,β-unsaturated carbonyl compound deta hai — isko aldol condensation kehte hain.
2CH3CHO --dil. NaOH--> CH3CH(OH)CH2CHO (aldol) --Δ, -H2O--> CH3CH=CHCHO (crotonaldehyde)
Do alag aldehydes/ketones ke beech hone waala aldol condensation crossed aldol condensation kehlata hai, jisme multiple products ban sakte hain agar dono me α-H ho.
6. Cannizzaro Reaction
Jin aldehydes me α-hydrogen nahi hota (jaise HCHO, benzaldehyde, aur other aromatic aldehydes ya trisubstituted aldehydes), unke do molecules concentrated NaOH/KOH ke saath react karke self oxidation-reduction se ek molecule alcohol me reduce hota hai aur doosra carboxylate salt me oxidise hota hai.
2HCHO --conc. NaOH--> CH3OH + HCOONa
Ye reaction sirf un aldehydes ke saath hota hai jinme alpha-hydrogen absent ho — kyunki agar alpha-H present ho to base uske saath aldol condensation pehle kara degi, Cannizzaro nahi ho paayega.
7. Haloform (Iodoform) Reaction
Jin carbonyl compounds me CH3CO- group (methyl ketone) ho, ya jo alcohols me CH3CH(OH)- group ho (kyunki oxidation se pehle CH3CO- ban jaata hai), unka I2/NaOH ke saath reaction karne par yellow precipitate iodoform (CHI3) banta hai. Ye test methyl ketones aur ethanol/secondary alcohols with CH3CH(OH)- group ko distinguish karne ke liye use hota hai. Formaldehyde, aur other aldehydes/ketones jinme ye group nahi hai (jaise benzaldehyde, acetone ke alawa other ketones), ye test negative dete hain — sirf ethanal (CH3CHO) aldehydes me se iodoform positive deta hai kyunki usme bhi CH3CO- group hai.
8. Distinguishing Tests
- Tollens' test: ammoniacal AgNO3 (Tollens' reagent) ke saath warm karne par aldehydes silver mirror bana dete hain (Ag⁺ → Ag). Ketones is test me react nahi karte.
- Fehling's test: Fehling's solution (Cu2+ tartrate complex) ke saath garam karne par aliphatic aldehydes reddish-brown precipitate (Cu2O) dete hain. Aromatic aldehydes (jaise benzaldehyde) Fehling's test nahi dete. Ketones bhi negative dete hain.
- Schiff's test: Schiff's reagent (decolourised fuchsin dye) aldehydes ke saath pink-magenta colour deta hai; ketones is test me generally negative rehte hain.
9. Oxidation aur Reduction
Aldehydes mild oxidising agents (Tollens', Fehling's) se hi carboxylic acid me oxidise ho jaate hain, jabki ketones sirf strong oxidising agents (jaise hot KMnO4) se hi C-C bond todkar oxidise hote hain (jisse chhote acids ka mixture banta hai). Reduction: aldehyde/ketone → alcohol NaBH4/LiAlH4 ya H2/Ni (catalytic hydrogenation) se hota hai. Clemmensen reduction (Zn-Hg/conc. HCl) aur Wolff-Kishner reduction (NH2NH2/KOH) se C=O poori tarah CH2 me reduce ho jaata hai (complete removal of carbonyl oxygen).
10. Carboxylic Acids — Preparation aur Acidity
Preparation: primary alcohols/aldehydes ka oxidation, nitriles/amides ka hydrolysis, Grignard reagent + CO2 phir hydrolysis.
Carboxylic acid acidic hota hai kyunki ionisation ke baad bana carboxylate ion resonance se stabilise ho jaata hai (dono C-O bonds equivalent ho jaate hain). Substituent effect on acidity: electron-withdrawing groups (EWG jaise -Cl, -NO2, -F) carboxylate ion ko aur stabilise karte hain (−I effect se negative charge disperse hota hai) — isliye acidity badhaate hain. Electron-donating groups (EDG jaise -CH3, alkyl groups, +I effect) negative charge ko concentrate karte hain, destabilise karte hain — isliye acidity ghataate hain.
Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > HCOOH > CH3COOH
Formic acid (HCOOH) acetic acid se zyada strong hai kyunki usme koi electron-donating alkyl group nahi hai. Halogen substituent jitna carboxyl group ke paas (α-position) hoga, uska −I effect utna strong hoga aur acid utna zyada strong hoga (distance badhne par effect kam ho jaata hai).
Poore Class 12 Chemistry ke handwritten colour notes
IITian & district toppers ke banaye short notes — revision-ready, diagram ke saath. Board se pehle poora syllabus 3 din me revise.
Exercise Questions — Solutions (Q1–Q20)
Q1. Ethanal ka IUPAC naam aur usse HCHO ka IUPAC naam likhiye. In dono me electrophilic addition ki reactivity compare kijiye.
Ethanal = CH3CHO (IUPAC naam ethanal hi hai). HCHO ka IUPAC naam methanal hai.
Nucleophilic addition me methanal (HCHO) zyada reactive hai ethanal se kyunki HCHO me sirf do H atoms hain (koi steric hindrance nahi, koi +I effect wala alkyl group bhi nahi), jabki ethanal me ek CH3 group hai jo +I effect se carbonyl carbon ki electrophilicity kam karta hai aur thoda steric hindrance bhi deta hai.
Q2. CH3-CH2-CO-CH3 ka IUPAC naam likhiye aur uski structure banaiye.
IUPAC naam: Butan-2-one (numbering aise ki carbonyl carbon ko lowest number mile — CH3-CO-CH2-CH3 ko C1 se C4 number karne par carbonyl C2 par aata hai).
CH3-CO-CH2-CH3
Q3. Rosenmund reduction se aldehyde kaise banaya jaata hai? Iska mechanism/importance samjhaiye.
Acid chloride (R-COCl) ko H2 gas ke saath Pd catalyst (jo BaSO4 se poisoned hota hai) ki presence me controlled reduction karte hain.
R-COCl + H2 --Pd/BaSO4--> R-CHO + HCl
BaSO4 catalyst ki activity kam kar deta hai (poison karta hai) taaki reduction sirf aldehyde tak ruke — agar catalyst poison na kiya jaaye to further reduction hokar alcohol tak ban jaata.
Q4. Gattermann-Koch reaction se benzaldehyde kaise banta hai?
Benzene ko CO aur HCl gas ke saath anhydrous AlCl3 (ya CuCl) catalyst ki presence me react karaya jaata hai — high pressure pe.
C6H6 + CO + HCl --anhyd. AlCl3/CuCl--> C6H5CHO + HCl
Q5. Aldehydes aur ketones ka boiling point corresponding alcohols se kam kyun hota hai?
Aldehyde/ketone me carbonyl group polar hai (dipole-dipole interaction hoti hai) lekin inme N-H ya O-H bond nahi hota, isliye molecules aapas me intermolecular hydrogen bonding nahi bana sakte. Alcohols me -OH group hone ki wajah se strong intermolecular H-bonding hoti hai, isliye unka boiling point aldehyde/ketone se zyada hota hai (same carbon count par).
Q6. Nucleophilic addition reaction ka mechanism samjhaiye (HCN ke example se).
Carbonyl carbon electrophilic hota hai (oxygen ke electronegativity ki wajah se δ+ carbon, δ− oxygen). CN⁻ nucleophile carbonyl carbon par attack karta hai, C=O ka π bond tootkar electrons oxygen par chale jaate hain — alkoxide intermediate banta hai. Phir ye intermediate H+ (HCN se ya medium se) le kar cyanohydrin (-OH group wala product) banata hai.
R2C=O + CN- → R2C(O-)CN --H+--> R2C(OH)CN
Q7. Aldehydes, ketones ki reactivity nucleophilic addition ki taraf kis order me hoti hai aur kyun?
Order: HCHO > other aliphatic aldehydes > ketones (aur cyclic/aromatic ketones sabse kam).
Do reasons: (1) Steric factor — bade alkyl/aryl groups nucleophile ke attack ko block karte hain, ketones me do bulky groups hote hain isliye sabse kam reactive. (2) Electronic factor — alkyl groups +I (electron donating) hote hain jo carbonyl carbon ka positive charge kam kar dete hain, jisse electrophilicity aur reactivity dono kam ho jaati hai. Ketones me do alkyl groups honese ye effect double ho jaata hai.
Q8. Aldol condensation kise kehte hain? Ethanal se ise samjhaiye.
Jin aldehydes/ketones me α-hydrogen ho, unke do molecules dilute base (NaOH) ki presence me react karke β-hydroxy aldehyde/ketone (aldol) banate hain, jo garam karne par dehydrate hokar α,β-unsaturated carbonyl compound deta hai — isko aldol condensation kehte hain.
2CH3CHO --dil. NaOH--> CH3CH(OH)CH2CHO --Δ, -H2O--> CH3CH=CHCHO
Q9. Cannizzaro reaction kya hai? Kaunse substrates ye reaction dete hain aur kyun?
Cannizzaro reaction ek self oxidation-reduction reaction hai jo un aldehydes me hota hai jinme α-hydrogen nahi hota (jaise HCHO, benzaldehyde, other aromatic ya highly substituted aldehydes bina α-H ke). Concentrated NaOH/KOH ki presence me do molecules react karke ek alcohol me reduce hota hai aur doosra carboxylate salt me oxidise hota hai.
2HCHO --conc. NaOH--> CH3OH + HCOONa
Wajah: agar α-hydrogen present ho, to strong base pehle us α-H ko remove karke enolate/aldol pathway follow karegi (aldol condensation), Cannizzaro nahi ho payega. Isliye ye reaction sirf α-H-free aldehydes tak limited hai.
Q10. Iodoform test kya hota hai? Ye kin compounds ke saath positive aata hai?
Iodoform test me compound ko I2 aur NaOH ke saath treat karte hain. Positive test me yellow precipitate CHI3 (iodoform) banta hai.
Positive dene waale compounds: jin me CH3CO- group ho (methyl ketones — jaise acetone, methyl ethyl ketone) ya jin alcohols me CH3CH(OH)- group ho (jaise ethanol, isopropanol) kyunki ye oxidise hokar CH3CO- group bana lete hain. Aldehydes me sirf ethanal (CH3CHO) positive deta hai, HCHO ya other aldehydes bina CH3CO- group ke negative dete hain.
Q11. Tollens' test aur Fehling's test se aldehyde aur ketone ko kaise distinguish karte hain? Aromatic aldehyde ka special behaviour bhi likhiye.
Tollens' test: ammoniacal AgNO3 ke saath warm karne par aldehydes (aliphatic aur aromatic dono) silver mirror bana dete hain. Ketones react nahi karte.
Fehling's test: Fehling's solution ke saath garam karne par sirf aliphatic aldehydes reddish-brown Cu2O precipitate dete hain. Aromatic aldehydes (jaise benzaldehyde) Fehling's test negative dete hain — ye ek exception hai jo exam me common trick point hai. Ketones bhi is test me negative rehte hain.
Q12. Clemmensen reduction aur Wolff-Kishner reduction me kya farak hai?
Dono reactions carbonyl group (C=O) ko poori tarah CH2 me convert karte hain (complete reduction), par conditions alag hain:
- Clemmensen reduction: Zn-Hg amalgam + conc. HCl — acidic medium me hota hai, isliye acid-sensitive groups wale compounds ke liye suitable nahi.
- Wolff-Kishner reduction: NH2NH2 (hydrazine) + KOH, high boiling solvent (jaise ethylene glycol) me garam karke — basic/alkaline medium me hota hai, isliye base-sensitive groups wale compounds ke liye suitable nahi.
Q13. Carboxylic acid alcohols aur phenols se zyada acidic kyun hota hai?
Carboxylic acid ionise hokar carboxylate ion (RCOO-) banata hai, jisme negative charge dono oxygen atoms par resonance se equally distribute ho jaata hai (dono C-O bonds equivalent ban jaate hain) — ye extra stabilisation deta hai. Alcohol ke conjugate base (alkoxide) ya phenol ke conjugate base (phenoxide) me itni effective resonance stabilisation nahi hoti (phenoxide me kuch resonance hai par carboxylate se kam), isliye carboxylic acid zyada readily H+ deta hai — zyada acidic hai.
Q14. Chloroacetic acid (ClCH2COOH) acetic acid se zyada strong acid kyun hai?
Chlorine electron-withdrawing group (−I effect) hai. Ye carboxylate ion (ClCH2COO-) ke negative charge ko apni taraf khींchकर disperse/stabilise karta hai, jisse ionisation aasan ho jaata hai aur acid strength badh jaati hai. Acetic acid me CH3 group electron-donating (+I) hai jo negative charge ko concentrate karke destabilise karta hai — isliye kam acidic.
Q15. In acids ko badhte hue acidic strength ke order me arrange kijiye: CH3COOH, ClCH2COOH, Cl2CHCOOH, Cl3CCOOH.
Increasing order: CH3COOH < ClCH2COOH < Cl2CHCOOH < Cl3CCOOH
Jitne zyada Cl atoms (electron-withdrawing, −I effect), utni zyada carboxylate ion ki stability, utni zyada acid strength.
Q16. Formic acid (HCOOH) acetic acid (CH3COOH) se zyada strong acid kyun hai?
Formic acid me carboxyl carbon ke saath H atom hai (koi electron-donating group nahi), jabki acetic acid me CH3 group hai jo +I (electron-donating) effect deta hai — ye carboxylate ion ke negative charge ko destabilise karta hai. Isliye HCOOH zyada aasani se ionise hota hai aur CH3COOH se zyada strong acid hai.
Q17. Ethanol, ethanal aur ethanoic acid ko unke boiling points ke increasing order me arrange kijiye, wajah sahit.
Increasing order: CH3CHO < CH3CH2OH < CH3COOH
Ethanal me sirf dipole-dipole interaction hai (H-bonding nahi), isliye boiling point sabse kam. Ethanol me O-H group hone se H-bonding hai. Carboxylic acid molecules dimer bana lete hain (do strong H-bonds ek saath), isliye ethanoic acid ka boiling point sabse zyada hai.
Q18. Benzaldehyde ko Fehling's test se distinguish nahi kiya ja sakta — to ise acetaldehyde se kaise distinguish karenge?
Tollens' test dono ke saath positive deta hai (dono aldehyde hain), isliye Tollens' se distinguish nahi ho sakta. Iske jagah Fehling's test use karte hain: acetaldehyde (aliphatic) positive deta hai (reddish-brown Cu2O precipitate), jabki benzaldehyde (aromatic) is test me negative rehta hai. Iodoform test bhi use ho sakta hai — acetaldehyde positive (CH3CO- group hai), benzaldehyde negative.
Q19. Ozonolysis se aldehyde/ketone kaise banaya jaata hai? Ek example dijiye.
Alkene ko ozone (O3) se treat karke ozonide banaya jaata hai, phir usko Zn dust/H2O (reductive workup) se hydrolyse karte hain — C=C bond tootkar do carbonyl compounds bante hain.
CH3-CH=CH-CH3 --(i) O3 (ii) Zn/H2O--> 2 CH3CHO
Q20. Ketone ko strong oxidising agent se oxidise karne par kya hota hai, aldehyde se kaise alag hai?
Aldehydes mild oxidising agents (Tollens', Fehling's) se hi aasani se carboxylic acid me oxidise ho jaate hain (kyunki carbonyl carbon par H attached hota hai jo remove hokar OH ban sakta hai). Ketones me carbonyl carbon ke dono taraf alkyl groups hote hain, koi H nahi — isliye ketones sirf strong oxidising agents (jaise hot KMnO4) se hi oxidise hote hain, aur us process me C-C bond todna padta hai jisse chhote carboxylic acids ka mixture banta hai.
Important Equations — Ek Nazar Me
| Test / Reaction | Positive with | Negative with / Note |
|---|---|---|
| Tollens' test | Aliphatic + aromatic aldehydes (silver mirror) | Ketones — no reaction |
| Fehling's test | Aliphatic aldehydes only (red Cu2O ppt) | Aromatic aldehydes aur ketones — no reaction |
| Iodoform test | CH3CO- group (methyl ketones) ya CH3CH(OH)- group waale alcohols | HCHO aur other aldehydes/ketones bina is group ke — negative |
| Cannizzaro reaction | Aldehydes bina α-hydrogen ke (HCHO, benzaldehyde etc.) | Alpha-H waale aldehydes — aldol condensation hoga, Cannizzaro nahi |
| Aldol condensation | Aldehydes/ketones jisme α-hydrogen present ho | Bina α-H waale carbonyl compounds — nahi hoga |
| Nucleophilic addition reactivity | HCHO > aliphatic aldehydes > ketones | Steric + electronic (+I) factors dono contribute karte hain |
| Acidity of carboxylic acids | EWG substituent (Cl, NO2, F) badhata hai | EDG substituent (alkyl, +I) ghataata hai |
| Clemmensen reduction | Zn-Hg / conc. HCl — acidic medium, C=O → CH2 | Acid-sensitive substrates ke liye unsuitable |
| Wolff-Kishner reduction | NH2NH2 / KOH — basic medium, C=O → CH2 | Base-sensitive substrates ke liye unsuitable |
| Rosenmund reduction | RCOCl + H2 / Pd-BaSO4 → RCHO | BaSO4 catalyst ko poison karta hai, aldehyde tak hi rukta hai |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Fehling's test har aldehyde ke saath positive maan lena. Yaad rakho — aromatic aldehydes (jaise benzaldehyde) Fehling's test NEGATIVE dete hain, sirf aliphatic aldehydes positive dete hain. Tollens' test dono ke saath positive hota hai.
- Cannizzaro reaction har aldehyde ke saath likh dena. Ye reaction sirf un aldehydes me hota hai jinme α-hydrogen NAHI hota (HCHO, benzaldehyde). Agar substrate me α-H ho (jaise CH3CHO), to woh aldol condensation karega, Cannizzaro nahi.
- Iodoform test ko sirf ketones ke saath jodna. Ye test un compounds ke saath positive hota hai jisme CH3CO- group ho (chahe ketone ho) YA CH3CH(OH)- group ho (secondary/ethanol type alcohol). Ethanal (aldehyde) bhi positive deta hai — ye bhool jaana common mistake hai.
- Nucleophilic addition ko electrophilic addition samajh lena. Carbonyl compound me carbon electron-deficient hota hai (electrophilic centre), isliye Nu⁻ uspar attack karta hai — ye nucleophilic addition hai, electrophilic addition nahi (jo alkenes me hota hai). Reagent (jaise HCN, NaHSO3) nucleophile hai, substrate nahi.
- Substituted acetic acids ki acidity order galat likhna. Zyada electronegative/electron-withdrawing substituent aur carboxyl ke jitna close ho, acidity utni zyada — Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH. Reverse order likhna ya formic acid ko acetic acid se kamzor bata dena dono galat hain.
- Aldol condensation aur aldol addition ko same maan lena. Base ke saath sirf addition hokar β-hydroxy carbonyl (aldol) banna 'aldol addition' hai. Usko garam karke dehydrate karne (water lose karke α,β-unsaturated compound banane) ka step hi 'aldol condensation' kehlata hai — pura process likhte waqt dono steps clearly dikhao.
Board-Style Important Questions
- 1 mark: Ethanal ka structural formula aur IUPAC naam likhiye.
- 2 marks: Tollens' test aur Fehling's test me kya farak hai? Benzaldehyde in dono tests me kaisa behave karega?
- 2 marks: Iodoform test kya hai aur ye kin compounds ke liye positive aata hai?
- 3 marks: Cannizzaro reaction ka mechanism ek example (HCHO) ke saath samjhaiye. Ye reaction kis type ke substrates ke saath hoti hai aur kyun?
- 3 marks: Nucleophilic addition me aldehydes ketones se zyada reactive kyun hote hain? Steric aur electronic dono factors explain kijiye.
- 5 marks: Carboxylic acid ki acidity par substituent effect samjhaiye. Chloroacetic acid, dichloroacetic acid, trichloroacetic acid aur acetic acid ko increasing acidic strength ke order me arrange karke reason dijiye.
Aksar Poochhe Jaane Wale Sawaal
Aldehyde aur ketone me main farak kya hai?
Aldehyde me carbonyl carbon ke saath ek H aur ek alkyl/aryl group attached hota hai (R-CHO). Ketone me carbonyl carbon ke dono taraf alkyl/aryl groups hote hain, koi H nahi (R-CO-R'). Isi wajah se aldehydes zyada reactive hote hain nucleophilic addition me aur mild oxidising agents (Tollens', Fehling's) se oxidise ho jaate hain, ketones nahi.
Cannizzaro reaction sirf kin aldehydes ke saath hoti hai?
Sirf un aldehydes ke saath jinme alpha-hydrogen nahi hota — jaise HCHO (formaldehyde), benzaldehyde, aur other aromatic ya highly substituted aldehydes. Agar alpha-H present ho to base pehle aldol condensation karayegi, Cannizzaro reaction nahi ho paayegi.
Iodoform test se kya distinguish kar sakte hain?
Iodoform test un compounds ko identify karta hai jinme CH3CO- group ho (methyl ketones) ya CH3CH(OH)- group ho (jaise ethanol, isopropanol). Ye ethanol ko methanol se, ya acetone ko other ketones se distinguish karne me useful hai.
Fehling's test aromatic aldehyde ke saath kyun negative aata hai?
Fehling's test ka mechanism ek specific oxidation pathway follow karta hai jo aliphatic aldehydes ke saath hi effectively kaam karta hai. Aromatic aldehydes (jaise benzaldehyde) is reaction condition me react nahi karte, isliye unke saath Fehling's test negative aata hai — ye NCERT me explicitly mentioned exception hai.
Formic acid acetic acid se zyada strong kyun hai jabki dono carboxylic acid hain?
Formic acid (HCOOH) me carboxyl carbon ke saath sirf H hai, koi electron-donating group nahi. Acetic acid (CH3COOH) me CH3 group hai jo +I effect se electron density carboxylate ion par badha deta hai, jisse negative charge destabilise hota hai aur acid weaker ho jaata hai. Isliye HCOOH zyada strong acid hai.
Clemmensen aur Wolff-Kishner reduction kab use karte hain?
Dono C=O ko CH2 me completely reduce karte hain, par medium alag hai. Agar substrate acid-sensitive hai (jaise usme koi acid-labile group hai), to Wolff-Kishner (basic medium) use karte hain. Agar substrate base-sensitive hai, to Clemmensen (acidic medium) use karte hain.
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