Questions: Aldehydes, Ketones and Carboxylic Acids

Q1. Why are aldehydes more reactive than ketones towards nucleophilic addition?

Answer (English): Two reasons work together. A ketone has two alkyl groups on the carbonyl carbon, and those groups crowd the carbon so an incoming nucleophile has less room; an aldehyde has one carbon group and one small hydrogen. The same two alkyl groups also push electron density towards the carbonyl carbon through the inductive effect, which reduces the partial positive charge that the nucleophile attacks. Aromatic aldehydes are slower still because the ring is conjugated with the carbonyl and spreads the charge.

Hinglish: Do wajah hain. Ketone mein carbonyl carbon par do alkyl groups hoti hain, jo jagah gher leti hain aur nucleophile ko aane mein rukawat deti hain; aldehyde mein ek carbon group aur ek chhota hydrogen hota hai. Wahi alkyl groups inductive effect se electron density carbonyl carbon ki taraf bhejti hain, jisse positive charge kam ho jata hai. Aromatic aldehyde aur dheema hota hai kyunki ring conjugation charge ko phaila deti hai.

Q2. Write the mechanism of nucleophilic addition to a carbonyl group.

Answer (English): The carbonyl carbon is electron poor, so the nucleophile approaches it. In step 1 the nucleophile's lone pair forms a bond with the carbon while the pi electrons of the carbon oxygen double bond move onto oxygen, which gives a tetrahedral alkoxide intermediate. In step 2 the negatively charged oxygen picks up a proton from the medium and the addition product results. In acidic conditions the oxygen is protonated first, which makes the carbon more electrophilic and speeds up step 1.

Hinglish: Carbonyl carbon electron poor hota hai, isliye nucleophile wahin aata hai. Step 1 mein nucleophile ka lone pair carbon se bond banata hai aur C=O ke pi electrons oxygen par chale jate hain, jisse tetrahedral alkoxide intermediate banta hai. Step 2 mein oxygen proton lekar neutral ho jata hai aur addition product ban jata hai. Acidic medium mein pehle oxygen protonate hota hai, isse carbon zyada electrophilic ban jata hai aur attack tez hota hai.

Q3. What is the aldol reaction and which compounds cannot undergo it?

Answer (English): Dilute sodium hydroxide removes an alpha hydrogen from one carbonyl molecule to give an enolate ion. That enolate attacks the carbonyl carbon of a second molecule, and the product is a beta hydroxy aldehyde or ketone. On heating it loses water and gives an alpha, beta unsaturated carbonyl compound. A compound can only start this reaction if it has at least one alpha hydrogen. Methanal has none, benzaldehyde has none, and so neither can act as the nucleophile. Benzaldehyde can still be attacked by the enolate of another compound, as in the crossed aldol reaction.

Hinglish: Dilute NaOH ek carbonyl molecule se alpha hydrogen nikaal kar enolate banata hai. Woh enolate doosre molecule ke carbonyl carbon par attack karta hai aur beta hydroxy aldehyde ya ketone banta hai. Garam karne par paani nikal kar alpha, beta unsaturated carbonyl milta hai. Jis compound mein alpha hydrogen hi nahi hota, woh yeh reaction start nahi kar sakta; methanal aur benzaldehyde mein alpha hydrogen nahi hota.

Q4. What is the Cannizzaro reaction? Give an example.

Answer (English): An aldehyde without an alpha hydrogen reacts with concentrated sodium hydroxide, and two molecules of the aldehyde react together. One molecule is oxidised to the carboxylate salt and the other is reduced to the alcohol. This disproportionation is the Cannizzaro reaction. Benzaldehyde with concentrated NaOH gives sodium benzoate and benzyl alcohol. Methanal gives sodium methanoate and methanol. The mechanism starts with hydroxide attacking the carbonyl carbon, and the intermediate then transfers a hydride ion to a second aldehyde molecule, which is why one molecule is oxidised as the other is reduced.

Hinglish: Jis aldehyde mein alpha hydrogen nahi hota, woh concentrated NaOH ke saath do molecule milkar react karta hai. Ek molecule carboxylate ban kar oxidise ho jata hai aur doosra alcohol ban kar reduce ho jata hai. Benzaldehyde se sodium benzoate aur benzyl alcohol bante hain, methanal se sodium methanoate aur methanol. Mechanism mein hydroxide attack karta hai aur intermediate ek hydride ion doosre molecule ko deta hai.

Q5. How do you distinguish an aldehyde from a ketone?

Answer (English): Use a mild oxidising agent that only aldehydes can reduce. Tollens reagent, which is ammoniacal silver nitrate, gives a silver mirror with an aldehyde and no change with a ketone. Fehling solution gives a red precipitate of copper(I) oxide with an aliphatic aldehyde; aromatic aldehydes and ketones do not respond. A third route is the iodoform test, which is positive for ethanal and for methyl ketones but not for other aldehydes or ketones, so it separates some pairs rather than the whole family.

Hinglish: Aisa mild oxidising agent lo jo sirf aldehyde reduce kar sake. Tollens reagent aldehyde ke saath silver mirror deta hai aur ketone ke saath kuch nahi. Fehling solution aliphatic aldehyde ke saath red precipitate deta hai. Iodoform test ethanal aur methyl ketone ke liye positive hota hai, isliye kuch jodon ko alag kar sakta hai.

Q6. Why is trichloroethanoic acid a much stronger acid than ethanoic acid?

Answer (English): Acid strength depends on how stable the anion left after losing a proton is. The three chlorine atoms are strongly electronegative and pull electron density away from the carboxylate through the inductive effect. That withdrawal spreads and stabilises the negative charge over the oxygen atoms and the carbon chain, so the carboxylate is easier to form. The measured pKa values show the effect: ethanoic acid 4.76, chloroethanoic acid 2.86, dichloroethanoic acid 1.48 and trichloroethanoic acid 0.64.

Hinglish: Acid ki strength is baat par hai ki proton nikalne ke baad banne wala anion kitna stable hai. Teen chlorine atoms electronegative hain aur inductive effect se electron density carboxylate se kheench lete hain. Isse negative charge oxygen atoms aur chain par phail kar stabilise hota hai. pKa values: ethanoic 4.76, chloroethanoic 2.86, dichloroethanoic 1.48, trichloroethanoic 0.64.

Q7. What happens when a carboxylic acid is heated with soda lime?

Answer (English): Soda lime is a mixture of sodium hydroxide and calcium oxide, and it removes carbon dioxide from the sodium salt of the acid, so the product is an alkane with one carbon fewer. Sodium ethanoate with soda lime gives methane, and sodium propanoate gives ethane. The reaction is a decarboxylation, and it is useful for stepping down the carbon chain. The salt must be dry, and the mixture is heated strongly.

Hinglish: Soda lime sodium hydroxide aur calcium oxide ka mixture hai, jo acid ke sodium salt se carbon dioxide nikaal deta hai. Product ek carbon kam wala alkane hota hai; sodium ethanoate se methane aur sodium propanoate se ethane banta hai. Ye decarboxylation hai aur chain ko ek carbon chhota karne ke liye kaam aata hai.

Q8. What are the Clemmensen and Wolff Kishner reductions, and when is each used?

Answer (English): Both replace a carbonyl group by CH2. Clemmensen reduction uses zinc amalgam with concentrated hydrochloric acid, and Wolff Kishner reduction uses hydrazine with potassium hydroxide in ethylene glycol at high temperature. The choice depends on the rest of the molecule. Clemmensen conditions are strongly acidic, so a group that is acid sensitive, such as an ester or a ketal, can be hydrolysed; Wolff Kishner conditions are strongly basic, so a base sensitive group is at risk. A chemist picks the one that leaves the other functional groups alone.

Hinglish: Dono C=O ko CH2 bana dete hain. Clemmensen mein zinc amalgam aur concentrated HCl, Wolff Kishner mein hydrazine aur KOH ethylene glycol mein garam karte hain. Chunav molecule ke baaki groups par hai: Clemmensen acidic hai isliye acid sensitive group kharab ho sakta hai, aur Wolff Kishner basic hai isliye base sensitive group par asar pad sakta hai.

Q9. Why is the alpha hydrogen of an aldehyde or ketone acidic?

Answer (English): The alpha carbon sits next to the carbonyl carbon. When a base removes an alpha hydrogen, the negative charge left on that carbon is delocalised into the carbon oxygen pi system, so the conjugate base, called an enolate, is resonance stabilised. A stabilised conjugate base means a stronger acid, and the pKa of an alpha hydrogen is low enough for dilute hydroxide to remove it. That is why aldol condensation and the haloform reaction both begin with the removal of an alpha hydrogen.

Hinglish: Alpha carbon carbonyl carbon ke bilkul paas hota hai. Jab base alpha hydrogen nikaalta hai, us carbon par bacha negative charge C=O ke pi system mein delocalise ho jata hai, isliye enolate resonance se stabilise hota hai. Stable conjugate base ka matlab zyada acidic hydrogen, aur pKa itna kam hota hai ki dilute hydroxide bhi hydrogen nikaal leta hai. Isi wajah se aldol aur iodoform reactions alpha hydrogen nikaalne se shuru hote hain.

Q10. How are aldehydes and ketones prepared from alcohols and alkynes?

Answer (English): From alcohols, the product depends on the class. A primary alcohol is oxidised to an aldehyde with pyridinium chlorochromate or with chromium trioxide in an anhydrous medium, which stops before the acid. A secondary alcohol gives a ketone with chromic acid or chromium trioxide. Passing the vapour of either alcohol over heated copper at 573 K dehydrogenates it to the same carbonyl compound. From alkynes, hydration with dilute sulphuric acid and mercury(II) sulphate adds water across the triple bond; ethyne gives ethanal, and a substituted alkyne gives a ketone because the OH forms on the more substituted carbon and then tautomerises to the carbonyl.

Hinglish: Alcohol se product class par depend karta hai. Primary alcohol se PCC ya anhydrous CrO3 se aldehyde milta hai jo acid tak nahi jata. Secondary alcohol se chromic acid ya CrO3 se ketone. Alcohol vapour ko Cu at 573 K par pass karne se dehydrogenation hota hai. Alkyne mein dilute H2SO4 aur HgSO4 se paani judta hai; ethyne se ethanal milta hai aur substituted alkyne se ketone banta hai kyunki OH zyada substituted carbon par banta hai aur phir carbonyl mein tautomerise ho jata hai.

Quiz below: five questions on reactivity, the addition mechanism, named reactions, tests and acidity. Revise the mechanism page before you begin.