Aldehyde, Ketone& Carboxylic Acid flash cards

1. 1.What is the functional group present in both aldehydes and ketones?

   The carbonyl group $>\text{C}=\text{O}$, in which carbon is $sp^2$ hybridised and doubly bonded to oxygen.

   Hint: Same group, different neighbours.

2. 2.How do aldehydes and ketones differ in the groups attached to the carbonyl carbon?

   In an aldehyde the carbonyl carbon bears at least one H ($\text{R}-\text{CHO}$); in a ketone it bears two carbon groups ($\text{R}-\text{CO}-\text{R}'$).

   Hint: Aldehyde has a C–H on carbonyl.

3. 3.Why is the carbonyl carbon electrophilic?

   Oxygen is more electronegative than carbon, so the $\pi$ electrons are pulled toward O. The carbon becomes electron-poor ($\delta+$) and O becomes $\delta-$, making C open to nucleophilic attack.

   Hint: O hogs the electrons.

4. 4.What is the shape and bond angle around the carbonyl carbon?

   Trigonal planar, with bond angles close to $120^\circ$ due to $sp^2$ hybridisation.

   Hint: Three groups, flat.

5. 5.Why do aldehydes and ketones have higher boiling points than hydrocarbons of comparable mass?

   The polar carbonyl gives rise to dipole–dipole interactions, which are stronger than the van der Waals forces in nonpolar hydrocarbons.

   Hint: Polar molecules stick together more.

6. 6.Why are lower aldehydes and ketones soluble in water?

   Their carbonyl oxygen forms hydrogen bonds with water molecules. Solubility falls as the hydrophobic carbon chain lengthens.

   Hint: H-bonding to water, until the tail gets too long.

7. 7.Why do aldehydes and ketones have lower boiling points than corresponding alcohols?

   Alcohols engage in intermolecular hydrogen bonding (O–H), which aldehydes/ketones cannot do among themselves. So alcohols boil higher.

   Hint: No O–H means no self H-bond.

8. 8.Name the general reaction that characterises the carbonyl group.

   Nucleophilic addition across the $\text{C}=\text{O}$ double bond.

   Hint: Nu attacks the electrophilic carbon.

9. 9.Why are aldehydes generally more reactive than ketones toward nucleophilic addition?

   Two reasons: (i) ketones have two electron-donating alkyl groups that reduce the $\delta+$ on carbon (electronic), and (ii) the two bulky groups on a ketone hinder the approaching nucleophile (steric).

   Hint: Electronic + steric, both favour aldehyde.

10. 10.Arrange for reactivity toward nucleophilic addition: HCHO, CH3CHO, CH3COCH3.

    $\text{HCHO} > \text{CH}_3\text{CHO} > \text{CH}_3\text{COCH}_3$. More alkyl groups = less reactive.

    Hint: Formaldehyde is the most reactive.

11. 11.How are aldehydes prepared by ozonolysis of alkenes?

    An alkene treated with $\text{O}_3$ then $\text{Zn}/\text{H}_2\text{O}$ cleaves the $\text{C}=\text{C}$ to give carbonyl compounds; a $=\text{CHR}$ end gives an aldehyde.

    Hint: Ozone snips the double bond.

12. 12.How is acetaldehyde manufactured from ethyne (Wacker-type / hydration)?

    Ethyne + water with $\text{HgSO}_4/\text{H}_2\text{SO}_4$ gives an unstable enol that tautomerises to acetaldehyde: $\text{CH}\equiv\text{CH} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CHO}$.

    Hint: Markovnikov hydration then tautomerise.

13. 13.What product forms when a primary alcohol is oxidised mildly, and with which reagent?

    An aldehyde. Mild reagents such as PCC (pyridinium chlorochromate) stop at the aldehyde without over-oxidising to acid.

    Hint: PCC = stops at aldehyde.

14. 14.How is an aldehyde prepared from an acid chloride (Rosenmund reduction)?

    $\text{R}-\text{COCl}$ is hydrogenated over Pd on BaSO4 (a poisoned catalyst) to give $\text{R}-\text{CHO}$.

    Hint: Poisoned Pd stops at the aldehyde.

15. 15.What is the Stephen reduction?

    A nitrile is reduced by $\text{SnCl}_2/\text{HCl}$ to an imine salt, then hydrolysed to an aldehyde: $\text{R}-\text{CN} \rightarrow \text{R}-\text{CHO}$.

    Hint: Nitrile to aldehyde via imine.

16. 16.How is a ketone made from a nitrile using a Grignard reagent?

    $\text{R}-\text{CN} + \text{R}'\text{MgX}$ gives an imine salt that on hydrolysis yields a ketone $\text{R}-\text{CO}-\text{R}'$.

    Hint: Grignard adds once, then hydrolyse.

17. 17.How is a ketone prepared from an acid chloride using a dialkylcadmium?

    $2\,\text{R}-\text{COCl} + \text{R}'_2\text{Cd} \rightarrow 2\,\text{R}-\text{CO}-\text{R}'$. Cadmium reagents are mild enough to stop at the ketone.

    Hint: R'2Cd is gentle, stops at ketone.

18. 18.What is the product of adding HCN to a carbonyl compound?

    A cyanohydrin: $>\text{C}=\text{O} + \text{HCN} \rightarrow >\text{C(OH)(CN)}$. The CN adds to carbon, H to oxygen.

    Hint: Hydroxy + nitrile on the same carbon.

19. 19.Why is a trace of base used in the HCN addition to carbonyls?

    Base generates $\text{CN}^-$, the actual nucleophile, which attacks the carbonyl carbon much faster than undissociated HCN.

    Hint: CN⁻ is the real attacker.

20. 20.What forms when a carbonyl compound reacts with sodium bisulphite?

    A crystalline bisulphite addition compound $>\text{C(OH)}(\text{SO}_3\text{Na})$, useful for purifying/separating aldehydes and methyl ketones.

    Hint: White crystals used for purification.

21. 21.Which carbonyl compounds readily form bisulphite addition products?

    Aldehydes and methyl ketones (and small cyclic ketones); bulkier ketones fail because of steric hindrance.

    Hint: Aldehydes and CH3CO– type.

22. 22.What product forms when an aldehyde/ketone reacts with a primary amine?

    A Schiff base (imine) $>\text{C}=\text{N}-\text{R}$, plus water. It is a nucleophilic addition–elimination.

    Hint: C=N with loss of water.

23. 23.What is the product of a carbonyl reacting with hydroxylamine, $\text{NH}_2\text{OH}$?

    An oxime $>\text{C}=\text{N}-\text{OH}$, with elimination of water.

    Hint: =N–OH.

24. 24.What is formed when a carbonyl reacts with hydrazine, $\text{NH}_2\text{NH}_2$?

    A hydrazone $>\text{C}=\text{N}-\text{NH}_2$, plus water.

    Hint: =N–NH2.

25. 25.What derivative does 2,4-dinitrophenylhydrazine (2,4-DNP) form, and why is it useful?

    A 2,4-dinitrophenylhydrazone, an orange/yellow crystalline solid — a classic test for the carbonyl group in aldehydes and ketones.

    Hint: Brady's reagent; orange precipitate.

26. 26.What is formed when a carbonyl reacts with semicarbazide?

    A semicarbazone $>\text{C}=\text{N}-\text{NH}-\text{CO}-\text{NH}_2$, with elimination of water.

    Hint: =N–NH–CO–NH2.

27. 27.At what pH is the rate of carbonyl condensation with ammonia derivatives optimal?

    Around pH 3.5 (weakly acidic). Too acidic protonates the amine (no nucleophile); too basic slows the dehydration step.

    Hint: Mildly acidic is the sweet spot.

28. 28.What is formed when an aldehyde reacts with one and then two equivalents of an alcohol (dry HCl)?

    One equivalent gives a hemiacetal $>\text{CH(OH)(OR)}$; a second gives an acetal $>\text{CH(OR)}_2$ plus water.

    Hint: Hemiacetal then acetal.

29. 29.Why are acetals useful in synthesis?

    They are stable to base and act as protecting groups for the carbonyl; the aldehyde is regenerated by aqueous acid hydrolysis.

    Hint: Carbonyl protection, removed by acid.

30. 30.What is an alpha-hydrogen and why is it special in carbonyl chemistry?

    A hydrogen on the carbon adjacent to the carbonyl ($\alpha$-carbon). It is acidic because the resulting carbanion (enolate) is resonance-stabilised by the C=O.

    Hint: Acidic H next to C=O.

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