Aldehyde AND Ketone flash cards

1. 1.What is the general structural feature of the carbonyl group in aldehydes and ketones?

   Both contain the carbonyl group $\text{C}=\text{O}$. In aldehydes at least one bond of carbonyl C is to $\text{H}$ (\text{RCHO}); in ketones both remaining bonds are to carbon (\text{RCOR}').

   Hint: C=O with H vs two C.

2. 2.Describe the hybridization and geometry of the carbonyl carbon.

   The carbonyl C is $sp^2$ hybridized, giving a trigonal planar arrangement with bond angles near $120^\circ$. The $\pi$ bond forms by sideways overlap of unhybridized p orbitals on C and O.

   Hint: $sp^2$, planar, $120^\circ$.

3. 3.Why is the carbonyl group polar, and what is the direction of polarization?

   Oxygen is more electronegative than carbon, so the $\pi$ electrons are pulled toward O: $\text{C}^{\delta+}=\text{O}^{\delta-}$. Carbon is electrophilic, oxygen nucleophilic. This polarity governs nucleophilic addition.

   Hint: $\text{C}^{\delta+}=\text{O}^{\delta-}$.

4. 4.Why do aldehydes and ketones have higher boiling points than comparable hydrocarbons/ethers but lower than alcohols?

   Polar $\text{C}=\text{O}$ gives dipole-dipole attraction (higher than nonpolar/ether). But they cannot self-H-bond (no O-H), so their b.p. is lower than alcohols of similar mass.

   Hint: Dipole yes, self H-bond no.

5. 5.Why are lower aldehydes and ketones fairly soluble in water?

   Carbonyl O has lone pairs and accepts hydrogen bonds from water. So lower members (up to ~4 C) are miscible/soluble; solubility falls as the hydrocarbon chain grows.

   Hint: H-bond acceptor with water.

6. 6.Why is the carbonyl carbon susceptible to nucleophilic addition rather than electrophilic addition (unlike C=C)?

   The electron-poor $\text{C}^{\delta+}$ attracts nucleophiles; the electronegative O stabilizes the negative charge formed as the $\pi$ bond breaks. Alkene C=C is electron-rich, favoring electrophiles instead.

   Hint: Electron-poor C, O stabilizes charge.

7. 7.Compare the reactivity of aldehydes vs ketones toward nucleophilic addition and give two reasons.

   Aldehydes are more reactive. (1) Electronic: ketones have two alkyl groups donating electron density, reducing $\delta+$ on C. (2) Steric: two bulky groups in ketones hinder nucleophile approach.

   Hint: Aldehyde > ketone: +I and sterics.

8. 8.Order these by reactivity toward nucleophilic addition: HCHO, CH3CHO, CH3COCH3.

   $\text{HCHO} > \text{CH}_3\text{CHO} > \text{CH}_3\text{COCH}_3$. Fewer/smaller electron-donating alkyl groups means more electrophilic carbonyl carbon and less steric hindrance.

   Hint: Formaldehyde most reactive.

9. 9.Preparation: how are aldehydes and ketones made by oxidation of alcohols?

   Primary alcohol $\rightarrow$ aldehyde (mild, e.g. \text{PCC}) $\rightarrow$ acid (strong oxidant). Secondary alcohol $\rightarrow$ ketone. \text{PCC} stops at aldehyde by avoiding water/over-oxidation.

   Hint: 1° → RCHO (PCC), 2° → ketone.

10. 10.How does ozonolysis of alkenes give aldehydes/ketones?

    Alkene + $\text{O}_3$ forms an ozonide, then reductive workup (\text{Zn}/\text{H}_2\text{O}) cleaves $\text{C}=\text{C}$ into two carbonyls. $\text{=CH}-$ gives aldehyde; $\text{=CR}_2$ gives ketone.

    Hint: O3 then Zn/H2O cleaves C=C.

11. 11.How does hydration of alkynes give carbonyl compounds?

    $\text{H}_2\text{O},\ \text{H}_2\text{SO}_4,\ \text{HgSO}_4$ adds across the triple bond (Markovnikov) forming an enol that tautomerizes. Ethyne $\rightarrow$ acetaldehyde; other terminal alkynes $\rightarrow$ methyl ketones.

    Hint: Markovnikov, enol → keto.

12. 12.What is the Rosenmund reduction and its purpose?

    $\text{RCOCl} + \text{H}_2 \xrightarrow{\text{Pd/BaSO}_4} \text{RCHO}$. The catalyst is poisoned (BaSO4/S) to stop at the aldehyde and prevent over-reduction to alcohol.

    Hint: Acyl chloride → aldehyde, poisoned Pd.

13. 13.How does the Stephen reduction convert nitriles to aldehydes?

    $\text{RCN} \xrightarrow{\text{SnCl}_2/\text{HCl}} \text{RCH=NH} \xrightarrow{\text{H}_3\text{O}^+} \text{RCHO}$. An imine (aldimine) intermediate is hydrolyzed to the aldehyde.

    Hint: Nitrile + SnCl2/HCl → imine → RCHO.

14. 14.How can DIBAL-H be used to make aldehydes?

    DIBAL-H (\text{(i-Bu)}_2\text{AlH}) at low temperature partially reduces nitriles or esters to aldehydes, stopping at the aldehyde stage instead of going to amine/alcohol.

    Hint: Partial reduction of ester/nitrile.

15. 15.How does a Grignard reagent give ketones and aldehydes?

    \text{RMgX} + nitrile $\rightarrow$ ketone (after hydrolysis). \text{RMgX} + HCHO $\rightarrow$ 1° alcohol; with other aldehydes $\rightarrow$ 2° alcohol; with ketones $\rightarrow$ 3° alcohol.

    Hint: RMgX + nitrile → ketone.

16. 16.What is the Friedel-Crafts acylation route to aromatic ketones?

    Arene + $\text{RCOCl}\ \xrightarrow{\text{anhyd. AlCl}_3}$ aryl ketone. e.g. benzene + acetyl chloride $\rightarrow$ acetophenone. AlCl3 generates the acylium electrophile $\text{RCO}^+$.

    Hint: ArH + RCOCl/AlCl3.

17. 17.What is the Gattermann-Koch reaction?

    Benzene + $\text{CO} + \text{HCl}\ \xrightarrow{\text{anhyd. AlCl}_3/\text{CuCl}}$ benzaldehyde. It formylates the ring, acting like Friedel-Crafts with formyl chloride formed in situ.

    Hint: CO + HCl → benzaldehyde.

18. 18.How does calcium salt of carboxylic acid (dry distillation) give aldehyde/ketone?

    Calcium formate alone $\rightarrow$ HCHO. (\text{RCOO})_2\text{Ca} $\rightarrow$ ketone $\text{RCOR}$. A formate mixed with another salt gives an aldehyde \text{RCHO}.

    Hint: Ca carboxylate dry distillation.

19. 19.What is the general mechanism of nucleophilic addition to a carbonyl?

    Nucleophile attacks $\text{C}^{\delta+}$, C rehybridizes $sp^2 \rightarrow sp^3$, forming a tetrahedral alkoxide $\text{C}-\text{O}^-$; protonation gives the addition product. Acid/base can catalyze.

    Hint: Nu attack → tetrahedral alkoxide → protonate.

20. 20.What is the product and use of HCN addition to a carbonyl?

    Forms a cyanohydrin: $\text{RR}'\text{C(OH)CN}$. The C-C bond adds a carbon; hydrolysis of -CN gives an $\alpha$-hydroxy acid. Base (CN$^-$) catalyzes as the nucleophile.

    Hint: Cyanohydrin, CN⁻ is nucleophile.

21. 21.Why is a trace of base needed for good yields in cyanohydrin formation?

    HCN is a weak acid and poor nucleophile; base generates the stronger nucleophile $\text{CN}^-$, which attacks the carbonyl. Pure HCN reacts very slowly.

    Hint: Base makes CN⁻.

22. 22.What is the product of sodium bisulphite addition, and why is it useful?

    $\text{NaHSO}_3$ adds to give a crystalline bisulphite adduct $\text{RR}'\text{C(OH)SO}_3\text{Na}$. Being solid/water-soluble, it is used to purify/separate aldehydes and methyl ketones; the carbonyl is regenerated with dilute acid or base.

    Hint: Crystalline adduct → purification.

23. 23.Which carbonyl compounds form bisulphite addition products?

    All aldehydes and only methyl ketones / cyclic ketones (sterically small). Bulky ketones do not react because of steric hindrance around the carbonyl carbon.

    Hint: Aldehydes + methyl ketones only.

24. 24.What products form from a carbonyl reacting with one and then two equivalents of alcohol (acid-catalyzed)?

    One equivalent gives a hemiacetal $\text{RCH(OH)(OR')}$; a second (with $\text{H}^+$, remove water) gives an acetal $\text{RCH(OR')}_2$. For ketones: hemiketal $\rightarrow$ ketal.

    Hint: Hemiacetal → acetal.

25. 25.Why are acetals used as protecting groups for carbonyls?

    Acetals are stable to base and nucleophiles (e.g. Grignard, hydride) but are readily hydrolyzed back to the carbonyl by dilute aqueous acid. So the carbonyl can be 'hidden' during a reaction.

    Hint: Stable to base, cleaved by acid.

26. 26.What is the general product of ammonia-derivative (H2N-Z) addition-elimination with carbonyls?

    They add, then eliminate water to form a $\text{C}=\text{N}-\text{Z}$ double bond: $\text{RR}'\text{C}=\text{O} + \text{H}_2\text{N-Z} \rightarrow \text{RR}'\text{C}=\text{N-Z} + \text{H}_2\text{O}$.

    Hint: Addition then dehydration to C=N.

27. 27.Name the products of carbonyl with (a) hydroxylamine, (b) hydrazine, (c) phenylhydrazine.

    (a) \text{H}_2\text{NOH} $\rightarrow$ oxime ($\text{C}=\text{NOH}$). (b) \text{H}_2\text{NNH}_2 $\rightarrow$ hydrazone. (c) \text{C}_6\text{H}_5\text{NHNH}_2 $\rightarrow$ phenylhydrazone.

    Hint: Oxime, hydrazone, phenylhydrazone.

28. 28.What is formed with 2,4-DNP reagent and why is it important?

    2,4-dinitrophenylhydrazine gives a yellow-orange 2,4-dinitrophenylhydrazone precipitate. It is a classic qualitative test to detect the presence of any aldehyde or ketone (carbonyl).

    Hint: Orange ppt = carbonyl present.

29. 29.At what pH is the rate of oxime/imine formation optimal, and why?

    Around mildly acidic pH ($\sim 4$–5). Too much acid protonates the amine (no nucleophile); too basic, no acid catalysis for dehydration. So a moderate acid is optimal.

    Hint: Weakly acidic; balance of two effects.

30. 30.What are $\alpha$-hydrogens and why are they acidic?

    H atoms on the carbon adjacent to the carbonyl. They are acidic ($pK_a \sim 20$) because the resulting carbanion (enolate) is resonance-stabilized by the carbonyl oxygen.

    Hint: Alpha to C=O; enolate resonance.

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