Aldehydes, Ketones, and Carboxylic Acids

Comprehensive practice flashcards covering preparations, chemical reactions, distinguishing tests, and mechanisms of aldehydes and ketones based on lecture notes.

What products are obtained from the oxidation of primary and secondary alcohols using $CrO_3$ or $PCC$?

Primary alcohols ($R\text{-}CH_2\text{-}OH$) are oxidised to aldehydes ($R\text{-}CHO$), whereas secondary alcohols ($R\text{-}CH(OH)\text{-}R$) are oxidised to ketones ($R\text{-}CO\text{-}R$).

What are the reaction conditions and products for the catalytic dehydrogenation of alcohols over copper?

Alcohol vapours are passed over a $Cu$ catalyst heated at $573\,K$. Primary alcohols dehydrogenate to give aldehydes ($R\text{-}CHO$), and secondary alcohols dehydrogenate to give ketones ($R\text{-}CO\text{-}R$).

What are the carbonyl products formed by the reductive ozonolysis ($O_3$ followed by $Zn/H_2O$) of ethene and propene?

Ethene ($CH_2=CH_2$) yields two molecules of methanal ($HCHO$), while propene ($CH_3\text{-}CH=CH_2$) yields an equimolar mixture of methanal ($HCHO$) and ethanal ($CH_3CHO$).

What products are formed upon the hydration of ethyne and propyne in the presence of $HgSO_4$ ($Hg^{2+}$) and $H_2SO_4$ ($H^+$)?

Ethyne undergoes hydration followed by isomerisation to give ethanal ($CH_3CHO$), whereas propyne undergoes Markovnikov addition and subsequent isomerisation to give propanone ($CH_3COCH_3$).

What is the Rosenmund reaction, and what catalyst is employed?

It is the selective catalytic hydrogenation of an acyl chloride (acid chloride) to an aldehyde over palladium supported on barium sulphate ($Pd/BaSO_4$).

How does the Stephen reaction convert nitriles into aldehydes?

Alkanenitriles are reduced by stannous chloride in hydrochloric acid ($SnCl_2 + HCl$) to form an imine intermediate ($RCH=NH$), which yields the corresponding aldehyde upon subsequent acid hydrolysis ($H_3O^+$).

How does diisobutylaluminium hydride ($DIBAL\text{-}H$) react with nitriles and esters?

$DIBAL\text{-}H$ selectively reduces nitriles and esters to imine/aldehyde precursors at low temperatures, which upon hydrolysis with $H_2O$ yield aldehydes without reducing carbon-carbon double bonds.

What is the Étard reaction, and what intermediate complex is formed?

The Étard reaction oxidises toluene to benzaldehyde using chromyl chloride ($CrO_2Cl_2$) in carbon disulphide ($CS_2$) solvent, proceeding through a brown chromium complex intermediate, $C_6H_5CH(OCrOHCl_2)_2$, that is hydrolysed by $H_3O^+$.

How is toluene converted into benzaldehyde using chromic oxide ($CrO_3$)?

Toluene is treated with $CrO_3$ in acetic anhydride at $273\text{--}278\,K$ to trap the aldehyde stage as benzylidene diacetate ($C_6H_5CH(OCOCH_3)_2$), which is subsequently hydrolysed with aqueous acid to yield benzaldehyde.

How is benzaldehyde prepared by side-chain chlorination of toluene?

Toluene undergoes free radical side-chain chlorination with $Cl_2$ under light ($h\nu$) to yield benzal chloride ($C_6H_5CHCl_2$), which on boiling with $H_2O$ at $373\,K$ hydrolyses to benzaldehyde.

What reagents are used in the Gattermann-Koch reaction to convert benzene to benzaldehyde?

Carbon monoxide ($CO$) and hydrogen chloride ($HCl$) in the presence of anhydrous aluminium chloride ($AlCl_3$) and cuprous chloride ($CuCl$).

Why is dialkylcadmium used instead of a Grignard reagent to prepare ketones from acyl chlorides?

Dialkylcadmium ($R_2Cd$) is less reactive than a Grignard reagent, so it reacts with acyl chlorides ($R'COCl$) to form ketones ($R'COR$) without further adding to the resulting ketone.

What ketone is produced by reacting propanenitrile with phenylmagnesium bromide followed by acid hydrolysis?

1-phenylpropanone ($CH_3\text{-}CH_2\text{-}CO\text{-}C_6H_5$).

What are the products of the Friedel-Crafts acylation of benzene with acetyl chloride and benzoyl chloride in the presence of anhydrous $AlCl_3$?

Reaction with acetyl chloride ($CH_3COCl$) yields acetophenone ($C_6H_5COCH_3$), while reaction with benzoyl chloride ($C_6H_5COCl$) yields benzophenone ($C_6H_5COC_6H_5$).

Why do carbonyl groups in aldehydes and ketones undergo nucleophilic addition reactions?

Due to the higher electronegativity of oxygen, the carbonyl bond is strongly polarised ($C^{\delta+}=O^{\delta-}$). A nucleophile attacks the planar, electrophilic $sp^2$ carbonyl carbon perpendicularly, converting it into a tetrahedral $sp^3$ alkoxide intermediate.

How can ethanal be converted into lactic acid (2-hydroxypropanoic acid)?

Ethanal is reacted with $HCN$ to form ethanal cyanohydrin ($CH_3\text{-}CH(OH)\text{-}CN$), which upon complete acid hydrolysis ($H^+/H_2O$) gives lactic acid ($CH_3\text{-}CH(OH)\text{-}COOH$).

Why is the addition of sodium hydrogen sulphite ($NaHSO_3$) used for the purification and separation of aldehydes?

Carbonyl compounds form water-soluble, crystalline bisulphite addition products via proton transfer, which can be easily reverted back to the pure carbonyl compound by treatment with dilute mineral acid.

What products are formed when aldehydes and ketones react with alcohols in the presence of dry $HCl$?

Aldehydes react with one alcohol equivalent to form a hemiacetal (alkoxy alcohol) and a second equivalent to yield an acetal. Ketones react with ethylene glycol to form cyclic ethylene glycol ketals.

What product is formed by the reaction of a carbonyl compound with hydroxylamine ($NH_2OH$)?

An oxime is formed via nucleophilic addition followed by rapid dehydration (e.g., ethanal forms ethanal oxime / acetaldoxime).

Why is 2,4-dinitrophenylhydrazine (2,4-DNP) used to test for aldehydes and ketones?

It condenses with carbonyl groups to form crystalline 2,4-dinitrophenylhydrazone derivatives that have a characteristic orange-red colour, allowing qualitative identification.

What is a Schiff's base, and how is it prepared from carbonyl compounds?

A Schiff's base is a substituted imine ($>C=NR'$) prepared by the reaction of an aldehyde or ketone with a primary amine ($R'NH_2$) accompanied by the elimination of water.

What are the reagents and characteristic transformation of the Clemmensen reduction?

Reagents: Zinc amalgam ($Zn\text{-}Hg$) and concentrated hydrochloric acid ($HCl$). It reduces the carbonyl group ($>C=O$) of an aldehyde or ketone directly to a methylene group ($-CH_2-$).

What are the reagents and steps involved in the Wolff-Kishner reduction?

The carbonyl group is first reacted with hydrazine ($NH_2NH_2$) to form a hydrazone, which is then heated with potassium hydroxide ($KOH$) in a high-boiling solvent (ethylene glycol) to liberate $N_2$ and form a hydrocarbon ($-CH_2-$).

What is Popoff's rule for the oxidation of unsymmetrical ketones?

During the destructive cleavage of an unsymmetrical ketone by strong oxidising agents like concentrated $HNO_3$, the keto group ($>C=O$) preferentially stays attached to the smaller (lower) alkyl group.

What constitutes Tollens' test, and what is observed with an aldehyde?

Tollens' reagent is ammoniacal silver nitrate ($[Ag(NH_3)_2]^+$ in alkaline medium). Warming an aldehyde reduces silver ions to metallic silver, depositing a bright silver mirror while oxidising the aldehyde to a carboxylate anion ($RCOO^-$). Ketones do not react.

What are Fehling's solutions A and B, and how do they react with aldehydes?

Fehling's A is aqueous copper sulphate ($CuSO_4$); Fehling's B is alkaline sodium potassium tartrate (Rochelle salt). On heating with aliphatic aldehydes, a reddish-brown precipitate of cuprous oxide ($Cu_2O$) forms. Aromatic aldehydes (e.g., benzaldehyde) do not give this test.

What structural feature is required for a compound to undergo the haloform (iodoform) reaction?

The compound must contain at least one methyl carbonyl group ($CH_3\text{-}C(=O)-$) or a group that can be oxidised to it (such as $CH_3\text{-}CH(OH)-$).

What products are formed when propanone reacts with sodium hypoiodite ($NaOI$ or $I_2 + NaOH$)?

A yellow precipitate of iodoform ($CHI_3$) and sodium acetate ($CH_3COONa$).

What is an aldol reaction, and what requirement must the starting carbonyl compound possess?

Aldehydes or ketones containing at least one $\alpha$-hydrogen react in the presence of dilute alkali to form a $\beta$-hydroxy aldehyde (aldol) or $\beta$-hydroxy ketone (ketol).

What is the final aldol condensation product obtained from ethanal upon heating with dilute $NaOH$?

Ethanal condenses to 3-hydroxybutanal, which on heating loses water to yield but-2-enal (crotonaldehyde, $CH_3\text{-}CH=CH\text{-}CHO$).

What is the major product of the crossed aldol condensation between benzaldehyde and acetophenone in dilute $NaOH$?

Benzaldehyde (lacking $\alpha$-H) and acetophenone (providing $\alpha$-H) condense to form 1,3-diphenylprop-2-en-1-one (benzalacetophenone).

What is the Cannizzaro reaction, and which carbonyl compounds undergo it?

Aldehydes lacking $\alpha$-hydrogens (such as $HCHO$ and $C_6H_5CHO$) undergo self-oxidation and reduction (disproportionation) upon heating with concentrated alkali, yielding one molecule of alcohol and one molecule of carboxylate salt.

Why do aromatic aldehydes and ketones undergo electrophilic aromatic substitution at the meta position?

The carbonyl group ($>C=O$) is an electron-withdrawing group due to resonance and inductive effects, which deactivates the benzene ring and directs incoming electrophiles to the meta position.