Study guide: Aldehydes Ketones and Carboxylic Acids

📋 Course Outline

1. Carbonyl Groups and Their Structure
2. Naming Aldehydes and Ketones
3. Preparation of Aldehydes and Ketones
4. Physical Properties of Carbonyl Compounds
5. Nucleophilic Addition Reactions
6. Reduction and Oxidation Reactions
7. Reactions at the Alpha Carbon
8. Uses of Aldehydes and Ketones
9. Carboxylic Acid Names and Structure
10. Preparation of Carboxylic Acids
11. Physical Properties of Carboxylic Acids
12. Acidity of Carboxylic Acids
13. Reactions of Carboxylic Acids
14. Uses of Carboxylic Acids

📖 1. Carbonyl Groups and Their Structure

🔑 Key Concepts & Definitions

• Carbonyl group: A carbon–oxygen double bond, >C=O.

★ Must-know

📌 In an aldehyde the carbonyl carbon is bonded to a carbon and a hydrogen, whereas in a ketone it is bonded to two carbon atoms.

• The carbonyl bond is polar because oxygen is more electronegative than carbon, making the carbonyl carbon electrophilic and the oxygen nucleophilic.

Further detail

• The carbonyl carbon is sp2-hybridised, trigonal coplanar, and has bond angles of approximately 120°.

💡 Memory Hook

Aldehyde: carbonyl carbon bonded to H; ketone: bonded to two carbons.

📖 2. Naming Aldehydes and Ketones

★ Must-know

📌 Open-chain aliphatic aldehydes and ketones are named from the corresponding alkane by replacing -e with -al and -one, respectively.

📌 Aldehyde chains are numbered from the aldehyde carbon, while ketone chains are numbered from the end nearest the carbonyl group.

Further detail

• When an aldehyde group is attached to a ring, the suffix carbaldehyde is used and numbering starts at the ring carbon attached to that group.

• Common aldehyde names are generally derived from corresponding carboxylic acid names by replacing -ic acid with aldehyde, and substituent positions may use Greek letters.

• Common ketone names identify the two groups bonded to the carbonyl, while alkyl phenyl ketones are commonly named with the acyl-group name followed by phenone.

📖 3. Preparation of Aldehydes and Ketones

★ Must-know

• Primary alcohols oxidize to aldehydes and secondary alcohols oxidize to ketones; passing their vapours over silver or copper also dehydrogenates them to these products.

Further detail

• Alkene ozonolysis followed by zinc and water produces aldehydes, ketones, or both, depending on alkene substitution.

• Hydration of ethyne with sulfuric acid and mercury sulfate gives ethanal, whereas other alkynes give ketones.

• Rosenmund reduction converts acyl chlorides to aldehydes by hydrogenation over palladium on barium sulfate.

• Stephen reduction converts nitriles to imines with stannous chloride and hydrochloric acid, followed by hydrolysis to aldehydes.

• DIBAL-H selectively reduces nitriles and esters to aldehydes after hydrolysis of the intermediate imine in the nitrile route.

• Aromatic aldehydes can be prepared from methylbenzenes by Etard oxidation, chromic oxide in acetic anhydride followed by hydrolysis, or side-chain chlorination followed by hydrolysis.

• Gatterman–Koch formylation treats benzene or a derivative with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride to produce an aromatic aldehyde.

• Ketones are prepared from acyl chlorides and dialkylcadmium, from nitriles and Grignard reagents followed by hydrolysis, or by Friedel–Crafts acylation of aromatic compounds.

📖 4. Physical Properties of Carbonyl Compounds

★ Must-know

• Aldehydes and ketones boil above hydrocarbons and ethers of comparable molecular mass because of dipole–dipole association, but below comparable alcohols because they lack intermolecular hydrogen bonding.

• Lower aldehydes and ketones are miscible with water in all proportions, but their water solubility decreases rapidly as the alkyl chain grows.

Further detail

📌 Methanal is a gas at room temperature, ethanal is a volatile liquid, and other aldehydes and ketones are liquids or solids at room temperature.

💡 Memory Hook

Dipole attraction raises boiling points above ethers; absent hydrogen bonding keeps them below alcohols.

📖 5. Nucleophilic Addition Reactions

★ Must-know

• In nucleophilic addition, a nucleophile attacks the electrophilic carbonyl carbon, forms a tetrahedral alkoxide as carbon changes from sp2 to sp3, and the alkoxide is protonated to give the product.

📌 Aldehydes are generally more reactive than ketones in nucleophilic addition because ketones have greater steric hindrance and two alkyl groups reduce carbonyl-carbon electrophilicity more strongly.

Further detail

• Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins, and base catalyses the reaction by generating the stronger nucleophile cyanide ion.

📌 Sodium hydrogensulfite addition products form readily with most aldehydes but the equilibrium lies to the left for most ketones, and the water-soluble products can be decomposed with dilute mineral acid or alkali to recover the carbonyl compound.

• Aldehydes react with alcohols to form hemiacetals and then acetals, while ketones react with ethylene glycol to form cyclic ketals; aqueous mineral acid hydrolyses acetals and ketals back to carbonyl compounds.

• C=N derivatives include:

  • imines
  • oximes
  • hydrazones
  • semicarbazones
  • 2,4-DNP derivatives

💡 Memory Hook

Attack → tetrahedral alkoxide → protonation.

📖 6. Reduction and Oxidation Reactions

★ Must-know

• Sodium borohydride, lithium aluminium hydride, or catalytic hydrogenation reduces aldehydes to primary alcohols and ketones to secondary alcohols.

• Clemmensen reduction uses zinc amalgam and concentrated hydrochloric acid, whereas Wolff–Kishner reduction uses hydrazine followed by heating with strong alkali to convert a carbonyl group to CH2.

• Aldehydes are readily oxidised to carboxylic acids by common oxidants, whereas ketones generally require vigorous oxidation that cleaves carbon–carbon bonds.

Further detail

• Tollens’ reagent gives a bright silver mirror with aldehydes as the aldehyde is oxidised to a carboxylate, while ketones generally do not respond.

• Fehling’s reagent gives a reddish-brown precipitate with aliphatic aldehydes, but aromatic aldehydes do not respond to this test.

📌 Methyl ketones react with sodium hypohalite to form haloform and a carboxylate with one fewer carbon atom, and the iodoform reaction detects CH3CO or CH3CH(OH) groups that oxidize to CH3CO.

💡 Memory Hook

Aldehydes oxidize readily; ketones resist mild oxidants.

📖 7. Reactions at the Alpha Carbon

★ Must-know

• The alpha hydrogens of aldehydes and ketones are acidic because the carbonyl group withdraws electrons and the conjugate base is resonance-stabilized.

• Aldehydes and ketones with at least one alpha hydrogen undergo base-catalysed aldol addition to form beta-hydroxy carbonyl compounds, which can lose water to give alpha,beta-unsaturated carbonyl compounds.

📌 Aldehydes without an alpha hydrogen undergo disproportionation with concentrated alkali, reducing one molecule to an alcohol while oxidising another to a carboxylate; this is the Cannizzaro reaction.

Further detail

• Crossed aldol condensation between two different carbonyl compounds that both have alpha hydrogens can produce a mixture of four products.

• In aromatic aldehydes and ketones, the carbonyl group deactivates the ring and directs electrophilic substitution to the meta position.

💡 Memory Hook

Alpha-hydrogen present: aldol; absent in aldehyde: Cannizzaro.

📖 8. Uses of Aldehydes and Ketones

📝 Essential Points

• Formaldehyde is used as a 40% formalin solution to preserve biological specimens and to manufacture Bakelite and urea-formaldehyde glues.

• Acetaldehyde is used to manufacture acetic acid, ethyl acetate, vinyl acetate, polymers, and drugs.

• Benzaldehyde is used in perfumery and dye industries, while acetone and ethyl methyl ketone are common industrial solvents.

📖 9. Carboxylic Acid Names and Structure

🔑 Key Concepts & Definitions

• Carboxylic acids: Compounds containing the carboxyl functional group, –COOH, and may be aliphatic or aromatic depending on the group attached to the carboxyl carbon.

★ Must-know

📌 Aliphatic carboxylic acids are named by replacing the final -e of the corresponding alkane with -oic acid, with the carboxyl carbon numbered one.

Further detail

📌 Compounds with multiple carboxyl groups use multiplicative terms such as dicarboxylic acid or tricarboxylic acid, with the positions of the groups indicated by numerals.

• Fatty acids are higher aliphatic carboxylic acids containing 12–18 carbon atoms that occur in natural fats as glycerol esters.

📖 10. Preparation of Carboxylic Acids

★ Must-know

• Primary alcohols and aldehydes are oxidised to carboxylic acids by suitable oxidising agents.

• Grignard reagents react with carbon dioxide and then acidification to give carboxylic acids containing one more carbon atom than the starting alkyl halide.

Further detail

📌 Vigorous oxidation converts primary and secondary alkyl groups on an aromatic ring to carboxyl groups, but does not affect tertiary alkyl groups.

• Nitriles and amides are hydrolysed under acidic or basic conditions to carboxylic acids, with mild conditions allowing nitrile hydrolysis to stop at the amide stage.

• Acyl halides, anhydrides, and esters produce carboxylic acids by hydrolysis, with basic hydrolysis followed by acidification converting carboxylates to acids.

📖 11. Physical Properties of Carboxylic Acids

★ Must-know

• Carboxylic acids have higher boiling points than aldehydes, ketones, and alcohols of comparable molecular mass because of extensive intermolecular hydrogen bonding and dimer formation.

Further detail

• Aliphatic carboxylic acids with up to nine carbon atoms are colourless liquids at room temperature, while higher acids are wax-like solids.

• Carboxylic acids with up to four carbon atoms are miscible with water, but solubility decreases as the carbon chain grows and higher acids are practically insoluble.

💡 Memory Hook

Hydrogen-bonded dimers → high boiling points.

📖 12. Acidity of Carboxylic Acids

★ Must-know

📐 Formula - The acidity measure is $\mathrm{p}K_a=-\log K_a$, and a smaller pKa indicates a stronger acid.

📌 Carboxylic acids are stronger acids than alcohols and simple phenols because the carboxylate ion is stabilized by two equivalent resonance structures with negative charge delocalized over two oxygen atoms, unlike the less-stabilized phenoxide ion.

📌 Electron-withdrawing substituents increase carboxylic acid strength by stabilizing the conjugate base, whereas electron-donating substituents decrease acid strength by destabilizing it.

Further detail

• The pKa values of trifluoroacetic acid, benzoic acid, and acetic acid are 0.23, 4.19, and 4.76, respectively.

• The increasing acidity effect of the listed substituents is Ph < I < Br < Cl < F < CN < NO2 < CF3.

💡 Memory Hook

Electron-withdrawing groups stabilize carboxylate → stronger acid.

📖 13. Reactions of Carboxylic Acids

★ Must-know

• Carboxylic acids react with electropositive metals to release hydrogen and with carbonates or hydrogencarbonates to release carbon dioxide, a reaction used to detect the carboxyl group.

• Carboxylic acids esterify with alcohols or phenols in the presence of a mineral acid catalyst, and the reaction proceeds through nucleophilic acyl substitution and loss of water.

Further detail

• Thionyl chloride, phosphorus pentachloride, or phosphorus trichloride replaces the carboxylic acid hydroxyl group with chlorine, and thionyl chloride is preferred because its other products are gases.

• Carboxylic acids react with ammonia to form ammonium salts, which give amides on strong heating.

• Lithium aluminium hydride or diborane reduces carboxylic acids to primary alcohols, whereas sodium borohydride does not reduce the carboxyl group.

• Heating sodium carboxylates with sodalime forms hydrocarbons with carbon dioxide loss, whereas Kolbe electrolysis forms hydrocarbons with twice the number of carbons in the acid’s alkyl group.

• Carboxylic acids with an alpha hydrogen undergo alpha-halogenation with chlorine or bromine and red phosphorus in the Hell–Volhard–Zelinsky reaction.

• Aromatic carboxylic acids undergo electrophilic substitution with the carboxyl group as a deactivating meta director and do not undergo Friedel–Crafts reactions.

📖 14. Uses of Carboxylic Acids

📝 Essential Points

• Methanoic acid is used in rubber, textile, dyeing, leather, and electroplating industries, and ethanoic acid is used as a solvent and as vinegar in the food industry.

• Hexanedioic acid is used to manufacture nylon-6,6, benzoic acid esters are used in perfumery, sodium benzoate is a food preservative, and higher fatty acids are used to manufacture soaps and detergents.