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Chemistry · Chapter

Aldehydes, Ketones and Carboxylic Acids

This chapter covers organic compounds containing the carbonyl group (>C=O) and the carboxyl group (-COOH). It is divided into two main parts: aldehydes and ketones, followed by carboxylic acids. For each family, you will learn their nomenclature, the structure of their functional group, and several methods for their preparation. The physical properties, such as boiling points and solubility, are also explained based on their molecular structure and intermolecular forces.

The core of the chapter is the chemical reactions. For aldehydes and ketones, the focus is on nucleophilic addition reactions at the carbonyl carbon. You will study important named reactions like the Aldol condensation and the Cannizzaro reaction, which depend on the presence or absence of an alpha-hydrogen. The chapter also details specific chemical tests, such as Tollens' and Fehling's tests, used to distinguish aldehydes from ketones. For carboxylic acids, the topics include their acidic nature, reactions involving the cleavage of O-H and C-OH bonds, and the formation of derivatives like esters and amides.

Students often find this chapter challenging due to the large number of named reactions, each with specific reagents and conditions that must be memorized. Success depends heavily on a strong foundation in General Organic Chemistry, particularly understanding electronic effects and reaction mechanisms. It also builds directly on concepts from the "Alcohols, Phenols and Ethers" chapter.

Formulas for Aldehydes, Ketones and Carboxylic Acids

Rosenmund Reduction
R−COCl+H2→Pd/BaSO4R−CHO+HClR-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl Preparation of an aldehyde from an acyl chloride.
Stephen Reaction
R−CN→1.SnCl2/HCl→2.H3O+R−CHOR-CN \xrightarrow{1. SnCl_2/HCl} \xrightarrow{2. H_3O^+} R-CHO Preparation of an aldehyde from a nitrile.
Tollens' Test
R−CHO+2[Ag(NH3)2]++3OH−→R−COO−+2Ag↓+4NH3+2H2OR-CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow R-COO^- + 2Ag \downarrow + 4NH_3 + 2H_2O Aldehydes give a silver mirror precipitate; ketones do not react.
Fehling's Test
R−CHO+2Cu2++5OH−→R−COO−+Cu2O↓+3H2OR-CHO + 2Cu^{2+} + 5OH^- \rightarrow R-COO^- + Cu_2O \downarrow + 3H_2O Aliphatic aldehydes give a red-brown precipitate of copper(I) oxide.
Aldol Condensation
2CH3−CHO→dil.NaOHCH3−CH(OH)−CH2−CHO2CH_3-CHO \xrightarrow{dil. NaOH} CH_3-CH(OH)-CH_2-CHO Reaction of aldehydes or ketones having at least one α-hydrogen.
Cannizzaro Reaction
2HCHO+conc.KOH→CH3OH+HCOOK2HCHO + conc. KOH \rightarrow CH_3OH + HCOOK Disproportionation reaction for aldehydes with no α-hydrogen.
Clemmensen Reduction
>C=O→Zn−Hg,conc.HCl>CH2>C=O \xrightarrow{Zn-Hg, conc. HCl} >CH_2 Reduces a carbonyl group to a methylene group in acidic medium.
Wolff-Kishner Reduction
>C=O→NH2NH2,KOH,ethyleneglycol,Δ>CH2+N2>C=O \xrightarrow{NH_2NH_2, KOH, ethylene glycol, \Delta} >CH_2 + N_2 Reduces a carbonyl group to a methylene group in basic medium.
Fischer Esterification
R−COOH+R′−OH⇌[H+]R−COO−R′+H2OR-COOH + R'-OH \rightleftharpoons[H^+] R-COO-R' + H_2O Reaction of a carboxylic acid and an alcohol to form an ester.
Hell-Volhard-Zelinsky (HVZ) Reaction
R−CH2−COOH→1.X2/RedP→2.H2OR−CH(X)−COOHR-CH_2-COOH \xrightarrow{1. X_2/Red P} \xrightarrow{2. H_2O} R-CH(X)-COOH α-halogenation of carboxylic acids. X is Cl or Br.