Nomenclature and Structure of Carbonyl Group

Nomenclature

Organic compounds containing the carbonyl group (>C=O) are called aldehydes, ketones, and carboxylic acids. Aldehydes have the carbonyl group bonded to a carbon and a hydrogen atom (R-CHO), while ketones have it bonded to two carbon atoms (R-CO-R'). Carboxylic acids feature a carboxyl group (-COOH).

Common Names: Derived from the name of the corresponding carboxylic acid by replacing '-ic acid' with '-aldehyde' (for aldehydes) or by naming the alkyl/aryl groups attached to the carbonyl carbon followed by 'ketone'. Greek letters (α, β, γ) denote positions relative to the carbonyl carbon.

IUPAC Names: Aldehydes are named 'alkanal' (replacing '-e' of alkane with '-al'), with the carbonyl carbon being C1. Ketones are named 'alkanone' (replacing '-e' with '-one'), numbered to give the carbonyl carbon the lowest possible number. Cyclic aldehydes use '-carbaldehyde' appended to the cycloalkane name, with the carbonyl carbon as C1.

Structure Of The Carbonyl Group

The carbonyl carbon is $sp^2$ hybridized, forming three sigma bonds in a trigonal planar geometry with bond angles around 120°. The fourth valence electron forms a $π$ bond with oxygen. The C=O bond is polar due to oxygen's higher electronegativity, making the carbonyl carbon electrophilic and the oxygen nucleophilic. This polarity results in significant dipole moments.

Preparation Of Aldehydes And Ketones

Aldehydes and ketones are synthesized through various methods:

Preparation Of Aldehydes And Ketones

• Oxidation of Alcohols: Primary alcohols yield aldehydes, and secondary alcohols yield ketones using oxidizing agents like PCC (mild), $KMnO_4$, or $K_2Cr_2O_7$ (strong).
• Dehydrogenation of Alcohols: Passing alcohol vapors over heated copper, silver, or platinum catalysts yields aldehydes (from primary alcohols) or ketones (from secondary alcohols).
• From Hydrocarbons:
• Ozonolysis of alkenes followed by reductive workup yields aldehydes and/or ketones.
• Hydration of alkynes: Ethyne yields acetaldehyde, while other alkynes yield ketones.

Preparation Of Aldehydes

• From Acyl Chlorides: Reduction with $LiAlH_4$ or catalytic hydrogenation of acid chlorides (Rosenmund reduction using $Pd/BaSO_4$).
• From Nitriles and Esters: Selective reduction using DIBAL-H or Stephen reaction (using $SnCl_2/HCl$ followed by hydrolysis for nitriles).
• From Hydrocarbons (Aromatic):
• Oxidation of methyl groups on arenes using chromyl chloride (Etard reaction) or $CrO_3$/acetic anhydride.
• Side-chain chlorination of toluene followed by hydrolysis yields benzaldehyde.
• Gattermann-Koch reaction: Formylation of arenes using CO, HCl, and $AlCl_3$/$CuCl$.

Preparation Of Ketones

• From Acyl Chlorides: Reaction with organocadmium reagents ($R_2Cd$).
• From Nitriles: Reaction with Grignard reagents followed by hydrolysis.
• From Benzene/Substituted Benzenes: Friedel-Crafts acylation using acyl chlorides or anhydrides with Lewis acids ($AlCl_3$).

Physical Properties

Lower aldehydes (up to C4) are gases with pungent odours. Higher members are liquids or solids with less pungent, often fragrant, odours. Boiling points increase with molecular mass and decrease with branching. Aldehydes and ketones have higher boiling points than hydrocarbons and ethers of similar mass due to dipole-dipole interactions but lower than alcohols due to the absence of intermolecular hydrogen bonding. Lower members (up to C4) are soluble in water due to hydrogen bonding with water molecules; solubility decreases with increasing hydrocarbon chain length.

Chemical Reactions

Aldehydes and ketones undergo reactions primarily involving the polar carbonyl group.

Nucleophilic Addition Reactions

The carbonyl carbon is electrophilic and susceptible to attack by nucleophiles. The reaction involves addition across the C=O bond, changing hybridization from $sp^2$ to $sp^3$. Aldehydes are generally more reactive than ketones due to less steric hindrance and greater electrophilicity of the carbonyl carbon.

• Addition of HCN: Forms cyanohydrins.
• Addition of Sodium Bisulfite ($NaHSO_3$): Forms bisulfite addition products, useful for purification.
• Addition of Alcohols: Forms hemiacetals (from aldehydes) and ketals (from ketones), especially with ethylene glycol.
• Addition of Ammonia & Derivatives: Reacts with ammonia and its derivatives (like hydroxylamine, hydrazines, semicarbazide) to form imines, oximes, hydrazones, semicarbazones, etc., often accompanied by dehydration.

Reduction

• Reduction to Alcohols: Can be reduced to primary alcohols (aldehydes) or secondary alcohols (ketones) using reducing agents like $LiAlH_4$, $NaBH_4$, or catalytic hydrogenation.
• Reduction to Hydrocarbons: The carbonyl group can be completely reduced to a methylene group ($CH_2$) via Clemmensen reduction (Zn-Hg/HCl) or Wolff-Kishner reduction (hydrazine/base).

Oxidation

Aldehydes are easily oxidized to carboxylic acids by mild oxidants (Tollens' reagent, Fehling's reagent) and strong oxidants. Methyl ketones (with $CH_3$ adjacent to C=O) undergo oxidation at the methyl group with hypohalites (haloform reaction), yielding a carboxylate and haloform ($CHI_3$). Ketones are generally resistant to oxidation, requiring vigorous conditions that cleave C-C bonds.

Reactions Due To A-Hydrogen

The hydrogen atoms on the carbon adjacent to the carbonyl group (α-hydrogens) are acidic due to resonance stabilization of the resulting carbanion. This facilitates:

• Aldol Condensation: Aldehydes and ketones with α-hydrogens undergo self-condensation in the presence of dilute base to form β-hydroxy aldehydes (aldols) or ketones (ketols). These can further dehydrate to α,β-unsaturated carbonyl compounds.
• Cross Aldol Condensation: Occurs between two different carbonyl compounds, potentially yielding a mixture of products.

Other Reactions

• Cannizzaro Reaction: Aldehydes lacking α-hydrogens (e.g., formaldehyde, benzaldehyde) undergo disproportionation in the presence of concentrated alkali, yielding an alcohol and a carboxylate salt.
• Electrophilic Substitution: Aromatic aldehydes and ketones undergo electrophilic substitution on the ring, with the carbonyl group acting as a meta-director and deactivator.

Uses Of Aldehydes And Ketones

Aldehydes and ketones have numerous industrial and commercial uses. Formaldehyde (formalin) is used for preserving specimens and in resins (bakelite) and adhesives. Acetaldehyde is a starting material for acetic acid and polymers. Benzaldehyde is used in perfumes and dyes. Acetone and methyl ethyl ketone are common industrial solvents. Many aldehydes and ketones contribute to fragrances and flavors.

Nomenclature And Structure Of Carboxyl Group

Nomenclature

Carboxylic acids contain the carboxyl group (-COOH). Common names are derived from their sources or Latin/Greek names, ending in '-ic acid'. IUPAC names are formed by replacing the '-e' of the parent alkane with '-oic acid'. Numbering starts from the carboxyl carbon (C1). For dicarboxylic acids, suffixes like '-edioic acid' are used, with locants indicating -COOH positions.

Structure Of Carboxyl Group

The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (-OH) group attached to the same carbon. The C=O bond is polar, and the O-H bond is acidic. Resonance occurs between the carbonyl group and the hydroxyl group, stabilizing the carboxylate anion formed upon deprotonation. The carboxyl carbon is $sp^2$ hybridized, resulting in a planar structure with bond angles around 120°.

Methods Of Preparation Of Carboxylic Acids

Carboxylic acids can be synthesized through several routes:

From Primary Alcohols And Aldehydes

Primary alcohols and aldehydes are readily oxidized to carboxylic acids using strong oxidizing agents like $KMnO_4$ or $K_2Cr_2O_7$. Mild oxidants (like Tollens' or Fehling's reagent) oxidize aldehydes but not alcohols.

From Alkylbenzenes

Alkyl side chains on aromatic rings are oxidized to carboxyl groups using strong oxidants ($KMnO_4$, $CrO_3$), regardless of the side chain's length (except tertiary alkyl groups).

From Nitriles And Amides

Hydrolysis of nitriles ($R-C \equiv N$) or amides ($R-CONH_2$) in acidic or basic conditions yields carboxylic acids.

From Grignard Reagents

Reaction of Grignard reagents ($R-MgX$) with carbon dioxide ($CO_2$, dry ice), followed by acid hydrolysis, yields carboxylic acids with one more carbon atom than the original alkyl halide.

From Acyl Halides And Anhydrides

Hydrolysis of acyl halides ($R-COCl$) or acid anhydrides ($ (RCO)_2O $) readily yields carboxylic acids.

From Esters

Hydrolysis of esters ($R-COOR'$) under acidic or basic conditions yields carboxylic acids (or their salts).

Physical Properties

Lower aliphatic carboxylic acids (up to C9) are liquids with unpleasant odours; higher acids are solids with low volatility. They have higher boiling points than alcohols of similar molecular mass due to stronger intermolecular hydrogen bonding and dimer formation. Lower members are miscible with water due to hydrogen bonding, but solubility decreases with increasing hydrocarbon chain length. They are soluble in organic solvents.

Chemical Reactions

Carboxylic acids exhibit reactions due to cleavage of O-H and C-O bonds, and reactions involving the -COOH group itself.

Reactions Involving Cleavage Of O–H Bond

• Acidity: React with active metals (Na, Al) to release $H_2$. React with bases ($NaOH$, carbonates like $Na_2CO_3$, bicarbonates like $NaHCO_3$) to form salts and $CO_2$. Carboxylic acids are more acidic than alcohols and phenols due to the resonance stabilization of the carboxylate anion. Electron-withdrawing groups increase acidity (lower $pK_a$), while electron-donating groups decrease it.
• Esterification: React with alcohols/phenols in the presence of acid catalysts to form esters.

Reactions Involving Cleavage Of C–O Bond

The C-O bond cleavage occurs in alcohols but is difficult in phenols due to resonance and $sp^2$ hybridization of the carbon atom.

Reactions Involving –COOH Group

• Formation of Anhydrides: Reaction with dehydrating agents ($P_2O_5$) or heating with mineral acids.
• Reaction with $PCl_5$, $PCl_3$, $SOCl_2$: Convert -OH group to -Cl, forming acyl chlorides, with $SOCl_2$ being preferred due to gaseous by-products.
• Reaction with Ammonia: Form ammonium salts, which on heating yield amides.
• Reduction: Reduced to primary alcohols using strong reducing agents like $LiAlH_4$ or diborane ($B_2H_6$).
• Decarboxylation: Loss of $CO_2$ upon heating their sodium salts with sodalime, or via electrolysis (Kolbe electrolysis).

Substitution Reactions In The Hydrocarbon Part

Hell-Volhard-Zelinsky Reaction: Carboxylic acids with an α-hydrogen undergo halogenation at the α-position using $Cl_2$ or $Br_2$ in the presence of red phosphorus, forming α-halo carboxylic acids.

Uses Of Carboxylic Acids

Carboxylic acids have various uses: Methanoic acid in rubber and textile industries; Ethanoic acid as a solvent and in vinegar; Hexanedioic acid for nylon-6,6 production; Esters of benzoic acid in perfumery; Sodium benzoate as a food preservative; Higher fatty acids for soaps and detergents.

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