Aldehydes and Ketones: Structure, Nomenclature, Properties, and Synthesis

18.1 Carbonyl Compounds

Introduction to Carbonyl Compounds

Carbonyl compounds are central to organic chemistry, serving as key intermediates in numerous biological and chemical transformations. The carbonyl group (C=O) is present in a variety of functional groups, including aldehydes, ketones, carboxylic acids, esters, and amides.

• Carbonyl Group: Consists of a carbon atom double-bonded to an oxygen atom.

• General Formulas:

  • Aldehyde: R-CHO

  • Ketone: R-CO-R'

  • Carboxylic Acid: R-COOH

  • Ester: R-COOR'

  • Amide: R-CONH2

• Examples: Retinal (an aldehyde) and Warfarin (a ketone-containing drug).

18.2 Carbonyl Structure

Bonding and Electronic Structure

The carbonyl carbon is sp2 hybridized, resulting in a planar structure with bond angles of approximately 120°. The C=O bond is highly polarized due to the electronegativity of oxygen and is stabilized by resonance.

• Bond Lengths and Energies:

  • Ketone C=O bond length: 1.23 Å

  • Alkene C=C bond length: 1.34 Å

  • Ketone C=O bond energy: 745 kJ/mol

  • Alkene C=C bond energy: 611 kJ/mol

• Resonance: The carbonyl group can be represented by resonance structures, with the major form being the neutral C=O and the minor form having a negative charge on oxygen and a positive charge on carbon.

• Dipole Moments: Carbonyl compounds have significant dipole moments (e.g., acetaldehyde μ = 2.7 D, acetone μ = 2.9 D), which influence their physical properties.

18.3 Nomenclature Overview

Systematic Naming of Aldehydes and Ketones

Nomenclature in organic chemistry uses the Prefix-Infix-Suffix system to indicate the number of carbons, the type of carbon-carbon bonds, and the functional group.

Prefix	Number of Carbons
meth-	1
eth-	2
prop-	3

Infix	Nature of C–C Bonds
-an-	All single bonds
-en-	One or more double bonds
-yn-	One or more triple bonds

Suffix	Class of Compound
-al	Aldehyde
-one	Ketone
-ol	Alcohol
-oic acid	Carboxylic acid

Naming Aldehydes

• Aldehydes are named by replacing the suffix of the parent alkane with -al.

• The carbonyl carbon in aldehydes is always carbon-1.

• Examples: Ethanal (acetaldehyde), 3-hydroxybutanal, cyclohexanecarbaldehyde.

Naming Ketones

• Ketones are named by replacing the suffix of the parent alkane with -one.

• The chain is numbered so that the carbonyl carbon gets the lowest possible number.

• Examples: 2-pentanone, 3-methyl-2-butanone, cyclohexanone.

Substituent Naming: 'Oxo' and 'Formyl'

• When a ketone or aldehyde is a substituent on a higher priority chain, it is named as oxo (for ketone) or formyl (for aldehyde).

• Examples: 3-oxohexanoic acid, 2-formylbenzoic acid.

18.4 Physical Properties

Polarity and Hydrogen Bonding

The polarity of the carbonyl bond and its ability to accept hydrogen bonds significantly affect the physical properties of aldehydes and ketones.

• Hydrogen Bonding: Aldehydes and ketones cannot form hydrogen bonds with themselves, but they can accept hydrogen bonds from donors such as water or alcohols.

• Boiling Points: Generally higher than alkanes but lower than alcohols of similar molecular weight.

• Water Solubility: Small aldehydes and ketones are soluble in water due to hydrogen bonding; solubility decreases with increasing molecular size.

Physical Properties Table: Aldehydes

IUPAC Name	Common Name	Structure	mp (°C)	bp (°C)	Density (g/cm³)	H₂O Solubility (%)
methanal	formaldehyde	HCHO or CH₂O	-92	-19	0.81	55
ethanal	acetaldehyde	CH₃CHO	-123	21	0.78	100
propanal	propionaldehyde	CH₃CH₂CHO	-81	49	0.81	100
butanal	butyraldehyde	CH₃CH₂CH₂CHO	-99	75	0.81	7.3
benzaldehyde	benzaldehyde	C₆H₅CHO	-26	179	1.04	0.3

Physical Properties Table: Ketones

IUPAC Name	Common Name	Structure	mp (°C)	bp (°C)	Density (g/cm³)	H₂O Solubility (%)
propan-2-one	acetone	CH₃COCH₃	-95	56	0.79	25.6
butan-2-one	methyl ethyl ketone	CH₃COCH₂CH₃	-86	80	0.81	29.4
pentan-2-one	methyl propyl ketone	CH₃COCH₂CH₂CH₃	-60	102	0.81	6.6
hexan-2-one	methyl butyl ketone	CH₃COCH₂CH₂CH₂CH₃	-39	127	0.81	1.6

18.7 Review of Syntheses

General Synthetic Methods for Aldehydes and Ketones

Aldehydes and ketones can be synthesized by several methods, many of which involve oxidation or addition reactions.

• Grignard Reaction: Reaction of Grignard reagents with carbonyl compounds followed by oxidation yields ketones.

• Oxidation of Alcohols: Primary alcohols can be oxidized to aldehydes; secondary alcohols to ketones.

• Ozonolysis: Cleavage of alkenes with ozone produces carbonyl compounds.

• Friedel-Crafts Acylation: Aromatic aldehydes and ketones can be synthesized via acylation of aromatic rings.

• Hydroboration of Alkynes: Terminal alkynes can be converted to aldehydes; internal alkynes to ketones.

18.8 Syntheses of Ketones

Ketone Formation from Carboxylates

Organolithium reagents react with carboxylate salts to form ketones. The carboxylate salt can be prepared with LiOH or by using a two-fold excess of the organolithium reagent.

• General Reaction: $RCO_2Li + R'Li \rightarrow RCO_2R' + Li_2O$ $RCO_2R' + H_2O \rightarrow RCOR' + H_2O$

• Example: Cyclohexyl phenyl ketone from cyclohexanecarboxylic acid and phenyllithium.

18.9 Syntheses of Aldehydes & Ketones

Grignard Reaction with Nitriles

Aldehydes and ketones can be synthesized by the reaction of Grignard reagents with nitriles, followed by hydrolysis.

• General Reaction: $R-MgX + R'-C\equiv N \rightarrow R'-C(MgX)=N-R$ $R'-C(MgX)=N-R + H_2O \rightarrow R'-CO-R + NH_3$

• Example: Benzophenone from benzonitrile and phenylmagnesium bromide.

Hydride Reductions

• Disobutylaluminum hydride (DIBAL): Reduces nitriles and esters to aldehydes.

• General Reaction: $R-C\equiv N \xrightarrow{DIBAL-H} R-CHO$ $RCOOR' \xrightarrow{DIBAL-H} RCHO$

• Example: Hexanenitrile to hexanal.

18.10 Syntheses of Aldehydes & Ketones

Selective Reductions and Acid Chloride Pathways

Lithium aluminum hydride is a powerful but unselective reducing agent, reducing carboxylic acids to primary alcohols. Selective reductions can be achieved using alternative hydrides such as lithium tri-tert-butoxyaluminum hydride, which reduces acid chlorides faster than aldehydes.

• Conversion to Acid Chlorides: $RCOOH + SOCl_2 \rightarrow RCOCl + SO_2 + HCl$

• Selective Reduction: $RCOCl \xrightarrow{LiAlH(OtBu)_3} RCHO$

• Example: Isovaleric acid to isovaleraldehyde via acid chloride intermediate.

Gilman Reagent (Organocuprate) Reactions

• Gilman reagents (R2CuLi) react with acid chlorides to form ketones selectively.

• General Reaction: $RCOCl + R'_2CuLi \rightarrow RCOR' + R'Cu + LiCl$

• Example: 2-octanone from octanoyl chloride and diethylcuprate.

Quiz Questions (Integrated)

Practice Problems

• Nomenclature: Assign IUPAC names to given aldehyde and ketone structures.

• Synthesis: Select plausible reagents for multi-step syntheses of aldehydes and ketones.

• Product Prediction: Identify products of reactions involving Grignard reagents, DIBAL-H, and organocuprates.

Additional info: The notes cover the structure, nomenclature, physical properties, and synthetic methods for aldehydes and ketones, corresponding to Chapter 18 of a standard organic chemistry curriculum.