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

18.1 Carbonyl Compounds

Introduction to Carbonyl Compounds

Carbonyl compounds are central to organic chemistry, forming the basis of many important functional groups and chemical transformations. The carbonyl group (C=O) is present in a variety of classes, including aldehydes, ketones, carboxylic acids, esters, and amides.

• Definition: A carbonyl group consists of a carbon atom double-bonded to an oxygen atom.

• General Formulas:

  • Aldehydes: R-CHO

  • Ketones: R-CO-R'

  • Carboxylic acids: R-COOH

  • Esters: R-COOR'

  • Amides: R-CONH2

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

Current Focus: The chemistry of aldehydes and ketones.

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: 1.23 Å, 745 kJ/mol

  • Alkene C=C bond: 1.34 Å, 611 kJ/mol

• Resonance: The carbonyl group exhibits resonance between the neutral and ionic forms: $R-C=O \leftrightarrow R-C^{+}-O^{-}$

• Polarity: The dipole moment ($\mu$) of carbonyl compounds is significant, e.g., acetaldehyde ($\mu = 2.7$ D), acetone ($\mu = 2.9$ D).

18.3 Nomenclature

Systematic Naming of Aldehydes and Ketones

Aldehydes and ketones are named using the IUPAC system, which employs a prefix-infix-suffix approach.

• Prefix: Number of carbon atoms in the parent chain (e.g., meth-, eth-, prop-).

• Infix: Type of carbon-carbon bonds (e.g., -an- for single bonds, -en- for double bonds).

• Suffix: Functional group or class of compound (e.g., -al for aldehyde, -one for ketone).

Prefix	Number of Carbons	Suffix	Class
meth-	1	-al	aldehyde
eth-	2	-one	ketone
prop-	3	-oic acid	carboxylic acid

Naming Aldehydes

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

• The aldehyde carbonyl carbon is always carbon-1.

• Example: CH3CH2CHO is named propanal.

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.

• Example: CH3COCH2CH3 is named butan-2-one.

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).

• Example: 3-oxopentanoic 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 influence the physical properties of aldehydes and ketones.

• Aldehydes and ketones cannot hydrogen bond to themselves, but they can accept H-bonds from donors (e.g., water).

• This affects boiling points and solubility.

• Example: Acetone (bp 56°C) is more soluble in water than butane (bp 0°C).

Physical Properties Table: Aldehydes

IUPAC Name	Common Name	Structure	bp (°C)	Density (g/cm³)	H2O Solubility (g/100 mL)
methanal	formaldehyde	HCHO or CH2O	-19	0.81	55
ethanal	acetaldehyde	CH3CHO	21	0.78	71
propanal	propionaldehyde	CH3CH2CHO	49	0.81	19
butanal	butyraldehyde	CH3CH2CH2CHO	75	0.81	7

Physical Properties Table: Ketones

IUPAC Name	Common Name	Structure	bp (°C)	Density (g/cm³)	H2O Solubility (g/100 mL)
propan-2-one	acetone	CH3COCH3	56	0.79	25.6
butan-2-one	methyl ethyl ketone	CH3COCH2CH3	80	0.81	16

18.7 Review of Syntheses

Synthetic Routes to Aldehydes and Ketones

There are several important synthetic methods for preparing aldehydes and ketones in organic chemistry.

• Grignard Reaction: Reaction of Grignard reagents with carbonyl compounds, followed by oxidation, yields ketones. $R-MgX + R'-CHO \xrightarrow{ether} R-CH(OH)-R' \xrightarrow{NaOCl, HOAc} R-CO-R'$

• Oxidation of Alcohols: Primary alcohols can be oxidized to aldehydes; secondary alcohols to ketones. $R-CH_2OH \xrightarrow{[O]} R-CHO$ $R_2CHOH \xrightarrow{[O]} R_2CO$

• Ozonolysis: Cleavage of alkenes with ozone produces carbonyl compounds. $RCH=CHR' \xrightarrow{O_3} RCHO + R'CHO$

• Friedel-Crafts Acylation: Aromatic aldehydes and ketones can be synthesized via acylation of aromatic rings. $Ar-H + RCOCl \xrightarrow{AlCl_3} Ar-CO-R$

• Acetylides: Alkynes can be hydrated to yield ketones. $RC\equiv CH \xrightarrow{H_2SO_4, HgSO_4} RC(O)CH_3$

• Hydroboration of Alkynes: Terminal alkynes can be converted to aldehydes. $RC\equiv CH \xrightarrow{(Sia)_2BH, H_2O_2, NaOH} RCHO$

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: $RCO_2Li + R'Li \xrightarrow{H_2O} RCO-R'$

• Example: Cyclohexanecarboxylic acid to cyclohexyl phenyl ketone.

18.9 Syntheses of Aldehydes & Ketones

Grignard Reaction with Nitriles

Aldehydes and ketones can be synthesized via Grignard reaction with nitriles. The electronegative nitrogen atom in nitriles polarizes the molecule, making it reactive toward Grignard reagents.

• General Reaction: $R-C\equiv N + R'MgX \xrightarrow{H_2O} R-CO-R'$

• Example: Benzonitrile to benzophenone.

Reduction of Nitriles to Aldehydes

• Disobutylaluminum hydride (DIBAL-H): Reduces nitriles to aldehydes. $R-C\equiv N \xrightarrow{DIBAL-H} R-CHO$

• Example: Hexanenitrile to hexanal.

18.10 Syntheses of Aldehydes & Ketones

Reduction of Carboxylic Acids and Derivatives

Lithium aluminum hydride (LiAlH4) is a powerful but unselective reducing agent, reducing carboxylic acids to primary alcohols. Selectivity can be achieved using alternative hydrides.

• Selective Reduction: Lithium tri-tert-butoxyaluminum hydride reduces acid chlorides to aldehydes faster than it reduces aldehydes.

• General Reaction: $RCOCl \xrightarrow{LiAlH(O-tBu)_3} RCHO$

• Disobutylaluminum hydride (DIBAL-H): Reduces esters to aldehydes. $RCOOR' \xrightarrow{DIBAL-H} RCHO$

Gilman Reagent and Grignard Reaction

• Gilman Reagent: Reacts with acid chlorides to form ketones selectively. $R_2CuLi + R'COCl \rightarrow R'COR + RCu + LiCl$

• Grignard Reagent: Reacts with acid chlorides until a tertiary alcohol is obtained.

Quiz Questions

Quiz-1: IUPAC Naming

• Practice naming compounds such as 5-oxo-hexan-3-one, 3-phenylbutanal.

Quiz-2: Synthesis Reagents

• Identify plausible reagents for multi-step syntheses of aldehydes and ketones.

Quiz-3: Propose Reagents

• Propose reagents for the conversion of carboxylic acids and alkyl halides to ketones.

Quiz-4: Reaction Products

• Select plausible products for reactions involving Grignard reagents and hydride reductions.

Quiz-5: Reaction Products

• Identify products from reactions of acid chlorides and hydride reductions.

Additional info: These notes are based on lecture slides for a college-level Organic Chemistry course, focusing on the structure, nomenclature, physical properties, and synthetic methods for aldehydes and ketones (Chapter 18A).