Ketones and Aldehydes: Structure, Nomenclature, and Synthesis

Ketones and Aldehydes

Introduction to Carbonyl Compounds

The carbonyl group (C=O) is a central functional group in organic chemistry, found in many important classes of compounds such as ketones, aldehydes, carboxylic acids, and acid chlorides. Understanding its structure and reactivity is essential for studying organic reactions and mechanisms.

• Ketones: Compounds with two alkyl (or aryl) groups bonded to the carbonyl carbon.

• Aldehydes: Compounds with one alkyl (or aryl) group and one hydrogen bonded to the carbonyl carbon.

General structures:

• Aldehyde: $\ce{R-CHO}$

• Ketone: $\ce{R-CO-R'}$

Structure of the Carbonyl Group

The carbonyl carbon is sp2 hybridized, resulting in a planar structure. The oxygen atom is also sp2 hybridized, with two lone pairs occupying sp2 orbitals and one electron in a p orbital. The C=O double bond consists of a sigma bond and a pi bond, making it shorter and stronger than a C=C double bond.

• Bond Lengths and Energies:

Bond	Length (Å)	Energy (kcal/mol)
Ketone C=O	1.23	178 (745 kJ/mol)
Alkene C=C	1.34	146 (611 kJ/mol)

• The carbonyl group has a large dipole moment due to the polarity of the double bond (oxygen is more electronegative than carbon).

• This polarization makes the carbonyl carbon electrophilic (Lewis acid) and the oxygen nucleophilic (Lewis base).

Resonance Structures of the Carbonyl Group

The carbonyl group can be represented by two resonance structures, with the major contributor being the neutral form and the minor contributor showing a formal negative charge on oxygen and a positive charge on carbon.

• Major: $\ce{R_2C=O}$

• Minor: $\ce{R_2C^+-O^-}$

This resonance helps explain the reactivity of carbonyl compounds.

Nomenclature of Ketones and Aldehydes

Ketone Nomenclature

Ketones are named by replacing the -e ending of the parent alkane with -one. The position of the carbonyl group is indicated by a number.

• Examples:

• Butan-2-one (2-butanone)

• 2,4-dimethyl-3-pentanone

• 1-phenyl-1-propanone

• 3-methylcycloheptanone

• 2-cyclohexenone

Aldehyde Nomenclature

Aldehydes are named by replacing the -e ending of the parent alkane with -al. The aldehyde group must be at the end of the chain and is always carbon number 1.

• Examples:

• Ethanal ($\ce{CH_3CHO}$)

• Pent-2-enal ($\ce{H_3C-CH=CH-CH=CHO}$)

• Cyclohexanecarbaldehyde (when attached to a ring)

Substituent Naming

• The ketone carbonyl as a substituent is named with the prefix oxo-.

• The aldehyde group as a substituent is named as a formyl- group.

• Examples: 3-oxopentanal, 2-formylbenzoic acid, 3-oxobutanoic acid.

Common Names

• Many carbonyl compounds have common names used in everyday chemistry.

Structure	Common Name
$\ce{CH_3COCH_3}$	acetone
$\ce{C_6H_5COCH_3}$	acetophenone
$\ce{C_6H_5COC_6H_5}$	benzophenone

Synthesis of Aldehydes and Ketones

Oxidation of Alcohols

• Secondary alcohols are oxidized to ketones using chromic acid ($\ce{Na_2Cr_2O_7}$, $\ce{H_2SO_4}$) or $\ce{KMnO_4}$.

• Primary alcohols are oxidized to aldehydes using PCC (pyridinium chlorochromate) to avoid further oxidation to carboxylic acids.

Ozonolysis of Alkenes

• Alkenes can be cleaved by ozone ($\ce{O_3}$) followed by reduction to generate aldehydes and/or ketones.

Friedel-Crafts Acylation

• Preparation of alkyl aryl ketones via acylation of aromatic rings using acyl chlorides and $\ce{AlCl_3}$.

• Example: para-nitrobenzophenone.

Gatterman-Koch Reaction

• Produces benzaldehyde systems from aromatic compounds using $\ce{CO}$, $\ce{HCl}$, $\ce{CuCl}$, and $\ce{AlCl_3}$.

Hydration of Alkynes

• Alkynes can be hydrated to form ketones or aldehydes via Markovnikov (acid/mercury catalyzed) or anti-Markovnikov (hydroboration-oxidation) mechanisms.

• Enols produced rearrange to more stable keto forms.

Use of 1,3-Dithiane

• 1,3-Dithiane can be deprotonated and alkylated to form thioacetals, which upon hydrolysis yield aldehydes or ketones.

• Useful for constructing unsymmetrical ketones.

Ketones from Carboxylic Acids

• Organolithium reagents react with carboxylic acids to form ketones via lithium carboxylate intermediates.

• Hydrates are formed and then lose water to produce ketones.

Ketones from Nitriles

• Nitriles ($\ce{R-CN}$) can be converted to ketones by nucleophilic attack of Grignard or organolithium reagents, followed by hydrolysis.

• The intermediate imine is hydrolyzed to a ketone.

Aldehydes and Ketones from Acid Chlorides

• Carboxylic acids are converted to acid chlorides using thionyl chloride ($\ce{SOCl_2}$).

• Acid chlorides can be selectively reduced to aldehydes using lithium tri-tert-butoxyaluminum hydride ($\ce{Li^+AlH(O-t-Bu)_3^-}$).

• Strong reducing agents like $\ce{LiAlH_4}$ reduce acid chlorides all the way to primary alcohols.

Summary Table: Key Reactions for Synthesis of Aldehydes and Ketones

Method	Starting Material	Reagent	Product
Oxidation	Primary alcohol	PCC	Aldehyde
Oxidation	Secondary alcohol	Chromic acid	Ketone
Ozonolysis	Alkene	$\ce{O_3}$, reduction	Aldehyde/Ketone
Friedel-Crafts Acylation	Aromatic ring	Acyl chloride, $\ce{AlCl_3}$	Ketone
Hydration of Alkynes	Alkyne	Acid/HgSO4 or Hydroboration	Keto/Aldehyde
1,3-Dithiane Route	Thioacetal	Alkyl halide, hydrolysis	Aldehyde/Ketone
Carboxylic Acid Route	Carboxylic acid	Organolithium	Ketone
Nitrile Route	Nitrile	Grignard, hydrolysis	Ketone
Acid Chloride Reduction	Acid chloride	LiAlH(O-t-Bu)3	Aldehyde

Conclusion

Ketones and aldehydes are fundamental carbonyl compounds in organic chemistry. Their structure, nomenclature, and synthetic methods are essential knowledge for understanding organic reactions and mechanisms. Mastery of these topics provides a strong foundation for further study in organic and general chemistry.