# Lecture 6: Aldehydes and Ketones: Structure and Synthesis
## Organic Chemistry II
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the structure and bonding of the carbonyl group in aldehydes and ketones
2. Name aldehydes and ketones using IUPAC and common nomenclature
3. Compare the physical properties of aldehydes and ketones with other functional groups
4. Describe common synthetic methods for preparing aldehydes and ketones
5. Predict the relative reactivity of various aldehydes and ketones toward nucleophilic addition
6. Identify aldehydes and ketones using spectroscopic methods (IR, NMR, mass spec)
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## Lecture Content
### I. Structure and Bonding of the Carbonyl Group
The carbonyl group (C=O) is the defining functional group of both aldehydes and ketones. In an aldehyde, at least one hydrogen is bonded to the carbonyl carbon (RCHO), while in a ketone, two carbon groups flank the carbonyl carbon (RCOR'). The carbonyl carbon is sp2 hybridized, giving it a trigonal planar geometry with bond angles of approximately 120 degrees. The C=O bond consists of one sigma bond and one pi bond. At about 1.22 angstroms, the carbon-oxygen double bond is shorter than a carbon-carbon double bond (approximately 1.34 angstroms), and at roughly 745 kJ/mol, it is considerably stronger than a C=C bond (approximately 614 kJ/mol).
The carbonyl group is strongly polar because oxygen is more electronegative than carbon. Formaldehyde, for example, has a dipole moment of about 2.7 D. The resonance structures of the carbonyl group, C=O and C(+)-O(-), reveal that the carbonyl carbon bears a significant partial positive charge, making it an electrophilic center that is highly susceptible to nucleophilic attack.
### II. Nomenclature
In IUPAC nomenclature, aldehydes receive the suffix -al, which replaces the -e ending of the parent alkane. The aldehyde carbon is always numbered C1 because it has the highest priority. Examples include methanal (formaldehyde), ethanal (acetaldehyde), propanal, and butanal. When an aldehyde group is attached to a ring, the suffix -carbaldehyde is used, as in cyclohexanecarbaldehyde.
Ketones are named with the suffix -one, and the chain is numbered to give the carbonyl carbon the lowest possible locant. Examples include propanone (acetone), butanone, 2-pentanone, and cyclohexanone. Many aldehydes and ketones also retain widely used common names such as formaldehyde, acetaldehyde, benzaldehyde, acetone, acetophenone, and benzophenone. In naming priority, the aldehyde group (-CHO) ranks above the ketone carbonyl (-C(=O)R), and both rank above hydroxyl and amino groups.
### III. Physical Properties
The boiling points of aldehydes and ketones are higher than those of alkanes and ethers of similar molecular weight because of substantial dipole-dipole interactions. However, they are lower than the boiling points of comparable alcohols because aldehydes and ketones lack O-H bonds and therefore cannot serve as hydrogen-bond donors to one another. Representative boiling points illustrate this trend: formaldehyde boils at -21 degrees C, acetaldehyde at 20 degrees C, and acetone at 56 degrees C.
Lower-molecular-weight aldehydes and ketones (up to about five carbons) are soluble in water because the carbonyl oxygen acts as a hydrogen-bond acceptor. Solubility decreases as the hydrocarbon portion of the molecule grows. Many aldehydes and ketones have distinctive and often pleasant odors. Formaldehyde is pungent, acetaldehyde is fruity, benzaldehyde smells of almonds, and cinnamaldehyde provides the flavor of cinnamon. Many members of this class are used in the perfume and flavoring industries.
### IV. Synthesis of Aldehydes
Several methods are available for preparing aldehydes. Controlled oxidation of primary alcohols is among the most common: PCC (pyridinium chlorochromate) in dichloromethane stops the oxidation at the aldehyde stage, as does Dess-Martin periodinane and the Swern oxidation (using oxalyl chloride with DMSO and then triethylamine). Jones reagent (CrO3/H2SO4), on the other hand, over-oxidizes primary alcohols to carboxylic acids and should be avoided when the aldehyde is the desired product.
The Rosenmund reduction converts an acyl chloride to an aldehyde by catalytic hydrogenation over palladium on barium sulfate (a poisoned catalyst that prevents further reduction to the alcohol). DIBAL-H (diisobutylaluminum hydride) at -78 degrees C can reduce esters to aldehydes, provided the temperature is carefully controlled to prevent over-reduction. Ozonolysis of alkenes, followed by a reductive workup with dimethyl sulfide or triphenylphosphine, cleaves double bonds to produce carbonyl compounds: terminal alkenes give formaldehyde plus another aldehyde, and internal alkenes give two aldehydes or ketones. Hydroboration-oxidation of terminal alkynes affords aldehydes through anti-Markovnikov addition of a water equivalent. Specialized formylation reactions exist for aromatic systems, including the Gattermann-Koch reaction (CO + HCl + AlCl3) and the Reimer-Tiemann reaction (CHCl3 + NaOH on phenol).
Synthetic methods summary diagram for aldehydes. A central aldehyde structure (RCHO) is shown with arrows pointing inward from five different starting materials: (1) primary alcohol with PCC arrow, (2) acyl chloride with H2/Pd-BaSO4 arrow, (3) ester with DIBAL-H/-78C arrow, (4) alkene with O3 then Me2S arrow, and (5) terminal alkyne with (sia)2BH then H2O2/NaOH arrow. Each starting material structure is clearly drawn.
### V. Synthesis of Ketones
Oxidation of secondary alcohols is the most straightforward route to ketones. Because a secondary alcohol cannot be over-oxidized beyond the ketone stage, a broad range of oxidizing agents can be used, including Jones reagent (Na2Cr2O7/H2SO4), PCC, Dess-Martin periodinane, the Swern oxidation, and KMnO4.
Friedel-Crafts acylation (ArH + RCOCl + AlCl3) provides aryl ketones directly, without the carbocation rearrangement problems that plague Friedel-Crafts alkylation. Ozonolysis of internal alkenes produces two ketone fragments (or a mixture of aldehyde and ketone). Hydration of alkynes under Markovnikov conditions (H2O/H2SO4/HgSO4) converts terminal alkynes to methyl ketones and internal alkynes to ketones.
Organometallic reagents offer additional routes. Treatment of an acyl chloride with a Gilman reagent (R'2CuLi) gives the ketone product selectively, without over-addition. In contrast, adding two equivalents of RLi or RMgBr to an ester produces a tertiary alcohol through over-addition, making this approach unsuitable for ketone synthesis. Finally, oxidative cleavage of vicinal diols with periodic acid (HIO4) or lead tetraacetate generates two carbonyl compounds.
### VI. Spectroscopic Identification
The carbonyl stretch in the IR spectrum is one of the most recognizable and diagnostic absorptions in organic spectroscopy. It appears as a strong, sharp peak between 1715 and 1740 cm-1, with aldehydes typically near 1725 cm-1 and ketones near 1715 cm-1. Conjugation with a pi system lowers the frequency to the 1680-1700 cm-1 range. Aldehydes display an additional diagnostic feature: two weak bands near 2720 and 2820 cm-1, arising from a Fermi resonance doublet of the aldehyde C-H stretch.
In proton NMR, the aldehyde proton resonates distinctively far downfield at delta 9.4 to 10.0 ppm, making it easy to identify. Alpha protons adjacent to the carbonyl appear at delta 2.0 to 2.5 ppm. In carbon-13 NMR, carbonyl carbons resonate at delta 190 to 220 ppm, with aldehydes typically in the 190-205 ppm range and ketones in the 195-220 ppm range. In mass spectrometry, alpha cleavage next to the carbonyl is a common fragmentation pattern, producing acylium ions (RC=O+). The McLafferty rearrangement, a gamma-hydrogen transfer to oxygen accompanied by beta-cleavage, is another characteristic fragmentation observed for carbonyl compounds.
Panel A: Annotated IR spectrum of an aldehyde showing the strong C=O stretch at ~1725 cm-1 and the characteristic Fermi resonance doublet of the aldehyde C-H stretch at 2720 and 2820 cm-1, with both peaks clearly labeled. Panel B: 1H NMR spectrum of butanal showing the aldehyde proton signal at approximately 9.7 ppm (singlet/triplet), the alpha-CH2 signal at approximately 2.4 ppm, and the remaining alkyl signals upfield.
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