Aldehydes and Ketones: Structure, Properties, and Reactions

Chapter 15: Aldehydes and Ketones

15.1 The Carbonyl Group

The carbonyl group is a functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O). It is a key feature in several classes of organic compounds, including aldehydes, ketones, carboxylic acids, esters, and amides.

• Aldehyde: Contains a carbonyl group bonded to at least one hydrogen atom. General formula: R-CHO.

• Ketone: Contains a carbonyl group bonded to two carbon atoms. General formula: R-CO-R'.

• Carboxylic Acid: Contains a carbonyl group bonded to an -OH group. General formula: R-COOH.

• Ester: Contains a carbonyl group bonded to an -OR group. General formula: R-COOR'.

• Amide: Contains a carbonyl group bonded to a nitrogen atom. General formula: R-CONH2.

Example: Formaldehyde (HCHO) is the simplest aldehyde; acetone (CH3COCH3) is the simplest ketone.

15.2 Naming Simple Aldehydes and Ketones

Aldehydes and ketones are named according to IUPAC rules, with common names also frequently used. The carbonyl group is always given the lowest possible number in the chain.

• Aldehydes: Suffix "-al" (IUPAC); common names often end with "aldehyde".

• Ketones: Suffix "-one" (IUPAC); common names often end with "ketone".

• Numbering starts from the end closest to the carbonyl group.

• For ketones, the order of naming R groups is from smallest to largest, not alphabetical.

Compound	IUPAC Name	Common Name
HCHO	Methanal	Formaldehyde
CH3CHO	Ethanal	Acetaldehyde
CH3COCH3	Propanone	Acetone
CH3COC2H5	Butanone	Methyl ethyl ketone
C6H5CHO	Benzaldehyde	Benzaldehyde

Example: 3-ethylhexanal is an aldehyde with an ethyl group at position 3 and the carbonyl at position 1.

15.3 Properties of Aldehydes and Ketones

Aldehydes and ketones are characterized by their polar carbonyl group, which influences their physical properties.

• Polarity: Both are polar due to the C=O bond.

• Boiling Points: Lower than alcohols, higher than alkanes, due to dipole-dipole interactions.

• Solubility: Typically liquids; water-soluble due to hydrogen bonding with water molecules.

• Ketones: Good solvents for both polar and non-polar solutes.

• Toxicity: Simple aldehydes and ketones can be toxic.

Example: Acetone is widely used as a solvent in laboratories and industry.

15.4 Some Common Aldehydes and Ketones

• Formaldehyde (HCHO): Used in disinfectants and resins.

• Acetaldehyde (CH3CHO): Used in the manufacture of acetic acid.

• Acetone (CH3COCH3): Common solvent.

• Benzaldehyde (C6H5CHO): Used in flavorings and perfumes.

15.5 Oxidation of Aldehydes and Ketones

Aldehydes can be oxidized to carboxylic acids, while ketones are generally resistant to oxidation.

• Aldehyde Oxidation: Removal of two hydrogens; forms carboxylic acid.

• Ketone Oxidation: No reaction under normal conditions.

• Tollens' Reagent: Distinguishes aldehydes (oxidized) from ketones (not oxidized).

• Benedict's Reagent: Also distinguishes, but less efficiently.

Equation:

$\mathrm{R{-}CHO} + [O] \rightarrow \mathrm{R{-}COOH}$

Example: Ethanal oxidized to ethanoic acid.

15.6 Reduction of Aldehydes and Ketones

Both aldehydes and ketones can be reduced to alcohols using reducing agents.

• Aldehydes: Reduced to primary alcohols.

• Ketones: Reduced to secondary alcohols.

Equations:

Aldehyde reduction:

$\mathrm{R{-}CHO} + 2[H] \rightarrow \mathrm{R{-}CH_2OH}$

Ketone reduction:

$\mathrm{R{-}CO{-}R'} + 2[H] \rightarrow \mathrm{R{-}CH(OH){-}R'}$

Example: Acetone reduced to isopropanol.

15.7 Addition of Alcohols: Hemiacetals and Acetals

Aldehydes and ketones react with alcohols to form hemiacetals and hemiketals, which can further react to form acetals and ketals.

• Hemiacetal: Compound with both an alcohol-like (OH) and an ether-like (OR) group bonded to the former aldehyde carbonyl carbon.

• Hemiketal: Compound with both an alcohol-like (OH) and an ether-like (OR) group bonded to the former ketone carbonyl carbon.

• Acetal: Formed from hemiacetal by addition of another alcohol; contains two OR groups.

• Ketal: Formed from hemiketal by addition of another alcohol; contains two OR groups.

Compound	Functional Groups	Origin
Hemiacetal	OH and OR	Aldehyde
Acetal	2 OR	Aldehyde
Hemiketal	OH and OR	Ketone
Ketal	2 OR	Ketone

Example: Reaction of ethanol with acetaldehyde forms a hemiacetal, which can further react to form an acetal.

15.8 Hydrolysis of Acetals

Acetals can be hydrolyzed (broken down) in the presence of acid to regenerate the original aldehyde or ketone and alcohol.

• Hydrolysis: Reverse of acetal formation; requires acidic conditions.

• Application: Used in protecting groups in organic synthesis.

Equation:

$\mathrm{Acetal} + \mathrm{H_2O} \xrightarrow{\text{acid}} \mathrm{Aldehyde/Ketone} + 2 \mathrm{Alcohol}$

Example: Hydrolysis of dimethoxyethane yields acetaldehyde and methanol.

Additional info: Academic context and examples were added to clarify and expand on the original notes, including full chemical equations and explanations of functional group transformations.