Resources | Subject Notes | Chemistry
Aldehydes and ketones are important classes of organic compounds containing a carbonyl group ($C=O$). They are fundamental in organic chemistry due to their characteristic reactivity and widespread occurrence in nature and synthetic chemistry.
Aldehydes contain a carbonyl group ($C=O$) bonded to at least one hydrogen atom. The general formula for an aldehyde is $RCHO$, where R can be an alkyl or aryl group.
Ketones contain a carbonyl group ($C=O$) bonded to two carbon atoms. The general formula for a ketone is $R1COR2$, where R1 and R2 can be alkyl or aryl groups.
IUPAC Nomenclature:
Generally, aldehydes and ketones have lower boiling points than alcohols of comparable molecular mass due to weaker intermolecular forces (dipole-dipole and van der Waals forces). Aldehydes tend to have slightly higher boiling points than ketones of similar molecular weight due to their greater polarity.
Small aldehydes and ketones are soluble in water due to their ability to form hydrogen bonds. As the size of the alkyl groups increases, solubility in water decreases.
Many aldehydes and ketones have characteristic odors. For example, ethanal (acetaldehyde) has a pungent odor, while acetone has a sweet odor.
| Property | Aldehydes | Ketones |
|---|---|---|
| Boiling Point | Generally higher than alcohols of similar molar mass | Generally lower than alcohols of similar molar mass |
| Solubility in Water | Good for small aldehydes, decreases with increasing alkyl group size | Limited for small ketones, decreases with increasing alkyl group size |
| Odor | Often pungent | Often sweet |
The carbonyl carbon in aldehydes and ketones is electrophilic and susceptible to nucleophilic attack. This leads to a variety of important reactions.
Aldehydes and ketones can be reduced to alcohols using catalytic hydrogenation (e.g., $H_2$ with $Pd/C$ catalyst). The aldehyde is reduced to a primary alcohol, while the ketone is reduced to a secondary alcohol.
In the presence of an acid catalyst, aldehydes and ketones react with alcohols to form diols (also known as acetals or ketals). This is an equilibrium reaction.
Grignard reagents ($RMgX$) react with aldehydes and ketones to form alcohols after protonation. With aldehydes, the reaction yields a primary alcohol. With ketones, it yields a secondary alcohol.
Aldehydes are readily oxidized to carboxylic acids. Ketones are generally resistant to oxidation under mild conditions because they lack a hydrogen atom on the carbonyl carbon.
Oxidation of Aldehydes:
$RCHO + [O]$ -> $RCOOH$
Oxidation of Ketones:
Ketones typically require strong oxidizing agents (e.g., potassium permanganate, $KMnO_4$) to be oxidized to carboxylic acids. These reactions often lead to cleavage of the carbon-carbon bond adjacent to the carbonyl group.
Aldehydes and ketones can be reduced to alcohols using reducing agents such as sodium borohydride ($NaBH_4$) or lithium aluminium hydride ($LiAlH_4$).
Reduction with NaBH4:
$RCHO + NaBH_4 + H_2O$ -> $RCH_2OH$
Reduction with LiAlH4:
$R1COR2 + LiAlH_4 + H_2O$ -> $R_1CHOHR_2$ (Secondary alcohol)
Aldehydes and ketones can undergo nucleophilic addition reactions with reagents like cyanide ($CN^-$) and ammonia ($NH_3$).
Aldehydes and ketones react with cyanide to form cyanohydrins. This reaction is important in organic synthesis.
$RCHO + HCN \rightleftharpoons RCH(OH)CN$
Aldehydes and ketones react with ammonia to form imines (Schiff bases). These are important intermediates in organic synthesis.
$RCHO + NH_3 \rightleftharpoons RCH=NH + H_2O$
This test detects the presence of aldehydes and ketones. A positive test involves the reduction of the Fehling's reagent (a blue complex ion) to a red precipitate of copper(I) oxide ($Cu_2O$).
This test also detects the presence of aldehydes and ketones. A positive test involves the reduction of silver ions ($Ag^+$) to metallic silver, forming a silver mirror on the test tube.
This test is specific for the presence of methyl ketones (ketones with a $CH_3$ group directly attached to the carbonyl carbon). A positive test involves the reaction of the ketone with iodine ($I_2$) in the presence of a base (e.g., NaOH) to form a pale yellow precipitate of iodoform ($CHI_3$).
This test is used to differentiate between aldehydes and ketones. A positive test involves a blue colour change.
Aldehydes and ketones are used as solvents, flavouring agents, and intermediates in the synthesis of various organic compounds, including pharmaceuticals, polymers, and perfumes.