The key difference between aldehyde and ketone is that the functional group of an aldehyde occurs always at a terminus whereas the functional group of a ketone always occurs in the middle of a molecule.
Aldehydes and ketones are organic molecules with a carbonyl group. In a carbonyl group, carbon atom has a double bond to oxygen. The carbonyl carbon atom is sp2 hybridized. So, aldehydes and ketones have a trigonal planar arrangement around the carbonyl carbon atom. The carbonyl group is a polar group; thus, aldehydes and ketones have higher boiling points compared to the hydrocarbons having the same weight. However, these cannot make stronger hydrogen bonds like alcohols; therefore, they have lower boiling points than the corresponding alcohols. As a result of the hydrogen bond formation ability, low molecular weight aldehydes and ketones are soluble in water. But when the molecular weight increases, they become hydrophobic.
What is Aldehyde?
Aldehyde has a carbonyl group. This carbonyl group binds with another carbon from one side, and from the other end, it connects with a hydrogen atom. Therefore, we can characterize aldehydes with the –CHO group. The simplest aldehyde is formaldehyde. However, this molecule deviates from the general formula by having a hydrogen atom instead of R group.
In the nomenclature of aldehyde, according to the IUPAC system we use the term “al” to denote an aldehyde. For aliphatic aldehydes, the “e” of the corresponding alkane is replaced with “al”. For example, we name CH3CHO as ethanal, and CH3CH2CHO is named as propanal.
For aldehydes with ring systems where the aldehyde group directly attaches to the ring, we use the term “carbaldehyde” as a suffix to name them. However, we name the compound C6H6CHO commonly as benzaldehyde rather than using benzenecarbaldehyde. We can synthesize aldehydes by various methods. One method is via oxidizing primary alcohols. Moreover, we can synthesize aldehydes by reducing esters, nitriles and acyl chlorides.
What is Ketone?
In a ketone, the carbonyl group occurs between two carbon atoms. We use the suffix “one” in ketone nomenclature. Instead of “–e” of the corresponding alkane we use the term “one”. Moreover, we number the aliphatic chain in a way that gives the carbonyl carbon the lowest possible number. For instance, we name the compound CH3COCH2CH2CH3 as 2-pentanone.
Furthermore, we can synthesize ketones via the oxidation of secondary alcohols, via ozonolysis of alkenes, etc. Apart from that, the ketones have the ability to undergo keto-enol tautomerism. This process happens, when a strong base takes up the α-hydrogen (hydrogen attached to the carbon, which is next to the carbonyl group). The ability to release the α-hydrogen, makes ketones more acidic than corresponding alkanes.
What is the Difference Between Aldehyde and Ketone?
Aldehyde is an organic compound having the general chemical formula R-CHO while ketone is an organic compound having the general chemical formula R-CO-R’. The key difference between aldehyde and ketone is that the functional group of an aldehyde occurs always at a terminus whereas the functional group of a ketone always occurs in the middle of a molecule. Furthermore, aldehydes are usually more reactive than ketones.
As another important difference between aldehyde and ketone, we can say that aldehydes can undergo oxidation to form carboxylic acids, but ketones cannot undergo oxidation unless we break down its carbon chains. The below infographic on difference between aldehyde and ketone presents a more detailed comparison.
Summary – Aldehyde vs Ketone
Both aldehydes and ketones are organic compounds. The key difference between aldehyde and ketone is that the functional group of an aldehyde occurs always at a terminus whereas the functional group of a ketone always occurs in the middle of a molecule.
1. Libretexts. “Nomenclature of Aldehydes & Ketones.” Chemistry LibreTexts, National Science Foundation, 2 Oct. 2018. Available here
2. “Ketone.” Wikipedia, Wikimedia Foundation, 7 Nov. 2018. Available here