Aldehydes and ketones are types of organic compounds that contain a carbonyl functional group. Although they share some similar chemical and physical properties, they have distinct chemical structures and their functional groups are in different locations. Aldehydes are also typically more chemically reactive than ketones.
Continue reading to learn more about aldehydes and ketones, their defining properties and the key differences between them.
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The differences between aldehyde and ketone
From a chemical point of view, aldehydes and ketones are very similar – they’re both organic compounds and they both contain a carbonyl functional group.
The carbonyl functional group is made up of a carbon that’s double-bonded to an oxygen. However, there are also some crucial differences between the two compounds. The main differences relate to the type of groups or atoms that are bonded to the central carbon and the position of the carbonyl functional group.
- Aldehydes have an R group chain of hydrocarbon substituents and a hydrogen attached to the central carbon
- Ketones have R and R’ substituents attached to the central carbon
- In aldehydes, the carbonyl functional group is at the end of the chain
- In ketones, the carbonyl functional group is attached to a carbon within the chain
Chemical structure and configurations
Aldehydes have the general formula CnH2n+2. CHO. Their structure can be generalised as follows:
As you can see, the carbonyl is located at the end of the carbon chain. The R group represents a radical hydrocarbon chain.
Ketones, meanwhile, have the general formula CnH2nO. Their generalised structure is represented as follows:
The R and R’ groups are two different hydrocarbon substituents.
5 differences between aldehydes & ketones
Although ketones and aldehydes are similar in many respects, they are chemically distinct from each other. Here are five key differences between them:
- Generalised forms – Aldehydes and ketones both have several examples but their generalised forms are different. Aldehydes have the generic form R-CHO, while ketones have the generic form R-CO-R’. This point of difference is crucial in determining certain properties such as their miscibility in water and reactivity with other chemicals.
- Reactions with other chemicals – Aldehydes and ketones have distinct molecular structures, which means they react differently with other chemicals. Aldehydes are typically more chemically reactive than ketones and can be easily oxidised. When aldehydes undergo oxidation they form carboxylic acid, which is often used as a precursor for synthesising other chemicals such as biopolymers, adhesives, and pharmaceutical drugs.
- Location of the functional group – Although carbonyl is the functional group present in both aldehydes and ketones, its location differs. In aldehydes, the functional group is at the end of the carbon chain, but in ketones, it’s in the middle.
- Common occurrence – Both aldehydes and ketones are biologically produced in nature as metabolic byproducts. However, aldehydes are more commonly present in fragrances and other volatile compounds, while ketones are typically found in sugars produced by plants. Mammals like humans may use ketones as an alternative source of energy when their blood sugar is low.
- Standardised naming – Chemicals are known by their common names and their internationally standardised names, the most universally accepted of which is the IUPAC nomenclature. Under the IUPAC nomenclature, aldehydes include the suffix -al while ketones have the suffix -one. For example, the simplest ketone is commonly known as acetone but its formal IUPAC name is Propan-2-one.
Acidity level differences
The acidity of a substance is typically determined by the pH level. A pH of less than 7 is considered acidic (it becomes more acidic as the value decreases). According to the Brønsted–Lowry definition of acids and bases, an acidic substance has a tendency to donate its proton (hydrogen ion).
Acidity is also measured in terms of the dissociation constant. The dissociation constant for an acid is equal to the multiplication products of the concentration of hydrogen ions and base conjugate over the concentration of the acid dissolved in water.
The pKa of an acid is the inverse of the dissociation constant. If we calculate the relative pKa values of ketones and aldehydes, the latter has a lower value, which means it’s more acidic. While both types of compounds are miscible in water, aldehydes have a lower dissociation constant than ketones.
Are aldehydes or ketones more acidic?
Aldehydes are more acidic than ketones because they have a lower pKa value (primarily due to the location of the carbonyl functional group). The carbonyl group in aldehydes has an alpha hydrogen at the end of the carbon chain. This hydrogen can be easily removed by a base substance, which forms a reactive carbanion.
Reasons for distinct classification
Although aldehydes and ketones both have a carbonyl functional group, its placement differs. As a result, the two compounds are chemically distinct and have different chemical and physical properties.
Role of the carbonyl functional group
The defining properties of aldehydes and ketones such as their volatility, miscibility with water and reactivity are primarily determined by the carbonyl functional group. Its location also affects which products are formed when the compounds react with other substances.
How a single hydrogen atom defines properties
Although it might not seem significant, the presence of a single hydrogen atom attached to the carbonyl group largely defines the properties of aldehydes and ketones. It also determines their propensity to donate or accept electrons (nucleophilic or electrophilic reactions). Aldehydes have a greater tendency to donate the hydrogen atom that’s attached to the functional group located at the end of the chain.
Summary
Despite having the same functional group and some similar chemical and physical properties, aldehydes and ketones are chemically distinct. The location of the carbonyl functional group is the main reason for this. The functional group is also responsible for other key differences such as their acidity level and reactivity with other chemicals.
