Physical and Chemical Properties of Ketones

Inorganic compounds such as ketones belong to a class known as heterocyclic compounds. On their surface is a carbonyl group, which is a double bond formed by the elements carbon and oxygen (-C=O). The absence of any other reactive groups in their structure, such as –OH or –Cl, distinguishes them from more complicated compounds. In the case of aldehydes and ketones, the presence of the carbonyl group has a significant impact on the chemistry of compounds.

Ketones

Ketones are carbonyl compounds that contain substituents that contain carbon on both sides of the carbon-oxygen double bond. Sp2 hybridisation occurs between the carbonyl carbon and the oxygen of the ketone group. Ketones have a trigonal planar structure that is centred on the carbonyl carbon as its centre of attraction. The approximate value of the bond angles in this structure is 1200. Ketones are nucleophilic at the oxygen atom and electrophilic at the carbon atom because the carbon-oxygen bond makes the carbonyl group polar (oxygen has a greater ability to pull electrons than carbon).

Physical Properties of Ketones

Physical State

Methane is a noxious gas, but ethanol is a volatile liquid that boils at around 21oC. Colourless liquids are aldehydes and ketones with 11 carbon atoms or more.

Smell

Aldehydes and ketones, except lower carbon number aldehydes, are generally pleasant to the nose. Aldehyde and ketone molecules grow less pungent and more aromatic as the molecule’s size increases.

Bonding

This functional group is carbon double-bonded to oxygen. Because oxygen is more electronegative than carbon, it draws electrons in a carbon-oxygen bond towards itself. There are diffuse positive and negative charges from the carbon and oxygen atoms, respectively. It becomes highly polarised. A nucleophile can attack the slightly positive carbon atom, while electrophiles attack the slightly negative oxygen atom.

The carbon-oxygen double bond is broken in carbonyl compound reactions. Each has a carbonyl group.

Hydrogen Bonding

Aldehydes and ketones have dipoles that limit their usefulness as hydrogen bond acceptors. Not because there is a hydrogen atom on the carbonyl oxygen atom, this is because these chemicals easily form H-bonds with polar molecules like water.

Because aldehydes and ketones lack hydrogen atoms that are directly attached to the carbonyl oxygen atom, intermolecular hydrogen bonding is absent. They can establish hydrogen bonds with water molecules because the carbonyl oxygen acts as a hydrogen bond acceptor. Their boiling points are higher than alkanes and ethers because of hydrogen bonding. Aldehydes and ketones are water-soluble due to hydrogen bonding.

Solubility

All aldehydes and ketones are miscible in water. For example, ethane, methane, and propanone are all miscible with water. Aldehydes and ketones can establish hydrogen bonds with water molecules but not with each other. An aldehyde or ketone atom’s oxygen attracts one of the lone pairs on its hydrogen atom to establish a hydrogen bond. Aldehydes and ketones are water-soluble due to a hydrogen connection between the polar carbonyl group and water molecules.

Boiling Point

Because of the polarity of the carbonyl (>C=O) group, the boiling temperatures of aldehydes and ketones are greater than those of their non-polar counterparts with comparable molecular masses. Their boiling point, on the other hand, is lower than that of their equivalent alcohols or carboxylic acids. This is due to the fact that aldehydes and ketones, being polar compounds, have sufficient intermolecular dipole-dipole interactions between the carbonyl dipoles at their opposing ends.

Chemical Properties of Ketones

Nucleophilic Addition

Nucleophilic addition reactions are a type of chemical reaction in which two nucleophiles combine to form a nucleophile. The following mechanisms characterise nucleophilic addition reactions:

In the polar carbonyl group, a nucleophile contacts the electrophilic carbon atom from a direction that is essentially perpendicular to the plane of the sp2 hybridised orbitals of the polar carbonyl group. A tetrahedral alkoxide intermediate is formed as a result of the hybridisation of carbon, which shifts from sp2 to sp3 in this reaction. A proton is captured by this intermediate and transferred to the reaction medium, yielding the electrically neutral product when you put Nu– and H+ together across the carbon-oxygen double bond.

Reactivity

Aspects of reactivity include steric and electronic differences, which make aldehydes more reactive than ketones in nucleophilic addition processes. Because ketones have two relatively big substituents, the approach of a nucleophile to carbonyl carbon is more difficult in ketones than it is in aldehydes, which contain only one of these substituents.

Reaction with Hydrogen Cyanide

Aldehydes and ketones react with hydrogen cyanide (HCN) to produce cyanohydrins, which are a type of cyanide compound. In the presence of pure HCN, this reaction takes place very slowly. It is therefore catalysed by a base, and the cyanide ion (CN–), which is a stronger nucleophile than carbonyl compounds, quickly reacts with them to produce the appropriate cyanohydrin. A class of synthetic intermediates known as cyanohydrins exists.

Addition of Sodium Hydrogen Sulphite

Sodium hydrogen sulfite reacts with aldehydes and ketones to create the compounds known as additional products.

Addition of Alcohol

Aldehydes, when combined with monohydric alcohol in the presence of dry HCl, generate hemiacetal, which, when combined with one additional molecule of alcohol, transforms into acetal.

Addition of Ammonia and its Derivatives

Nucleophiles such as ammonia and its derivatives H2N-Z react with the carbonyl group of aldehydes and ketones to form a bond. Here, Z can be any of the following: alkyl, aryl, OH, NH₂, C₆H₅NH, NHCONH₂, or any other combination.

Reduction

When aldehydes and ketones react with sodium borohydride (NaBH₄) or lithium aluminium hydroxide, they form primary and secondary alcohols, respectively (LiAlH4).

Conclusion

The carbonyl group (>C=O) is present in ketone. The nature of the carbonyl group has a significant impact on the physical properties of ketones. This article provides an overview of the physical properties of ketones, with a focus on the solubility and boiling points of these organic substances. It was also revealed that the boiling point of these compounds is higher than that of their hydrocarbon equivalents. We also learned why alcohols and carboxylic acids have greater boiling temperatures than their comparable aldehyde and ketone acid counterparts.