Aldehydes and ketones are simple compounds consisting of the carbonyl group – a carbon-oxygen double bond. These compounds cannot have any other reactive groups like \( – {\rm{OH}}\) or \( – {\rm{Cl}}\) attached directly to the carbon atom in the carbonyl group.

In this article, let us learn everything about the physical properties of aldehydes and ketones in detail. Read on to find more.

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Aldehydes

In aldehydes, one end of the carbonyl group \(( > {\rm{C}} = {\rm{O}})\) has a hydrogen atom attached. The other end of the carbonyl group is attached to a second hydrogen atom or a hydrocarbon group, an alkyl or aryl group. The general structural formula of aldehydes is \(\left( {{\rm{R}} – {\rm{CHO}}} \right)\), where R refers to the alkyl or aryl group.

An aldehyde group always lies at the end of a carbon chain. The general molecular formula of the homologous aldehyde series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}} + 1}}{\rm{CHO}}\), where \({\rm{n}} = 1,\,2,\,3 \ldots \)

Some examples of aldehydes are:

The aldehyde group \(\left( { – {\rm{CHO}}} \right)\) is always present at the end of an organic compound. In all the examples above, the compounds have precisely the same end to the molecule. All that differs is that the complexity of the alkyl group \(\left( { – {\rm{R}}} \right)\) attached.

While writing the formulae for these, the aldehyde group (the carbonyl group with the hydrogen atom attached) is always written as \( – {\rm{CHO}}\) and never as \( – {\rm{COH}}\). That could easily be confused with alcohol. Methanal, for example, is written as \({\rm{HCHO}}\); Ethanal as \({\rm{C}}{{\rm{H}}_3}{\rm{CHO}}\). The name counts the total number of carbon atoms in the longest chain, including the carbonyl group. If side groups are attached to the chain, the count should always begin from the carbon atom in the carbonyl group.

Ketones

In ketones, both the ends of the carbonyl group have two hydrocarbon groups attached: either alkyl or aryl groups. The general structural formula of ketones is \(\left( {{\rm{R – CHO}}} \right){\rm{,}}\) as shown below:

The ketonic functional group always lies within the carbon chain. Hence, the simplest compound with the ketonic functional group is acetone or propanone.

Propanone is normally written \({\rm{C}}{{\rm{H}}_3}{\rm{COC}}{{\rm{H}}_3}\). As the carbonyl group of a ketone is always present within the carbon chain, the position of the carbonyl \(( > {\rm{C}} = {\rm{O}})\) group is necessary to mention. For example, in pentan-\(3\)-one, the carbonyl \(( > {\rm{C}} = {\rm{O}})\) group is present at \({\rm{C}} – 3\) of the five-carbon chain. Hence, the name pentan-\(3\)-one.

Where Do Aldehydes and Ketones Differ?

A ketone differs from an aldehyde by attaching an alkyl group to the carbonyl group in place of the hydrogen atom. The presence of this hydrogen enables easy oxidation of aldehydes.

The absence of this hydrogen makes ketones resistant to oxidation. They are only oxidised by powerful oxidising agents who can break carbon-carbon bonds.

Physical Properties of Aldehydes and Ketones

The physical properties of aldehydes and ketones are explained below:

1. Physical State

Methanal is a pungent-smelling gas, whereas Ethanal is a volatile liquid and boils at a temperature close to \({21^{\rm{o}}}{\rm{C}}\). Other aldehydes and ketones continuing up to eleven carbon atoms are colourless liquids. However, the higher members are solids.

2. Smell

The lower aldehydes and ketones have a sharp pungent smell; however, as the size of the alkyl group increases, the odour becomes less unpleasant and more fragrant. In fact, many naturally occurring aldehydes and ketones are widely used in perfumes (Jasmone, acetophenone) and flavouring agents (benzaldehyde, vanillin).

3. Bonding

The carbonyl \(( > {\rm{C}} = {\rm{O}})\) functional group comprises a carbon atom double-bonded to the oxygen atom. Oxygen is more electronegative than carbon, and due to this electronegativity difference, oxygen has a strong tendency to pull electrons in a carbon-oxygen bond towards itself. This results in dipole formation with a slight positive charge over the carbon atom and a slight negative charge over the oxygen atom. Hence, the carbon-oxygen double bond becomes highly polar.

The slightly positive carbon atom in the carbonyl group can be attacked by a nucleophile, and a slight negative oxygen atom is attacked by electrophiles.

In a reaction that involves carbonyl compounds, the carbon-oxygen double bond gets broken. Both ketones and aldehydes contain a carbonyl group. Hence, their reactions are very similar in this respect.

4. Hydrogen Bonding

The dipoles present in aldehydes and ketones allow these compounds to function only as Hydrogen bond acceptors. This is because no hydrogen atom is directly attached to the carbonyl oxygen atom. However, these compounds readily participate in energetically favourable hydrogen bonding (\({\rm{H}}\)-bonding) interactions with polar molecules like water.

Due to the absence of hydrogen atoms bonded directly to the carbonyl oxygen atom, aldehydes and ketones can’t hydrogen bond with themselves, resulting in the absence of intermolecular hydrogen bonding. However, due to a carbonyl oxygen atom that acts as a hydrogen bond acceptor, these carbonyl compounds can form a hydrogen bonds with water molecules.

Effect of hydrogen bonding in aldehydes and ketones

1. The hydrogen bonding leads to strong intermolecular forces of attraction, increasing their boiling points compared to their analogous alkanes and ethers.
2. The presence of hydrogen bonding accounts for the solubility of aldehyde and ketones in water.

5. Solubility

Aldehydes and ketones (four carbon atoms) are miscible in water. For example, ethanal, methanal, and propanone are miscible with water in all proportions. Aldehydes and ketones cannot form a hydrogen bond with themselves but can hydrogen bond with water molecules. One of the slightly positive hydrogen atoms in a water molecule is attracted to one of the lone pairs on the oxygen atom of an aldehyde or ketone to form a hydrogen bond.

The presence of a hydrogen bond association between the polar carbonyl group and water molecules accounts for the solubility of aldehyde and ketones in water.

However, as the length of the alkyl chain (carbon chain) increases, the solubility of aldehydes and ketones in water decreases rapidly. This is because the greasy alkyl chain starts interfering with water solubility. They break the relatively strong hydrogen bonds between water molecules without replacing them with anything, making the process energetically less profitable, so solubility decreases.

Hence, the higher members (more than four carbon atoms) are insoluble in water. However, aldehydes and ketones are soluble in organic solvents (like dissolves like) such as benzene, ether, chloroform, and alcohol. 

However, there exist dispersion forces and dipole-dipole attractions between the aldehyde or ketone and the water molecules. Due to these attractions, sufficient energy is released. This energy is used to separate the water molecules and aldehyde or ketone molecules from each other before mixing.

6. Boiling Point

Due to the polarity of the carbonyl \(( > {\rm{C}} = {\rm{O}})\) group, the boiling points of aldehydes and ketones are higher than their corresponding non-polar compounds of comparable molecular masses. However, their boiling point is lower than their analogous alcohols or carboxylic acids. This is because aldehydes and ketones being polar, have sufficient intermolecular dipole-dipole interactions between the opposite ends of the carbonyl dipoles.

Methanal is a gas (boiling point around \( – {21^{\rm{o}}}{\rm{C}}\)), whereas Ethanal has a boiling point of \( + {21^{\rm{o}}}{\rm{C}}\)). Hence, the boiling point rises as the molecules get more prominent. The strengths of the intermolecular forces govern the trend of the boiling point.

1. Van der Waals dispersion forces: These attractions get more robust with an increased number of electrons. As the molecules get longer, the number of electrons increases, increasing the sizes of the temporary dipoles. Due to this reason, the boiling points increase as the number of carbon atoms in the chains increases.
2. Van der Waals dipole-dipole attractions: The presence of the carbon-oxygen double bond imparts polarity to Both aldehydes and ketones. Besides dispersion forces, there are attractions between the permanent dipoles on neighbouring molecules. Hence, the boiling points of polar molecules will be higher than those of similarly sized hydrocarbons – which only have dispersion forces.

Comparison between similarly sized molecules that have similar lengths and a similar number of electrons.

	Type	Boiling Point \(^{\rm{o}}{\rm{C}}\)
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}\)	alkane	\(-42\)
\({\rm{C}}{{\rm{H}}_3}{\rm{CHO}}\)	aldehyde	\(+21\)
\({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\)	alcohol	\(+78\)

From the table above, we can conclude that the aldehyde (with dipole-dipole attractions as well as dispersion forces) has a higher boiling point than the similarly sized alkane, which only has dispersion forces. However, the aldehyde’s boiling point is lower than that of the alcohols. This is because, in alcohol, there is hydrogen bonding as well as dipole-dipole attractions and dispersion forces.

Summary

Aldehyde and ketones both consist of the carbonyl group \(( > {\rm{C}} = {\rm{O}})\). The physical properties of these carbonyl compounds primarily depend on the nature of the carbonyl group. In this article, we learned some of the physical properties of aldehydes and ketones especially pertaining to their solubility and boiling points. We also learned how these compounds have an elevated boiling point than their hydrocarbon counterparts. We also learned why alcohols and carboxylic acids have higher boiling points than analogous aldehyde and ketone acids.

FAQs on Physical Properties of Aldehydes and Ketones

Q.1. What are the physical properties of aldehydes and ketones?
Ans: The physical properties of aldehydes and ketones are:
Physical state: Lower aldehydes and ketones are volatile in nature. However, aldehydes and ketones up to eleven carbon atoms are colourless liquids. The higher members are solids.
Odour: The lower aldehydes and ketones have a sharp pungent smell; however, as the size of the alkyl group increases, the odour becomes less unpleasant and more fragrant.
Solubility: Aldehydes and ketones upto four carbon atoms are miscible in water. The hydrogen bond between the polar carbonyl group and water molecules accounts for the solubility of aldehyde and ketones in water. However, as the length of the alkyl chain (carbon chain) increases, the solubility of aldehydes and ketones in water decreases rapidly.
Boiling point: The polarity of the carbonyl \(( > {\rm{C}} = {\rm{O}})\) group accounts for the higher boiling points of aldehydes and ketones are than their corresponding non-polar compounds of comparable molecular masses. However, their boiling point is lower than those of their analogous alcohols or carboxylic acids.

Q.2. What is the functional group for aldehydes and ketone?
Ans: Aldehydes and ketones are organic compounds comprising of the carbonyl functional group \(( > {\rm{C}} = {\rm{O}})\).

Q.3. Explain Three Physical Properties of Aldehydes and Ketone.
Ans: Odour: The lower aldehydes and ketones have a sharp pungent smell; however, as the size of the alkyl group increases, the odour becomes less unpleasant and more fragrant.
Solubility: Aldehydes and ketones upto four carbon atoms are miscible in water. The hydrogen bond between the polar carbonyl group and water molecules accounts for the solubility of aldehyde and ketones in water. However, as the length of the alkyl chain (carbon chain) increases, the solubility of aldehydes and ketones in water decreases rapidly.
Boiling point: Due to the polarity of the carbonyl \(( > {\rm{C}} = {\rm{O}})\) group, the boiling points of aldehydes and ketones are higher than their corresponding non-polar compounds of comparable molecular masses. However, their boiling point is lower than their analogous alcohols or carboxylic acids.

Q.4. What are the properties of a ketone?
Ans: Ketones are highly reactive but less than aldehydes, to which they are closely related. The physical properties and chemical activity in ketones result from the nature of the carbonyl group. Ketones readily undergo a wide variety of chemical reactions that involve the cleavage of the carbonyl group.

Q.5. What happens when acetaldehyde reacts with iodine?
Ans: Acetaldehyde reacts with iodine to form pale yellow precipitate iodoform along with sodium formate.

Q.6. What are the tests to distinguish aldehydes and ketones?
Ans:

Test	Observation
Schiff’s Test:	An aldehyde group is confirmed by the appearance of a pink, red or magenta colour.
Fehling’s Test:	An aldehyde group is confirmed by the formation of a red precipitate.
Tollen’s Test: (Silver Mirror Test)	A shiny silver mirror confirms the presence of aldehydes.
Test with Chromic Acid:	An aldehyde group is confirmed by the formationof green or blue colour precipitate.
Sodium Nitroprusside Test:	The appearance of red colouration shows the presence of ketone.

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