Aldehydes, Ketones and Carboxylic Acids

Physical Properties of Aldehydes, Ketones and Carboxylic Acids

All of us have had a vanilla cake or vanilla flavoured ice-cream at some point in our life. How many of us have thought how the particular flavour and fragrance is found in vanilla beans. Aldehydes and ketones help in addition of flavour and fragrance to nature.

Few examples include cinnamaldehyde that adds flavour and fragrance to cinnamon, vanillin adds flavour and fragrance to vanilla beans, salicylaldehyde adds flavour and fragrance meadowsweet. Aldehydes and ketones are an essential component of many industrial processes such as solvent, polymer precursors, food, perfumes, and pharmaceuticals.

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They form an essential part of biochemical processes such as photosynthesis and Krebs cycle. Do you know medical conditions such as “inborn errors of metabolism” requires consumption of ketone associated foods? Moreover, most sugars are aldehyde derivatives. Even few sugars are the ketone. All of us have heard about fructose. Let us study about aldehydes and ketones which form an integral part of many industrial as well as natural processes.

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Aldehydes and Ketones

Aldehydes and ketones are one of the classes of organic compounds. They have carbonyl group, a double bond between carbon-oxygen (-C=O), attached to them. They are simple compounds as they lack any other reactive groups such as –OH or -Cl in their structure.  Presence of carbonyl group highly influences the chemistry of aldehydes and ketones.

Physical Properties of Aldehydes & Ketones

1) Boiling Point

At room temperature, methanol behaves as a gas whereas ethanol is in liquid form that is volatile in nature. The boiling point of methanol and ethanol is -19o C and +21o C. Thus, the boiling point of ethanol is nearly at room temperature. Moreover, all other aldehydes and ketones are either liquid or solid at room temperature.

The boiling point of these compounds increases with increase in molecular weight. Additionally, the strength of intermolecular forces is also responsible for the boiling point of aldehydes and ketones. However, the boiling points of these organic compounds are higher in comparison to hydrocarbons or ethers having nearly similar molecular masses.

The reason for such behaviour is the weak molecular association of these compounds occurring due to dipole-dipole interactions. Similarly, the boiling of aldehydes and ketones are lower than alcohol of nearly same molecular masses. The reason is lack of intermolecular hydrogen bonding.

Vander Waals Dispersion Force

The boiling point of aldehydes and ketones depends on the numbers of the carbon atom. It increases with increase in the number of atoms of carbon. The longer the molecules become and with the increase in the number of electrons, the attraction between the compounds increases.

Vander Waals Dipole-Dipole Attraction

Aldehydes and ketones are polar in nature due to the presence of the carbon-oxygen double bond. This creates an attraction between the permanent dipoles and with the nearby present molecules. Hence, the reason why this compound has a higher boiling point in comparison to the hydrocarbons of similar size.

Refer to the table below to note the arrangement of boiling points in the increasing order of the compounds having molecular masses from 58 to 60.

Name of the Compound Molecular mass Boiling Point
n-Butane 58 273
Methoxymethane 60 281
Propanal 58 322
Acetone 58 329
Propan-1-ol 60 370

2) The Solubility of Aldehydes and Ketones

Generally, these aldehydes and ketones are soluble in nature with respect to water. However, the solubility gradually decreases with the increase in the alkyl chain length. Therefore, lower members such as methanal, ethanal, and propanone demonstrate miscible nature with all proportions of water.

This happens due to the ability of the lower members of the aldehydes and ketones to develop hydrogen bong with water. However, these compounds are unable to form hydrogen bonds with themselves. The reason for such behaviour is dispersion forces and dipole-dipole interaction.

Usually, all aldehydes and ketones are relatively soluble in organic solvents such as ether, methanol, benzene, chloroform, etc. The lower members of these classes of compounds demonstrate the characteristic sharp pungent odours but the odour converts to more fragrant smell with an increase in the size of molecules.

Hence, aldehydes and ketones are used in different industrial applications. In fact, there are certain naturally occurring aldehydes and ketones that help in the blending of perfumes and also act as flavouring agents.

Physical Properties of Carboxylic Acids

Aliphatic carboxylic acids which consist of nine carbon atoms or less are colourless liquids at room temperature. They are characterized by very unpleasant smell/ odour. The higher members of this class of compounds are odourless and are present in the form of wax-like solids because of their low volatile nature.

The boiling points of carboxylic acids are higher than the comparable molecular masses of aldehydes, ketones, and alcohols. The reason for such behaviour is the ability of carboxylic acids molecules to extensively associate with each other through intermolecular hydrogen bonding. As a result of which, the hydrogen bonds are not broken entirely and remain intact even during the vapour state. Most of this class of compounds are present as dimers during the vapour stage or in the aprotic solvents.

The simple aliphatic members of this class having up to four carbon atoms can dissolve in water because of the ability of these members to develop hydrogen bonds with water. However, the solubility gradually decreases with the increase in the increase in the numbers of atoms of carboxylic acids.

The reason behind the insolubility of higher members of carboxylic acids is the hydrophobic interaction in the hydrocarbon part of the carboxylic acid. Therefore, higher carboxylic acids are insoluble in water. However, they are soluble in less polar organic solvents such as alcohol, benzene, chloroform, ether, etc. Even the simplest aromatic carboxylic acid “Benzoic acid” is almost insoluble in cold water.

Aldehydes and ketones

Dimer of Carboxylic Acid (Source: Wikipedia)

A Solved Question for You

Q. Arrange the given compounds according to the increasing order of their boiling points.


Solution: The arrangement orders of the compounds according to the boiling points are CH3CH2CH2CH3 < H5C2-O-C2H5 < CH3CH2CH2CHO < CH3CH2CH2CH2OH

Explanation: The molecular mass of all the compounds is in between the range 72 to 74. However, the only compound having an extensive intermolecular hydrogen bonding is butan-1-ol will be the highest. We know that Butanal is more polar in nature in comparison to ethoxyethane.

Therefore, the dipole-dipole interaction between the molecules will be greater in case of butanal. N-pentane molecules are having just the weak Vander Waals forces. Therefore, the arrangement of the compounds in increasing order will be CH3CH2CH2CH3 < H5C2-O-C2H5 < CH3CH2CH2CHO < CH3CH2CH2CH2OH

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