Physical Properties of Aldehydes, Ketones and Carboxylic Acids

Aldehydes and ketones are the simple compounds that involve a carbonyl group (a carbon-oxygen double bond). They are simple in a way that they don not have other reactive groups such as -OH or -Cl attached directly to the carbon atom in the Carbonyl group. In this article, we will consider the simple physical properties of Aldehydes, Ketones and Carboxylic acids such as boiling points, solubility in water.

Key Terms: Organic chemistry, Alcohol, Ketones, aldehydes, carbon, compounds, water, atom, carbonyl group, carbon-oxygen double bond, carboxylic acids

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Definition of Aldehydes and Ketones

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Aldehydes and Ketones are part of the organic compound class. Organic compounds are characterised by the presence of a carbonyl group or carbon-oxygen double bond (-C=O). Since the electronegativity is different in carbon and oxygen, the bond in the carbonyl group is polar. They are particularly simple compounds because they do not have reactive groups like -OH or -Cl attached to the carbon atom in -C=O.

Bond in the carbonyl group 

In aldehydes, the carbonyl group is attached to carbon and hydrogen. As opposed to this, in ketones, the carbonyl atom is attached to two carbon atoms.

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Physical Properties of Aldehydes and Ketones

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The physical properties of aldehydes and ketones are as below.

Physical State

While Ethanal is a volatile liquid, Methanal is a gas at room temperature. Similarly, other aldehydes and ketones are either gas or liquid at room temperature.

Boiling Point

The boiling point of methanal is -19°C and for ethanol it is +21°C. For aldehydes and ketones, the boiling point increases with molecular weight and is also dependent on the strength of the intermolecular forces.
The compounds have higher boiling points than hydrocarbons and ethers of comparable molecular masses because of the weak molecular connection in aldehydes and ketones. But the boiling points are lower in comparison to that of alcohols with the same molecular masses because there is no intermolecular hydrogen bond.

Vander Waals Dispersion Force

Vander Waals Dispersion Force: The boiling point of aldehydes and ketones depend on the carbon atoms. When the molecules lengthen and the number of electrons increases, the bond increases and the boiling point increases.

Van Der Waals dipole-dipole attraction: Due to the presence of the double bond between carbon and oxygen, aldehydes and ketones are polar to each other. This leads to an attraction between the permanent dipoles and other molecules near it. This is what makes the boiling point of these compounds high.

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The table below lists the boiling points of compounds in increasing order of molecular mass.

Solubility

Aldehydes and ketones are soluble in water. The solubility decreases when the length of the alkyl chain increases. Aldehydes and ketones like methanol, ethanal and propanone are miscible in water of all quantities. These compounds cannot form hydrogen bonds on their own but are able to do so with water due to the dipole-dipole attraction.

Solubility

Smell

All lower aldehydes have a strong and unpleasant smell. Other aldehydes and ketones have a pleasant smell. When the molecule size of the compounds increases, the smell becomes less pungent. Naturally occurring aldehydes and ketones are used regularly in flavouring agents and perfumes like vanilla flavouring.

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Definition Carboxylic Acids

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Carboxylic Acids are a part of the -COOH group or also called the carboxyl group. Carboxylic acids have a (C=O)OH group fixed to an R group. Here, R refers to the remaining part of the molecule. The carbonyl group attaches to a hydroxyl group thus making the name, carboxylic acids. These acids occur in nature but can also be manufactured in laboratories.

Carboxylic Acids​

Carboxylic acids are denoted by the formula R-COOH. The acids are polar in nature and also can be aliphatic (RCOOH) or fragrant (ArCOOH). Carboxylic acids are used in the manufacture of pharmaceuticals, food additives, polymers, solvents etc. Other examples of carboxylic acids include citric acids, acetic acids, adipic acids etc.

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Physical Properties of Carboxylic Acids

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Physical State

Carboxylic acids, at room temperature, are colourless liquids. These acids have nine carbon atoms or less. Higher acids are more waxy and are solid.

Boiling Point

Compared to aldehydes, ketones and other compounds of similar molecular masses, carboxylic acids have high boiling points. The reason behind such high boiling points is for the acid molecules ability to substantially associate with each other through intermolecular hydrogen bonding. The hydrogen bonds do not break up completely even in the vapour state as a result. Most carboxylic acids are present as dimers in the vapour state.

Acetic acid and dimer​

Smell

At room temperature, carboxylic acids have shown to possess unpleasant odours.

Solubility

The more simple aliphatic components of carboxylic acids have four carbon atoms and are miscible or soluble in water. This is due to its ability to form hydrogen bonds with water. When the number of atoms in the carboxylic acids is increased, the solubility nature decreases. For higher members of the same group, the immiscibility nature can be attributed to its hydrophobic communication of the hydrocarbon part of the acid. However, they are able to become soluble in organic solvents that are less polar like alcohol, benzene, ether etc.

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