Aldehydes are organic compounds that possess a carbonyl group in which the carbon atom is located bonded to an oxygen atom via a double bond and at least one hydrogen atom through a single bond ( H-C=O).

E.g. Formaldehyde(HCHO)

Many aldehydes have a characteristic odour. Aldehydes on oxidation yield acids, and if they undergo a reduction reaction, they produce alcohols. They are used in numerous chemical reactions and readily undergo polymerization. Because of this property of causing polymerization, aldehydes in combination with other molecules are used to make plastics.

In addition, they are used as large-scale industrial materials as solvents, monomers, perfume ingredients, and intermediates. Various sugar, natural and synthetic hormones, and compounds like retinal, a by-product of vitamin-A, and pyridoxal phosphate (a form of vitamin B6) are aldehydes.

 

Both the physical and chemical properties of aldehydes differ from hydrocarbons because of the presence of the carbonyl group in aldehydes.

The carbonyl group is intrinsically polar like the electrons, making the C=O bond more proximate to the oxygen atom than the carbon atom. Because of this, oxygen acquires a partial negative charge, and the carbon atom obtains a partial positive charge. This polarity is represented through a greek letter - Delta.

 

Hydrocarbons have low melting and boiling points as they are non-polar. But the carbonyl compounds have high melting and boiling points as they are polar. 

E.g., Butane (Hydrocarbon), Propanal (Aldehyde), and acetone (Ketone) have the same molecular weight (58), but their boiling points differ.

The boiling point of Butane is zero degrees centigrade; Propanal has forty-nine degrees centigrade, and acetone has fifty-six degrees centigrade. This significant difference in the boiling points is because polar molecules have a more substantial attraction than non-polar molecules; therefore, more energy or higher temperature is required to separate them.

Polar molecules don't mix easily with non-polar molecules; thus, hydrocarbons are insoluble in water. Likewise, though low molecular weight carbonyl compounds (with up to 5 carbon atoms) are soluble in water, high molecular weight compounds are insoluble.

Dipole moment is a number used to quantify the polarity of the molecules. Hydrocarbons have no or insignificant dipole moment, whereas, for aldehydes, it is considerably high.

 

Ketones are also organic compounds with a characteristic carbonyl group, in which a carbon atom is bonded with an oxygen atom through a covalent bond. The other two carbon bonds are either with other carbon atoms or hydrocarbon radicals.( -C=O).

E.g., Acetone (CH₃COCH₃)

Ketones have many physiological properties and are also part of many sugars and medicinal compounds like steroid hormones. For example, Cortisone, a steroid hormone with anti-Inflammatory properties, contains three ketone groups.

They are ideal chemical intermediaries as they are easy to prepare and relatively stable with high reactivity. Ketones are the building blocks of many complex organic compounds. For example, enterprises manufacturing explosives, dyes, paints, and fabrics use solvents. They are also used in hydraulic fluids, tanning, and preservatives.

 

As Ketones have a polar carbonyl group, they are polar in nature. Because of this, they have higher boiling points than non-polar compounds. Ketones cannot construct any intermolecular hydrogen bond-like alcohols as no hydrogen is connected to an oxygen atom. They have more effective dipole moments than alcohols or ethers because of the movement of electrons between carbon and oxygen atoms.

They respond with hydrogen cyanide to create cyanohydrins. The reaction usually happens in the existence of a base, serving as a catalyst when a base is missing; the reaction rolls gradually. Most of the ketones constitute bisulphite addition products when added to sodium bisulphite.

 

Carboxylic Acids are named so that they contain carbonyl and hydroxyl groups both. They are organic compounds in which a carbon atom is bonded with an oxygen atom with a double bond and a hydroxyl group (-OH) with a single bond. The fourth bond of carbon links with a hydrogen atom or some other combining group. (OH-C=O)

 

E.g., Formic acid (HCOOH)

 

Their Acidity is the main chemical characteristic. Though they are generally weaker than the mineral acids like hydrochloric acid sulphuric acid, they are more acidic than other hydroxyl groups ( -OH) containing compounds.

When the -OH of the carboxyl group gets replaced by some other groups, it gives rise to compounds known as carboxylic acid derivatives. The most important ones are -

1. Acid anhydrides

2. Ester

3. Acyl halides

4. Amides

These derivatives have wide and varied applications. E.g., Formic acid is used as an acid-reducing agent in textile treatment. Cellulose plastics are extensively produced by using acetic acid. Aspirin, an ester of salicylic acid, is also made from acetic acid.

Palmitic and stearic acid is crucial in manufacturing soaps, makeups, medicines, candles, and protective coatings. Stearic acid is also used in the manufacturing of rubber.

 

Acidity

It is the essential property of carboxylic acids and is responsible for naming them as acids. An acid is a compound that grants a hydrogen ion or proton, H+, to another compound called a base. They do it much more quickly than the other classes of organic compounds, therefore are more potent acids, even though they are much weaker than the other most important mineral acids—sulphuric (H₂SO₄), nitric (HNO₃), and hydrochloric (HCl).

Alcohols are neutral in an aqueous solution. When alcohol donates its hydrogen ion, it becomes a negative ion known as alkoxide ion, RO−. When a carboxylic acid donates its hydrogen ion, it becomes a carboxylate ion - a negatively charged ion, RCOO−

This carboxylate ion is much more steady than the related alkoxide ion because of the presence of resonance structures for the carboxylate ion that disbands its negative charge. You should know that you can draw two for a carboxylate ion, whereas you can draw just one structure for an alkoxide ion. When for a molecule or ion, two or more forms that differ only in the position of valence electrons can be drawn, which means its valence electrons are delocalized or dispersed over two atoms.

This procedure is called resonance, and the created structures are called resonance forms. Resonance secures a molecule or ion even if there is no charge collusion. The stability of the consequent anion demarcates the power of its parent acid. Therefore, the carboxylic acid is a more robust acid than the corresponding alcohol because a more steady anion loses its hydrogen ion.

Solubility

The water solubility of carboxylic acids is identical to aldehydes, alcohols, and Ketones. You need to know that carboxylic acids with less than five carbons dissolve in water. In contrast, higher molecular weight carboxylic acids are insoluble owing to the more significant hydrocarbon portion, which is hydrophobic. Carboxylic acids sodium, ammonium, and potassium salts are soluble in water.

Thus, nearly any carboxylic acid can liquefy in water by transforming it to salt by adding a powerful base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). For preserving cheese, bread, and other baked products, sodium and calcium salts of propanoic (propionic) acid are used.

Boiling Point

They have pretty high boiling points compared to hydrocarbons, alcohols, aldehydes, ethers, or ketones of equivalent molecular weight. Let us understand this by an example; the formic acid boils at 101 °C (214 °F), which is higher than the boiling point of ethanol, that boils at 78.5 °C (173 °F), although both of them have nearly equal molecular weights.

The dissimilarity is that two carboxylic acid molecules constitute two hydrogen bonds; on the other hand, two alcohol molecules can only construct one. Therefore, carboxylic acids prevail as dimers (pairs of molecules), not only in the liquid state but also in the gaseous form.

Carboxylic acid boiling demands the proliferation of more heat than boiling the connected alcohol because -

1.  If the dimer prevails in the gaseous state, the molecular weight in effect is doubled.

2.  If the dimer is busted upon boiling, extra energy is needed to break the two hydrogen bonds.

Carboxylic acids are solids at room temperature (e.g., benzoic and palmitic acids).

Odour

Fatty acids (Unbranched-chain carboxylic acids) are liquids at room temperature, particularly those from propanoic (C3) to decanoic (C10) acid, which have very nasty, unpleasant odours. E.g., Butanoic (Butyric) acid (C4), the main ingredient in stale perspiration, leading to a "locker-room" odour.

This was about Aldehydes, Ketones, and Carboxylic Acids, and if you want to understand them better, it is recommended to opt for the fantastic application MVSgo.