Difference Between Aldehydes and Ketones for JEE Main 2024

The important distinction between aldehyde and ketone is that the practical establishment of an aldehyde happens always at a terminus while the functional organization of a ketone constantly takes place in the middle of a molecule.

Aldehydes and ketones are natural molecules with a carbonyl institution. In a carbonyl group, the carbon atom has a double bond to oxygen. The carbonyl carbon atom is sp2 hybridized. Thus, aldehyde and ketones have a trigonal planar structure throughout the carbonyl carbon atom. aldehyde and ketones with better boiling points compared to hydrocarbons of equal weight. However, these cannot make strong hydrogen bonds like alcohol; therefore, they have lower boiling properties than concomitant alcohol. Due to the ability of the hydrogen bond to form, low-molecular-weight aldehyde and ketones dissolve in water. However as the weight of the cells grows, they become hydrophobic.

Aldehydes and ketones are chemical carbonyl compounds from the carbonyl group comprising a double bond between the Carbon and Oxygen atoms (C=O). Despite both having a carbon atom at the centre, the fundamental difference between an aldehyde and ketone lies in their distinct chemical structure. An aldehyde combines to an alkyl on one side and a Hydrogen atom on the other, while the ketones are known for their double alkyl bonds on both sides. 

What are Aldehydes?

Aldehyde has a carbonyl group. This carbonyl family binds with another carbon from one side, and from the opposite case, it connects with a hydrogen atom. Therefore, we can signify aldehydes with the –CHO organization. The simplest aldehyde is formaldehyde. However, this molecule deviates from the general formulation by using a hydrogen atom rather than an R organization.

The aldehydes can be defined as the compounds that have a double bond between Carbon atoms to that of Oxygen atoms, and are generally represented as: 

(R-(C=O)-H), where R represents the alkyl group, and H is the hydrogen atom. 

Aldehydes can be distinguished because of their must-have Hydrogen bond and are generally found at the extreme sides of a Carbon chain. Some useful aldehydes are formaldehyde. 

Compared to Ketones, aldehydes are more reactive and can be reduced to result in alcohol. These alcohols further undergo a reduction to form a carboxylic acid. The IUPAC system distinctly names aldehydes with a suffix 'al' and forms into acetal, propanal, etc. From Tollen's test to Fehling's Test, there are many ways to differentiate between aldehyde and ketone easily, thanks to its distinct chemical composition and super-reactiveness. 

What are Ketones?

In a ketone, the carbonyl group occurs among two carbon atoms. We use the suffix “one” in ketone nomenclature. In preference to “–e” of the corresponding alkane, we use the term “one”. Furthermore, we number the aliphatic chain in a manner that gives the carbonyl carbon the lowest viable variety. As an example, we call the compound CH₃COCH₂CH₂CH₃ 2-pentanone.

Ketones are organic compounds having the carbonyl group C=0 and have alkyl groups on both sides, making them less reactive to that of aldehydes due to the absence of Hydrogen atoms. They are represented in the form of:  

R-(C=0)-R', where R and R' are alkyl groups, present on the left and right sides of the compound. 

They occur in the middle of a Carbon chain because of the presence of alkyl on both ends. The IUPAC approves the naming of ketones with the suffix 'one' like acetone, pentanone, and can undergo reduction to yield respective alcohols. They are generally used as industrial solvents across many manufacturing processes. 

Aldehyde vs Ketone 

The aldehydes are more natural to undergo oxidation because of their Hydrogen atom in one of its sides. The aldehydes form when the primary alcohol compounds are oxidized and can be removed from the mixture via distillation before it forms carboxylic acid. 

The ketones are less reactive to the oxidation process since it lacks the Hydrogen atom, unlike the aldehydes. But once they're exposed to overheating, they can be oxidized with powerful oxidizing agents. It is because of this unique differentiability, that it can help to distinguish between aldehyde and ketone. Both aldehydes and ketones are natural compounds. Both aldehydes and ketones are natural compounds. The main difference between aldehyde and ketone is that the active group of aldehyde occurs most frequently in the terminus while the active association of ketone occurs continuously between molecules.

How to Differentiate Aldehyde and Ketone?

The aldehydes can get easily oxidized with the help of mild oxidizing agents like alkaline solutions of Fehling's Solutions and (Ag⁺) Tollens' Reagent. Because of the difference in aldehyde vs ketone structure, the following tests shall only yield results for the reactive aldehydes. It is through these reactions or tests that one can tell the difference between aldehyde and ketone. 

Fehling’s Test or Benedict's Solution

They are both reagents containing complex copper (II) in an alkaline solution.

• For Fehling's solution, the copper (II) ions are complexed with that of the tartrate ions in a sodium hydroxide solution. The complexing of the copper (II) ions with that of the tartrate ions restrict the formation of a precipitate - Copper (II) hydroxide.

• For Benedict's solution, the copper (II) ions are complex with the citrate ions in a sodium-carbonate solution. Here the copper (II) ions don't lead to the formation of copper (II) carbonate. 

The alkaline solutions contain the complex Copper ions, the colour of the solution is blue. The reagent when comes in contact with the aldehyde upon heating (via water bath), aldehyde breaks down to carboxylic acid, the Cu(II) ions leave a brick-red precipitate known as Copper Oxide,

The reaction is as follows:

RCHO + 2(Cu²⁺)(aq.) + 2H₂O → RCOOH + Cu₂O(s) + 4H+(aq)

Results:

For Ketones, there's no change observed in the natural blue solution of the reagents. 

Tollens' Reagent Test:

The Tollens' reagent comprises complex silver(I) ions, made from the silver nitrate(I) solution. When drops of sodium hydroxide are added, it leads to the formation of silver(I) oxide precipitate, which can further be redissolved by adding dilute ammonia. The resulting solution gives Tollen's reagent.

Aldehydes upon reacting with the Tollen's reagent gives: 

RCHO + 2Ag⁺ (aq) + H₂O → RCOOH + 2Ag(s) + 2H⁺

The aldehyde helps in the reduction of the diamine silver ion [Ag(NH3)2]+ to metallic silver and oxidized into salt and carboxylic acid. 

Results: 

For ketones, no change was observed in the colourless solution of the reagent.

For aldehydes, the colourless solution yields a grey precipitate of silver, also known as the ‘silver mirror test’.

Nomenclature Rule for Aldehyde 

• Aldehydes get their name from the alkane chains that they come from. The -e is eliminated and replaced with -al at the end.

• The numbering location for the aldehyde functional group is #1, and this number is not mentioned in the name.

• Start with the common parent chain name and add the suffix -aldehyde for the common name of aldehydes. Greek letters are used to represent substituent locations.

• The suffix -carbaldehyde is added to the -CHO functional group when it is linked to a ring, and the carbon associated with that group is C1.

Nomenclature Rule for Ketone

Ketones get their name from the alkane chains that they come from. The -e ending has been replaced with -one.

The alphabetical list of substituent groups + ketone is the popular name for ketones.

The generic names of certain common ketones are well-known. For example, propanone is frequently referred to as acetone.

Aldehyde and Ketones Applications

Aldehyde Applications

• Formaldehyde is a substance that is commonly encountered in biological laboratories. The most common usage of formaldehyde is to make formalin, which is a 40 percent formaldehyde solution in water. The preservation of biological specimens is aided by this solution.

• Bakelite is a phenol-formaldehyde resin that is widely used in plastics, coatings, and adhesives.

• Photography and drug testing both employ formaldehyde.

• Acetaldehyde is a chemical compound that is used to make acetic acid and pyridine derivatives.

• Perfumes and scents, as well as the cosmetic and dye industries, employ aldehydes extensively.

• Aldehydes are also employed in the food industry as artificial flavouring agents. It is a necessary component in the manufacture of perfumes, cosmetics, and colours.

Ketones Applications

• Acetone is the most common and basic of all ketones. It's most typically used as a paint thinner and nail paint remover.

• Acetone dissolves various chemical compounds and is infinitely soluble in water in all amounts. It may be easily eliminated by evaporation when no longer needed because of its low boiling point (56°C).

• Certain synthetic fibres and plastics respond well to ketones as a solvent.

• Ketones are frequently employed in the beauty sector as well as for medical applications such as chemical peeling and acne treatments.

• Butanone, also known as methyl ethyl ketone, is a common solvent used in the production of textiles, varnishes, paint thinners, paraffin wax, and plastics.