Difference Between Aldehydes and Ketones

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?

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. 


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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?

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 side of the compound. 


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They occur in the middle of a Carbon chain because of the presence of alkyl on both the 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. 

Difference Between Aldehyde and Ketone 

The aldehydes are more natural to undergo oxidation because of its 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. 


How to Differentiate Aldehyde and Ketone?

The aldehydes can get easily oxidized with the help of mild oxidizing agents like alkaline solutions of \[(Cu^{2+})\] 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), the aldehyde breaks down to carboxylic acid, the Cu(II) ions leave a brick-red precipitate known as \[Cu_{2}O\] Copper Oxide,


The reaction is as follows:


RCHO + \[2(Cu^{2+})\](aq.) + \[2H_{2}O\] → RCOOH + \[Cu_{2}O\](s) + 4H+(aq)


Results:

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


For aldehydes, the blue solution gets oxidized to leave a brick-red precipitate of \[Cu_{2}O\] (Copper(I) oxide). 


  • 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, that 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 + \[2Ag^{+} (aq) + H_{2}O \rightarrow RCOOH + 2Ag(s) + 2H^{+}\]


The aldehyde helps in the reduction of the diamminesilver 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’.

FAQ (Frequently Asked Questions)

Q1. Where are Aldehydes and Ketones Found Naturally?

A. The aldehydes are found in volatile compounds like perfume, plants, animals, microorganisms and the human body. The ketones, on the other hand, are found in sugar and get produced by our liver. Cinnemaldehyde (present in cinnamon), citral (in lemongrass) are some of the naturally-occurring aldehydes. For ketones, carvone (present in spearmint and caraway), cortisone (adrenal hormone), are some naturally found ketones. There are also certain chiral compounds that are found in nature in their enantiomerically pure forms. The carvone synthesized from the spearmint oil is generally from the (R) enantiomer form, while the one present in caraway seeds contains (S) - enantiomer.

Q2. Can Ketones be Oxidized Like Aldehydes?

A. Ketones cannot be oxidized like aldehydes as they are resistant to oxidizing because of the lack of Hydrogen atom compared to that of aldehydes. Therefore, ketones can only be oxidized with the help of strong oxidizing agents like the potassium manganate solution. However, it can only take place when there's a breaking down of the Carbon bonds present in the ketones, destroying their shape completely. Therefore, it's better to have a hot bath before you start experimenting on different organic compounds of aldehydes and ketones for faster, effective results. While ketones do not show any observational changes, it’s the aldehydes that take all the credit.