Main menu

Pages

Aldehydes and Ketones: Preparation - Nomenclature - Properties

 

Aldehydes and Ketones

Aldehydes and ketones have the same functional group. Its name is carbonyl group. This group is formed by oxygen atom attached with a double bond to carbon atom. Compounds which include this <C=O  functional group are called as carbonyl compounds. Their general formula is CnH2nO. In aldehydes, the car- bon of carbonyl is bonded to a hydrogen and an alkyl group, whereas in ketones, carbon of carbonyl group is bonded to the same or different two alkyl groups.

 

Aldehydes and Ketones: Preparation - Nomenclature - Properties


 

Nomenclature of Aldehydes and Ketones

Aldehydes and ketones are named according to IUPAC nomenclature system as in the following steps:

 

A) Nomenclature of Aldehydes

 

1.      The longest carbon chain is chosen. The carbon atom of carbon group is numbered with 1.

2.     -al suffix is added to the end of the name of the corresponding alkane.

3.     Alkyl groups are mentioned with their numbers.

 

As numbering starts from the side of carbonyl group, the number of carbonyl group doesn’t need to be mentioned. For example:

Nomenclature of Aldehydes and Ketones


 

B) Nomenclature of Ketones

 

We apply the steps in nomenclature of aldehydes.

1.      We choose the longest carbon chain and start numbering from the closest carbon to the carbonyl group.

2.     We add –one suffix to the end of the corresponding alkane.

3.     If there is branching, alkyl groups are mentioned with their numbers.

 

Nomenclature of Aldehydes and Ketones


 

Preparation of Aldehydes and Ketones

Aldehydes and ketones are synthesized through many methods in industry and laboratories. Here, we will explain one of the laboratory methods.

 

Oxidation of Alcohols

Aldehydes and ketones are prepared by oxidation of primary (1°) and secondary (2°) alcohols with acidic solution of potassium dichromate (K2Cr2O7 ) or potas- sium permanganate (KMnO4 ).

 

1- Primary alcohols form aldehydes as shown in the following equation.

1- Primary alcohols form aldehydes as shown in the following equation.


 

The aldehyde remaining in the reaction mixture above is readily oxidized and it forms carboxylic acid.

Therefore, oxidation reaction must be controlled.

1- Primary alcohols form aldehydes as shown in the following equation.


 

2- Ketones are prepared through oxidation of secondary (20) alcohols.

2- Ketones are prepared through oxidation of secondary (20) alcohols.


 

3- Tertiary alcohols cannot be oxidized.

 

 

Example 1

Write down oxidation products of the following compounds.

1.      -1propanol

2.     -2pentanol

 

Solution:

Write down oxidation products of the following compounds.  1.      -1propanol  2.     -2pentanol


 

Physical Properties of Aldehydes and Ketones

All aldehydes and ketones are in liquid form at room temperature except mathanal (formaldehyde, in gas form).

Although ketones have pleasant odors, aldehydes have unplaeasant odors. Densities of ketones are lower than that of water. Owing to the polar property of carbonyl group, these compounds are accepted as polarized. They can dis- solve in water and organic solvents like ether. Boiling points of aldehydes and ketones are higher than those of alkanes, but lower than alcohols which have the same molecular weight with them. For example:

Physical Properties of Aldehydes and Ketones


 

 

Chemical Properties of Aldehydes and Ketones

The carbonyl group in aldehydes and ketones has high ionization property as shown below:

Chemical Properties of Aldehydes and Ketones


Carbon in the carbonyl group is positive charged, as nucleophile approaches it, electrophile approaches to negative charged oxygen.

Chemical Properties of Aldehydes and Ketones

 

Aldehydes and ketones can undergo two-step nucleophilic reactions. These are:

 

A) 1st step: Nucleophile approaches carbon of carbonyl group and forms a new bond. This bond breaks double bond between oxygen and carbon and pushes a pair of electrons to oxygen atom.

A) 1st step: Nucleophile approaches carbon of carbonyl group and forms a new bond. This bond breaks double bond between oxygen and carbon and pushes a pair of electrons to oxygen atom.


 

B) 2nd step: approach of an electrophile such as H+ ion:

B) 2nd step: approach of an electrophile such as H+ ion:


 

a) Reduction with hydrogen

Aldehydes are reduced to primary alcohols and ketones are reduced to second- ary alcohols when they react with H2 with Ni and Pt as catalysts.

Aldehydes are reduced to primary alcohols and ketones are reduced to second- ary alcohols when they react with H2 with Ni and Pt as catalysts.


 

b) Reduction to Alkanes

Aldehydes and ketones are reduced to alkanes via Clemmensen reduction method. In this method, solution of zinc and mercury in hydrochloric acid is used as catalyst.

Aldehydes and ketones are reduced to alkanes via Clemmensen reduction method. In this method, solution of zinc and mercury in hydrochloric acid is used as catalyst.


 

c) Reaction with Hydrazine

Aldehydes and ketones react with hydrazine (H2N-NH2) and produce hydrazone which is known as Schiff base.

Aldehydes and ketones react with hydrazine (H2N-NH2) and produce hydrazone which is known as Schiff base.


 

This reaction is used to examine the presence of carbonyl group in aldehydes and ketones. Hydrazone (orange or yellow color) shows the presence of the carbonyl group.

 


d) Oxidation

Presence of hydrogen bonded to carbonyl group in aldehydes causes some re- actions different from those of ketones. The most important reactions of aldehydes and ketones are oxidation reactions.

Aldehydes are oxidized into carboxylic acids by the help of some oxidizing agents. But in ketones, this does not occur. We can differentiate aldehydes and ketones using following reactions:

 

1-Tollens’ Reagent

In order to differentiate aldehydes, silver ammonium hydroxide is used. In this reaction, silver ion is reduced in solution and silver metal covers the tube’s sur- face as mirror showing the presence of aldehyde. This reaction is shown below:

1-Tollens’ Reagent

 

1-Tollens’ Reagent


 

C) Reaction with Fehling’s Solution

Fehling’s solution is a solution of copper sulfate salt. It’s basic and dark blue. It is used in oxidation of aldehydes. In this reaction, copper (II) ions transforms into red copper (I) oxide. This red copper (I) oxide precipitate shows presence of aldehyde in mixture. As shown below, ketones do not react with this reagent.

C) Reaction with Fehling’s Solution


  

See also

1.      Alcohols: nomenclature – synthesis - Properties

2.     Alkyl halides: nomenclature - synthesis - Properties

 

 

References

1.      K. J. Denniston c J. J.Topping c and R. L.Caretc “General Organic and Biochemistry”c Mc-Graw- Hillc New York   (2004).

2.     K.W. Whittenc R.E. Davis  and L. M. Peckc “General Chemistry” 7th ed. Holt Rinehart and Winstonc New York (2010).

3.     Clayden, J.; Greeves, N. and Warren, S. (2012) Organic Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.

4.     Streitwieser, Andrew; Heathcock, Clayton H.; Kosower, Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4: Medtech (Scientific International, reprint of revised 4th edition, Macmillan, 1998). ISBN 978-93-85998-89-8.


Comments

contents title