What are Aldehydes and Ketones?
Aldehydes and ketones are organic compounds that have a carbonyl group, C = O, as their functional group. They both differ in the placement of this carbonyl group in their structures.
The carbonyl group in aldehydes is placed at the end of the molecular structure, which means, the C=O is attached to one hydrogen atom and an alkyl group or a benzene ring. Just like all the other homologous series in organic chemistry, the naming of aldehydes is done using a common suffix “-al”. The general molecular formula is CnH2nO.
If carefully observed they all have the same end molecule that is -COH.
The carbonyl group in ketones is placed somewhere in the middle of the molecular structure, which means, the C=O is attached to two alkyl groups or benzene rings. Just like all other homologous series, the naming of ketones is done using a common suffix “-one”. The general molecular formula is CnH2nO.
Uses of Aldehydes
- Formaldehyde, a gas, when mixed with water can be used to preserve biological specimens.
- Formaldehyde is also used in pesticides and glue making.
- Benzaldehyde is used in the manufacturing of perfumes and other cosmetic products.
Uses of Ketones
- Acetone is used as a nail paint remover and paint thinner.
- Acetone is also an excellent solvent for plastics and synthetic fibers.
- They are used in the manufacturing of medicines as well.
- Cyclohexanone is used primarily in the production of nylon.
Preparation of Aldehydes and Ketones
Oxidation of alcohols : Aldehydes and ketones are most commonly prepared by the oxidation of primary and secondary alcohols respectively.
Dehydration of alcohols : Primary and secondary alcohol vapors are passed over heavy metal catalyst to form aldehydes and ketones respectively.
Ozonolysis of alkenes : Ozonolysis is a process used to determine the position of the carbon-carbon double bond in compounds. Ozonolysis of alkenes followed by reaction with zinc and water gives us a mixture of both aldehydes and ketones.
Hydration of alkynes : A specific reaction of ethyne with water in the presence of sulfuric acid and mercury sulfate gives acetaldehyde. The rest of the alkynes give ketones as their end products.
Boiling points : The boiling points of aldehydes and ketones are higher compared to other hydrocarbons with the same molecular mass but it is lower than that of alcohols due to the missing hydrogen bonding which is present in the former.
Solubility : The smaller members of aldehydes and ketones are soluble in water as they are capable of forming a hydrogen bond with water. The solubility of the compounds decreases as the length of the alkyl chain increases. Moreover, they are mostly soluble in organic solvents.
Odor : The lower members have a very pungent smell but as one moves upwards towards higher members, the pungent smell fades away to give a more fragrant one. This property is often used for making perfumes.
Nucleophilic addition reaction
Aldehydes and ketones react and allow nucleophiles to add across the carbon-oxygen double bond in the carbonyl group. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions.
Step 1: C=O is polarised due to the greater electronegativity of the oxygen atom because of which the electrons are drawn closer to the oxygen atom, giving it a partial negative charge and carbon, a partial positive charge. The nucleophile attacks the carbon of the polarised carbonyl group to form an intermediate.
Step 2: This negatively charged intermediate is highly reactive. Thus, it (the oxygen atom) quickly combines to a proton from the solution to give a neutral product.
Examples of nucleophilic addition reactions are:
- Addition of hydrogen cyanide (HCN).
- Addition of sodium hydrogen sulfite.
- Addition of alcohol.
- Addition of ammonia.
Aldehydes can be reduced to primary alcohols and ketones to secondary alcohols respectively. The reducing agent commonly used is sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4).
They can also be reduced to hydrocarbons (CH2 group) by reacting with zinc and hydrochloric acid.
Aldehydes and ketones can be distinguished by their oxidation properties. Aldehydes can easily be oxidised under mild oxidising agents whereas ketones need vigorous conditions to undergo this reaction.
This is why mild reagents like Tollens’ reagent (aqueous silver nitrate in excess of ammonia) is used as a distinguishing test. When an aldehyde is heated with this reagent, reduction of silver ions to silver atoms takes place and a silver mirror is formed on the inside of the test tube indicating a positive result. The aldehyde gets oxidised to carboxylate ion under the present alkaline conditions. When repeated with ketones, there is no change observed as no reaction takes place.
Another reagent used is the Fehling’s solution (alkaline Copper (II) ions). When added to aldehydes, the clear blue solution turns to an opaque red-orange colored one. This confirms that the aldehyde got oxidized to a carboxylate ion. There is no reaction with ketones.
Identification of a Carbonyl Group
o identify a carbonyl group, one of the most common reagents, 2,4–dinitrophenylhydrazine, commonly abbreviated as 2,4-DNPH is used. When the reagent is added to the solution to be tested, formation of an orange precipitate confirms the presence of a carbonyl group in the mixture.
- Students often get confused between the two.
- Remember it is a nucleophilic addition reaction and not an electrophilic addition reaction.
- It is important to remember the general mechanism of the addition reactions of aldehydes and ketones.
Context and Applications
This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.
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