Aldehydes and Ketones

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What are Aldehydes and Ketones?

Aldehydes and Ketones are both simple organic compounds that contain a carbonyl group. A carbonyl group contains a carbon-oxygen double bond. The aldehydes and ketones organic compounds are quite simple due to the carbon atom present in the carbonyl group lacking reactive groups like Cl or OH. 

Both organic compounds incorporate a carbonyl functional group, as C=O. These are the organic compounds, having the structures RC(=O)R’ and -CHO, where R and R’ represents the carbon-containing substituents, respectively.


What is Aldehyde?

Aldehydes contain the carbonyl group, having one hydrogen atom attached to it together with either a hydrogen group or a 2nd hydrogen atom, which can be the one containing a benzene ring or an alkyl group.

Aldehyde examples can be given as follows.

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We can notice that all the compounds given above have the exact same end to the molecule. The one and only difference is the complexity of the other attached group.


What are Ketones?

Ketones contain the carbonyl with 2 hydrocarbon groups attached to it. These are either the ones containing either the alkyl groups or the benzene rings. They do not have any hydrogen atom attached to the carbonyl group.

Ketone examples can be given as follows:

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In general, propane is written as CH3COCH3. The carbonyl group in pentanone could be in the middle of the chain or next to the end by giving either pentane-2-one or pentane-3-one.


Occurrence of Aldehydes and Ketones

Combined with the other functional group, aldehydes and ketones are widespread in nature. The compounds such as vanillin (vanilla bean), Citra (lemongrass), cinnamaldehyde (cinnamon bark), helminthosporal (a fungal toxin), camphor (camphor trees), and carvone (spearmint and caraway) are found chiefly in plants or microorganisms. Whereas the compounds such as testosterone (male sex hormone), progesterone (female sex hormone), cortisone (adrenal hormone), and muscone (musk deer) have a human and animal origin.


Preparation of Aldehydes and Ketones

Aldehydes and Ketone compounds can be prepared by various methods. Let us discuss those below:


Formation by Oxidation of Alcohols

The primary and secondary oxidation of alcohol leads to the formation of aldehydes and ketones. Oxidation becomes possible, using the common oxidizing agents such as K2Cr2O7, KMnO4, and CrO3. The strong oxidizing agents help in the oxidation of primary alcohol to aldehyde and then to a carboxylic acid.

Primary alcohols that have low molecular weight can undergo oxidation and form aldehydes. The reaction mixture after the formation of aldehyde can avoid further oxidation if the temperature of the reaction is modulated so that the aldehyde’s boiling point is lower than the alcohol, which helps in the aldehyde distillation from the reaction mixture soon after the formation of aldehyde. Thus, it is essential to maintain a reaction temperature of more than 349K slightly. Let us look at the reaction given below.

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The preparation of Aldehyde and Ketone is possibly done by the oxidation of primary and secondary alcohol by agents like Collins reagents (Chromium trioxide-pyridine complex), PCC (pyridinium chlorochromate), and Cu at 573 K.

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Collin’s Reagents (Chromium Trioxide-Pyridine Complex)

Collin’s reagent, which is also called the chromium trioxide-pyridine complex, is a good oxidizing reagent for the conversion of a primary alcohol to aldehydes. In addition, Collin’s reagent has an advantage, which is, it helps to cease the further oxidation of aldehydes to carboxylic acids. However, the reaction with Collin’s reagent becomes possible in a non-aqueous medium like CH2Cl2.


PCC (Pyridinium Chlorochromate)

The pyridine mixture, along with the HCl and CrO3 in dichloromethane, leads to the formation of PCC (C5H5NH+CrO3 Cl–) or Pyridine chlorochromate.

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We can prepare the ketones by using similar oxidizing agents from the secondary alcohols.

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Formation by Alcohols Dehydrogenation

This method of preparation applies in case of the conversion of volatile alcohols to aldehydes. In general, it is used in industrial applications. The alcohol vapors are passed through the heavy metal catalysts like Ag or Cu in this respective technique. Besides, primary alcohol produces aldehyde, and secondary alcohol produces ketones, respectively.

As an example, alcohols undergo dehydrogenation when vapors of either primary alcohol or secondary alcohol pass through the copper gauze at a temperature of 573 K. The example given below represents how n-propyl alcohol leads to the propionaldehyde formation in the process of dehydrogenation.

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It is also possible to use various metal catalysts like silver or copper under heating conditions during the dehydrogenation of alcohol. However, this particular technique is apt for the conversion of valuable alcohols into aldehydes. Furthermore, it is much useful in industrial applications.

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Also, this is one of the better methods for the preparation of aldehydes and ketones because further oxidation is not possible for aldehydes. Therefore, there is no conversion risk of aldehydes to carboxylic acids.

This is one of the better methods used for the preparation of Aldehyde and Ketone. In addition, there are also other methods used based on the requirements.

FAQ (Frequently Asked Questions)

1. Explain the Uses of Aldehydes and Ketones?

Formaldehyde is one of the simplest aldehydes, and acetone is the smallest ketone. There are numerous counts of aldehydes and ketones that find application because of their chemical properties. A few uses of them are listed below.


Uses of Aldehydes

  • Acetaldehyde is largely used for acetic acid production and pyridine derivatives.

  • Benzaldehyde can be used in perfumes, cosmetic products, and dyes. It is also added to provide almond flavor to the food products and is used as a bee repellent.

Uses of Ketones

  • The most common ketone is acetone, an excellent solvent for a number of synthetic fibers and plastics.

  • Acetone is used in the household as a nail paint remover and as a paint thinner.

2. Why are aldehydes more reactive than ketones towards Nucleophilic Substitutions?

The two alkyl or aryl groups present in the ketones offer steric hindrance during the substitution reactions. Since the hydrogen atom is relatively small, it offers any steric hindrance. This is the main reason why the aldehydes are more susceptible to nucleophilic substitutions. Moreover, the partially positive charge on the carbonyl carbon is stabilized by the two R groups in ketones.

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