Relationship Between Structure And Properties Of Oxygen Containing Organic Compounds

In nucleophilic addition reaction nucleophile attacks and adds to an electrophilic center within a molecule. Carbonyl compounds such as Aldehydes and ketones undergo nucleophilic addition reaction. Can you tell what carbonyl compounds are? The compounds that contain C=O functional group are called carbonyl compounds. The nucleophile attacks the carbonyl carbon of aldehyde or ketone. This results in the addition of the nucleophile to the molecule.

Let us discuss the detailed mechanism of this reaction. In the first step, nucleophile interacts with the electrophilic carbon atom of the carbonyl group. Unlike nucleophilic substitution reactions, aldehydes and ketones lack a leaving group. Instead, pi electrons in the pi bond of the carbonyl group are shifted towards the oxygen atom. This is because oxygen is more electronegative. Now, tetrahedral alkoxide intermediate is formed.

In the next step, alkoxide is protonated. Protonation means the addition of proton. The acid catalyst is added. Negatively charged oxygen of alkoxide ion attacks the hydrogen atom of acid catalyst to form alcohol. The final product contains a nucleophile added to molecule. Alcohol group is also seen in the molecule.

When an aldehyde is reacted with an alcohol in the presence of an acid catalyst, hemiacetal is formed. Hemiacetal consists of a carbon atom of alkyl group bonded to a hydroxyl group and an alkoxy group. The carbon atom attached to the hydroxyl group and the alkoxy group is called the hemiacetal carbon. The formation of hemiacetal is also a nucleophilic addition reaction.

Hemiketals are formed by the reaction of a ketone with an alcohol in the presence of an acid catalyst. The formation of hemiketals is also a nucleophilic addition reaction like hemicaetals. In hemiketals two alkyl groups are attached to carbon atom to which hydroxyl group and alkoxy group are attached.

Brady's reagent is a chemical reagent used for the detection and identification of carbonyl compounds. This reagent consists of the compound 2,4-dinitrophenylhydrazine. It has the chemical formula C₆H₆N₄O₄. When some amount of aldehyde or ketone are added to Brady’s reagent, a bright orange or yellow precipitate is formed. This confirms the presence of aldehydes or ketones.

Bright orange or yellow precipitate is due to formation of 2,4-diitrophenylhydrazone. When a ketone is reacted with Brady’s reagent, both molecules combine to form 2,4-dinitrophenylhydrazone. The water molecule is also lost in this reaction. The formation of 2,4-dinitrophenylhydrazone confirms the presence of aldehyde or ketone.

The Clemmensen reduction is a chemical reaction used to convert aldehydes or ketones, into their corresponding hydrocarbons. It involves the reduction of the carbonyl group of aldehyde or ketone to a methylene group. Clemmensen reduction utilizes amalgamated zinc as the reducing agent. Concentrated hydrochloric acid is used as the reaction medium for this reaction.

Let us understand the mechanism of clemmenson reduction. The aldehyde or ketone, interacts with the amalgamated zinc in the presence of concentrated hydrochloric acid. The carbonyl oxygen coordinates with the zinc. This results in formation of an organozinc intermediate. After that the cleavage of the carbon oxygen bond of the carbonyl group takes place. Zinc is now coordinated with carbon atom. After this, the double bond between carbon atom and zinc attacks hydrogen ions. Hydrogen ions are added to the carbon atom. Zinc ions are removed from the molecule. Alkane is formed as a product.

The Wolf Kishner reduction is also used to convert aldehydes or ketones, into their corresponding hydrocarbons. In Wolf Kishner reduction an aldehyde or ketone reacts with hydrazine in the presence of a strong base. Heat is provided for carrying out the reaction. As a result, aldehyde or ketone is converted to alkane.

The mechanism of this reaction actually takes place in several steps. In the first step, carbonyl compound reacts with hydrazine to form a hydrazone. This step involves the nucleophilic addition of the hydrazine to the carbonyl carbon. The nitrogen atom of hydrazine replaces the oxygen atom of the carbonyl group. Now the nitrogen atom of hydrazine is double bonded with the carbon atom of the carbonyl compound.

In the second step, nitrogen atom of hydrazone is deprotonated. Deprotonation means removal of hydrogen atom. The hydroxide ion removes the hydrogen atom attached to the nitrogen atom. This deprotonation results in the formation of a double bond between the two nitrogen atoms in the molecule. The electron density of the second bond between carbon atom and nitrogen atom is shifted toward the carbon atom. The carbon atom is now negatively charged.

In the third step, the carbon atom is protonated. As we know, the oxygen atom of the water molecule is more electronegative. It withdraws the electron density from the hydrogen atom towards itself. The hydrogen atom of water molecule becomes partially positive charged. The negative charged carbon attacks the partial positive charged hydrogen of water molecule. In this way, hydrogen atom is attached to the carbon atom.

In the fourth step, deprotonation of nitrogen atom takes place again. The hydroxide ion removes the hydrogen atom attached to nitrogen atom and becomes a water molecule. The electron density of nitrogen hydrogen bond is shifted between two nitrogen atoms. The electron density of carbon nitrogen bond is shifted towards carbon atom. In this way carbon nitrogen bond breaks. This results in the formation of carbanion and the nitrogen molecule.

The final step involves the protonation of carbon atom of carbanion. The carbanion attacks the hydrogen atom of the water molecule. Hydrogen atom is attached to the carbon atom of carbanion. Carbanion is now converted into an alkane.