Investigate The Relationship Between Structure And Properties Of Oxygen Containing Organic Compounds

Aldehydes and ketones can be reduced to alcohols by using Grignard reagent. We know that Grignard reagent is a strong nucleophile. It attacks the electrophilic carbonyl carbon of the aldehyde or ketone. This nucleophilic addition of the Grignard reagent to the carbonyl carbon leads to the formation of a new carbon carbon bond. The pi electron density between the carbon atom and the oxygen atom in carbonyl group shifts to the oxygen atom. This nucleophilic attack results in the formation of Alkoxide.

In the next step, a suitable acid is added. The acid protonates the alkoxide ion. This protonation results in the formation of desired alcohol. Magnesium salt is also formed as a side product. The reaction of aldehydes with Grignard reagent results in the formation of secondary alcohol. Meanwhile the ketones react with Grignard reagent to form tertiary alcohol.

Aldehydes and ketones can be reduced into alcohols in the presence of Lithium aluminum hydride. Lithium aluminum hydride is a reducing agent. Molecular formula of Lithium aluminum hydride is LiAlH₄LL I A L H four.It reduces aldehydes into primary alcohol. Ketones get reduced into secondary alcohol in the presence of Lithium aluminum hydride.

Aldehydes can be oxidized into carboxylic acids in the presence of acidified potassium permanganate. Potassium permanganate is a strong oxidizing agent. For example, when ethanal is reacted with acidified potassium permanganate, it is converted into ethanoic acid. In the presence of an aldehyde, the vibrant pink color of a solution containing potassium permanganate fades and turns colorless. This color change confirms the presence of aldehyde.

Aldehydes can also be converted into carboxylic acid in the presence of acidified potassium dichromate. On addition of acidified potassium dichromate to an aldehyde, the orange color of potassium dichromate solution is changed into green. This color change confirms the presence of aldehyde.

The Tollens test is a chemical reaction used to detect the presence of an aldehyde compound. It involves the reaction between an aldehyde and Tollens' reagent. The reaction results in the formation of silver mirror or a silver precipitate. The chemical formula of Tollens reagent is [Ag(NH₃)₂]OH.

Tollens reagent is prepared by first dissolving silver nitrate in water to form a clear, colorless solution. After that Sodium hydroxide is dissolved in water separately. The silver nitrate solution is then added slowly to the sodium hydroxide solution while stirring, until a brown precipitate of silver oxide forms. This brown precipitate is then dissolved by adding a few drops of ammonia. This results in the formation of Tollens' reagent.

Now we shall react the aldehyde with Tollens reagent. The aldehyde undergoes oxidation in the presence of Tollens reagent. Tollens reagent acts as an oxidizing agent. It converts the aldehyde into a carboxylic acid. During the oxidation of the aldehyde, the silver ions in Tollens reagent are reduced to metallic silver. As the oxidation reaction proceeds, metallic silver is deposited as a thin mirror like layer on the inner surface of the reaction vessel. The silver mirror appearance is a visual indicator that confirms the presence of an aldehyde.

The Fehling test is a chemical test used to detect the presence of aldehydes and particular ketones. It involves the reaction between the aldehyde or ketone and Fehling's solution. Fehling's solution is a mixture of copper sulfate solution and alkaline tartrate solution. Fehling test results in the formation of a reddish brown precipitate of copper(I) oxide.

Let us first prepare the Fehling’s solution. Fehling's solution is prepared by mixing equal volumes of Fehling's A and Fehling's B solutions. Fehling's A solution contains copper sulfate. Fehling's B solution contains alkaline tartrate which is sodium potassium tartrate dissolved in sodium hydroxide solution.

Let us react Aldehyde with Fehling’s solution. The aldehyde group is oxidized to a carboxylate ion. As the aldehyde is oxidized, the Cu+2 ions in Fehling's solution are reduced to reddish brown precipitate of copper(I) oxide. The presence of this precipitate confirms the presence of an aldehyde.

In Aldol condensation two carbonyl compounds combine to form aldol. Then, aldol undergoes dehydration to form Alpha beta unsaturated carbonyl compound. Carbonyl compounds that undergo aldol condensation can be aldehyde or ketone.

Let us understand the mechanism of aldol condensation of aldehyde. The first step in aldol condensation is the removal of alpha hydrogen from aldehyde. Alpha hydrogen is the hydrogen atom attached to the carbon atom next to the carbonyl group. The removal of hydrogen atom is facilitated by the presence of a strong base, such as sodium hydroxide. The base abstracts the alpha hydrogen. This results in the formation of an enolate ion. The enolate ion is resonance stabilized. It has negative charge on the oxygen atom and a double bond between the alpha carbon and carbonyl carbon.

The enolate ion acts as a nucleophile and attacks the electrophilic carbon of another aldehyde. After the nucleophilic attack, an intermediate is formed. Protonation of intermediate results in the formation of beta hydroxy butanal. This beta hydroxy butanal is called aldol.

Now a water molecule is removed from aldol. This removal of water molecule is called dehydration. The elimination of water results in the formation of but-2-enal. but-2-enal is alpha beta unsaturated aldehyde.