How does the reactivity of propanal compare to propanone in nucleophilic addition reactions?

Propanal (an aldehyde) is more reactive than propanone (a ketone). This is due to less steric hindrance from the single alkyl group and a larger partial positive charge on the carbonyl carbon because it has fewer electron-donating alkyl groups.

What is the difference between the products formed when an aldehyde and a ketone are reduced?

Reduction of an aldehyde produces a primary alcohol, whereas reduction of a ketone produces a secondary alcohol. Both reactions involve the addition of hydrogen across the $C=O$ bond.

When using Fehling's solution, why do aldehydes give a positive result while ketones do not?

Aldehydes have a hydrogen atom attached to the carbonyl carbon, making them easily oxidizable to carboxylic acids. Ketones lack this hydrogen and cannot be oxidized by mild agents like $Cu^{2+}$ ions.

What is a common error when describing the reaction between an aldehyde and Tollens' reagent?

A common error is stating that the aldehyde is reduced. In reality, the aldehyde is oxidized to a carboxylate ion, while the silver ions ($Ag^+$) in the reagent are reduced to metallic silver ($Ag$).

Why is it incorrect to use only HCN for the synthesis of hydroxynitriles without a catalyst?

HCN is a weak acid and does not dissociate sufficiently to provide enough $CN^-$ nucleophiles. A catalyst like KCN or a small amount of alkali is needed to increase the concentration of $CN^-$ to initiate the attack.

What mistake is often made when identifying a carbonyl group using 2,4-DNPH?

Students often assume 2,4-DNPH identifies only aldehydes. It actually reacts with both aldehydes and ketones to form a precipitate; further tests like Tollens' are needed to distinguish between them.

Define a 'Cyanohydrin' (Hydroxynitrile).

A cyanohydrin is a functional group found in organic compounds where a hydroxyl group ($-OH$) and a cyano group ($-CN$) are attached to the same carbon atom, typically formed by the addition of HCN to a carbonyl.

What is the composition of Tollens' reagent?

Tollens' reagent is an ammoniacal silver nitrate solution, specifically containing the complex ion $[Ag(NH_3)_2]^+$. It acts as a mild oxidizing agent.

What is the 'Iodoform Test' used for?

The iodoform test is used to detect the presence of a $CH_3CO-$ group (methyl ketones) or a $CH_3CH(OH)-$ group. A positive result is indicated by the formation of a yellow precipitate of $CHI_3$.

Why does the addition of HCN to ethanal result in a racemic mixture?

The carbonyl carbon in ethanal is planar, allowing the $CN^-$ nucleophile to attack from either side with equal probability. This creates a chiral center with both enantiomers produced in equal amounts.

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Chemistry

17. Carbonyl Compounds

Reactions of Aldehydes & Ketones

Summary

The chemical reactivity of aldehydes and ketones is primarily governed by the polar nature of the carbonyl group ( $C=O$ ). Because oxygen is more electronegative than carbon, the carbonyl carbon becomes electrophilic, making it a prime target for nucleophilic addition. These compounds undergo a variety of transformations, including addition of hydrogen cyanide, reduction to alcohols, and selective oxidation, which serves as a critical diagnostic tool for distinguishing between the two functional groups.

1. Nature of the Carbonyl Group

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom, where the oxygen is significantly more electronegative than the carbon.

This electronegativity difference creates a permanent dipole, resulting in a partial positive charge ( $\delta^+$ ) on the carbon and a partial negative charge ( $\delta^-$ ) on the oxygen.

The electrophilic nature of the carbonyl carbon allows it to be attacked by nucleophiles, which are electron-rich species looking for positive centers.

The planar geometry of the $sp^2$ hybridized carbonyl carbon provides an unhindered path for nucleophilic attack from above or below the plane.

Diagram showing the nucleophilic attack on a polarized carbonyl carbon. The nucleophile (Nu-) approaches the electrophilic carbon (delta+) while the pi electrons move toward the electronegative oxygen (delta-).

2. Mechanism of Nucleophilic Addition

3. Relative Reactivity: Aldehydes vs. Ketones

4. Reduction and Oxidation Reactions

5. Diagnostic Chemical Tests

6. Exam Strategy & Tips

Reactions of Aldehydes & Ketones

Summary

1. Nature of the Carbonyl Group

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom, where the oxygen is significantly more electronegative than the carbon.

This electronegativity difference creates a permanent dipole, resulting in a partial positive charge ( $\delta^+$ ) on the carbon and a partial negative charge ( $\delta^-$ ) on the oxygen.

The electrophilic nature of the carbonyl carbon allows it to be attacked by nucleophiles, which are electron-rich species looking for positive centers.

The planar geometry of the $sp^2$ hybridized carbonyl carbon provides an unhindered path for nucleophilic attack from above or below the plane.

2. Mechanism of Nucleophilic Addition

Step 1: Nucleophilic Attack: The nucleophile uses its lone pair of electrons to form a new covalent bond with the carbonyl carbon, causing the $\pi$ bond of the $C=O$ to break.

This step converts the carbon from $sp^2$ (planar) to $sp^3$ (tetrahedral) hybridization, forming a negatively charged alkoxide intermediate.

Step 2: Protonation: The negatively charged oxygen atom in the intermediate attracts a proton ( $H^+$ ) from the solvent or an added acid to form the final addition product.

A classic example is the addition of Hydrogen Cyanide (HCN), which requires a catalytic amount of alkali or cyanide salt (KCN) to generate sufficient $CN^-$ ions for the initial attack.

3. Relative Reactivity: Aldehydes vs. Ketones

Aldehydes are generally more reactive than ketones toward nucleophilic addition due to both electronic and steric factors.

Electronic Factor: Alkyl groups are electron-donating (+I effect). Ketones have two alkyl groups that reduce the partial positive charge on the carbonyl carbon more than the single alkyl group in aldehydes.

Steric Factor: The presence of two relatively large alkyl groups in ketones hinders the approach of the nucleophile compared to the smaller hydrogen atom in aldehydes.

Consequently, formaldehyde ( $HCHO$ ) is the most reactive carbonyl compound, followed by other aldehydes, with ketones being the least reactive.

4. Reduction and Oxidation Reactions

Reduction: Aldehydes and ketones can be reduced to alcohols using reagents like Sodium Borohydride ( $NaBH_4$ ) or Lithium Aluminium Hydride ( $LiAlH_4$ ).

Aldehydes yield primary alcohols, while ketones yield secondary alcohols through the addition of hydride ions ( $H^-$ ) to the carbonyl carbon.

Oxidation: Aldehydes are easily oxidized to carboxylic acids because of the presence of a hydrogen atom attached to the carbonyl carbon.

Ketones lack this hydrogen and do not undergo oxidation under mild conditions, allowing for chemical tests to distinguish between the two.

5. Diagnostic Chemical Tests

Tollens' Reagent: Also known as the 'silver mirror test', it uses ammoniacal silver nitrate. Aldehydes reduce $Ag^+$ to metallic silver, forming a mirror on the test tube wall.

Fehling's Solution: Contains $Cu^{2+}$ ions in an alkaline medium. Aliphatic aldehydes reduce the blue $Cu^{2+}$ to a brick-red precipitate of Copper(I) oxide ( $Cu_2O$ ).

2,4-Dinitrophenylhydrazine (2,4-DNPH): This reagent reacts with both aldehydes and ketones to form yellow or orange crystalline precipitates, serving as a general test for the carbonyl group.

Iodoform Test: Specifically identifies methyl ketones (and ethanol/secondary alcohols with a methyl group) by forming a yellow precipitate of triiodomethane ( $CHI_3$ ) when reacted with iodine and sodium hydroxide.

6. Exam Strategy & Tips

Chain Extension: Always remember that the reaction with $HCN$ is a key method for increasing the carbon chain length by one atom.
Chirality: Note that the addition of $HCN$ to an aldehyde (except formaldehyde) or an unsymmetrical ketone often creates a chiral center, resulting in a racemic mixture.
Reagent Specificity: Be careful with Fehling's solution; it typically gives a positive result with aliphatic aldehydes but may not react with aromatic aldehydes like benzaldehyde.
Visual Cues: In exam questions, look for keywords like 'silver mirror' (Tollens'), 'red precipitate' (Fehling's), or 'orange crystals' (2,4-DNPH) to identify the functional group present.