Aldehydes, Ketones and Carboxylic Acids

Definition & Structure

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom ($C=O$). This bond is polar, where oxygen pulls electrons toward itself, leaving the carbon slightly positive ($\delta+$) and the oxygen slightly negative ($\delta-$).

• Aldehydes: Carbonyl carbon is attached to at least one hydrogen ($R-CHO$). Found at the end of chains.
• Ketones: Carbonyl carbon is attached to two carbon groups ($R-CO-R'$). Found in the middle of chains.
• Geometry: The carbon is $sp^2$ hybridized, resulting in a trigonal planar shape.

Preparation Methods

• Oxidation of Alcohols: Primary alcohols $\rightarrow$ Aldehydes; Secondary alcohols $\rightarrow$ Ketones.
• Ozonolysis of Alkenes: Cleaving double bonds using $O_3$ to create carbonyl groups.

Examples

• Methanal (Formaldehyde): $HCHO$ (Simplest aldehyde).
• Propanone (Acetone): $CH_3COCH_3$ (Simplest ketone).

Real-Life Use

Formaldehyde is used in "formalin" to preserve biological specimens. Acetone is widely used as a nail polish remover and an industrial solvent.

The Step-by-Step Mechanism

Because the carbonyl carbon is electrophilic ($\delta+$), it is attacked by a nucleophile ($Nu^-$):

1. Step 1: Nucleophilic Attack: The nucleophile attacks the carbon, breaking the $\pi$-bond and moving electrons to oxygen, forming a tetrahedral intermediate.
2. Step 2: Protonation: The negative oxygen ($O^-$) picks up a proton ($H^+$) to form a stable hydroxyl ($-OH$) group.

General Equation:
R-CHO + Nu⁻ → R-CH(O⁻)Nu → R-CH(OH)Nu

Reactivity Comparison

Aldehydes are generally more reactive than ketones due to:

• Steric Hindrance: Ketones have two bulky $R$ groups blocking the attack.
• Inductive Effect (+I): Alkyl groups in ketones donate electrons, reducing the $\delta+$ charge on the carbon.

Comparison Table

Meaning and Acidity

Carboxylic acids contain the $-COOH$ group. They are stronger acids than alcohols due to the stability of the Carboxylate Ion ($R-COO^-$). The negative charge is delocalized between two oxygen atoms via resonance.

Preparation

• Oxidation: Primary alcohols using $KMnO_4$.
• Hydrolysis: Nitriles ($R-CN$) + $H_2O \rightarrow R-COOH$.

Examples & Uses

• Ethanoic Acid: Component of vinegar (food preservation).
• Methanoic Acid: Found in ant venom (causes stinging).

1. Which functional group is present in Acetone?
   Ans: b) Ketone
2. The hybridization of the carbonyl carbon is:
   Ans: b) $sp^2$
3. Which is more reactive towards nucleophilic addition?
   Ans: a) Formaldehyde
4. The reaction of an aldehyde with HCN produces:
   Ans: b) Cyanohydrin
5. Carboxylic acids are acidic due to the stability of:
   Ans: b) Carboxylate ion
6. Which reagent is used to oxidize a primary alcohol to an aldehyde?
   Ans: b) PCC (Pyridinium chlorochromate)
7. The functional group $-COOH$ is called:
   Ans: b) Carboxyl
8. What happens when an aldehyde reacts with Tollens' reagent?
   Ans: b) Silver mirror forms
9. Which of the following is a ketone?
   Ans: b) $CH_3COCH_3$
10. The +I effect of alkyl groups in ketones ________ the reactivity of the carbonyl carbon.
    Ans: b) Decreases

1. Q: Explain why aldehydes are more reactive than ketones towards nucleophilic addition.
   Ans: Due to steric hindrance (ketones have two bulky groups blocking the attack) and the inductive effect (+I effect of two alkyl groups in ketones reduces the electrophilicity of the carbon).
2. Q: Write the mechanism for the addition of $HCN$ to Methanal.
   Ans: (Step 1) $H-CN$ dissociates to $CN^-$. $CN^-$ attacks $HCHO$ carbon. (Step 2) $O^-$ picks up $H^+$ to form $HO-CH_2-CN$.
3. Q: Why is the carboxylate ion more stable than an alkoxide ion?
   Ans: The carboxylate ion is stabilized by resonance, where the negative charge is delocalized over two oxygen atoms, whereas in an alkoxide ion, the charge is localized on one oxygen.
4. Q: Distinguish between Formaldehyde and Acetone using a chemical test.
   Ans: Use Tollens' Test. Formaldehyde (aldehyde) will form a silver mirror, while Acetone (ketone) will not react.
5. Q: How can you prepare Ethanoic acid from Ethanol?
   Ans: By vigorous oxidation of ethanol using acidified potassium dichromate ($K_2Cr_2O_7/H^+$).
6. Q: What is the role of PCC in organic synthesis?
   Ans: PCC is a mild oxidizing agent used to stop the oxidation of a primary alcohol at the aldehyde stage.
7. Q: Define Nucleophile. Give an example.
   Ans: A nucleophile is an electron-rich species that attacks an electron-deficient center. Example: $OH^-$ or $CN^-$.
8. Q: Arrange in increasing order of acidity: $CH_3COOH$, $ClCH_2COOH$, $Cl_2CHCOOH$.
   Ans: $CH_3COOH < ClCH_2COOH < Cl_2CHCOOH$ (Electron-withdrawing Cl atoms increase acidity).
9. Q: What is Ozonolysis?
   Ans: A reaction where an alkene reacts with ozone to cleave the $C=C$ bond, producing carbonyl compounds.
10. Q: Predict the product: $CH_3CHO + CH_3MgBr$ followed by $H_3O^+$.
    Ans: The product is Propan-2-ol ($CH_3CH(OH)CH_3$).

• Reactivity Order: Aldehydes > Ketones (A > K).
• Acid Strength: More Chlorine atoms = Stronger Acid (Think of Cl as a "Vacuum Cleaner" sucking away electrons).
• Structure Mnemonics:
  • Aldehyde = At the end (Terminal).
  • Ketone = Kore (Core/Middle) of the chain.