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Organic Chemistry - Synthetic routes (HL)

Grade 11IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Retrosynthetic Analysis: A technique for planning organic syntheses by working backward from the target molecule to simpler precursors using the disconnection approach, symbolized by ⇒\Rightarrow.

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Carbon Chain Elongation: The reaction of a halogenoalkane with cyanide ions (CN−CN^-) in ethanol increases the carbon chain length by one, forming a nitrile (R−CNR-CN).

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Reduction of Nitriles: Nitriles (R−CNR-CN) can be reduced to primary amines (R−CH2NH2R-CH_2NH_2) using LiAlH4LiAlH_4 in dry ether or hydrogen gas with a NiNi catalyst.

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Hydrolysis of Nitriles: Heating a nitrile under reflux with dilute acid (H(aq)+H^+_{(aq)}) produces a carboxylic acid (R−COOHR-COOH) and an ammonium salt.

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Synthesis of Phenylamine: Benzene is first nitrated to nitrobenzene (C6H5NO2C_6H_5NO_2) using a mixture of conc. HNO3HNO_3 and conc. H2SO4H_2SO_4, then reduced to phenylamine (C6H5NH2C_6H_5NH_2) using SnSn and conc. HClHCl followed by NaOHNaOH.

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Reduction of Carbonyls: Aldehydes and ketones are reduced to primary and secondary alcohols respectively using NaBH4NaBH_4 or LiAlH4LiAlH_4. Carboxylic acids require the stronger reducing agent LiAlH4LiAlH_4.

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Conversion of Alcohols to Halogenoalkanes: Using reagents like PCl5PCl_5, PCl3PCl_3, or SOCl2SOCl_2 for chloroalkanes, and KBr/H2SO4KBr/H_2SO_4 for bromoalkanes.

📐Formulae

R−X+CN−→ethanolR−CN+X−R-X + CN^- \xrightarrow{ethanol} R-CN + X^-

R−CN+2H2→NiR−CH2NH2R-CN + 2H_2 \xrightarrow{Ni} R-CH_2NH_2

R−CN+H++2H2O→refluxR−COOH+NH4+R-CN + H^+ + 2H_2O \xrightarrow{\text{reflux}} R-COOH + NH_4^+

C6H6+HNO3→H2SO4,50∘CC6H5NO2+H2OC_6H_6 + HNO_3 \xrightarrow{H_2SO_4, 50^\circ C} C_6H_5NO_2 + H_2O

C6H5NO2→2.OH−1.Sn/conc.HClC6H5NH2C_6H_5NO_2 \xrightarrow[2. OH^-]{1. Sn/conc. HCl} C_6H_5NH_2

R−COOH→2.H+1.LiAlH4 in etherR−CH2OHR-COOH \xrightarrow[2. H^+]{1. LiAlH_4 \text{ in ether}} R-CH_2OH

💡Examples

Problem 1:

Outline a reaction sequence to convert CH3CH2BrCH_3CH_2Br into CH3CH2CH2NH2CH_3CH_2CH_2NH_2.

Solution:

Step 1: CH3CH2Br+KCN→ethanol,ΔCH3CH2CN+KBrCH_3CH_2Br + KCN \xrightarrow{ethanol, \Delta} CH_3CH_2CN + KBr. Step 2: CH3CH2CN+2H2→NiCH3CH2CH2NH2CH_3CH_2CN + 2H_2 \xrightarrow{Ni} CH_3CH_2CH_2NH_2.

Explanation:

The target molecule has one more carbon than the starting material. Step 1 introduces a nitrile group to extend the chain. Step 2 involves the complete reduction of the nitrile to a primary amine.

Problem 2:

Identify the reagents and conditions for the conversion of nitrobenzene to phenylamine.

Solution:

  1. SnSn (tin) and concentrated HClHCl, heating under reflux. 2. NaOH(aq)NaOH_{(aq)}.

Explanation:

The reduction of the nitro group (−NO2-NO_2) to an amino group (−NH2-NH_2) requires a strong reducing agent. The reaction initially forms the phenylammonium ion (C6H5NH3+C_6H_5NH_3^+), which must be deprotonated by a base (OH−OH^-) to release the free amine.

Problem 3:

Predict the product of the reaction between CH3COCH3CH_3COCH_3 and NaBH4NaBH_4 followed by water.

Solution:

CH3CH(OH)CH3CH_3CH(OH)CH_3 (Propan-2-ol)

Explanation:

NaBH4NaBH_4 provides the hydride ion (H−H^-) which acts as a nucleophile attacking the carbonyl carbon of the ketone (propanone), reducing it to a secondary alcohol.