21.2 Key Reagents and Functional Group Interconversions | Organic Synthesis | Chemistry 11

This section explores essential chemical reagents used in organic synthesis, categorizing them by their primary function (oxidizing, reducing, or dehydrating agents). It also illustrates how these reagents facilitate the interconversion of various organic functional groups.

Oxidizing Agents

Oxidizing agents gain electrons and cause the oxidation of other substances. In organic chemistry, they typically increase the number of bonds to oxygen or decrease the number of bonds to hydrogen.

Acidified Potassium Dichromate ( $K_{2} C r_{2} O_{7} / H^{+}$ ) or Acidified Potassium Permanganate ( $KMn O_{4} / H^{+}$ )

These are strong oxidizing agents.

Primary Alcohols → Aldehydes → Carboxylic Acids:

Primary alcohols are initially oxidized to aldehydes. With excess strong oxidizing agent, oxidation proceeds further to carboxylic acids.

$R C H_{2} O H [O] R C H O [O] R C O O H$

Example: Oxidation of ethanol:

$C H_{3} C H_{2} O H K_{2} C r_{2} O_{7} / H^{+} C H_{3} C H O K_{2} C r_{2} O_{7} / H^{+} C H_{3} C O O H$

Secondary Alcohols → Ketones:

Secondary alcohols are oxidized to ketones. Ketones resist further oxidation because the carbonyl carbon lacks a hydrogen atom.

$R C H (O H) R^{'} [O] R C O R^{'}$

Example:

$C H_{3} C H (O H) C H_{3} KMn O_{4} / H^{+} C H_{3} C O C H_{3}$

Reducing Agents

Reducing agents lose electrons and cause the reduction of other substances. In organic chemistry, they decrease the number of bonds to oxygen or increase the number of bonds to hydrogen.

Sodium Borohydride ( $NaB H_{4}$ )

A mild, selective reducing agent. It reduces aldehydes to primary alcohols and ketones to secondary alcohols. It does not reduce carboxylic acids, esters, or $C = C$ double bonds.

$R C H O NaB H_{4} R C H_{2} O H$

$R C O R^{'} NaB H_{4} R C H (O H) R^{'}$

This selectivity makes $NaB H_{4}$ ideal when other reducible groups (e.g., alkenes) must remain intact.

Lithium Aluminum Hydride ( $LiAl H_{4}$ )

A strong, versatile reducing agent. The $Al - H$ bond is weaker and more polar than the $B - H$ bond in $NaB H_{4}$ , making hydride ( $H^{-}$ ) more available for nucleophilic attack.

Reduces: aldehydes, ketones, carboxylic acids, esters, and amides.

$R C O O H LiAl H_{4} R C H_{2} O H$

Hydrogen Gas with Nickel Catalyst ( $H_{2} /Ni$ )

Used for catalytic hydrogenation. Reduces $C = C$ double bonds (alkenes → alkanes) and can also reduce alkynes.

$R_{2} C = C R_{2} H_{2} / N i R_{2} C H - C H R_{2}$

Tin/Hydrochloric Acid ( $Sn/HCl$ )

Used for the reduction of nitro groups to amino groups. This is the standard method for synthesizing primary aromatic amines from nitroarenes.

$R - N O_{2} S n / H C l R - N H_{2}$

Example: Nitrobenzene → Aniline ( $C_{6} H_{5} N H_{2}$ )

Dehydrating Agents

Dehydrating agents remove water from a molecule. In organic reactions, they form double bonds from alcohols (elimination reactions) or facilitate condensation reactions.

Aluminum Oxide ( $A l_{2} O_{3}$ )

Used at high temperatures for the dehydration of alcohols to alkenes.

Concentrated Sulfuric Acid ( $H_{2} S O_{4}$ )

A strong dehydrating agent used for acid-catalyzed elimination of alcohols to alkenes and in condensation reactions.

$R - C H_{2} - C H_{2} - O H Conc. H_{2} S O_{4}, Δ R - C H = C H_{2} + H_{2} O$

Summary Table

Reagent	Type	Functional Group Converted	Product
$K_{2} C r_{2} O_{7} / H^{+}$ or $KMn O_{4} / H^{+}$	Oxidizing	Primary alcohol	Aldehyde → Carboxylic acid
$K_{2} C r_{2} O_{7} / H^{+}$ or $KMn O_{4} / H^{+}$	Oxidizing	Secondary alcohol	Ketone
$NaB H_{4}$	Mild reducing	Aldehyde / Ketone	Primary / Secondary alcohol
$LiAl H_{4}$	Strong reducing	Carboxylic acid / Ester / Amide	Primary alcohol / Amine
$H_{2} /Ni$	Reducing	Alkene ( $C = C$ )	Alkane
$Sn/HCl$	Reducing	Nitro group ( $- N O_{2}$ )	Amino group ( $- N H_{2}$ )
Conc. $H_{2} S O_{4}$ / $A l_{2} O_{3}$	Dehydrating	Alcohol	Alkene

Functional Group Interconversion Flowchart

The following diagram provides a visual representation of how different functional groups can be interconverted using various reagents and reaction types.

<PracticeQuestions questionsString="[{"question":"Which reagent is specifically used for the selective reduction of carbonyl groups in the presence of carbon-carbon

Oxidizing Agents

Oxidizing agents gain electrons and cause the oxidation of other substances. In organic chemistry, they typically increase the number of bonds to oxygen or decrease the number of bonds to hydrogen.

Acidified Potassium Dichromate ( $K_{2} C r_{2} O_{7} / H^{+}$ ) or Acidified Potassium Permanganate ( $KMn O_{4} / H^{+}$ )

These are strong oxidizing agents.

Primary Alcohols → Aldehydes → Carboxylic Acids:

Primary alcohols are initially oxidized to aldehydes. With excess strong oxidizing agent, oxidation proceeds further to carboxylic acids.

$R C H_{2} O H [O] R C H O [O] R C O O H$

Example: Oxidation of ethanol:

$C H_{3} C H_{2} O H K_{2} C r_{2} O_{7} / H^{+} C H_{3} C H O K_{2} C r_{2} O_{7} / H^{+} C H_{3} C O O H$

Secondary Alcohols → Ketones:

Secondary alcohols are oxidized to ketones. Ketones resist further oxidation because the carbonyl carbon lacks a hydrogen atom.

$R C H (O H) R^{'} [O] R C O R^{'}$

Example:

$C H_{3} C H (O H) C H_{3} KMn O_{4} / H^{+} C H_{3} C O C H_{3}$

Reducing Agents

Reducing agents lose electrons and cause the reduction of other substances. In organic chemistry, they decrease the number of bonds to oxygen or increase the number of bonds to hydrogen.

Sodium Borohydride ( $NaB H_{4}$ )

A mild, selective reducing agent. It reduces aldehydes to primary alcohols and ketones to secondary alcohols. It does not reduce carboxylic acids, esters, or $C = C$ double bonds.

$R C H O NaB H_{4} R C H_{2} O H$

$R C O R^{'} NaB H_{4} R C H (O H) R^{'}$

This selectivity makes $NaB H_{4}$ ideal when other reducible groups (e.g., alkenes) must remain intact.

Lithium Aluminum Hydride ( $LiAl H_{4}$ )

A strong, versatile reducing agent. The $Al - H$ bond is weaker and more polar than the $B - H$ bond in $NaB H_{4}$ , making hydride ( $H^{-}$ ) more available for nucleophilic attack.

Reduces: aldehydes, ketones, carboxylic acids, esters, and amides.

$R C O O H LiAl H_{4} R C H_{2} O H$

Hydrogen Gas with Nickel Catalyst ( $H_{2} /Ni$ )

Used for catalytic hydrogenation. Reduces $C = C$ double bonds (alkenes → alkanes) and can also reduce alkynes.

$R_{2} C = C R_{2} H_{2} / N i R_{2} C H - C H R_{2}$

Tin/Hydrochloric Acid ( $Sn/HCl$ )

Used for the reduction of nitro groups to amino groups. This is the standard method for synthesizing primary aromatic amines from nitroarenes.

$R - N O_{2} S n / H C l R - N H_{2}$

Example: Nitrobenzene → Aniline ( $C_{6} H_{5} N H_{2}$ )

Dehydrating Agents

Dehydrating agents remove water from a molecule. In organic reactions, they form double bonds from alcohols (elimination reactions) or facilitate condensation reactions.

Aluminum Oxide ( $A l_{2} O_{3}$ )

Used at high temperatures for the dehydration of alcohols to alkenes.

Concentrated Sulfuric Acid ( $H_{2} S O_{4}$ )

A strong dehydrating agent used for acid-catalyzed elimination of alcohols to alkenes and in condensation reactions.

$R - C H_{2} - C H_{2} - O H Conc. H_{2} S O_{4}, Δ R - C H = C H_{2} + H_{2} O$

Summary Table

Reagent	Type	Functional Group Converted	Product
$K_{2} C r_{2} O_{7} / H^{+}$ or $KMn O_{4} / H^{+}$	Oxidizing	Primary alcohol	Aldehyde → Carboxylic acid
$K_{2} C r_{2} O_{7} / H^{+}$ or $KMn O_{4} / H^{+}$	Oxidizing	Secondary alcohol	Ketone
$NaB H_{4}$	Mild reducing	Aldehyde / Ketone	Primary / Secondary alcohol
$LiAl H_{4}$	Strong reducing	Carboxylic acid / Ester / Amide	Primary alcohol / Amine
$H_{2} /Ni$	Reducing	Alkene ( $C = C$ )	Alkane
$Sn/HCl$	Reducing	Nitro group ( $- N O_{2}$ )	Amino group ( $- N H_{2}$ )
Conc. $H_{2} S O_{4}$ / $A l_{2} O_{3}$	Dehydrating	Alcohol	Alkene

Functional Group Interconversion Flowchart

The following diagram provides a visual representation of how different functional groups can be interconverted using various reagents and reaction types.

<PracticeQuestions questionsString="[{"question":"Which reagent is specifically used for the selective reduction of carbonyl groups in the presence of carbon-carbon