15.6 Types of Organic Chemical Reactions | Organic Chemistry | Chemistry 11

Organic compounds undergo a variety of chemical reactions based on reaction conditions and the types of reagents used. This section covers the most important types of organic reactions tested in FBISE FSc Chemistry.

Types of Organic Reactions

i. Free Radical Substitution Reactions of Alkanes

In this reaction type, a hydrogen atom in an alkane is successively replaced by a halogen atom via free radical intermediates. A free radical is a species with one or more unpaired electrons. The reaction requires UV light ( $h ν$ ) and proceeds through three steps: initiation, propagation, and termination.

Example: Chlorination of Methane

$C H_{4} + C l_{2} h ν C H_{3} C l + H C l$

ii. Electrophilic Addition Reactions

An electrophile (electron-deficient species) attacks a multiple bond ( $C = C$ or $C \equiv C$ ), breaking it and adding across the unsaturated compound to form a saturated or less-unsaturated product. This is characteristic of alkenes and alkynes.

Example: Addition of HBr to Propene (Markovnikov's Rule)

$C H_{3} C H = C H_{2} + H B r \to C H_{3} C H (B r) C H_{3}$

iii. Elimination Reactions

Atoms or groups are removed from adjacent carbon atoms, forming a new $π$ bond (double bond) and a small molecule such as $H_{2} O$ or $H C l$ . This is the reverse of addition.

Example: Dehydration of Ethanol

$C H_{3} C H_{2} O H co n c . H_{2} S O_{4}, Δ C H_{2} = C H_{2} + H_{2} O$

iv. Nucleophilic Substitution Reactions

A nucleophile (electron-rich species, e.g., $O H^{-}$ , $C N^{-}$ , $N H_{3}$ ) attacks an electron-deficient carbon atom and replaces another atom or group — the leaving group (usually a halide ion).

Example: Hydrolysis of Bromoethane

$C H_{3} C H_{2} B r + N a O H \to C H_{3} C H_{2} O H + N a B r$

Here $O H^{-}$ is the nucleophile and $B r^{-}$ is the leaving group.

v. Nucleophilic Addition Reactions

A nucleophile adds to a polar double bond, typically the carbonyl group ( $C = O$ ) in aldehydes and ketones. The $π$ bond breaks and the nucleophile bonds to the electrophilic carbon.

Example: Addition of HCN to Acetaldehyde

$C H_{3} C H O + H C N N a C N / H C l C H_{3} C H (C N) O H$

vi. Hydrolysis

A water molecule breaks one or more chemical bonds in an organic compound, splitting it into smaller substances. Often catalyzed by acids or bases.

Example: Acid-catalyzed Hydrolysis of an Ester

$C H_{3} C O O C_{2} H_{5} + H_{2} O H_{2} S O_{4} C H_{3} C O O H + C_{2} H_{5} O H$

vii. Condensation

Two organic molecules join to form a larger molecule with elimination of a small molecule ( $H_{2} O$ or $H C l$ ). This is the reverse of hydrolysis.

Example: Esterification

$C H_{3} C O O H + C_{2} H_{5} O H H_{2} S O_{4} C H_{3} C O O C_{2} H_{5} + H_{2} O$

viii. Oxidation Reaction

In organic chemistry, oxidation involves:

Gain of oxygen atoms
Loss of hydrogen atoms
An increase in the number of C–O bonds

Example: Oxidation of Methanol

$C H_{3} O H + [O] K_{2} C r_{2} O_{7} / H_{2} S O_{4} H C O O H (Formic Acid)$

ix. Reduction Reaction

In organic chemistry, reduction involves:

Loss of oxygen atoms
Gain of hydrogen atoms
A decrease in the number of C–O bonds

Example: Reduction of Acetaldehyde

$C H_{3} C H O + 2 [H] Reducing Agent C H_{3} C H_{2} O H$

x. Rearrangement Reactions

The carbon skeleton or functional group of a molecule is reorganized to yield a structural isomer of the original compound without adding or removing atoms.

Example: Isomerization of n-Butane

$n - C_{4} H_{10} A l C l_{3} / H C l i so - C_{4} H_{10}$

Reaction Mechanisms

15.6.1 Free Radical Substitution Reaction of Alkanes

This mechanism explains how halogens substitute hydrogen atoms on alkanes in the presence of UV light. Free radicals are species with one or more unpaired electrons, represented by a dot (e.g., $C l^{∙}$ ).

Step 1: Initiation

UV light provides energy for homolytic fission of the halogen molecule — each atom retains one electron, forming two highly reactive free radicals.

$C l - C l h ν C l^{∙} + C l^{∙}$

Step 2: Propagation

A chain reaction where radicals are consumed and regenerated:

A chlorine radical abstracts a hydrogen from methane: $C H_{4} + C l^{∙} \to C H_{3}^{∙} + H C l$
The methyl radical attacks a chlorine molecule: $C H_{3}^{∙} + C l_{2} \to C H_{3} C l + C l^{∙}$

If excess $C l_{2}$ is present, further substitution produces $C H_{2} C l_{2}$ , $C H C l_{3}$ , and finally $C C l_{4}$ .

Step 3: Termination

The chain ends when two free radicals combine to form a stable neutral molecule:

$C l^{∙} + C l^{∙} \to C l_{2}$ $C H_{3}^{∙} + C l^{∙} \to C H_{3} C l$ $C H_{3}^{∙} + C H_{3}^{∙} \to C_{2} H_{6}$

15.6.2 Elimination Reactions

In the elimination mechanism for bromoethane, a base ( $O H^{-}$ ) removes an acidic hydrogen from the carbon adjacent to the C–Br bond. Simultaneously, the bromine atom leaves with the bonding pair of electrons as $B r^{-}$ , and a $C = C$ double bond forms:

$C H_{3} C H_{2} B r + O H^{-} \to C H_{2} = C H_{2} + H_{2} O + B r^{-}$

15.6.3 Nucleophilic Substitution Reactions

A nucleophile attacks the partially positive ( $δ^{+}$ ) carbon atom of a halogenoalkane. This displaces the halogen atom, which leaves as a halide ion (the leaving group).

General mechanism:

$N u^{-} + R - X \to R - N u + X^{-}$

Where $N u^{-}$ is the nucleophile and $X^{-}$ is the leaving group (halide ion).

Example: Reaction of bromoethane with $C N^{-}$

$C H_{3} C H_{2} B r + K C N \to C H_{3} C H_{2} C N + K B r$

The carbon bonded to bromine is electron-deficient ( $δ^{+}$ ) due to the electronegativity of bromine, making it susceptible to nucleophilic attack.

Types of Organic Reactions

i. Free Radical Substitution Reactions of Alkanes

Example: Chlorination of Methane

$C H_{4} + C l_{2} h ν C H_{3} C l + H C l$

ii. Electrophilic Addition Reactions

Example: Addition of HBr to Propene (Markovnikov's Rule)

$C H_{3} C H = C H_{2} + H B r \to C H_{3} C H (B r) C H_{3}$

iii. Elimination Reactions

Atoms or groups are removed from adjacent carbon atoms, forming a new $π$ bond (double bond) and a small molecule such as $H_{2} O$ or $H C l$ . This is the reverse of addition.

Example: Dehydration of Ethanol

$C H_{3} C H_{2} O H co n c . H_{2} S O_{4}, Δ C H_{2} = C H_{2} + H_{2} O$

iv. Nucleophilic Substitution Reactions

Example: Hydrolysis of Bromoethane

$C H_{3} C H_{2} B r + N a O H \to C H_{3} C H_{2} O H + N a B r$

Here $O H^{-}$ is the nucleophile and $B r^{-}$ is the leaving group.

v. Nucleophilic Addition Reactions

A nucleophile adds to a polar double bond, typically the carbonyl group ( $C = O$ ) in aldehydes and ketones. The $π$ bond breaks and the nucleophile bonds to the electrophilic carbon.

Example: Addition of HCN to Acetaldehyde

$C H_{3} C H O + H C N N a C N / H C l C H_{3} C H (C N) O H$

vi. Hydrolysis

A water molecule breaks one or more chemical bonds in an organic compound, splitting it into smaller substances. Often catalyzed by acids or bases.

Example: Acid-catalyzed Hydrolysis of an Ester

$C H_{3} C O O C_{2} H_{5} + H_{2} O H_{2} S O_{4} C H_{3} C O O H + C_{2} H_{5} O H$

vii. Condensation

Two organic molecules join to form a larger molecule with elimination of a small molecule ( $H_{2} O$ or $H C l$ ). This is the reverse of hydrolysis.

Example: Esterification

$C H_{3} C O O H + C_{2} H_{5} O H H_{2} S O_{4} C H_{3} C O O C_{2} H_{5} + H_{2} O$

viii. Oxidation Reaction

In organic chemistry, oxidation involves:

Gain of oxygen atoms
Loss of hydrogen atoms
An increase in the number of C–O bonds

Example: Oxidation of Methanol

$C H_{3} O H + [O] K_{2} C r_{2} O_{7} / H_{2} S O_{4} H C O O H (Formic Acid)$

ix. Reduction Reaction

In organic chemistry, reduction involves:

Loss of oxygen atoms
Gain of hydrogen atoms
A decrease in the number of C–O bonds

Example: Reduction of Acetaldehyde

$C H_{3} C H O + 2 [H] Reducing Agent C H_{3} C H_{2} O H$

x. Rearrangement Reactions

The carbon skeleton or functional group of a molecule is reorganized to yield a structural isomer of the original compound without adding or removing atoms.

Example: Isomerization of n-Butane

$n - C_{4} H_{10} A l C l_{3} / H C l i so - C_{4} H_{10}$

Reaction Mechanisms

15.6.1 Free Radical Substitution Reaction of Alkanes

Step 1: Initiation

UV light provides energy for homolytic fission of the halogen molecule — each atom retains one electron, forming two highly reactive free radicals.

$C l - C l h ν C l^{∙} + C l^{∙}$

Step 2: Propagation

A chain reaction where radicals are consumed and regenerated:

A chlorine radical abstracts a hydrogen from methane: $C H_{4} + C l^{∙} \to C H_{3}^{∙} + H C l$
The methyl radical attacks a chlorine molecule: $C H_{3}^{∙} + C l_{2} \to C H_{3} C l + C l^{∙}$

If excess $C l_{2}$ is present, further substitution produces $C H_{2} C l_{2}$ , $C H C l_{3}$ , and finally $C C l_{4}$ .

$N u^{-} + R - X \to R - N u + X^{-}$

Where $N u^{-}$ is the nucleophile and $X^{-}$ is the leaving group (halide ion).

Example: Reaction of bromoethane with $C N^{-}$

$C H_{3} C H_{2} B r + K C N \to C H_{3} C H_{2} C N + K B r$

The carbon bonded to bromine is electron-deficient ( $δ^{+}$ ) due to the electronegativity of bromine, making it susceptible to nucleophilic attack.