Comprehensive Guide to Organic Reactions: Alkanes, Alkenes, Haloalkanes, and Alcohols

This comprehensive guide details various organic reactions involving alkanes, alkenes, haloalkanes, and alcohols, covering their oxidation, substitution, elimination, and addition reactions, along with specific conditions, reactants, products, and important rules like Markovnikov's rule and esterification principles.

Reactions of Alkanes

Alkanes, characterized by single carbon-carbon bonds, undergo several fundamental reactions, including oxidation (combustion), substitution, and elimination.

1. Oxidation (Combustion)

Oxidation, specifically combustion, is a highly exothermic reaction where alkanes react with oxygen to produce carbon dioxide, water, and a significant amount of energy. This property makes alkanes valuable as fuels.

General Reaction: Alkane + Oxygen → Carbon Dioxide + Water + Energy
Example: The combustion of hexane (C₆H₁₄) is represented by the balanced chemical equation: 2C_6H_{14} + 19O_2 \rightarrow 12CO_2 + 14H_2O. This reaction has a negative enthalpy change (\Delta H < 0), indicating its exothermic nature.
Nature: The reaction is highly exothermic, meaning it releases a large amount of heat, which is why alkanes are widely utilized as fuels.

2. Substitution Reactions

Alkanes can undergo substitution reactions, where one or more hydrogen atoms are replaced by other atoms, typically halogens. This process is known as halogenation.

General Reaction: Alkane → Haloalkane
Example (Bromination): A hydrogen atom in an alkane, such as ethane, can be substituted by a bromine atom. The reaction involves ethane (CH_3CH_3) reacting with bromine (Br_2) to form bromoethane (CH_3CH_2Br) and hydrogen bromide (HBr). The structural representation shows a C-H bond breaking and a C-Br bond forming, along with the formation of H-Br.
Conditions: This reaction requires either heat or ultraviolet (UV) light (sunlight) to initiate.
Reactants: An alkane reacts with a halogen molecule (X₂), where X can be Fluorine (F), Chlorine (Cl), Bromine (Br), or Iodine (I).
Type of Substitution: Halogenation, specifically bromination when bromine is used.
Products: The reaction yields a haloalkane and a hydrogen halide (HX).
Continuation: Substitution reactions can continue until all hydrogen atoms on the alkane molecule are replaced by halogen atoms, depending on the reaction conditions and stoichiometry.

3. Elimination Reactions (Cracking)

Elimination reactions in alkanes typically involve the breaking of larger alkane molecules into smaller alkane and alkene molecules, a process known as cracking. This can also lead to the formation of alkenes and hydrogen.

General Reactions:
- Alkane → Alkene(s) + Alkane with shorter chain
- Alkane → Alkene(s) + Hydrogen
Example: A long-chain alkane molecule can break down, for instance, into a shorter alkane and an alkene, or an alkene and hydrogen. The structural diagram illustrates a long carbon chain breaking, forming a double bond in one fragment (an alkene) and a shorter alkane, or an alkene and a hydrogen molecule.
Types of Elimination:
- Thermal Cracking:
  - Conditions: This process requires very high temperatures (around 800 °C) and high pressure, carried out in the absence of oxygen to prevent complete combustion.
- Catalytic Cracking:
  - Conditions: This method uses a catalyst, typically a mixture of silicon dioxide (SiO₂) and aluminium oxide (Al₂O₃), which allows the reaction to proceed at lower temperatures (around 500 °C) and lower pressures compared to thermal cracking.
Products: The products of alkane elimination (cracking) are typically a mixture of alkanes and alkenes, or alkenes and hydrogen gas.

Reactions of Alkenes

Alkenes, characterized by the presence of at least one carbon-carbon double bond, are highly reactive and primarily undergo addition reactions, where the double bond breaks to form new single bonds.

Bromine Test to Distinguish Alkanes and Alkenes

The bromine test is a simple chemical test used to differentiate between saturated alkanes and unsaturated alkenes.

Procedure: Bromine water, which has an orange-brown color, is added to an unknown organic substance.
Result: If the bromine water rapidly discolors (loses its orange-brown color), it indicates the presence of an alkene. Alkanes, being saturated, do not react with bromine water under normal conditions and thus do not cause discoloration.
Reaction: For example, ethene (CH_2CH_2) reacts with bromine (Br_2) to form 1,2-dibromoethane (CH_2BrCH_2Br). The bromine molecule adds across the double bond, breaking it and forming two new C-Br single bonds.

Addition Reactions of Alkenes

Alkenes readily undergo addition reactions across their double bond.

1. Halogenation (Addition of Halogens)

In this reaction, a halogen molecule adds across the double bond of an alkene.

General Reaction: Alkene + X₂ → Dihaloalkane
Example: Ethene (CH_2CH_2) reacts with chlorine (Cl_2) to form 1,2-dichloroethane (CH_2ClCH_2Cl). The structural diagram shows the double bond of ethene breaking, and a chlorine atom adding to each carbon atom that was part of the double bond.
Conditions: The reaction is typically carried out in an unreactive solvent.
Type of Addition: Halogenation.
Reactants: An alkene reacts with a halogen molecule (X₂), where X is typically Chlorine (Cl) or Bromine (Br).
Products: The product is a dihaloalkane, where two halogen atoms are attached to adjacent carbon atoms.

2. Hydrogenation (Addition of Hydrogen)

Hydrogenation involves the addition of hydrogen gas across the double bond of an alkene, converting it into a saturated alkane.

General Reaction: Alkene + H₂ → Alkane
Example: Ethene (CH_2CH_2) reacts with hydrogen gas (H_2) to form ethane (CH_3CH_3). The structural diagram shows the double bond of ethene breaking, and a hydrogen atom adding to each carbon atom that was part of the double bond.
Conditions: This reaction requires a metal catalyst such as Platinum (Pt), Palladium (Pd), or Nickel (Ni).
Type of Addition: Hydrogenation.
Reactants: An alkene reacts with hydrogen gas (H₂).
Product: The product is an alkane.
Application: A significant industrial application of hydrogenation is the conversion of unsaturated vegetable oils into saturated fats, which is used in the manufacture of margarine.

3. Hydrohalogenation (Addition of Hydrogen Halides)

Hydrohalogenation involves the addition of a hydrogen halide (HX) across the double bond of an alkene, forming a haloalkane. For unsymmetrical alkenes, this reaction follows Markovnikov's rule.

General Reaction: Alkene + HX → Haloalkane
Example: Propene (CH_3CH=CH_2) reacts with hydrogen bromide (HBr). According to Markovnikov's rule, the major product is 2-bromopropane (CH_3CHBrCH_3), where the bromine atom attaches to the central carbon. A minor product, 1-bromopropane (CH_3CH_2CH_2Br), can also form. The structural diagrams illustrate the addition of H and Br across the double bond, showing both major and minor products.
Conditions: The reaction is typically carried out in an unreactive solvent and in the absence of water.
Type of Addition: Hydrohalogenation.
Reactants: An alkene reacts with a hydrogen halide (HX), where X can be Iodine (I), Bromine (Br), or Chlorine (Cl).
Product(s): Haloalkane(s).
Major Product Rule (Markovnikov's Rule): In the addition of HX to an unsymmetrical alkene, the hydrogen atom preferentially attaches to the carbon atom of the double bond that already has the greater number of hydrogen atoms. Consequently, the halogen atom attaches to the carbon atom with fewer hydrogen atoms.

4. Hydration (Addition of Water)

Hydration involves the addition of water across the double bond of an alkene, forming an alcohol. Similar to hydrohalogenation, this reaction also follows Markovnikov's rule for unsymmetrical alkenes.

General Reaction: Alkene + H₂O → Alcohol
Example: Propene (CH_3CH=CH_2) reacts with water (H_2O). The major product is propan-2-ol (CH_3CH(OH)CH_3), where the hydroxyl group (-OH) attaches to the central carbon. A minor product, propan-1-ol (CH_3CH_2CH_2OH), can also form. The structural diagrams show the addition of H and OH across the double bond, illustrating both major and minor products.
Conditions: The reaction requires an excess of water and an acid catalyst, such as concentrated sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄).
Type of Addition: Hydration.
Reactants: An alkene reacts with water (H₂O).
Product: Alcohol(s).
Major Product Rule (Markovnikov's Rule): In the addition of water to an unsymmetrical alkene, the hydrogen atom from water attaches to the carbon atom of the double bond that already has the greater number of hydrogen atoms. The hydroxyl (-OH) group then attaches to the carbon atom with fewer hydrogen atoms.

Reactions of Haloalkanes

Haloalkanes, organic compounds containing a halogen atom bonded to a saturated carbon atom, can undergo both elimination and substitution reactions.

1. Elimination Reactions (Dehydrohalogenation)

Haloalkanes can undergo elimination reactions to form alkenes by removing a hydrogen atom and a halogen atom from adjacent carbon atoms.

General Reaction: Haloalkane → Alkene
Example: 2-bromopropane (CH_3CHBrCH_3) reacts with a concentrated strong base like sodium hydroxide (NaOH) under heat. The major product is propene (CH_3CH=CH_2), along with sodium bromide (NaBr) and water (H₂O). The structural diagrams show the removal of H and Br to form a double bond, illustrating both major and minor product possibilities.
Conditions: This reaction requires a concentrated strong base (e.g., NaOH, KOH, LiOH) dissolved in ethanol as a solvent, and heat.
Type of Elimination: Dehydrohalogenation (removal of hydrogen and halogen).
Reactants: A haloalkane reacts with a concentrated strong base.
Products: The reaction yields an alkene, a metal halide (e.g., NaBr), and water.
Major Product Rule: The hydrogen atom is preferentially removed from the carbon atom that has the least number of hydrogen atoms. This leads to the formation of the most substituted double bond (i.e., the double bond with the most alkyl groups attached), which is generally the more stable alkene.

2. Substitution Reactions (Hydrolysis/Hydration)

Haloalkanes can undergo substitution reactions where the halogen atom is replaced by a hydroxyl (-OH) group, leading to the formation of an alcohol. This is a type of hydrolysis or hydration.

2.1. Using a Diluted Strong Base

This method involves reacting a haloalkane with a diluted strong base.

General Reaction: Haloalkane → Alcohol
Example: 1-bromopropane (CH_3CH_2CH_2Br) reacts with a diluted strong base like sodium hydroxide (NaOH) under mild heat. The product is propan-1-ol (CH_3CH_2CH_2OH) and sodium bromide (NaBr). The structural diagrams show the bromine atom being replaced by a hydroxyl group.
Conditions: A diluted strong base (e.g., NaOH, KOH, LiOH) is used, typically in ethanol as a solvent, along with mild heat.
Type of Substitution: Hydrolysis/hydration.
Reactants: A haloalkane reacts with a diluted strong base.
Products: The reaction yields an alcohol and a metal halide (e.g., NaBr, KBr, LiBr).

2.2. Using Water

Haloalkanes can also be hydrolyzed by reacting directly with water, especially under mild heat.

General Reaction: Haloalkane → Alcohol
Example: 1-bromopropane (CH_3CH_2CH_2Br) reacts with excess water (H_2O) under mild heat. The products are propan-1-ol (CH_3CH_2CH_2OH) and hydrogen bromide (HBr). The structural diagrams illustrate the replacement of the bromine atom by a hydroxyl group.
Conditions: Excess water is used, along with mild heat.
Type of Substitution: Hydrolysis.
Reactants: A haloalkane reacts with water (H₂O).
Products: The reaction yields an alcohol and a hydrogen halide (HBr).

Reactions of Alcohols

Alcohols, characterized by the presence of a hydroxyl (-OH) group, can undergo elimination, substitution, and esterification reactions.

1. Elimination Reactions (Dehydration)

Alcohols can undergo elimination reactions to form alkenes by removing a molecule of water (dehydration).

General Reaction: Alcohol → Alkene
Example: Propan-1-ol (CH_3CH_2CH_2OH) or propan-2-ol (CH_3CH(OH)CH_3) can undergo dehydration to form propene (CH_3CH=CH_2) and water (H₂O). The structural diagrams show the removal of an -OH group and a hydrogen atom from adjacent carbons to form a double bond, illustrating both major and minor product possibilities.
Conditions: This reaction requires a dehydrating agent, typically concentrated sulfuric acid (H₂SO₄), and heat.
Type of Elimination: Dehydration (removal of water).
Reactants: An alcohol reacts with concentrated sulfuric acid.
Products: The reaction yields alkene(s) and water.
Major Product Rule: The hydrogen atom is removed from the carbon atom that has the least number of hydrogen atoms, leading to the formation of the most substituted (and generally more stable) alkene.

2. Substitution Reactions (Formation of Haloalkanes)

Alcohols can undergo substitution reactions where the hydroxyl (-OH) group is replaced by a halogen atom, forming a haloalkane.

General Reaction: Alcohol → Haloalkane
Example: Propan-1-ol (CH_3CH_2CH_2OH) reacts with hydrogen bromide (HBr) under heat to form 1-bromopropane (CH_3CH_2CH_2Br) and water (H₂O). The structural diagrams show the replacement of the -OH group by a bromine atom.
Conditions: The reaction requires heat.
Reactants Needed: An alcohol reacts with a hydrogen halide (HX).
Specific Preparations:
- For primary and secondary alcohols, hydrogen halides like HBr are often generated in situ in the reaction flask by reacting sodium bromide (NaBr) with concentrated sulfuric acid (H₂SO₄).
- For tertiary alcohols, hydrogen halides such as HBr or HCl can be applied directly, as tertiary alcohols are more reactive in substitution reactions.
Products: The reaction yields a haloalkane and water.

3. Esterification

Esterification is a condensation reaction where an alcohol reacts with a carboxylic acid to form an ester and water.

Process Definition: Esterification is the chemical process by which esters are formed.
Reaction Overview: Esters are formed when an alcohol reacts with a carboxylic acid, typically when heated. This is a condensation reaction because a small molecule (water) is eliminated.
Example: Ethanol (CH_3CH_2OH) reacts with ethanoic acid (CH_3COOH) to form ethyl ethanoate (CH_3COOCH_2CH_3) and water (H₂O). The structural diagrams illustrate the combination of the alcohol and carboxylic acid, with the elimination of water.
Conditions: The reaction requires concentrated sulfuric acid (H₂SO₄) as a catalyst and heat.
Role of Catalyst: Concentrated sulfuric acid acts as a dehydrating agent, effectively extracting the water molecule formed during the reaction, which helps to drive the equilibrium towards ester formation.
Reactants: An alcohol reacts with a carboxylic acid in the presence of concentrated sulfuric acid.
Products: The reaction yields an ester and water.
Mechanism of Water Formation:
- The alcohol molecule, which contains one oxygen atom in its hydroxyl group, provides a hydrogen atom (H) for the formation of the water molecule.
- The carboxylic acid molecule, which contains two oxygen atoms (one in the carbonyl group and one in the hydroxyl group), provides the hydroxyl group (-OH) for the formation of the water molecule.
Ester Structure and Origin of Atoms:
- The central oxygen atom within the C-O-C linkage of the ester molecule is derived from the alcohol molecule.
- The oxygen atom that is double-bonded to a carbon atom (part of the carbonyl group, C=O) in the ester is obtained from the carboxylic acid.
Nomenclature of Esters:
- The first part of the ester's name is determined by the alcohol used. For example, if butanol is used, the first part of the name is "butyl-".
- The second part of the ester's name is derived from the carboxylic acid used. For example, if ethanoic acid is used, the second part of the name is "ethan-".
- Esters belong to a homologous series, and their names consistently end in "-oate".
- Therefore, an ester formed from butanol and ethanoic acid would be named butylethanoate.