Reactions of different homologous series

Organic compounds undergo various reactions depending on their functional groups. The main reaction types include:

✅ Oxidation

✅ Substitution

✅ Addition

✅ Elimination

1. Reactions of the Alkanes

Alkanes are saturated hydrocarbons and are generally unreactive, except under specific conditions.

1.1 Oxidation (Combustion) of Alkanes

• Complete combustion: Produces carbon dioxide $(CO₂)$ and water $(H₂O).$
• Incomplete combustion: Produces carbon monoxide $(CO)$ or soot $(C)$ due to limited oxygen.
• Exothermic reaction – releases energy. 💡 Example:

$\text{C}_3\text{H}_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O + \text{energy}$

1.2 Substitution Reactions

• Occur when an H atom in an alkane is replaced by another atom (e.g., halogen).
• Requires UV light or heat.
• Example: Halogenation (reaction with $Cl₂, Br₂,$ etc.)

(Methane + Chlorine → Chloromethane + Hydrogen chloride) $CH_4 + Cl_2 \xrightarrow{\text{UV light}} CH_3Cl + HCl$

1.3 Cracking of Alkanes

• Breaks large alkane molecules into smaller, more useful molecules.
• Two types:
  • Thermal cracking (high temperature, no catalyst).
  • Catalytic cracking (low temp & pressure, catalyst used).
• Produces alkenes and alkanes.

(Decane → Hexane + Butene) $C_{10}H_{22} \xrightarrow{\text{heat}} C_6H_{14} + C_4H_8$

2. Reactions of the Alkenes

Alkenes are unsaturated hydrocarbons with at least one double bond $(C=C).$

2.1 Addition Reactions

• Breaks the double bond and adds atoms across it.
• Faster than substitution reactions. | Reaction | Conditions | Example | |---|---|---| | Hydrogenation (adding $H₂$) | Catalyst: $Pt, Pd,$ or $Ni$ | $Ethene + H₂ → Ethane$ | | Halogenation (adding $X₂$) | No catalyst needed | $Ethene + Br₂ → 1,2-Dibromoethane$ | | Hydrohalogenation (adding $HX$) | Follows Markovnikov's Rule | $Propene + HCl → 2-Chloropropane$ | | Hydration (adding $H₂O$) | $H₂SO₄$ catalyst | $Ethene + H₂O → Ethanol$ |

🔹 Markovnikov's Rule: The hydrogen atom from HX attaches to the carbon with more hydrogens.

3. Reactions of the Haloalkanes

Haloalkanes contain a halogen atom $(F, Cl, Br, I)$ attached to a carbon chain.

3.1 Substitution Reactions

• Hydrolysis: $Haloalkane + H₂O → Alcohol + HX.$ $CH_3CH_2Br + H_2O \rightarrow CH_3CH_2OH + HBr$

(Bromoethane → Ethanol)

• Reaction with $NaOH$ (in ethanol): Produces alcohol and $NaX.$ $CH_3CH_2Cl + NaOH \rightarrow CH_3CH_2OH + NaCl$

(Chloroethane → Ethanol)

3.2 Elimination Reactions (Dehydrohalogenation)

• Forms alkenes by removing $HX.$
• Requires concentrated $NaOH$ in ethanol + heat.

$CH_3CH_2Br + NaOH \rightarrow CH_2=CH_2 + HBr$

(Bromoethane → Ethene)

🔹 Zaitsev's Rule: The hydrogen is removed from the carbon with fewer hydrogens.

4. Reactions of Alcohols

Alcohols contain the $-OH$ functional group.

4.1 Substitution Reactions

• React with HX to form haloalkanes.

$CH_3CH_2OH + HBr \rightarrow CH_3CH_2Br + H_2O$

(Ethanol + $HBr$ → Bromoethane)

4.2 Elimination Reactions (Dehydration)

• Produces alkenes by removing $H₂O.$
• Requires $H₂SO₄$ catalyst + heat.

$CH_3CH_2OH \xrightarrow{H_2SO_4} CH_2=CH_2 + H_2O$

(Ethanol → Ethene)

5. Reactions of Esters (Esterification)

• Formed when an alcohol and carboxylic acid react in the presence of $H₂SO₄.$ $\text{Alcohol} + \text{Carboxylic Acid} \rightarrow \text{Ester} + \text{Water}$

• Example: (Ethanol + Ethanoic acid → Ethyl ethanoate) $CH_3CH_2OH + CH_3COOH \xrightarrow{H_2SO_4} CH_3COOCH_2CH_3 + H_2O$

Key Takeaways

✅ Saturated compounds (alkanes, haloalkanes) undergo substitution or elimination.

✅ Unsaturated compounds (alkenes, alkynes) undergo addition reactions.

✅ Alcohols undergo substitution, elimination, or oxidation.

✅ Esterification forms esters from alcohols and carboxylic acids.