Prep and Properties of Alkane, alkene and alkyne

Preparation of Alkanes

1. Soda-Lime Process

• A method to synthesize alkanes from carboxylic acids and their salts.

• Reaction:CaO + R-COONa + NaOH → Na2CO3 + R-HCaO + CH3CH2COONa + NaOH → NaCO + CH3CH3 (Ethane)

2. Wurtz Synthesis

• Reaction that produces alkanes from alkyl halides.

• Mechanism:

  • 2 CH3CH2-Cl + 2 Na + Dry ether → R-R (Alkane) + 2 NaCl

  • Example:

    • 2 CH3CH2-Cl = 2 Na + Dry ether → CH3CH2-CH2CH3 (Butane)

3. Grignard Reaction

• Reaction involving alkyl halides and magnesium to form alkanes.

• Steps: R-X + Mg → R-Mg-X (Grignard reagent) R-Mg-X + H2O → R-H + Mg(OH)X

Preparation of Alkenes

1. Dehydration of Alcohol (Intramolecular Dehydration)

• Removal of water to form alkenes from alcohols by using an acid catalyst (H2SO4).

• Examples:

  • CH3CH2CH2OH (Propan-1-ol) → CH3CH=CH2 (Propene)

  • CH3CH2OH → CH2=CH2 + H2O (Ethene)

2. Dehydrohalogenation of Alkyl Halides

• Formation of alkenes by removing HX from alkyl halides.

• Reaction:RCH2CH2Cl + KOH → RCH=CH2 (Alkene) + HCl

3. Dehalogenation of Alkyl Dihalides

• Reaction that converts alkyl dihalides into alkenes.

• Mechanism:

  • R-CHClCH2Cl + Mg → R-CH=CH2 + 2MgCl2

Preparation of Alkynes

1. Laboratory Preparation of Acetylene

• Reaction: CaC2 + 2H2O → Ca(OH)2 + HC≡CH

2. Dehydrohalogenation of Alkyl Dihalides

• Yields alkynes from dihalides through elimination of hydrogen halides.

• Equation:

• R-CHClCH2Cl + 2 K-OH → R-C≡CH + 2 KCl + 2H2O

3. Dehalogenation of Alkyl Tetrahalides

• Example Reaction:R-CHCl2 + 2 Mg → R-C≡CH + 2 MgCl2

Properties of Alkanes, Alkenes, and Alkynes

1. Combustion Reactions

• General reactions of alkanes, alkenes, and alkynes with oxygen to form CO2 and H2O.

2. Baeyer's Test for Unsaturation

• Alkane:KMnO4 + Alkane → No Reaction

• Alkene:KMnO4 + Alkene → Diol + Brown Precipitate

• Alkyne:KMnO4 + Alkyne → Carboxylic Acid + Brown Precipitate

3. Substitution Reactions (Alkane only)

• Typical substitution reactions include:

  • A. Halogenation (e.g., chlorination)

  • B. Sulfonation (Introduction of SO3H group)

  • C. Nitration (Introduction of NO2 group)

4. Addition Reactions (Alkenes and Alkynes)

• A. Hydrogenation (addition of H2)

• B. Halogenation (addition of halogens)

• C. Hydrohalogenation (addition of HX, following Markovnikov's rule)

5. Hydration of Alkenes

• Addition of water to alkenes to form alcohols.