Alkenes & Alkynes Notes for NEET

🌟 Key NEET Concepts:

  1. Structure of Double Bond
    Sigma (σ) bond: Strong, head-on overlap
    Pi (π) bond: Weak, sideways overlap → Makes alkenes reactive!
  2. Geometrical Isomerism
    Requires different groups on each carbon of C=C
    cis (same side) vs. trans (opposite sides)
  3. Markovnikov’s Rule
    “Rich get richer!” H⁺ adds to carbon with more H atoms
    Peroxide reverses this for HBr (Anti-Markovnikov)
  4. Ozonolysis
    Cuts double bond → Reveals original alkene structure
  5. Preparation of Alkenes
    From alkynes (Lindlar’s catalyst → cis; Na/NH₃ → trans)

📚 Detailed Notes:

9.3 Alkenes (CnH2n)

Why “olefins”? First member (ethene) formed oily liquid with Cl₂
Structure of Double Bond:
– σ-bond (397 kJ/mol): sp² hybrid orbitals overlap head-on
– π-bond (284 kJ/mol): p-orbitals overlap sideways → weak & reactive!
– Bond length: 134 pm (shorter than C-C single bond: 154 pm)

Naming (IUPAC):
– Find longest chain with double bond
– Number from end closer to double bond
– Suffix: -ene (e.g., CH3-CH=CH2 = propene)

🔍 Isomerism in Alkenes

1. Structural Isomerism:
– Chain: Different carbon skeletons (e.g., but-1-ene vs. 2-methylpropene)
– Position: Double bond in different locations (e.g., but-1-ene vs. but-2-ene)

2. Geometrical (cis-trans) Isomerism:
– Needs two different groups on each carbon of C=C
– Example:
– cis-But-2-ene (CH3 groups same side, b.p. 277 K)
– trans-But-2-ene (CH3 opposite, b.p. 274 K)
– cis is polar (dipole moment > 0), trans is non-polar

⚗️ Preparation of Alkenes

  1. From Alkynes:
    cis-Alkene: H₂ + Lindlar’s catalyst (Pd/C + quinoline)
    $$ \ce{RC#CR’ + H2 ->[Pd/C] R\overset{\displaystyle H}{\underset{\displaystyle |}{C}}=\overset{\displaystyle H}{\underset{\displaystyle |}{C}}R’} $$
    trans-Alkene: Na in liquid NH₃
    $$ \ce{RC#CR’ + H2 ->[Na/NH_3] R\overset{\displaystyle R’}{\underset{\displaystyle |}{C}}=\overset{\displaystyle H}{\underset{\displaystyle |}{C}}H} $$
  2. From Alkyl Halides:
    Dehydrohalogenation with alcoholic KOH (β-elimination)
    $$ \ce{CH3-CH2Br ->[alc. KOH][\Delta] CH2=CH2} $$
  3. From Vicinal Dihalides:
    Dehalogenation with Zn dust
    $$ \ce{CH2Br-CH2Br + Zn -> CH2=CH2 + ZnBr2} $$
  4. From Alcohols:
    Dehydration with conc. H₂SO₄ (β-elimination)
    $$ \ce{CH3-CH2-OH ->[conc. H2SO4][\Delta] CH2=CH2} $$

🔥 Chemical Reactions of Alkenes

  1. Addition of H₂:
    Forms alkane (Ni/Pt/Pd catalyst)
    $$ \ce{CH2=CH2 + H2 ->[Ni] CH3-CH3} $$
  2. Addition of Halogens (X₂):
    Test for unsaturation (decolorizes Br₂ in CCl₄)
    $$ \ce{CH2=CH2 + Br2 -> CH2Br-CH2Br} $$
  3. Addition of HX (Markovnikov’s Rule):
    H⁺ adds to carbon with more H atoms
    $$ \ce{CH3-CH=CH2 + HBr -> CH3-CHBr-CH3} $$
    Peroxide Effect (Anti-Markovnikov):
    Only for HBr → Forms 1° alkyl bromide
    $$ \ce{CH3-CH=CH2 + HBr ->[(PhCOO)2] CH3-CH2-CH2Br} $$
  4. Ozonolysis:
    Cuts double bond → Carbonyl compounds
    $$ \ce{CH3-CH=CH2 ->[1. O3][2. Zn/H2O] CH3CHO + HCHO} $$
  5. Polymerization:
    Forms plastics (e.g., polythene)
    $$ \ce{n CH2=CH2 ->[high T/P, catalyst] [-CH2-CH2-]_n} $$

9.4 Alkynes (CnH2n-2)

IUPAC Naming: Replace -ane with -yne (e.g., HC≡CH = ethyne)
Isomerism:
– Position isomers (e.g., but-1-yne vs. but-2-yne)
– Chain isomers (e.g., pent-1-yne vs. 3-methylbut-1-yne)

💡 Quick Tips:

  • Pi (π) bonds make alkenes electron-rich → attacked by electrophiles!
  • cis-trans isomers have different physical properties (b.p., polarity)
  • Markovnikov vs. Anti-Markovnikov: Watch for peroxides!

Good luck with your studies! 🚀✨