ch 3. Alkenes and Alkynes

Chapter 3: Alkenes and Alkynes

Section 1: Alkenes & Alkynes

Introduction to Alkenes and Alkynes

• Definitions:
  • Alkenes: Hydrocarbons that contain at least one carbon-carbon double bond, general formula: $C<em>nH</em>{2n}$.
  • Alkynes: Hydrocarbons that contain at least one carbon-carbon triple bond, general formula: $C<em>nH</em>{2n-2}$.
• Polyunsaturated Hydrocarbons: Compounds with more than one double or triple bond.

Structure of Alkenes

Hybridization

• Carbon atoms in alkenes are sp² hybridized.
• Hybridization involves mixing one s orbital and two p orbitals:
  • The third p orbital remains unchanged, leading to a trigonal planar geometry.

Bond Formation

• Sigma (σ) bonds form from head-to-head overlap of sp² orbitals.
• Pi (π) bonds form from side-to-side overlap of unhybridized p orbitals.
• A double bond consists of one σ bond and one π bond.
• The π electrons are not symmetrical around the bond axis and are located above and below the plane of the molecule:
  • Double bonds (C=C) are rigid and do not allow for rotation.

Physical Properties of Alkenes

• Isomerism:
  • Carbon chain isomers and functional isomers exist.
  • Isomers include cis-trans isomers (geometric isomers) based on the spatial arrangement around the double bond.

Nomenclature of Alkenes

1. Basic Naming: Replace the suffix -ane with -ene (e.g., methane becomes ethene).

2. IUPAC Rules:

   • Longest carbon chain with the highest number of double bonds.
   • Number the chain starting from the end closest to the double bond for the lowest possible number.
   • Name substituents alphabetically.
   • For multiple double bonds, use suffixes -diene, -triene, etc.

3. Naming Examples:

   • Ethylene (ethene), Propylene (propene), Acetylene (ethyne).
   • Example: 1-Hexene has the double bond between C1 and C2.

Structural Characteristics of Alkynes

• Hybridization: Sp hybridization, resulting in a linear structure with bond angles of 180°.
• Alkynes can be terminal or internal, with terminal alkynes being more acidic than alkenes.

Inductive Effects

• The inductive effect (I-effect) relates to electron withdrawal or donation through sigma bonds.
  • Atoms with higher electronegativity exert a negative inductive effect (-I) while those that release electrons exhibit a positive inductive effect (+I).

Properties and Reactions of Alkenes & Alkynes

Electrophilic Addition Reactions

1. Mechanism Overview:

   • Electrophilic addition involves an electrophile attacking a nucleophilic site at the double bond.
   • Example: The addition of HBr to alkenes follows Markovnikov's rule where the hydrogen adds to the carbon with more hydrogen atoms, leading to stable carbocation intermediates.

2. Reaction Examples:

   • Hydrogenation: Alkenes react with H2 in the presence of catalysts to form alkanes.
   • Halogenation: Alkenes react with halogens (Br2, Cl2) to produce dibromoalkanes.
   • Hydrohalogenation and hydration reactions can also occur, producing alcohols or haloalkanes depending on conditions.

Properties of Dienes

• Classification: Dienes can be classified into three main groups: cumulated, isolated, and conjugated.
• Stability: Conjugated dienes are more stable due to the delocalization of π electrons; they have lower energy states.
• Reactivity: Dienes react differently compared to simple alkenes; they can undergo 1,2-addition and 1,4-addition reactions, with products depending on the stability of cation intermediates.