Ch07 Alkenes - Structure & reactivity

Chapter Overview

7.1 Industrial Preparation and Use of Alkenes

• Alkenes, specifically ethylene and propylene, are significant industrial organic chemicals.

  • Annual production: 220 million tons of ethylene and 138 million tons of propylene.

  • Uses include synthesis of polyethylene, polypropylene, ethylene glycol, acetic acid, etc.

• Ethylene, propylene, and butene are produced from C2-C8 alkanes via steam cracking at 900 °C.

7.2 Calculating the Degree of Unsaturation

• An alkene has fewer hydrogens than its corresponding alkane due to the presence of a double bond.

  • Alkene formula: CnH2n (e.g., ethylene C2H4) vs. Alkane formula: CnH2n+2 (e.g., ethane C2H6).

• Each ring or double bond decreases the number of hydrogens by 2.

  • Example: C6H10 has a degree of unsaturation of 2 (C6H14 saturated vs C6H10).

• Different classes of compounds affect unsaturation degree:

  • Organohalogen compounds (C,H,X): Add halogens, subtract hydrogens for equivalent hydrocarbons.

  • Organooxygen compounds (C,H,O): Oxygen can be ignored in calculations.

  • Organonitrogen compounds (C,H,N): Subtract nitrogens from hydrogens for equivalent hydrocarbons.

7.3 Naming

• Named similarly to alkanes, with the suffix -ene for double bonds.

  1. Identify and name the longest carbon chain containing the double bond.

  2. Number carbons nearest the double bond to assign the lowest numbers.

  3. Write the full name with substituents listed alphabetically and the position of the double bond indicated.

• Changes to naming conventions by IUPAC are acknowledged but older conventions persist in common usage.

7.4 Cis-Trans Isomerism

• Cis-trans isomers arise from restricted rotation around the carbon-carbon double bond.

• For disubstituted alkenes, one isomer has substituents on the same side (cis) and another on opposite sides (trans).

• Cis-counted compounds are less stable due to steric strain from larger groups on the same side.

7.5 E,Z Designation

• The E,Z designation is used for alkenes with more complicated substitution patterns where cis-trans nomenclature is insufficient.

  • Higher topology groups on the same side = Z; on opposite sides = E.

  • Rule 1: Rank substituents according to high to lowest atomic number of the first atom in each. If there’s no difference move to the next atom.

  • Rule 2: Multiple bonded atoms are equivalent to the same number of single-bonded atoms.

7.6 Stability

• Stability comparison shows trans isomers are generally more stable than cis isomers due to steric hindrance.

• Alkene stability depends on substitution level; more substituted alkenes (more alkyl groups) are more stable due to hyperconjugation and bond strength considerations.

7.7 Electrophilic Addition Reactions

• Alkenes act as nucleophiles due to their electron-rich π bonds, allowing reactions with electrophiles.

  • Example reaction of 2-methylpropene with HBr forms a carbocation intermediate.

• The reaction occurs in two main steps: protonation and nucleophilic addition from bromide.

7.8 Orientation of Electrophilic Additions: Markovnikov's Rule

• Markovnikov's Rule states that in the addition of HX to an alkene, H attaches to the carbon with fewer alkyl substituents.

  • This focuses on the stability of the intermediate carbocation formed during the reaction.

7.9 Carbocation Structure and Stability

• Carbocations are planar and sp2 hybridized.

• Stability increases with substitution (tertiary > secondary > primary > methyl).

• Inductive effects and hyperconjugation contribute to carbocation stability.

7.10 The Hammond Postulate

• The stability of the transition states resembles either reactants or products depending on whether the reaction is endergonic or exergonic.

7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

• Structural rearrangements during reactions support the carbocation mechanism, allowing a shift leading to more stable carbocations.

  • Example: A hydride shift converts a less stable secondary to a more stable tertiary carbocation.

Key Terms

• Alkene: R!C=CR!

• Electrophilic Addition Reaction: A type of reaction involving electrophiles adding to alkenes.

• Markovnikov’s Rule: Rule predicting product distribution based on alkyl substituents.

• Cis-Trans Isomerism: Isomerism that involves different spatial arrangements of substituents around double bonds.