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5A Alkenes - Edexcel IAS Chemistry (Unit 1)

Miss Natalie Chemistry・25 minutes read

Alkenes are more reactive than alkanes due to the carbon-carbon double bond, allowing for various addition reactions like hydrogenation and halogenation. The naming systems for geometric isomers in alkenes, such as cis-trans and E-Z, rely on group positioning around the double bond to determine configuration and product formation based on carbo-cation stability.

Insights

  • Alkenes are more reactive than alkanes due to the presence of a carbon-carbon double bond, which contains both a sigma bond and a pi bond, with pi bonds being more reactive.
  • Understanding the E-Z naming system and the mechanism of addition reactions for alkenes involving electrophilic additions is crucial, as stability of carbo-cations determines major product formation in asymmetric molecules.

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Recent questions

  • What distinguishes alkenes from alkanes?

    Alkenes contain a carbon-carbon double bond.

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Summary

00:00

Alkenes: Reactivity, Isomerism, and Addition Reactions

  • Topic 5e in the IES chemistry course focuses on alkenes, comparing them to cyclo alkanes and exploring their general formula.
  • Alkenes contain a carbon-carbon double bond, making them more reactive than alkanes due to the presence of the double bond.
  • Nomenclature for alkenes follows similar rules to alkanes, with the double bond positioned on the lowest possible carbon.
  • The structure of a carbon-carbon double bond consists of a sigma bond and a pi bond, with pi bonds being more reactive due to their electrons being further away from the carbon atoms.
  • Geometric isomerism in alkenes arises due to the restricted rotation caused by the carbon-carbon double bond, leading to different positioning of groups on the molecule.
  • Geometric isomers can be named using the cis-trans system or the E-Z system based on the positioning of groups around the double bond.
  • The E-Z naming system relies on priority rules based on atomic numbers to determine the configuration of groups around the double bond.
  • Addition reactions of alkenes involve breaking the pi bond to form saturated compounds, with examples including hydrogenation, halogenation, hydration, and oxidation.
  • Oxidation of alkenes can lead to the formation of diols, containing two hydroxyl groups, with acidified potassium permanganate commonly used as an oxidizing agent.
  • Understanding the mechanism of addition reactions involves using curly arrows to show the movement of electron pairs, particularly in the addition of hydrogen halides to alkenes.

18:13

Electrophilic Addition: Curly Arrows and Carbo-Cations

  • Electrophile is attracted to pi bond due to high electron density, described as Delta positive end of HBr.
  • Curly arrows used to show movement in electrophilic addition reaction.
  • Reaction begins heterolytically, with electrons from pi bond moving to hydrogen in HBr.
  • Formation of bond between carbon and hydrogen, creating carbo-cation and bromide ion.
  • Lone pair on bromide ion donates electrons to carbo-cation, forming C-Br bond.
  • Mechanism is the same for halogens and hydrogen halides, induced dipole causes polarity in halogen molecule.
  • Asymmetric molecules can result in two products, major and minor, based on stability of carbo-cations.
  • Electrophilic additions proceed via carbo-cations, with stability determining major product formation.
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