5A Alkenes - Edexcel IAS Chemistry (Unit 1)
Miss Natalie Chemistry・2 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
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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.
