Alkenes - Lecture notes 1 PDF

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Summary

Alkenes  Naming- always choose the lowest number when locating the double bond  Name the parent compound as you would an alkane but change ending to ‘ene’  Number the carbon chain of parent starting at end nearer double bon  Use the smaller number to indicate position of the double bondDouble bo...

Description

Alkenes    

Naming- always choose the lowest number when locating the double bond Name the parent compound as you would an alkane but change ending to ‘ene’ Number the carbon chain of parent starting at end nearer double bon Use the smaller number to indicate position of the double bond

Double bond overrides side chains 2 double bonds= …”diene” Isolated double bonds Conjugated double bonds Bonding in alkanes 

Sp2 hybridised

Constitutional isomers 

Alkenes can have many constitutional isomers (also called structural isomers)

Fixed π Bond  

Π bond will not rotate and therefore you can have geometric isomers Π Bond needs to break in order for rotation to occur

Geometric isomer  

A type of stereoisomer Same molecular order but different arrangement of atom in space

Cis-Trans Isomer (Same as E-Z isomers)    

2 different groups attached to each carbon atom Z isomer (2 groups with the highest priority on the same side) E isomer (2 groups with the highest priority on different sides) Highest atomic number I given to the highest priority

Steric interactions   

In space are the 2 groups interacting with each other or not Cis isomer has greater steric interaction between substituents across the double bond Trans isomer are therefore favoured

Cis-Trans Isomers and Vision   

11-cis-Retinal derived from Vitamin A combines with protein (opsin) to form visual pigment Rhodopsin When light absorbed the cis double bond converted to a trans double bond (no longer fits protein) Conversion begins a cascade of events which brain interprets as ‘seeing’

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Phytochromobilin in plants can convert between two forms in response to light Two forms are cis- trans- isomers Conversion triggers light responses

Cis-Trans Isomers in Fatty Acids  

Most fatty acids are cis isomers and are converted to trans isomers during cooking Trans fatty acids can disrupt normal fatty acids processing increasing risk of stroke and heart attack

Reactivity of the Double Bond     

Π bond results in a high electron density above and below the double bond Alkenes act as nucleophiles They attack electrophiles More reactive than the corresponding alkane because of the polarisable p electrons Most characteristic reaction is Electrophilic Addition (attack by electron deficient species)

Addition to the Double Bond    

Halogenation Hydration Hydrohalogenation Hydrogenation

Addition to symmetrical double bond 1. Halogen gas approaches the electron rich double bond a dipole is formed on the halogen 2. Those electrons are donated and a bond between the halogen and carbon molecule is formed 3. A carbocation s formed and then the remaining negative halogen attack the carbo cation

Addition to an unsymmetrical double bond

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Secondary carbo cations readily donate electrons to increase stability Increasing substitution by carbon stabilises carbocations

Markovniko’s Rule Oxidation of Alkenes...