AQA Chemistry A-level 3.3.4: Alkenes Detailed Notes PDF
Document Details
Uploaded by FelicitousPortland1685
AQA
Tags
Summary
This document provides detailed notes on alkenes, including their structure, reactivity, electrophilic addition reactions, and addition polymers. It covers topics like the identification of alkenes using bromine water, and the formation of various polymers.
Full Transcript
AQA Chemistry A-level 3.3.4: Alkenes Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...
AQA Chemistry A-level 3.3.4: Alkenes Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.3.4.1 - Structure and Reactivity Alkenes are unsaturated hydrocarbons meaning they contain a carbon-carbon double bond. This is an area of high electron density making it susceptible to attack from electrophiles (species that are attracted to ∂- areas). It consists of a normal covalent bond and a π bond. Example: Bromine water is used to identify this double bond and other unsaturated compounds. It turns the solution from orange-brown to colourless if a double bond is present in the substance. 3.3.4.2 - Electrophilic Addition Alkenes undergo electrophilic addition about the double bond. Electrophiles These are electron acceptors and are attracted to areas of high electron density. Some of the most common electrophiles are: HBr Br2 H2SO4 They can be used to form alkyl hydrogensulphates or halogenoalkanes from alkenes. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Electrophilic Addition This is the reaction mechanism that shows how electrophiles attack the double bond in alkenes. When the double bond is broken, a carbocation forms. This is a carbon atom with only three bonds, meaning it has a positive charge. Carbocations can have varying stability, with tertiary being the most stable and primary the least. The more stable the carbocation, the more likely it is to form. Therefore in an addition reaction, multiple products can form but the major product will always be the most stable possible. Mechanism - Halogenoalkanes The π bond causes the bromine molecule to gain a temporary dipole so that electrons are transferred. Mechanism - Sulphuric Acid Alkene reacts with sulphuric acid to produce ethyl hydrogensulphate. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.3.4.3 - Addition Polymers Addition polymers are produced from alkenes where the double bond is broken to form a repeating unit. Example: The repeating unit must always be shown with extended bonds through the brackets showing that it bonds to other repeating units on both sides. Reaction Conditions The reaction conditions used in the production of these polymer chains can be altered to give the plastics produced different properties. High pressures and temperatures produce branched chain polymers with weak intermolecular forces. Whereas lower pressures and temperatures produce straight chain polymers with strong intermolecular forces. Uses of Polymers Polymers are unreactive hydrocarbon chains with multiple strong, non-polar covalent bonds. This makes them useful for manufacturing many everyday plastic products such as shopping bags (poly(ethene)). However, the unreactive nature of the bonds in addition polymers means they are not biodegradable and cannot be broken down by species in nature. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc PVC Poly(chloroethene) more commonly known as PVC is an addition polymer with waterproof properties. It gains these properties by the addition of plasticisers during the reaction. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc
