AQA Chemistry A-level 3.3.3 Halogenoalkanes Detailed Notes PDF
Document Details
Uploaded by FelicitousPortland1685
AQA
Tags
Summary
These detailed notes cover AQA Chemistry A-level 3.3.3 Halogenoalkanes. Concepts like nucleophilic substitution, elimination reactions, and ozone depletion are explored in depth. The notes are suitable for secondary school students studying A-level Chemistry.
Full Transcript
AQA Chemistry A-level 3.3.3: Halogenoalkanes 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 3.3.3: Halogenoalkanes 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 3.3.3.1 - Nucleophilic Substitution Halogenoalkanes contain polar bonds as the halogens are more electronegative than carbon atoms. This means electron density is drawn towards the halogen forming ∂+ and ∂- regions. Example: Nucleophiles These species are ‘positive liking’. They contain a lone electron pair that is attracted to ∂+ regions of molecules. Some of the most common nucleophiles are: CN:- :NH3 - :OH They must be shown with the lone electron pair and often a negative sign indicating they are nucleophiles. Nucleophilic Substitution This is the reaction mechanism that shows how nucleophiles attack halogenoalkanes. It can be used to produce alcohols or amines from halogenoalkanes. Mechanism - Alcohol The nucleophile attacks the ∂+ carbon and the electrons are transferred to the chlorine. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Mechanism - Amines The intermediate formed has an N+ atom, so electrons are transferred to it causing a hydrogen to be lost too. The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy meaning it can be broken more easily. Therefore the rate of reaction for these halogenoalkanes is faster. Nucleophilic substitution reactions can only occur for 1o (primary) and 2o (secondary) halogenoalkanes. 3.3.3.2 - Elimination When a halogenoalkane is heated to high temperatures under alcoholic conditions, elimination occurs. In this reaction, the nucleophile acts as a base and accepts a proton, removing a hydrogen atom from the molecule. This results in the elimination of the halide too producing a carbon-carbon double bond, an alkene. Mechanism Elimination reactions can only occur from 2o and 3o (tertiary) halogenoalkanes. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.3.3.3 - Ozone Depletion Ozone in the atmosphere absorbs UV radiation. CFCs (chloro-fluoro carbons) also absorb UV radiation, breaking down the carbon-halogen bonds to form free radicals that can catalyse ozone depletion. Example: CFC-free solvents are now being produced to prevent them entering the atmosphere. This helps minimise ozone depletion and global warming. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc
