OCR A Chemistry A-level Alcohols, Haloalkanes & Analysis Detailed Notes PDF

OCR A Chemistry A-level Module 4.2: Alcohols, Haloalkanes and Analysis 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...

OCR A Chemistry A-level Module 4.2: Alcohols, Haloalkanes and Analysis 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 4.2.1 Alcohols Properties of Alcohols Alcohols contain an ​-OH group​ and follow the general formula ​C​n​H​2n+1​OH​. They can be produced via two main methods of fermentation or hydration. Alcohols are named according to IUPAC​ rules and have the suffix -ol. Alcohols can be ​primary (1​o​), secondary (2​o​) ​or ​tertiary (3​o​),​ depending on the position of the hydroxyl group. Alcohols are ​polar molecules​ since there is a large difference in ​electronegativity​ between carbon and oxygen. The oxygen is very electronegative, so can ​hydrogen bond​ to water molecules. This ability means that alcohols are ​water soluble​. Hydrogen bonds​ are a much stronger intermolecular force than van der Waals forces so more energy is required to overcome them. Alcohols have both types of intermolecular force present between molecules, so are much ​less volatile​ than alkanes which only have van der Waals forces present. Reactions of Alcohols Combustion When burned in air, alcohols react with ​oxygen ​to form​ carbon dioxide ​and ​water​. Alcohols make good ​fuels​ by reacting in this way as lots of ​energy ​is also released. Example: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Reactions with Halogenating Agents Alcohols can react with ​halogenating agents​ via ​nucleophilic substitution. ​The ​-OH​ group is replaced by a ​halogen​, producing a ​haloalkane​. PCl​5​ is used to produce ​chloroalkanes​. This can be used as a ​test for alcohols ​because their reaction with PCl​5​ produces ​white steamy fumes​ that turn damp blue litmus paper red. A reaction mixture of concentrated ​sulfuric acid​ and ​potassium bromide​ can be used to produce ​bromoalkanes. ​The potassium bromide reacts with the sulfuric acid to form HBr. This then reacts with the alcohol to produce the bromoalkane. CH​3​CH​2​OH + HBr → CH​3​CH​2​Br + H​2​O A reaction mixture of ​red phosphorus​ with iodine can be used to produce ​iodoalkanes​. First, the phosphorus reacts with the iodine to produce ​phosphorus(III) iodide​. This then reacts with the alcohol to form the iodoalkane. 2P + 3I​2​ → 2PI​3 3CH​3​CH​2​OH + PI​3​ → 3CH​3​CH​2​I + H​3​PO​3 Elimination Reactions Alkenes can be formed from the ​dehydration of alcohols​, where a molecule of ​water is removed​ from the molecule. In order to do this, the alcohol is heated with ​concentrated phosphoric acid​. Oxidation of Alcohols Primary ​and​ secondary ​alcohols can be ​oxidised​ to produce various products but​ tertiary alcohols are​ not easily oxidised​. When primary alcohols are heated in the presence of ​acidified potassium dichromate(VI)​, they are ​oxidised ​to​ ​aldehydes​.​ ​Distillation​ is required to separate the ​aldehyde​ product. Example: ​Oxidation of ethanol to ethanal https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc When an aldehyde is heated further with acidified potassium dichromate(VI) under ​reflux conditions, the aldehyde is ​oxidised ​to produce ​carboxylic acids​. This shows primary alcohols are oxidised to aldehydes and then to carboxylic acids. Example: ​Oxidation of ethanal to ethanoic acid Secondary alcohols​ can be oxidised to ​ketones ​when heated in the presence of ​acidified potassium dichromate(VI)​. Example: ​Oxidation of propan-2-ol to propanone Potassium Dichromate(VI) (K​2​Cr​2​O​7​) Potassium dichromate(VI) is used in the oxidation of alcohols as the ​oxidising agent​. It is reduced as the alcohol is oxidised. A colour change from ​orange to green​ is observed when the alcohol is oxidised with potassium dichromate(VI). This ​colour change​ occurs due to a change in ​oxidation state​ of the ​chromium ion​. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 4.2.2 Haloalkanes Introduction to Haloalkanes Haloalkanes contain ​polar bonds​ since the halogens are more ​electronegative​ than the carbon atom. This means electron density is drawn towards the halogen, forming ​∂+ and ∂- regions​. Example: ​The electronegative carbon-halogen bond here X indicates the halogen atom. Haloalkanes can be classed as ​primary, secondary or tertiary ​haloalkanes depending on the position of the halogen within the carbon chain. Relative Reactivity Reactivity varies depending on the halogen present in the molecule. ​Electronegativity​ of the halogens ​decreases down the group,​ meaning that a carbon-fluorine bond is much more polar​ than a carbon-iodine bond. The increased polarity, along with the fact that the carbon-fluorine bond is ​shorter​, means that the carbon-fluorine bond is much stronger than the carbon-iodine bond. The ​greater the Mr​ of the halogen in the polar bond, the ​lower the bond enthalpy. ​A​ ​lower bond enthalpy means the bond can be ​broken​ more easily. Therefore, the ​rate of reaction increases​ for haloalkanes as you move ​down ​the group. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Substitution Reactions of Haloalkanes To Produce Alcohols Haloalkanes can react with an ​aqueous alkali​, such as aqueous sodium or potassium hydroxide, to produce ​alcohols ​in a ​nucleophilic substitution ​reaction. The ​hydroxide ion acts as a ​nucleophile​. To Produce Alkenes Haloalkanes can react with ​ethanolic potassium hydroxide​ (KOH) to produce ​alkenes ​in an elimination ​reaction. The ​hydroxide ion​ acts as a ​base​. Hydrolysis with Silver Nitrate Haloalkanes can be​ broken down​ in their reaction with ​aqueous silver nitrate and ethanol. The ​water ​in the solution acts as a ​nucleophile ​which leads to formation of the alcohol and releases the halide ions into the solution. The ​halide ions​ then react with the ​silver ions​ from silver nitrate to form ​silver precipitates​. The ​colour ​of the precipitate allows you to identify the halide ion present. The rate at which the precipitates form allows you to identify the ​relative stability​ of the haloalkanes, because the faster the precipitate forms, the ​less stable​ the haloalkane, and therefore the more quickly it is hydrolysed​. Cl​- Br​- I​- White precipitate (AgCl) Cream precipitate (AgBr) Yellow Precipitate (AgI) Reactivity ​depends on the ​strength ​of the ​C-X bond ​(where X is a halogen atom) and not the bond polarity. Bond strength ​decreases ​with ​increasing Mr​. Therefore, iodoalkanes react faster than bromoalkanes and chloroalkanes, and bromoalkanes react faster than chloroalkanes. Nucleophilic Substitution Nucleophiles A nucleophile is an ​electron pair donor​. 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​:-​ :​NH​3 -​ :​OH https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Nucleophilic Substitution Mechanism Nucleophilic substitution​ is the reaction mechanism that shows how ​nucleophiles​ attack haloalkanes. Aqueous potassium hydroxide is used to produce ​alcohols, ​potassium cyanide is used to produce ​nitriles​ and ammonia is used to produce​ amines​. Mechanism - Producing Alcohols The nucleophile attacks the ∂+ carbon and the electrons are transferred to the chlorine. Mechanism - Producing Amines The intermediate has a positively charged nitrogen (N+​ ​). Electrons are transferred to the nitrogen by the loss of a hydrogen atom. The ​greater the Mr​ of the halogen in the polar bond, the ​lower the bond enthalpy.​ This means the bond can be broken more easily. Therefore the rate of reaction for these haloalkanes is faster. Nucleophilic substitution reactions can only occur for ​1o​​ (primary) and 2​o​ (secondary) haloalkanes. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Environmental Concerns from Use of Organohalogen Compounds Ultraviolet (UV) radiation ​in the upper atmosphere can cause ​CFCs​ to produce halogen radicals​. These radicals catalyse the breakdown of the Earth’s protective ​ozone​ layer. CFCs are ​chlorofluorocarbons​ - haloalkanes containing carbon, chlorine and fluorine atoms only. The radical mechanism for the breakdown of ozone, ​O​3​, is shown below. CF​3​Cl → CF​3​ + Cl ​Initiation Cl + O₃ → ClO + O₂ Propagation 1 ClO + O₃ → Cl + 2O₂ ​Propagation 2 Overall equation for the breakdown of ozone: ​2O₃ → 3O₂ The chlorine radical is ​regenerated​ in the second propagation step, so is ​catalytic​ in the breakdown of ozone. This means a small amount of CFC released can cause a lot of damage. 4.2.3 Organic Synthesis Practical Skills There are many different techniques that can be used to ​prepare ​and then ​purify ​an organic compound. Heating under Reflux Reflux apparatus is used to ​continually heat​ the contents of the flask to allow reactions like the oxidation​ of primary alcohols to proceed all the way to the formation of carboxylic acids. The condenser​ helps ensure the vapours condense and ​return ​to the flask for further heating. This ensures the product vapours can not escape. Distillation Distillation apparatus is used to separate liquids with ​different boiling points​. The round-bottomed flask is heated and the liquid with the lower boiling point will ​evaporate ​first. It rises out of the flask and into the attached tubing which is surrounded by a condenser. The condenser causes the vapour to ​cool and condense​ back into a liquid, which is then collected in a separate flask. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Diagrams:​ Reflux apparatus (left) and distillation apparatus (right) Separating Funnel A separating funnel is used to separate two liquids with​ different densities​. The mixture is added to the flask, and then the flask is stoppered and inverted to mix the contents. The liquids are allowed to ​separate ​into​ two layers​. The tap can then be opened to collect the bottom, denser liquid in one flask and the second, less dense liquid in a second flask. Usually these layers will be distinguished to be an aqueous and an organic layer. Redistillation Subsequent distillations can be carried out to obtain a purer product. Boiling point determination Determining the boiling point of a compound and comparing it to a databook value is a way of testing the ​purity ​of a substance. The purer a substance, the ​closer ​to the databook boiling point value it will be. If a sample has a low purity, the melting or boiling point will take place over a ​range​ of temperatures. To determine the boiling point, the substance is packed into a ​Thiele tube​ which has an inverted capillary tube​ in it. The substance is heated to above its boiling point and allowed to cool. When it ​condenses ​into a liquid it will be drawn into the capillary tube and the temperature at which this change occurs is taken to be the boiling point. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Drying A compound can be dried by the addition of an ​anhydrous ​(contains no water) ​salt.​ The anhydrous salt will ​absorb moisture and water​ present, thus drying and purifying the compound. Common anhydrous salts used for drying are ​magnesium sulphate ​and​ calcium chloride. Synthetic Routes Synthetic routes are the routes which can be used to produce a ​certain product from a starting organic compound​. It is important that you understand the different methods and conditions​ required to convert compounds to other products. Multi-Stage Synthesis Some organic molecules can be prepared using a ​multi-stage synthesis​. Typically, this involves two stages: reactant → intermediate → product. It can cover more stages. Example 1 Below is a diagram showing how ethanoic acid can be formed from chloroethane: Example 2 2-propylamine can be formed from propene as follows: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Analysing Synthetic Routes When ​synthesising​ an organic compound, several factors are considered before deciding which synthetic route to use: Type of reaction​ - addition reactions are more sustainable than substitution or elimination reactions as there are no waste products. Reagents​ - renewable reagents with few safety concerns are preferred. By-products ​- less harmful by-products are favoured as there would be fewer safety and environmental concerns. If the by-products can be used in another industry, the process is more sustainable. Conditions​ - choose the reaction with the most energy efficient and safe conditions. Identification of Functional Groups Individual ​functional groups​ covered in this module can be identified through various tests as described in their sections above. These include: Test for unsaturation - bromine water Test for 1°/2° alcohols - acidified potassium dichromate(vi) Test for aldehydes - Tollens’ reagent Test for haloalkanes - aqueous silver nitrate with ethanol 4.2.4 Analytical Techniques Infrared Spectroscopy Infrared (IR) radiation causes ​covalent bonds​ to ​vibrate​ and absorb energy. Infrared spectroscopy is an analytical technique that uses ​infrared (IR) radiation​ to determine the ​functional groups​ present in organic compounds. The IR radiation is passed through a sample where the different types of bonds ​absorb​ the radiation in different amounts. These varying amounts of absorbance are ​measured and recorded,​ allowing certain bonds, and thus functional groups, to be identified. A ​spectrum​ is produced from the measurements, which has ​characteristic curves​ for the different functional groups: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc -OH Alcohol Group ​ ​. ​ 230 - 3550 cm-1 The characteristic -OH alcohol group peak is in the range 3 It is a broad, curved peak. -OH Acid Group ​ ​. The characteristic -OH acid group peak is in the range ​2500 - 3300 cm-1 It overlaps with the C-H region, hence the peak is not a smooth curve. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc C=C Unsaturated Group ​ ​. The characteristic C=C peak is in the range ​1620-1680 cm-1 It is a sharp peak. C=O Carbonyl Group ​.​ The characteristic C=O peak is in the range ​1680-1750 cm-1 It is a sharp peak. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Interpreting IR Spectra An infrared spectrum of an organic compound can be used to ​identify​ any ​functional groups present: An ​alcohol​ from an absorption peak of the O–H bond. An ​aldehyde​ or ​ketone​ from an absorption peak of the C=O bond. A ​carboxylic acid​ from an absorption peak of the C=O bond and a broad absorption peak of the O–H acid bond. Most organic compounds will produce a peak at approximately 3000 cm⁻¹ due to absorption by C–H bonds. Fingerprint Region Each IR spectrum has a ​fingerprint region​ on the right-hand side, from 500-1500 cm​-1​. This is unique for each species, containing ​tiny differences​ between each species. This means it acts as a molecules’ ‘fingerprint’, allowing it to be ​identified​. Environmental There is a link between the absorption of infrared radiation by ​atmospheric gases​ containing C=O, O–H and C–H bonds (e.g. CO₂, H₂O and CH₄), and ​global warming​. As a result, there is a need for changes to be made to ​renewable​ energy resources. Uses of Infrared Spectroscopy IR spectroscopy can be used to monitor gases causing ​air pollution​ (e.g. CO and NO from car emissions) and in modern breathalysers to measure ethanol in people’s breath. Mass Spectrometry Mass spectrometry is an ​analytical technique​ used to identify different molecules and find the overall relative molecular mass. Time of Flight (TOF) Mass Spectrometry TOF mass spectrometry records the time it takes for ions to reach a detector. Using this, spectra​ can be produced showing ​each isotope present along with their relative abundances​. 1. Ionisation​ - A sample is ​vaporised​ and injected into the mass spectrometer where a high voltage​ is passed over the chamber. This causes electrons to be removed from the atoms (they are ionised) leaving ​+1 charged ions​ in the chamber. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  2. Acceleration​ ​- These positively charged ions are then ​accelerated​ towards a negatively charged ​detection plate​. 3. Ion Drift​ ​- The ions are then deflected by a ​magnetic field​ into a ​curved path​. The radius of the path is dependent on the charge and mass of the ion. 4. Detection​ ​- When the positive ions hit the negatively charged detection plate, they ​gain an electron,​ producing a ​flow of​ ​charge​. The greater the current produced, the greater the abundance of that particular ion. 5. Analysis​ ​- The current values are then used in combination with the ​flight times​ to produce a ​spectra print-out​ with the relative abundance of each isotope displayed. Using this print-out spectra, the ​Mr ​(relative molecular mass)​ can be calculated​ by looking at the m/z value of the ​molecular ion peak​. This is the peak that is furthest to the right on the spectrum. The mass spectra of organic compounds may contain a very small ​M+1 peak​ (one greater than the molecular ion peak) from the small proportion of ​carbon-13​ isotopes present. Fragmentation Sometimes organic compounds​ fragment ​in the mass spectrometer. This means that peaks at smaller m/z ​values than the molecular ion peak appear on the ​spectrum​. The molecular ions break down into a ​fragment​ ion and a ​radical​. The radical is uncharged so is not detected. The m/z value of a fragment peak can be used to suggest the ​Mr​ and ​structure​ of a fragment ion. Combined Techniques The ​analytical techniques​ covered in this chapter can be used together to ​predict​ the structure of unknown compounds. A combination of ​functional group ​tests, ​infrared spectroscopy​ and ​mass spectrometry​ can be used to identify organic structures. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc