AQA A Level Chemistry- Organic Chemistry Questions and Answers

AQA A Level Chemistry- Organic Chemistry Questions and Answers General formula The simplest algebraic formula that can describe any member of a homologous series Molecular formula The actual number of atoms of each element in a molecule Structural formula Uses letter symbols and bonds to show the arrangement of atoms in a compound Displayed formula A formula showing the relative positioning of all the atoms in a molecule and the bonds between them Skeletal formula A simplified organic formula which only shows the carbon skeleton and functional groups Homologous series Family of chemical that differ from one another by a repeating unit, with the same general formula and similar chemical properties IUPAC rules of nomenclature order Longest carbon chain Unsaturation Functional groups Position of any functional groups Free radical substitution Conditions: UV light Steps: Initiation Propagation Termination Free radical An uncharged molecule, typically highly reactive and short-lived, having an unpaired electron Nucleophile A reactant that provides a pair of electrons form a new covalent bond Lewis base Electrophile A reactant that accepts a pair of electrons top form a new covalent bond Lewis acid Substitution Reaction in which one functional group is replaced by another Addition A reaction where one molecule combines with another to form a larger molecule No other products Elimination A reaction in which a molecule loses atoms or groups of atoms Nucleophilic substitution w/OH- Reagent: NaOH Conditions: aqueous, warm What happens: Halogen atom replaced by OH group Nucleophilic substitution w/ CN- Reagent: KCN Conditions: ethanolic, warm What happens: Halogen atom replaced by CN group Nucleophilic substitution w/NH3 Reagent: NH3 Conditions: excess conc. ammonia dissolved in ethanol at pressure in a sealed container What happens: first molecule of NH3 -halogen atom replaced by NH2 group, second molecule of NH3 -leads to formation of NH4X Elimination of halogenoalkanes Reagent: KOH Conditions: ethanolic, hot What happens: the halogen atom and one H atom from an adjacent C is removed to form an alkene Electrophilic addition w/Br2 Reagent: Br2 Conditions: aqueous (like bromine water) What happens: double bond on alkene opens and alkane is produced Electrophilic addition w/H2SO4 Reagent: H2SO4 Conditions: conc H2SO4, cold (room temp) What happens: double bond on alkene opens and alkane is produced Electrophilic addition w/HBr Reagent: HBr Conditions: - What happens: double bond on alkene opens and alkane is produced Formation of alcohol: Hydration of alkenes Reagent: H2O and strong acid Conditions: ACID CATALYST (e.g. H2SO4, H3PO4) What happens: double bond opens up, H and OH are added onto the molecule Elimination (of water from alcohols) Reagent: excess of conc. H2SO4 or H3PO4 Conditions: heat (150-200C) What happens: lone pair of electrons on the O of the alcohol picks up a H from the acid. Water molecule removed leaving a carbocation. Negative acid ion removes a H leaving a C=C double bond and acid molecule is regenerated Structural isomerism Same molecular formula but different structural formula Stereoisomerism Same structural formula, different arrangement of atoms in space Chain isomerism STRUCTURAL The same molecular formula but a different arrangement of carbon atoms in the chain Position isomerism STRUCTURAL Same molecular formula but functional groups in different places Functional group isomerism STRUCTURAL Same molecular formula but different functional groups E-Z isomerism STEREO Occurs when there is restricted rotation about the planar C=C bond Alkanes The main constituent of crude oil (important raw material for the chemical industry) Saturated hydrocarbons Saturated hydrocarbons Hydrocarbons containing only single-bonded carbon atoms Petroleum A mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation Fractional distillation A process of separating crude oil into its components by boiling and condensing Cracking Breaking up of alkanes into smaller compounds, alkanes, alkenes and hydrogen by breaking the C-C bond Thermal cracking High pressure and high temperature Produces a high percentage of alkenes Catalytic cracking Slight pressure, high temperature In the presence of a zeolite catalyst Used mainly to produce motor fuels and aromatic hydrocarbons Economic reasons for cracking alkanes More use for shorter chains so larger market and greater profit How to reduce incomplete combustion emissions Use of catalytic converters which can remove unto 90% of CO and NO in exhaust gases The internal combustion engine Produces a number of pollutants including NOx, CO, carbon and unburned hydrocarbons Combustion of hydrocarbons containing sulfur Produces sulfur dioxide that causes air pollution Flue gas desulfurisation How sulfur dioxide can be removed from waste gases using calcium oxide or calcium carbonate which both react with the SO2 to produce calcium sulfate which can be used in the building industry to make plaster Reaction of methane with chlorine CH4(g) + Cl2(g) - CH3Cl(g) + HCl(g) Can occur until CHCl3(g) + Cl2(g) - CCl4(g) + HCl(g) Methane-chlorine reaction as a free radical substitution mechanism Initiation: Cl2 splits in presence of UV light Propagation: Chlorine radical can react with methane molecule Termination: Where two radicals collide Halogenoalkenes Contain polar bonds Undergo substitution reactions with the nucleophiles OH-, CN- and NH3 Why the carbon-halogen bond enthalpy influences the rate of reaction Ozone O3 Formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation How chlorine atoms are formed in the upper atmosphere Ultraviolet radiation causes C-Cl bonds in chlorofluorocarbons (CFCs) to break Effect of chlorine atoms in the upper atmosphere Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer (this is why chemists have now developed chlorine free alternatives) How use of chlorofluorocarbons was prevented Research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants Decomposition of ozone equations Cl• + O3 → ClO• + O2 ClO• + O3 → 2O2 + Cl• Alkenes Unsaturated hydrocarbons Unsaturated hydrocarbons Hydrocarbons with one or more carbon-carbon double bonds C=C double bond A centre of high electron density that leads to attack on the molecule by electrophiles Major product Product from the most stable carbocation of an unsymmetrical alkene. The most likely product to be formed (e.g. in stability: tertiary carbocation secondary carbocation primary carbocation) Minor product Product from the less stable carbocation(s) of an unsymmetrical alkene (e.g. in stability: tertiary carbocation secondary carbocation primary carbocation) Test for unsaturation Bromine water If double bonds present, solution turns colourless Polymer A large molecule consisting of many repeating chemical units linked together Addition polymers Formed from alkenes and substituted alkenes Unreactive Typical uses and properties of poly(chloroethene) (aka PVC) Very hard and rigid due to dipole-dipole interactions due to the polarity of the carbon-chlorine bonds as chlorine is more electronegative and so attracts the electrons in the bond towards itself Often used to make window frames, credit cards, footwear and leather-look fabric as it is very durable How properties of PVC can be modified using a plasticiser Using a plasticiser softens the PVC making it more suitable to use in construction automotive and wire and cables Why addition polymers are unreactive No double C=C bonds Alcohols Organic compounds containing hydroxyl(OH) groups with many scientific, medicinal and industrial uses Production of ethanol Reaction of ethene and steam in the presence of a phosphoric acid catalyst Production of ethanol by fermentation Fermentation of glucose to produce ethanol which is separated by fractional distillation for use as biofuel Fermentation of glucose conditions Conditions: Absence of oxygen Presence of yeast and sugar Temperature of 37-40C (optimum for the enzymes used) Enzyme catalyst (zymase) to increase rate of reaction Production of and use ethanol(biofuel) equations C6H12O6(aq) - 2CH5OH(aq) + 2CO2(g) 2C2H5OH(g) + 6O2(g) - 4CO2(g) + 6H2O(l) Biofuel Liquid fuel made from processed or refined biomass Environmental/ethical issues of biofuel +Fermentation uses renewable resources -Land used for growing biomass for fermentation is also needed for food production -Large amount of water used -Destruction of habitats Carbon neutral use of biofuel equations 6CO2 + 6H2O → C6H12O6 + 6O2 Production of glucose uses 6C C6H12O6(aq) - 2CH5OH(aq) + 2CO2(g) 2C2H5OH(g) + 6O2(g) - 4CO2(g) + 6H2O(l) Use of ethanol gives out 6C Why biofuel is not carbon neutral Energy used up to obtain the ethanol is not taken into account, power required to run machinery may have come from non renewable sources e.g. burning fossil fuels Primary alcohol The -OH group of an alcohol is attached to a C-atom that in turn is bonded to one C-atom. Secondary alcohol The -OH group of an alcohol is attached to a C-atom that in turn is bonded to two C-atoms. Tertiary alcohol The -OH group of an alcohol is attached to a C-atom that in turn is bonded to three C-atoms. Not easily oxidised Tests for aldehyde Tollens' reagent Warm Silver mirror produced Fehling's solution Warm Brick-red precipitate forms Primary alcohols oxidise to Aldehydes And carboxylic acids with further oxidation Secondary alcohols oxidise to Ketones Suitable oxidising agent for alcohols Potassium dichromate(VI) K2Cr2O7