AQA A level - Organic Chemistry Questions and Answers

AQA A level - Organic Chemistry Questions and Answers Organic compounds Compounds that contain a chain of carbon atoms Number of electrons in carbons outer shell 4 so can form 4 covalent bonds Carbon-Carbon bonds are... non-polar and relatively strong Empirical formula the simplest whole number ratio of atoms of each element in a compound molecular formula a formula that tells us the actual number of atoms of each different element in a compound structural formula a way of writing the formulas of an organic compound in which bonds aren't shown but each carbon is written seperately. skeletal formula a way of showing the structure of a complex molecule. only labels the carbon and side groups. nomenculture the universl rules about naming compounds why use nomenculture? so all scientists use the same names and others can understand what compounds they have used. IUPAC ruling system the way that the IUPAC has developed a naming convention that chemists all follow. side chains are shown by... a prefix of the root of the longest carbon chain in the side chain. Alkanes general formula CnH2n+2 Alkene FG C=C double bond anywhere alcohol FG -OH at end or on a branch Aldehydes FG - CHO at the end of the chain ketones FG - CO - carbonyl group in the middle of the chain Carboxylic acids FG - COOH terminates the chain Esthers FG - COO - in the middle Nitriles FG C triple bonded to N terminates the chain Alkynes FG C triple bonded to C Homologous series a family of organic compounds with the same functional group, but different carbon chain length. Similar rules of homologous series? have the same general formula and their formula will differ from the next member of the series. Isomers molecules with the same molecular formula but different arrangements of their atoms. Structural Isomers have the same molecular but different structural formulae. positional isomers have the same functional group but they are attached to the hydrocarbon chain in different places. functional group isomers have the same molecular formula but arranged as different functional groups chain isomers have the same molecular formula but the chain is arranged differently. Alkane uses fuels, lubricants and important in industry Crude Oil a mixture of lots of different alkanes and so they must be separated before they can be used. alkanes are.. unreactive but release a lot of heat energy when burned. Alkanes saturated hydrocarbons and un-polar. they are held tigether by weak van der waals forces. hydrocarbon a compound made up of hydrogen and carbon atoms only. suffix for alkanes -ane Methane CH4 Ethane C2H6 propane C3H8 Butane C4H10 Pentane C5H12 to name a branched alkane 1. find the longest carbon chain 2. the branches are added as prefixes with number to show what carbon they are attached to. Polarity of alkanes non-polar as the electro-negativity of hydrogen and carbon are so similar. Melting and boiling point of alkanes as the chain length increases, the size of the melting and boiling point increases due to the increasing strength of van der waals forces in the larger molecules. Solubility of Alkanes not soluble in water due to the hydrogen bonds that hold water together are stronger than van der waals forces. Reactivity of alkanes low reactivity due to their strong carbon-carbon and carbon-hydrogen bonds. they will burn and react with halogens. Fractional Distillation Method to separate a liquid mixture into its individual components using the boiling and condensing point of the liquids crude oil is non-renewable because the process takes a very long time. the shorter the hydrocarbon chain the lower its boiling point different sized hydrocarbons condense at different levels so each fraction is a mixture of hydrocarbons of similar sizes. Cracking the process of splitting longer chain hydrocarbons into shorter more useful hydrocarbons Economic reasons for cracking more demand for shorter hydrocarbons, more valuable. 2 types of cracking Thermal and Catalytic Combustion of alkanes burn with a less smoky flame and combust completely when enough oxygen fuel a substance that releases heat energy when it is burnt. this energy can be used to power machines. why are alkanes good fuels? they release a large amount of energy when they burn. negative enthalpies of combustion - the more carbon in the alkane the greater the amount of energy released. Halogenoalkanes reaction between alkanes and halogens. free-radical substitution reaction when a halogenoalkane is formed Free radical atom with an unpaired electron and are very reactive Initiation stage homolytic breaking of covalent bonds and electrons in the bonds between halogen atoms Propagation stage the halogen free radical takes a hydrogen ion from the alkane and forms a stable H-halogen and a free radical. this reacts with another halogen free radical to form a stable halogenoalkane molecule. termination step the step where the free radicals are removed. halogenoalkanes organic compounds that consist of a hydrocarbon chain with one or more halpgen atoms attached. Halogenalkanes aren't natural but they are the basis of many synthetic compounds. general formula for halogenoalkanes CnH2n+1X (X being the halogen) naming halogenoalkanes named with the prefix that identifies the halogen halogenoalkane reactivity the C-X bond is polar therefore they are more reactive than alkanes. Halogenoalkanes polarity polar as the halogen is more electronegative than the carbon - the carbon is slightly positive which attracts nucleophiles. nucleophiles electron-rich molecules Bond enthalpy of halogenalkanes the enthalpy decreases down group 7. this is because the higher up the group, the more polar so less energy needed to break the bonds. reactivity of teh bonds increases down the halogen group showing the bond enthalpy is more important than the bond polarity alkenes unsaturated hydrocarbons. they are not saturated with hydrogen and have at least one C=C bond. gneral formula for alkenes CnH2n position isomers of alkenes isomers have different positions of the double bond in the hydrocarbon chain. the longer the chain, the more isomers there are. Geometrical Isomers the same structural formula but different arrangement of the bonds around teh C=C bond. Reactivity of alkenes very reactive because of the double bonds. double bonds have a higher bond enthalpy, but they are nucleophiles so can be easily attacked by positive electrophiles. Polymers long molecules formed of lots of the same molecules, called a monomer. Addition polymerisation the joining of monomers 3 steps of fractional distillation evaporation, condensation and collection Fractional Distillation the process of separating crude oil into groups of hydrocarbons with similar number of carbon atoms long chain hydrocarbons higher boiling point so evaporate and condense earlier on in the column short chain hydrocarbons lower boiling point so evaporate and condense further up the column Temperature is highest at the bottom of the column why cracking? shorter chain alkanes are more valuable and useful thermal cracking uses harsh conditions like high temperature and high pressure. Catalytic Cracking uses lower temperatures and pressure to reduce costs and make the reaction faster Conditions of Catalytic Cracking 450 degrees C and 1-2atm Catalyst used in catalytic cracking zeolite zeolite An iconic compound containing aluminium, silicon and oxygen Combustion the burning of materials complete combustion results blue flame and produces CO2 and H2O Incomplete combustion results orange smoky flame and produces H2O CO2 and Carbon Why orange flame? due to small particles of pure carbon glowing red hot Combustion in enines products nitrous oxides, unburned hydrocarbons, carbon monoxide Flue gases gases that come out of chimneys. like Sulfur Dioxide. Removing sulfur dioxide by reacting it with calcium oxide or calcium carbonate CaO + SO2 = CaSO3