8Lesson 1: Homologous Series Objectives:Reflect on previous knowledge of organic chemistryUnderstand the term ‘homologous series’Conduct the fractional distillation of crude oilUnderstand and use the variety of different types of formula used in organic chemistry
9Organic ChemistryOrganic chemistry is the chemistry of carbon containing compounds.From the very simple: methaneTo the very complex: Haem B
10Homologous SeriesA homologous series is a family of compounds that differs only by the length of its hydrocarbon chainMembers share:General formulaChemical propertiesThree such series are the:AlkanesAlkenesAlcohols
11Homologous Series and Boiling Points What do you think will be the trend in melting/boiling points as you go down a homologous series?Why?
12FormulasDraw the compound with the formula C4H8O
13What did you get?Clearly a molecular formula is not enough!
14Types of Formula Empirical Formula C4H8O C4H8O Molecular Formula C4H8O C4H8OFull Structural FormulaAka displayed formulaCondensed Structural FormulaNote the ‘=‘ used for the C=C double bondSkeletal formulaNot required but v. usefulUsed in data booklet for complicated structuresDo not use in exam answers!CH2=CHCH2CH2OH CH2=C(CH3)CH2OH
15Thinking About Formulas Produce a table to summarise each of the formulas. Include columns for:What they showProsConsHow you make themDraw full structural, condensed structural and skeletal formulas for at least 5 of the C4H8O compounds (not the cyclic ones)
16Key PointsOrganic chemistry is the chemistry of carbon containing compoundsA homologous series is a family of organic compounds differing only by the length of their carbon chainsThe melting and boiling point increases as you go down a homologous seriesDisplayed formulas show the unambiguous arrangement of atoms in a compound
19Lesson 2: Isomers Objectives: Describe the term structural isomer Draw a name the non-cyclic alkanesDraw and name the straight-chain alkenes
20IsomersCompounds with the same molecular formula but different structural formulaThe 20 different C4H8O compounds from last lesson are isomers of each otherThese are all structural isomersSame number of each atom, but bonded in a different orderYou would have even more if you included geometric and optical isomers
21Structural Isomers of the Alkanes The (non-cyclic) alkanes have the general formula CnH2n+2Draw full and condensed structural formulas for every isomer of every one of the alkanes up to n = 6If you finish early, draw each as a skeletal formula
24Naming Straight-chain alkanes Suffix:Tells us the functional group of the moleculeFor alkanes it is ‘-ane’Prefix:Tells us the length of the longest carbon chain:1 carbon: meth-2 carbons: eth-3 carbons: prop-4 carbons: but-5 carbons: pent-6 carbons: hex-Example 1: ethaneExample 2: butane:Task: write in the names of the 4 straight chain alkanes next to your diagrams from last slide
25Naming branched-chain alkanes Start by naming the longest chainAdd extras to say the size of a branch, its position and how many of that branchBranch Size:1 carbon: methyl-2 carbons: ethyl-3 carbons: propyl-Position:Number the carbons in the longest chainChoose numbers to minimise the total numbers usedNumber of same branchesOne branch – nothingTwo branches – di-Three branches – tri-Four branches – tetra-Example 1: 2-methylpropaneExample 2: 2,3-dimethylbutaneTask: name the remaining alkanes
26The straight-chain alkenes Alkenes are the same as alkanes but have one C=C double bond.The suffix for the alkene homologous series is ‘-ene’Task: draw full structural and skeletal formulas for each of the straight-chain alkenes up to C6 and name them.Do the branched ones as well if you have timeHint: you need to state the position of the double bond, but only if there is the possibility of multiple isomers:i.e. ‘but-2-ene’ or ‘hex-1-ene’ but only ‘ethene’ not ‘eth-1-ene’
28Key PointsStructural isomers have the same number of each atom but they are connected differentlyWhen naming compoundsThe longest carbon chain forms the prefixThe functional group tells you the suffixSometimes numbers need to be used to tell you where this functional group isSide chains and other groups are named according to what they are, how many there are and their position
31Lesson 3: Meet the Families Objectives:Meet and learn to recognise the 7 functional groups required for the SL courseProduce a mind-map summarising each of the homologous series
32Functional Groups Table (landscape) You need to research and produce a mind- map summarising the following functional groups:AlkaneAlkeneAlcoholAldehydeKetoneCarboxylic acidHalide/HalogenoalkaneYour table should have four columns including:Name of functional groupGeneral structural formula (use ‘R’ to signify a carbon chain)Rules for naming them (including the position where relevant)A named exampleRelative volatilityRelative solubility in waterFor alcohols and halides you should include a branch to explain the difference between 1o, 2o and 3oYou should also have a branch called ‘Other Functional Groups’ that just allows you to recognise the groups:AmineEsterBenzeneIf HL you should leave space for four more functional groups
33Building Organic Compounds Use molecular models to make any of the compounds mentioned in your mind-map:Draw it (structural and skeletal)Name itGive it to a friend and challenge them to do the sameOnly go up to 6 carbonsOnly include branched-chains for the alkanes
34Key PointsThere are 7 functional groups we need to know in detail and 3 extra we need to be able to recogniseWe will look at each in detail over the rest of the unit
36Your notes and mind-map must be ready for me to inspect. RefreshReviewing Your NotesYou should spend 60 seconds reviewing your notes from last lesson before attempting this.Your notes and mind-map must be ready for me to inspect.The following is a computer-generated representation of the molecule, methyl 2- hydroxy benzoate, better known as oil of wintergreen.Deduce the empirical formula of methyl 2-hydroxy benzoate and draw the full structural formula, including any multiple bonds that may be present…The computer-generated representation shown does not distinguish between single and multiple bonds.Name all the functional groups present in the molecule.HCO
38Lesson 4: Alkanes Objectives: Explain the stability of the alkanes Observe the combustion of alkanesDescribe the free-radical substitution reactions of alkanes and its mechanismObserve the free-radical substitution of hexane
39Combustion of Alkanes The alkanes really don’t do much Combustion is of one of two notable reactions (this is why we use them for fuels)Complete combustion:alkane + oxygen carbon dioxide + waterIncomplete combustion:Alkane + oxygen carbon + carbon monoxide + carbon dioxide + waterThe amounts of C, CO and CO2 will vary depending on conditionsTask: Observe the combustion of the gas from the gas taps (propane/butane mix) and of a small amount hexane (in spirit burners). Hold the end of a clean boiling tube just over the flame for 15 seconds, this will collect soot from the flame.Record all observations clearly and try to account for themInclude balanced equations to describe the (complete) combustion
40Why so boring stable?There are at least two reasons why alkanes are so unreactiveTask: Think back to your knowledge of molecular structure, and look at the tables of bond-enthalpies in the data booklet to see if you can work out why.
41HalogenationAlkanes will undergo halogenation if reacted with a halide in the presence of u.v. light.For example:C2H6(g) + Cl2(g) CH3CH2Cl(g) + HCl(g)ethane chloroethaneThis reaction is an example of free radical substitutionu.v.
42Radicals Radicals are species with unpaired electrons They are crazy reactiveHalogens form radicals when hit by uv light of the right frequency:Cl Cl•The dot after the Cl represents the unpaired electron and tells us we have a radicalThis process is called homolytic fission – the bond breaks equally with one electron going to each chlorineTask: draw Lewis structures for the Cl2 molecule and each of the Cl• radicalsu.v.
43Reaction Mechanism: Free Radical Substitution Cl Cl•Cl• + C2H6 C2H5• + HClC2H5• + Cl2 C2H5Cl + Cl•Cl• + Cl• Cl2Cl• + C2H5• C2H5ClC2H5• + C2H5• C4H10InitiationRadicals formed by homolytic fissionPropagationThese steps feed each other the radicals needed to continueTerminationAny two radicals can combine to terminate the reactionConcentration of radicals is low so this is a rare eventu.v.A single radical can cause thousands of cycles of the propagation stage before it reaches terminationThis same mechanism applies to all of the halogensThe alkane can be substituted multiple times, until every H has been replaced
44Key Points Alkanes are unreactive They release a lot of energy on combustion, and are easy to handle which makes them good fuelsUndergo free radical substitution to form halogenoalkanes and a hydrogen halide in the presence of UV light
47Lesson 5: Alkenes Objectives: Describe the main addition reactions of the alkenesExtract an alkene from a citrus fruit
48Reactivity of AlkenesAlkenes are considerably more reactive than alkanes and are a major industrial feedstockThe reactivity is due to the double bond:The double bond contains 4 electronsThis is a significant amount of charge which:Makes it attractive to electrophilesEnables it to polarise approaching moleculesMost reactions of alkenes are addition reactions where two molecules come together to make one new one
49Alkenes and hydrogen Alkene + hydrogen alkane Reaction conditions: HotNi catalystThis is an addition reaction, in which the hydrogen adds across the double bond
50Alkenes and hydrogen halides Alkene + hydrogen halide halogenoalkaneReaction conditions:This reaction occurs very readily and needs no special conditionsThis is an addition reaction, in which the hydrogen halide adds across the double bond
51Alkenes and halogens Alkene + halogen dihalogenoalkane Reaction conditions:This reaction occurs very readily and needs no special conditionsIf the halogen used is an aqueous solution of bromine (bromine water), the orange-brown colour of bromine solution is decolourised.This is the standard test for alkenes.
52Alkenes and water Alkene + water alcohol Reaction conditions: Water must be steamPhosphoric or sulphuric acid catalystThis is the process used to make industrial ethanolFermentation from sugar would be far too expensive!
53PolymerisationUnder the right conditions, alkene molecules will add to each other creating a polymerIn this case, 1-bromo-2-fluoroethene polymerises to form poly-1- bromo-2-fluroetheneConditions:Vary from alkene to alkene but often include high pressure, temperature and a catalystThe carbons in the C=C double bonds form the carbon chain, everything else hangs off this chain
54Drawing polymers Draw three-monomer lengths of the polymers formed by: PropeneStyrenePent-2-ene
55Key Points Alkenes undergo addition reactions with: HydrogenHydrogen halidesHalogensWater (steam)Alkenes undergo addition polymerisationAlkenes are very economically important due to the range of products they can make
58Lesson 6: Alcohols Objectives: Explain the relative ease of combustion of the alcoholsDescribe the oxidation reactions of the alcoholsInvestigate the oxidation reactions of the alcohols
59Alcohols as FuelsAlcohols combust more readily than equivalent alkanes but release less energy since they are already partially oxidisedAlcohol + oxygen carbon dioxide + waterAlcohols are used as fuels:As a fuel for cars – either pure or blended with petrolMethanol as fuel for competitive motorsports including dragsters and monster trucksMuch fuel ethanol is fermented from crops…crops that could otherwise be eaten, forcing up food prices. Is this ok?
60Oxidation of alcoholsThe most important reactions of the alcohols are their oxidationsA range of compounds will oxidise them so the oxidiser is often represented as [O]One oxidising agent you need to know is potassium dichromate, K2Cr2O7.When using this, orange Cr (VI) is reduced to green Cr (III)More on what this means in the oxidation and reduction unitSee next slide for details
62Key Points Alcohols are highly combustible Primary alcohols oxidise to form aldehydes, which oxidise to form carboxylic acidsSecondary alcohols oxidise to form ketonesTertiary do not oxidise due to the 3 strong C-C bonds surrounding the –OH carbon
65Lesson 7: Halogenoalkanes Objectives:Describe the substitution reactions of halogenoalkanes with a strong baseUnderstand the SN1 and SN2 mechanisms for nucleophilic substitutionProduce an animation showing the two different mechanisms
66Nucleophilic Substitution One of the most important reactions undergone by halogenoalkanes is nucleophilic substitutionA nucleophile is a ‘nucleus-loving’ species that is attracted to positive charges.Nucleophiles have either full negative charges or delta-negative chargesWater and hydroxide are both nucleophilesIn this case we can also call the reaction ‘hydrolysis’The carbon in the carbon-halogen bond has a + charge due to the greater electronegativity of the halogenThis makes it susceptible to attack by nucleophiles
67Halogenoalkanes and strong bases A substitution reaction takes place, where the halogen atom is displaced by the hydroxide ionhalogenoalkane + sodium hydroxide alcohol + sodium chlorideConditions:Aqueous baseGently warmed (can at room temperature, but may be quite slow)This is a nuclephilic substitution.The C attached to the halogen is + due to the high electronegativity of the halogenThe OH- ion (our nucleophile) is attracted to the + carbonA nucleophile is a species with a negative charge or a lone pair that is attracted to positive/delta-positive atoms
68SN1 – Unimolecular nucleophilic substitution – animation here Unimolecular because only one molecule is involved in the rate determining stepThe rate determining step involves the spontaneous breaking of the carbon-halogen bond and is a heterolytic fission, forming a halide ion and a carbocation intermediateThe stability of the carbocation intermediate is a key factor in SN1The attack by the nucleophile (OH-) is very fast, but does need the carbocation to be formed firstThe rate is only dependent on the concentration of the halogenoalkane:Rate = k[halogenoalkane]Note: the curly arrows show the movement of pairs of electrons
69SN2 – Bimolecular nucleophilic substitution – animation here Bimolecular because two molecules are involved in the rate determining stepIn the rate determining step, the nucleophile (OH-) attacks at the same time as the carbon-halogen bond breaks.The reaction passes through a negative transition state where the carbon has a ‘half-bond’ to both the –OH and the –Br with an overall negative chargeThe rate is dependent on both the concentration of the halogenoalkane and the nucleophileRate = k[halogenoalkane][nucleophile]
70SN1 or SN2? 1o halogenoalkanes predominantly undergo SN2 2o halogenoalkanes undergo a mix of SN1 and SN23o halogenoalkanes predominantly undergo SN1You do not need to know why at SL, but will find out more at HL
71RefreshHalogenoalkanes undergo substitution with strong bases to form alcoholsThe reaction has two possible mechanisms:SN1: the C-X bond breaks and then the nucleophile attacksSN2: the nucleophile attacks at the same time as the C-X bond breaksThe mechanism depends on the halogenoalkane:1o - SN22o - SN1 and SN23o - SN1