Presentation on theme: "Bonding in methane, ethane and ethene"— Presentation transcript:
1 Bonding in methane, ethane and ethene AS ChemistryBonding in methane, ethane and ethene and bonds
2 Learning Objectives Candidates should be able to: describe covalent bonding in terms of orbital overlap, giving and bonds.explain the shape of, and bond angles in, ethane and ethene molecules in terms of and bonds.
5 Hybridisation of orbitals The electronic configuration of a carbon atom is 1s22s22p211s22s2p
6 HYBRIDISATION OF ORBITALS If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p311s22s2pThe extra energy released when the bonds form more than compensates for the initial input.
7 Hybridisation of orbitals in alkanes The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.Because one s and three p orbitals are used, it is called sp3 hybridisation.2s22p22s12p34 x sp3
8 Hybridisation of orbitals in alkanes In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.sp3 orbitals
11 Bonding in etheneAlternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.2s22p22s12p33 x sp22p
15 Learning Objectives Candidates should be able to: describe cis-trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds.deduce the possible isomers for an organic molecule of known molecular formula.identify cis-trans isomerism in a molecule of given structural formula.
17 What is stereoisomerism? In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different arrangement in spaceISOMERISMSTRUCTURAL ISOMERISMSTEREOISOMERISMGEOMETRIC ISOMERISMOPTICAL ISOMERISM
19 GEOMETRIC ISOMERISM RESTRICTED ROTATION OF C=C BONDS Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same.ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATIONAnimation doesn’t work in old versions of Powerpoint
23 GEOMETRIC ISOMERISM How to tell if it exists Two different atoms/groups attachedTwo different atoms/groups attachedGEOMETRICAL ISOMERISMTwo similar atoms/groups attachedTwo similar atoms/groups attachedOnce you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerismTwo similar atoms/groups attachedTwo different atoms/groups attachedTwo different atoms/groups attachedTwo different atoms/groups attachedGEOMETRICAL ISOMERISM
24 GEOMETRIC ISOMERISM Isomerism in butene There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.BUT-1-ENEcis BUT-2-ENEtrans BUT-2-ENE2-METHYLPROPENE* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING
25 Summary To get geometric isomers you must have: restricted rotation (involving a carbon-carbon double bond for A-level purposes);two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.
26 The effect of geometric isomerism on physical properties melting point (°C)boiling point (°C)cis-8060trans-5048You will notice that:the trans isomer has the higher melting point;the cis isomer has the higher boiling point.
27 Why is the boiling point of the cis isomers higher? The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.
28 Why is the melting point of the cis isomers lower? In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid.Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.
30 Learning Objectives Candidates should be able to: explain what is meant by a chiral centre and that such a centre gives rise to optical isomerism.deduce the possible isomers for an organic molecule of known molecular formula.identify chiral centres in a molecule of given structural formula.
32 Optical isomerism Chiral centre Chiral molecule When four different atoms or groups are attached to a carbon atom, the molecules can exist in two isomeric forms known as optical isomers. These are non-superimposable mirror images.
33 Optical Isomerism What is a non-superimposable mirror image? Animation doesn’t work in old versions of Powerpoint
34 Optical isomerismAmino acids (the building blocks of proteins) are optically active. They affect plane polarised light differently.
36 Optical Isomerism The polarimeter A B C D E F A Light source produces light vibrating in all directionsB Polarising filter only allows through light vibrating in one directionC Plane polarised light passes through sampleD If substance is optically active it rotates the plane polarised lightE Analysing filter is turned so that light reaches a maximumF Direction of rotation is measured coming towards the observerIf the light appears to have turned to the right turned to the leftDEXTROROTATORY LAEVOROTATORY
37 Enantiomers – how do they differ? Usually have the same chemical and physical properties – but behave differently in presence of other chiral compounds.
39 TYPES OF ISOMERISM STRUCTURAL ISOMERISM CHAIN ISOMERISMSTRUCTURAL ISOMERISMPOSITION ISOMERISMSame molecular formula but different structural formulaeFUNCTIONAL GROUP ISOMERISMGEOMETRICAL ISOMERISMOccurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANSSTEREOISOMERISMSame molecular formula but atoms occupy different positions in space.OPTICAL ISOMERISMOccurs when molecules have a chiral centre. Get two non- superimposable mirror images.
40 Electrophilic Addition to Alkenes AS ChemistryElectrophilic Addition to Alkenes
41 Learning Objectives Candidates should be able to: describe the mechanism of electrophilic addition in alkenes, using bromine/ethene as an example.describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: addition of hydrogen, steam, hydrogen halides and halogens.
48 Addition reactions of alkenes EQUATIONTEMPERATURE(OC)PRESSURECATALYSTPHASENOTEShydrogenCH2=CH2 + H2 → CH3CH3~150Finely divided nickel on support materialGasNever carried out industrially. Analogous reaction used to produce some margarines from oils (see later).steamCH2=CH2 + H2O→ CH3CH2OH3306MPaPhosphoric (V) acid (H3PO4) adsorbed onto the surface of silica.Major industrial process for the manufacture of ethanol.hydrogen halides(e.g. HBr)CH2=CH2 + HBr → CH3CH2BrRoom temperatureAqueous solutionReactivity increases from HF to HI.halogensCH2=CH2 +Br2→ CH2BrCH2BrLiquid bromine or solution (both aqueous and non-polar solvent.Chlorine and iodine produce similar addition products. Fluorine is too powerful an oxidizing agent.Addition reactions of alkenes
49 Addition to unsymmetrical alkenes Electrophilic addition to propene2-bromopropane1-bromopropane
50 Addition to unsymmetrical alkenes In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion)least stable most stablemethyl < primary (1°) < secondary (2°) < tertiary (3°)
51 Addition to unsymmetrical alkenes SECONDARYCARBOCATIONPATH AMAJOR PRODUCTPRIMARYCARBOCATIONPATH BMINOR PRODUCT
53 Learning Objectives Candidates should be able to: describe the chemistry of alkenes including polymerisation.describe the characteristics of addition polymerisation as exemplified by poly(ethene) and PVC.Recognize the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products.
58 LDPE or HDPESandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in constructionFreezer bags, water pipes, wire and cable insulation, extrusion coating
59 Polymerisation of alkenes ETHENEPOLY(ETHENE)CHLOROETHENEPOLY(CHLOROETHENE)POLYVINYLCHLORIDE PVCPROPENEPOLY(PROPENE)TETRAFLUOROETHENEPOLY(TETRAFLUOROETHENE)PTFE “Teflon”
60 Disposal of polymers Method Comments Landfill Emissions to the atmosphere and water; vermin; unsightly. Can make use of old quarries.IncinerationSaves on landfill sites and produces energy. May also release toxic and greenhouse gases.Recyclinghigh cost of collection and re- processing.Feedstock recyclingUse the waste for the production of useful organic compounds. New technology can convert waste into hydrocarbons which can then be turned back into polymers.
62 Learning ObjectivesCandidates should be able to describe the oxidation of alkenes by:cold, dilute, acidified manganate(VII) ions to form the diol, andhot, concentrated, acidified manganate(VII) ions leading to the rupture of the carbon-to-carbon double bond in order to determine the position of alkene linkages in larger molecules.
64 Oxidation of alkenesIn the presence of dilute (acidified or alkaline) potassium manganate (VII).Alkenes react readily at room temperature (i.e. in the cold).The purple colour disappears and a diol is formed.CH2=CH H2O [O] HOCH2CH2OHethane – 1,2-diol
65 Oxidation of alkenesIn the presence of a hot, concentrated solution of acidified potassium manganate (VII), any diol formed is split into two fragments which are oxidized further to carbon dioxide, a ketone or a carboxylic acid.FragmentProduct=CH2CO2R-CH=Aldehyde → carboxylic acidR2C=Ketone
66 Oxidation of alkenes CH2=CH2 CH3CH=CH2 (CH3)2C=CH2 2 products – both contain ketone1 product only2 products – one contains 2 ketone groups and one contains 2 acid groups.
68 Learning ObjectivesCandidates should be able to recall the chemistry of halogenoalkanes as exemplified by the following nucleophilic substitution reactions of bromoethane:hydrolysis;formation of nitriles;formation of primary amines by reaction with ammonia.
71 Physical Properties1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in non-polar butane.1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in non-polar butane.
73 Nucleophilic substitution This is known as an SN2 reaction.S stands for substitution,N for nucleophilic, and2 because the initial stage of the reaction involves two species.
74 ANIMATION SHOWING THE SN2 MECHANISM Nucleophilic substitution - mechanismAttack by nucleophile is to the back of the molecule – away from the negatively charged halogen atom.ANIMATION SHOWING THE SN2 MECHANISM
75 Rate of reaction Halogen F Cl Br I Electronegativity 4.0 3.0 2.8 2.5 Bond strength (C-X) kJ mol-1484338276238You may expect the fluoroalkane to react more quickly as the C-F bond is the most polar and therefore more susceptible to attack by nucleophiles. However, the C-F bond is the strongest. A nucleophile may be more attracted more strongly to the carbon atom but, unless it forms a stronger bond to carbon, it will not displace the halogen.Actually the reaction with the iodoalkane is the most rapid. This suggests that the strength of the C-X bond is more important than its polarity. Note that the C-I bond is not polar. However, it is easily polarisable.
76 Measuring the rate of reaction ExperimentWater is a poor nucleophile but it can slowly displace halide ionsC2H5Br(l) H2O(l) C2H5OH(l) H+(aq) + Br¯(aq)If aqueous silver nitrate is shaken with a halogenoalkane (they are immiscible) the displaced halide combines with a silver ion to form a precipitate of a silver halide. The weaker the C-X bond the quicker the precipitate appears.
77 Nucleophilic substitution hydroxide ion with bromoethanewarmCH3CH2Br+ OH-(aqueous)CH3CH2OH + Br-ethanolWater with bromoethanewarmCH3CH2Br+ H2O(aqueous)CH3CH2OH + HBrethanolThis is a slower reaction – water is not such a good nucleophile.
78 hydroxide ion with bromoethane Nucleophilic substitution mechanismhydroxide ion with bromoethaneCH3HBrCCH3HOHC+-Br--OHethanol
79 water with bromoethane Nucleophilic substitution mechanismwater with bromoethaneCH3HBrCHCH3OHC+Br-+-H2OCH3HOHCHBrethanol
80 Nucleophilic substitution propanenitrileCH3CH2Br+ CN-(ethanol)CH3CH2CN + Br-cyanide ion with bromoethanerefluxammonia with bromoethaneHeat /pressureaminoethaneCH3CH2Br+ NH3(ethanol)CH3CH2NH2+ HBrHeat /pressureCH3CH2Br+ NH3(ethanol)CH3CH2NH22+ NH4+Br-
81 cyanide ion with bromoethane Nucleophilic substitution mechanismcyanide ion with bromoethaneCH3HBrCCH3HCNC+-Br-CN-propanenitrile
82 ammonia with bromoethane Nucleophilic substitution mechanismammonia with bromoethaneCH3HBrCHCH3NH2C+Br-+-NH3NH3CH3HNH2CH NH3+Br-aminoethane
83 Past paper question Cl2 U.V. /sunlight Ethanolic KCN reflux Br2
84 Substitution vs. Elimination AS ChemistrySubstitution vs. Elimination
85 Learning Objectives Candidates should be able to: recall the chemistry of halogenoalkanes as exemplified by the elimination of hydrogen bromide from 2-bromopropane.describe the mechanism of nucleophilic substitution (by both SN1 and SN2 mechanisms) in halogenoalkanes.
87 Type of halogenoalkane Position of halogeno- group Exampleprimaryat end of chain:bromoethanesecondaryin middle of chain:2-bromopropanetertiaryattached to a carbon atom which carries no H atoms:2-bromo-2-methylpropane
88 SN1 – tertiary halogenoalkanes Nucleophilic attack at the back of the molecule is hindered by bulky CH3 groups. Tertiary carbocation is stabilised by electron donating effect of CH3 groups.
89 SN1 or SN2 ? Halogenoalkane Mechanism Primary SN2 Secondary SN1 and SN2TertiarySN1
90 CH3CH2Br + NaOH CH2=CH2 + NaBr + H2O EliminationYou need to be aware that the hydroxide ion can act as a strong base as well as a nucleophile.An alternative reaction can take place in which HBr is removed and an alkene is formed. This is known as elimination.CH3CH2Br + NaOH CH2=CH NaBr + H2O
91 Elimination of HX from haloalkanes Elimination of HBr from 2-bromopropaneCH3CHBrCH3+ OH-CH3CH=CH2 + H2O + Br-(in ethanol)CH3HCBrCH3HCpropeneBr-H OHOH-acting as a base
92 Substitution or Elimination? alcoholnucleophilic substitutionRCH3CH2OH + Br-+ OH-(aqueous)hydroxide acts as a nucleophileRCH2CH2X+ OH-(ethanol)hydroxide acts as a baseeliminationRCH=CH2 + H2O + X-alkene
94 Learning Objectives Candidates should be able to: interpret the different reactivities of halogenoalkanes e.g. CFCs; anaesthetics; flame retardants; plastics with particular reference to hydrolysis and to the relative strengths of the C-Hal bonds;explain the uses of fluoroalkanes and hydrofluorooalkanes in terms of their relative chemical inertness;recognise the concern about the effect of chlorofluoroalkanes on the ozone layer.
101 ReplacementsHydrochlorofluorocarbons, HCFCs: shorter life in the atmosphere.Hydrofluorocarbons, HFCs: don’t contain chlorine so zero affect on ozone layer.Hydrocarbons: zero effect on ozone layer but flammable and lead to photochemical smog.
102 C. Why is BCF good at extinguishing fires? The presence of a bromine confers flame – retarding qualities on the product.The high temperature in fires break this compound down, producing free radicals such as Br∙. These react with other free radicals produced during combustion, quenching the flames.