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Bonding in methane, ethane and ethene  and  bonds AS Chemistry.

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Presentation on theme: "Bonding in methane, ethane and ethene  and  bonds AS Chemistry."— Presentation transcript:

1 Bonding in methane, ethane and ethene  and  bonds AS Chemistry

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.

3 Starter activity

4 Alkenes pent-2-eneCH 3 CH=CHCH 2 CH 3 hex-3-ene CH 3 CH 2 CH=CHCH 3 2,3-dimethylpent-2-ene cyclopenta-1,3-diene 3-ethylhept-1-ene CH 2 =CHCH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 3

5 Hybridisation of orbitals The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 1 1s 2 2s 2p

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 1s 2 2s 1 2p 3 1 1s 2 2s 2p The 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 sp 3 hybridisation. 2s 2 2p 2 2s 1 2p 3 4 x sp 3

8 sp 3 orbitals In ALKANES, the four sp 3 orbitals repel each other into a tetrahedral arrangement. Hybridisation of orbitals in alkanes

9 Bonding in methane

10 Bonding in ethane

11 Bonding in ethene Alternatively, 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. 2s 2 2p 2 2s 1 2p 3 3 x sp 2 2p

12 sp 2 hybrids What about ethene?

13  - bonds

14 Geometric Isomerism AS Chemistry

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.

16 Starter activity

17 ISOMERISM STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRIC ISOMERISM OPTICAL ISOMERISM 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 space

18 Geometric 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’ ROTATION Animation doesn’t work in old versions of Powerpoint

20 Geometric Isomerism?

21 Geometric isomers of but-2-ene

22 X Geometric Isomerism?

23 GEOMETRIC ISOMERISM How to tell if it exists   Two different atoms/groups attached Two similar atoms/groups attached Two different atoms/groups attached GEOMETRICAL ISOMERISM Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism

24 GEOMETRIC ISOMERISM Isomerism in butene There are 3 structural isomers of C 4 H 8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism. BUT-1-ENE2-METHYLPROPENE trans BUT-2-ENEcis BUT-2-ENE * YOU CAN GET ALKANES WITH FORMULA C 4 H 8 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 isomer melting point (°C) boiling point (°C) cis-8060 trans-5048 You 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.

29 Optical Isomerism AS Chemistry

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.

31 Starter activity

32 Optical isomerism 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. Chiral centre Chiral molecule

33 Optical Isomerism What is a non-superimposable mirror image? Animation doesn’t work in old versions of Powerpoint

34 Optical isomerism Amino acids (the building blocks of proteins) are optically active. They affect plane polarised light differently.

35 Butan-2-ol

36 Optical Isomerism The polarimeter If the light appears to have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY A Light source produces light vibrating in all directions B Polarising filter only allows through light vibrating in one direction C Plane polarised light passes through sample D If substance is optically active it rotates the plane polarised light E Analysing filter is turned so that light reaches a maximum F Direction of rotation is measured coming towards the observer AB C D E F

37 Enantiomers – how do they differ? Usually have the same chemical and physical properties – but behave differently in presence of other chiral compounds.

38 Enantiomers – how do they differ?

39 TYPES OF ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRICAL ISOMERISM OPTICAL ISOMERISM CHAIN ISOMERISM Same molecular formula but different structural formulae Occurs when molecules have a chiral centre. Get two non- superimposable mirror images. Same molecular formula but atoms occupy different positions in space. POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM

40 Electrophilic Addition to Alkenes AS Chemistry

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.

42 Starter activity

43 CH 2 =CH 2 + Br 2  CH 2 BrCH 2 Br Electrophilic addition

44 CH 2 =CH 2 + Br 2 CH 2 BrCH 2 Br bromine with ethene hydrogen bromide with ethene CH 2 =CH 2 + HBrCH 3 CH 2 Br bromoethane 1,2-dibromoethane

45 Br Electrophilic addition mechanism H H H H C C ++ -- H H H H C C Br + - carbocation H H H H C C Br 1,2-dibromoethane bromine with ethene

46 Electrophilic addition mechanism H H H H C C H H H H C C H + carbocation H H H H C C BrH bromoethane hydrogen bromide with ethene -- ++ Br H -

47 Electron flow during electrophilic addition Electron flow during electrophilic addition

48 EQUATIONTEMPERATURE ( O C) PRESSURECATALYSTPHASENOTES hydrogenCH 2 =CH 2 + H 2 → CH 3 CH 3 ~150 Finely divided nickel on support material Gas Never carried out industrially. Analogous reaction used to produce some margarines from oils (see later). steamCH 2 =CH 2 + H 2 O → CH 3 CH 2 OH 3306MPa Phosphoric (V) acid (H 3 PO 4 ) adsorbed onto the surface of silica. Gas Major industrial process for the manufacture of ethanol. hydrogen halides (e.g. HBr) CH 2 =CH 2 + HBr → CH 3 CH 2 Br Room temperature Aqueous solution Reactivity increases from HF to HI. halogensCH 2 =CH 2 +Br 2 → CH 2 BrCH 2 Br Room temperature Liquid 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 propene 2-bromopropane 1-bromopropane

50 In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion) least stable most stable methyl < primary (1°) < secondary (2°) < tertiary (3°) Addition to unsymmetrical alkenes

51 PATH A PATH B MAJOR PRODUCT PRIMARY CARBOCATION SECONDARY CARBOCATION MINOR PRODUCT Addition to unsymmetrical alkenes

52 Polymerisation AS Chemistry

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.

54 Starter activity

55 Poly(ethene) Temperature: about 200°C Pressure: about 2000 atmospheres Initiator: often a small amount of oxygen as an impurity Conditions

56 Free radical addition Initiation Propagation Termination Propagation

57 LDPE or HDPE

58 Freezer bags, water pipes, wire and cable insulation, extrusion coating Sandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction

59 Polymerisation of alkenes ETHENEPOLY(ETHENE) TETRAFLUOROETHENE POLY(TETRAFLUOROETHENE) PTFE “Teflon” PROPENEPOLY(PROPENE) CHLOROETHENE POLY(CHLOROETHENE) POLYVINYLCHLORIDE PVC

60 MethodComments LandfillEmissions 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 recycling Use the waste for the production of useful organic compounds. New technology can convert waste into hydrocarbons which can then be turned back into polymers. Disposal of polymers

61 Oxidation of alkenes AS Chemistry

62 Learning Objectives Candidates should be able to describe the oxidation of alkenes by:  cold, dilute, acidified manganate(VII) ions to form the diol, and  hot, 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.

63 Starter activity

64 Oxidation of alkenes In 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. CH 2 =CH 2 + H 2 O + [O]  HOCH 2 CH 2 OH ethane – 1,2-diol

65 Oxidation of alkenes FragmentProduct =CH 2 CO 2 R-CH= Aldehyde → carboxylic acid R 2 C= Ketone In 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.

66 Oxidation of alkenes 1.CH 2 =CH 2 2.CH 3 CH=CH 2 3.(CH 3 ) 2 C=CH 2 2 products – both contain ketone 2 products – one contains 2 ketone groups and one contains 2 acid groups. 1 product only

67 Halogenoalkanes AS Chemistry

68 Learning Objectives Candidates 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.

69 Starter activity

70 a.CHCl 3 trichloromethane b.CH 3 CHClCH 3 2-chloropropane c.CF 3 CCl 3 1,1,1-trichloro-2,2,2-trifluoroethane Naming Halogenoalkanes F F Cl F

71 Physical Properties a.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. b.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.

72 Nucleophilic substitution negotiate clever alp or cadet tart eat given enticed if chenille soup had lie stubs tuition electronegative polar attracted negative deficient nucleophiles halide substitution

73 Nucleophilic substitution This is known as an S N 2 reaction.  S stands for substitution,  N for nucleophilic, and  2 because the initial stage of the reaction involves two species.

74 Nucleophilic substitution - mechanism ANIMATION SHOWING THE S N 2 MECHANISM Attack by nucleophile is to the back of the molecule – away from the negatively charged halogen atom.

75 Rate of reaction You 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. HalogenFClBrI Electronegativity Bond strength (C-X) kJ mol

76 Experiment Water is a poor nucleophile but it can slowly displace halide ions C 2 H 5 Br (l) + H 2 O (l)  C 2 H 5 OH (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. Measuring the rate of reaction

77 hydroxide ion with bromoethane ethanol CH 3 CH 2 Br+ OH - CH 3 CH 2 OH + Br - (aqueous) Nucleophilic substitution Water with bromoethane ethanol CH 3 CH 2 Br+ H 2 OCH 3 CH 2 OH + HBr (aqueous) This is a slower reaction – water is not such a good nucleophile. warm

78 ++ -- CH 3 H Br C H - OH CH 3 H OH C H Br - hydroxide ion with bromoethane Nucleophilic substitution mechanism ethanol

79 water with bromoethane Nucleophilic substitution mechanism ethanol ++ -- CH 3 H Br C H - H CH 3 H OH C H + CH 3 H OH C H HBr H2OH2O

80 propanenitrile CH 3 CH 2 Br + CN - (ethanol) CH 3 CH 2 CN + Br - cyanide ion with bromoethane ammonia with bromoethane CH 3 CH 2 Br + NH 3(ethanol) CH 3 CH 2 NH 2 2+ NH 4 + Br - Nucleophilic substitution aminoethane CH 3 CH 2 Br + NH 3(ethanol) CH 3 CH 2 NH 2 + HBr reflux Heat / pressure Heat / pressure

81 ++ -- CH 3 H Br C H CN - CH 3 H CN C H Br - cyanide ion with bromoethane Nucleophilic substitution mechanism propanenitrile

82 ammonia with bromoethane Nucleophilic substitution mechanism aminoethane ++ -- CH 3 H Br C H - H CH 3 H NH 2 C H + CH 3 H NH 2 C H NH 3 H NH 3 + Br - NH 3

83 Past paper question Cl 2 U.V. /sunlight Ethanolic KCN reflux Br 2 U.V. /sunlight

84 Substitution vs. Elimination AS Chemistry

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 S N 1 and S N 2 mechanisms) in halogenoalkanes.

86 Starter activity

87 Type of halogenoalkane Position of halogeno- group Example primary at end of chain:bromoethane secondary in middle of chain:2-bromopropane tertiary attached to a carbon atom which carries no H atoms: 2-bromo-2-methylpropane

88 S N 1 – tertiary halogenoalkanes Nucleophilic attack at the back of the molecule is hindered by bulky CH 3 groups. Tertiary carbocation is stabilised by electron donating effect of CH 3 groups.

89 S N 1 or S N 2 ? HalogenoalkaneMechanism PrimarySN2SN2 Secondary S N 1 and S N 2 TertiarySN1SN1

90 Elimination You 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. CH 3 CH 2 Br + NaOH  CH 2 =CH 2 + NaBr + H 2 O

91 Elimination of HBr from 2-bromopropane CH 3 H H H C C OH - CH 3 H H H C C BrH propene H OH Br - CH 3 CHBrCH 3 + OH - CH 3 CH=CH 2 + H 2 O + Br - ( in ethanol ) acting as a base Elimination of HX from haloalkanes

92 92 elimination + OH - RCH=CH 2 + H 2 O + X - (ethanol) nucleophilic substitution alcohol + OH - RCH 3 CH 2 OH + Br - (aqueous) RCH 2 CH 2 X alkene hydroxide acts as a base hydroxide acts as a nucleophile Substitution or Elimination?

93 AS Chemistry Pros and Cons

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.

95 Starter activity

96 . Properties:  Non-flammable  Low toxicity  Unreactive  Liquefy easily when compressed

97  Refrigerants Propellants for aerosols Solvents (including dry-cleaning) Degreasers

98

99 Natural ozone layer

100

101 Replacements Hydrochlorofluorocarbons, 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.


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