AS Chemistry Bonding in methane, ethane and ethene and bonds 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. Starter activity Alkenes pent-2-ene CH3CH=CHCH2CH3 hex-3-ene CH3CH2CH=CHCH3 2,3-dimethylpent-2-ene cyclopenta-1,3-diene 3-ethylhept-1-ene CH2=CHCH2CH(CH2CH3)CH2CH2CH3 Hybridisation of orbitals The electronic configuration of a carbon atom is 1s22s22p2 2p 2 2s 1 1s 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 1s22s12p3 2p 2 2s 1 1s The extra energy released when the bonds form more than compensates for the initial input. 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. 2s22p2 2s12p3 4 x sp3 Hybridisation of orbitals in alkanes In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement. sp3 orbitals Bonding in methane Bonding in ethane 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. 2s22p2 2s12p3 3 x sp2 2p What about ethene? sp2 hybrids - bonds AS Chemistry Geometric Isomerism 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. Starter activity 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 ISOMERISM STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRIC ISOMERISM OPTICAL ISOMERISM Geometric Isomerism? 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 Geometric Isomerism? Geometric isomers of but-2-ene Geometric Isomerism? X GEOMETRIC ISOMERISM How to tell if it exists Two different atoms/groups attached Two different atoms/groups attached Two similar atoms/groups attached Two similar atoms/groups attached Two similar atoms/groups attached Two different atoms/groups attached Two different 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 GEOMETRICAL ISOMERISM GEOMETRIC ISOMERISM Isomerism in butene There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism. BUT-1-ENE cis BUT-2-ENE trans BUT-2-ENE 2-METHYLPROPENE * YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING 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. The effect of geometric isomerism on physical properties isomer melting (°C) point boiling (°C) cis -80 60 trans -50 48 point You will notice that: the trans isomer has the higher melting point; the cis isomer has the higher boiling point. 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. 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. AS Chemistry Optical Isomerism 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. Starter activity 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. Optical Isomerism What is a non-superimposable mirror image? Animation doesn’t work in old versions of Powerpoint Optical isomerism Amino acids (the building blocks of proteins) are optically active. They affect plane polarised light differently. Butan-2-ol Optical Isomerism The polarimeter A B C D E F A B C D E F Light source produces light vibrating in all directions Polarising filter only allows through light vibrating in one direction Plane polarised light passes through sample If substance is optically active it rotates the plane polarised light Analysing filter is turned so that light reaches a maximum Direction of rotation is measured coming towards the observer If the light appears to have turned to the right DEXTROROTATORY turned to the left LAEVOROTATORY Enantiomers – how do they differ? Usually have the same chemical and physical properties – but behave differently in presence of other chiral compounds. Enantiomers – how do they differ? TYPES OF ISOMERISM CHAIN ISOMERISM STRUCTURAL ISOMERISM Same molecular formula but different structural formulae POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM GEOMETRICAL ISOMERISM STEREOISOMERISM Same molecular formula but atoms occupy different positions in space. Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS OPTICAL ISOMERISM Occurs when molecules have a chiral centre. Get two nonsuperimposable mirror images. AS Chemistry Electrophilic Addition to Alkenes 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. Starter activity Electrophilic addition CH2=CH2 + Br2 CH2BrCH2Br bromine with ethene CH2=CH2 + Br2 CH2BrCH2Br 1,2-dibromoethane hydrogen bromide with ethene CH2=CH2 + HBr CH3CH2Br bromoethane Electrophilic addition mechanism H H C H C H Br H + H carbocation C + C H Br Br - H Br Br- Br 1,2-dibromoethane H H H C C Br Br H Electrophilic addition mechanism H H C C H H + H Br- H H H carbocation C + C H Br- H H H H C C Br H H bromoethane Electron flow during electrophilic addition EQUATION TEMPERATURE (OC) hydrogen CH2=CH2 + H2 → CH3CH3 ~150 steam CH2=CH2 + H2O→ CH3CH2OH 330 hydrogen halides (e.g. HBr) CH2=CH2 + HBr → CH3CH2Br Room temperature halogens CH2=CH2 +Br2→ CH2BrCH2Br Room temperature PRESSURE CATALYST Finely divided nickel on support material 6MPa Phosphoric (V) acid (H3PO4) adsorbed onto the surface of silica. PHASE Gas Gas Aqueous solution Liquid bromine or solution (both aqueous and non-polar solvent. NOTES Never carried out industrially. Analogous reaction used to produce some margarines from oils (see later). Major industrial process for the manufacture of ethanol. Reactivity increases from HF to HI. Chlorine and iodine produce similar addition products. Fluorine is too powerful an oxidizing agent. Addition reactions of alkenes Addition to unsymmetrical alkenes Electrophilic addition to propene 2-bromopropane 1-bromopropane Addition to unsymmetrical alkenes In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion) least stable methyl < primary (1°) most stable < secondary (2°) < tertiary (3°) Addition to unsymmetrical alkenes SECONDARY CARBOCATION PATH A MAJOR PRODUCT PRIMARY CARBOCATION PATH B MINOR PRODUCT AS Chemistry Polymerisation Learning Objectives Candidates should be able to: describe the polymerisation. chemistry of alkenes including 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. Starter activity Poly(ethene) Conditions Temperature: Pressure: Initiator: about 200°C about 2000 atmospheres often a small amount of oxygen as an impurity Free radical addition Initiation Propagation Propagation Termination LDPE or HDPE LDPE or HDPE Sandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction Freezer bags, water pipes, wire and cable insulation, extrusion coating Polymerisation of alkenes ETHENE CHLOROETHENE PROPENE TETRAFLUOROETHENE POLY(ETHENE) POLY(CHLOROETHENE) POLYVINYLCHLORIDE PVC POLY(PROPENE) POLY(TETRAFLUOROETHENE) PTFE “Teflon” Disposal of polymers Method Landfill Incineration Recycling Feedstock recycling Comments Emissions to the atmosphere and water; vermin; unsightly. Can make use of old quarries. Saves on landfill sites and produces energy. May also release toxic and greenhouse gases. high cost of collection and reprocessing. 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. AS Chemistry Oxidation of alkenes 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. Starter activity 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. CH2=CH2 + H 2O + [O] HOCH2CH2OH ethane – 1,2-diol Oxidation of alkenes 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. Fragment =CH2 Product CO2 R-CH= Aldehyde R2C= Ketone → carboxylic acid Oxidation of alkenes 1. CH2=CH2 2. CH3CH=CH2 3. (CH3)2C=CH2 2 products – both contain ketone 1 product only 2 products – one contains 2 ketone groups and one contains 2 acid groups. AS Chemistry Halogenoalkanes 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. Starter activity Naming Halogenoalkanes a. CHCl3 b. CH3CHClCH3 c. CF3CCl3 trichloromethane 2-chloropropane 1,1,1-trichloro-2,2,2-trifluoroethane F Cl F Cl F Cl Physical Properties a. 1-chloropropane is polar and has permanent dipoledipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in nonpolar butane. b. 1-chloropropane is polar and has permanent dipoledipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in nonpolar butane. Nucleophilic substitution negotiate clever electronegative alp or polar cadet tart attracted eat given negative enticed if deficient chenille soup nucleophiles had lie halide stubs tuition substitution Nucleophilic substitution This is known as an SN2 reaction. S stands for substitution, N for nucleophilic, and 2 because the initial stage of the reaction involves two species. Nucleophilic substitution - mechanism Attack by nucleophile is to the back of the molecule – away from the negatively charged halogen atom. ANIMATION SHOWING THE SN2 MECHANISM Rate of reaction Halogen Electronegativity F 4.0 Cl 3.0 Br 2.8 I 2.5 Bond strength (C-X) kJ mol-1 484 338 276 238 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. Measuring the rate of reaction Experiment Water is a poor nucleophile but it can slowly displace halide ions C2H5Br(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. hydroxide ion with bromoethane CH3CH2Br + OH- (aqueous) warm CH3CH2OH + Brethanol Water with bromoethane CH3CH2Br + H2O (aqueous) warm CH3CH2OH + HBr ethanol This is a slower reaction – water is not such a good nucleophile. Nucleophilic substitution mechanism H + CH3 C H OH- Br H CH3 C H ethanol OH - Br Nucleophilic substitution mechanism H + CH3 C Br H CH3 H C + OH H H - Br H2O H CH3 ethanol C H OH HBr Nucleophilic substitution cyanide ion with bromoethane CH3CH2Br + CN-(ethanol) reflux CH3CH2CN + Brpropanenitrile ammonia with bromoethane CH3CH2Br + NH3(ethanol) CH3CH2Br + 2 NH3(ethanol) Heat / pressure Heat / pressure CH3CH2NH2 + HBr aminoethane CH3CH2NH2 + NH4+Br- Nucleophilic substitution mechanism H + CH3 C H CN- Br H CH3 C CN H propanenitrile - Br Nucleophilic substitution mechanism H + CH3 C Br H CH3 H C + NH2 H H - Br NH3 NH3 H CH3 aminoethane C H NH2 H NH3+Br - Past paper question Cl2 U.V. /sunlight Ethanolic KCN reflux Br2 U.V. /sunlight AS Chemistry Substitution vs. Elimination 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. Starter activity 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 2-bromo-2-methylpropane carries no H atoms: 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. SN1 or SN2 ? Halogenoalkane Primary Secondary Tertiary Mechanism SN2 SN1 and SN2 SN1 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. CH3CH2Br + NaOH CH2=CH2 + NaBr + H2O Elimination of HBr from 2-bromopropane CH3CHBrCH3 + OH(in ethanol) CH3 H H C C Br H CH3CH=CH2 + H2O + Br- H H H CH3 OH acting as a base propene C C H Br - H OH nucleophilic substitution + OH- (aqueous) alcohol RCH3CH2OH + Br- hydroxide acts as a nucleophile RCH2CH2X + OH- (ethanol) elimination hydroxide acts as a base RCH=CH2 + H2O + Xalkene 92 AS Chemistry Pros and Cons 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 hydrofluorooalkanes in chemical inertness; of fluoroalkanes and terms of their relative recognise the concern about the chlorofluoroalkanes on the ozone layer. effect of Starter activity Properties: . Non-flammable Low toxicity Unreactive Liquefy easily when compressed Refrigerants Propellants for aerosols Solvents (including dry-cleaning) Degreasers Natural ozone layer 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. 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.