Presentation on theme: "Bonding in methane, ethane and ethene and bonds AS Chemistry."— Presentation transcript:
Bonding in methane, ethane and ethene and bonds AS Chemistry
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.
Hybridisation of orbitals The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 1 1s 2 2s 2p
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.
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
sp 3 orbitals In ALKANES, the four sp 3 orbitals repel each other into a tetrahedral arrangement. Hybridisation of orbitals in alkanes
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. 2s 2 2p 2 2s 1 2p 3 3 x sp 2 2p
sp 2 hybrids What about ethene?
Geometric Isomerism AS Chemistry
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.
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
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 isomers of but-2-ene
X Geometric Isomerism?
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
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
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 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.
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.
Optical Isomerism AS Chemistry
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.
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
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.
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
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 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
Electrophilic Addition to Alkenes AS Chemistry
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.
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
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 -
Electron flow during electrophilic addition Electron flow during electrophilic addition
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
Addition to unsymmetrical alkenes Electrophilic addition to propene 2-bromopropane 1-bromopropane
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
PATH A PATH B MAJOR PRODUCT PRIMARY CARBOCATION SECONDARY CARBOCATION MINOR PRODUCT Addition to unsymmetrical alkenes
Polymerisation AS Chemistry
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.
Poly(ethene) Temperature: about 200°C Pressure: about 2000 atmospheres Initiator: often a small amount of oxygen as an impurity Conditions
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
Oxidation of alkenes AS Chemistry
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.
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
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.
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
Halogenoalkanes AS Chemistry
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.
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
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.
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
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.
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.
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
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
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
++ -- CH 3 H Br C H - OH CH 3 H OH C H Br - hydroxide ion with bromoethane Nucleophilic substitution mechanism ethanol
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
++ -- CH 3 H Br C H CN - CH 3 H CN C H Br - cyanide ion with bromoethane Nucleophilic substitution mechanism propanenitrile
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
Past paper question Cl 2 U.V. /sunlight Ethanolic KCN reflux Br 2 U.V. /sunlight
Substitution vs. Elimination AS Chemistry
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.
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
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.
S N 1 or S N 2 ? HalogenoalkaneMechanism PrimarySN2SN2 Secondary S N 1 and S N 2 TertiarySN1SN1
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
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 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?
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 of fluoroalkanes and hydrofluorooalkanes in terms of their relative chemical inertness; recognise the concern about the effect of chlorofluoroalkanes on the ozone layer.
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.