Topic 10 – Organic Chemistry Section 10.2 Functional Group Chemistry 1

Why Do Hydrocarbons Make Good Fuels? To do: Use Table 12 in the data booklet to help you determine the trend in energy released per gram by combustion of the alkanes. Use bond enthalpies to help you explain the trend noted above. What do you think should be the characteristics of a good fuel? Use the above to decide and explain which out of methane and octane is a better fuel. 2

Functional Group Chemistry Alkanes relatively inert used as fuel – easily combust with O2 and release a lot of energy lower volatility as the carbon chain gets longer alkane + O2 → CO2 + H2O Complete combustion: alkane + oxygen  carbon dioxide + water Incomplete combustion: Alkane + oxygen  carbon + carbon monoxide + carbon dioxide + water The amounts of C, CO and CO2 will vary depending on conditions 3

Types of Reactions Substitution Oxidation Addition Elimination Free radical substitution Nucleophilic substitution Electrophilic substitution Oxidation Combustion Redox Addition Elimination 4

Classifying Molecules Halogenoalkanes – look at the carbon attached to the halogen and see how many carbons it’s attached to. Alcohols – same as halogens Amines – how many carbons are directly bonded to the nitrogen 5

Halogenation Alkanes are relatively stable and difficult to react, but 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 chloroethane This reaction is an example of free radical substitution. 6

Free Radicals Free radicals are species with unpaired electrons formed by homolytic fission. They are crazy reactive Halogens form radicals when hit by uv light of the right frequency: Cl2 2 Cl• The dot after the Cl represents the unpaired electron and tells us we have a radical Homolytic fission – the bond breaks equally with one electron going to each chlorine Task: draw Lewis structures for the Cl2 molecule and each of the Cl• radicals u.v. 7

Reaction Mechanism: Free Radical Substitution Cl2 2 Cl• Cl• + C2H6 → C2H5• + HCl C2H5• + Cl2 → C2H5Cl + Cl• Cl• + Cl• → Cl2 Cl• + C2H5• → C2H5Cl C2H5• + C2H5• → C4H10 Initiation Radicals formed by homolytic fission Propagation These steps feed each other the radicals needed to continue Termination Any two radicals can combine to terminate the reaction Concentration of radicals is low so this is a rare event u.v. A single radical can cause thousands of cycles of the propagation stage before it reaches termination This same mechanism applies to all of the halogens The alkane can be substituted multiple times, until every H has been replaced 8

Free Radical Substitution Remember the role of free radical reactions in the depletion of the ozone layer Initiation CF2Cl2(g) → CF2Cl•(g) + Cl•(g) Propagation Cl•(g) + O3(g) → ClO•(g) + O2(g) ClO•(g) + O3(g) → Cl•(g) + 2O2(g) Termination Overall 2O3(g) → 3O2(g) 9

Demo Place approx 1 cm3 of hexane into two test tubes Add roughly 1 cm3 of bromine water to each and stopper them, then give them a good shake. Leave one test-tube in the classroom but take the other outside and shake it in direct sunlight. Record and explain all observations Write equations showing the mechanism of the reaction Draw full structural and skeletal formulas of at least 6 possible products, and name each one. 10

Key Points Alkanes are pretty unreactive They release a lot of energy on combustion, and are easy to handle which makes them good fuels. Undergo free radical substitution to form halogenoalkanes and a hydrogen halide in the presence of UV light. 11

Nucleophilic Substitution Reactions Carbon is e‾ deficient, so is ripe for attack by a nucleophile Example: KOH attacking C Electrophilic Substitution electrophile is something that needs e‾ happens with benzene 12

Nucleophilic Substitution SN1 Weak nucleophile Leaving group controls rate of reaction Stereo-isomer configurations (>50% inversion; <50% retention) Rate = k[R-X] C X •• Nu / Solvent Nu Nu ̶ + carbo-cation X ̶ slow fast 13

Nucleophilic Substitution SN1 tert-carbon more reactive primary carbon less reactive Rule ̶ methyl < n ̶ < sec ̶ < tert ̶ C Cl •• CH3 + C Cl •• H CH3 + 14

Nucleophilic Substitution SN2 Strong nucleophile Always a primary carbon Leaving group and nucleophile control rate of reaction 100% inversion isomer Rate = k[R-X][Nu] C Nu C X •• ƍ + ƍ ̶ •• X ̶ Nu / Solvent Nu ̶ C X •• ƍ + 15

Oxidation Reactions Combustion Reactions Hydrocarbon + O2 → CO2 + H2O Incomplete reaction Products include C + CO + H2O 16

Chromate goes from 6+ (orange) to 3+ (green) Oxidation Reactions Alcohols Using acidified K2Cr2O7 or KMnO4 Chromate goes from 6+ (orange) to 3+ (green) Primary alcohol results in an aldehyde. Secondary alcohol results in a ketone. H ̶ C ̶ C ̶ C ̶ H H OH O → H2SO4 K2Cr2O7 17

Oxidation of Alcohols → Alcohols Using acidified K2Cr2O7 or KMnO4 Primary alcohol – two-stage process that goes first to an aldehyde and then a carboxylic acid → H ̶ C ̶ C ̶ OH H H ̶ C ̶ C O H2SO4 K2Cr2O7 H2SO4 K2Cr2O7 ethanol ethanal ethanoic acid 18

Oxidation Reactions Esterification → Using concentrated H2SO4 Carboxylic acid + alcohol → ester conc. H2SO4 → H ̶ C ̶ C ̶ C ̶ OH H O H ̶ C ̶ C ̶ C ̶ O ̶ CH2CH3 + CH3CH2OH 19

Functional Group Chemistry Alkenes Alkenes are considerably more reactive than alkanes and are a major industrial feedstock The reactivity is due to the double bond: The double bond contains 4 electrons This is a significant amount of charge which: Makes it attractive to electrophiles Enables it to polarise approaching molecules Most reactions of alkenes are addition reactions where two molecules come together to make one new one 20

Addition Reactions alkene + water → alcohol Reaction conditions: Water must be steam Phosphoric or sulphuric acid catalyst This is the process used to make industrial ethanol Fermentation from sugar would be far too expensive! 21

Addition Reactions alkene + hydrogen  alkane Reaction conditions: Hot Ni catalyst This is an addition reaction, in which the hydrogen adds across the double bond 22

Addition Reactions Alkene + hydrogen halide → halogenoalkane Reaction conditions: This reaction occurs very readily and needs no special conditions This is an addition reaction, in which the hydrogen halide adds across the double bond 23

Addition Reactions + Br2 → C = C H H ̶ C ̶ C ̶ H Br alkene + halogen → dihalogenoalkane Reaction conditions: This reaction occurs very readily and needs no special conditions If 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. C = C H + Br2 → H ̶ C ̶ C ̶ H Br 24

Homework Design and construct a functional groups table (landscape) You need to research and produce a mind-map summarising the following functional groups: Alkane Alkene Alcohol Aldehyde Ketone Carboxylic acid Ester Halide/Halogenoalkane Your table should have six columns including: Name of functional group General structural formula (use ‘R’ to signify a carbon chain) Rules for naming them (including the position where relevant) A named example Relative volatility Relative solubility in water For alcohols and halides you should include a branch to explain the difference between 1o, 2o, and 3o You should also have a branch called ‘Other Functional Groups’ that just allows you to recognise the groups: Amine Benzene 25

Functional Group Chemistry Polymerization Reactions of many small monomers with double bonds to form long chains CH2 = CH CH3 → ̶ CH2 ̶ CH ̶ CH3 n 26

Polymerisation Under the right conditions, alkene molecules will add to each other creating a polymer In this case, 1-bromo-2-fluoroethene polymerises to form poly-1-bromo-2-fluroethene Conditions: Vary from alkene to alkene, but often include high pressure, temperature and a catalyst The carbons in the C=C double bonds form the carbon chain, everything else hangs off this chain 27

Drawing Polymers Draw three-monomer lengths of the polymers formed by: Propene Styrene Pent-2-ene 28

Drawing Polymers Draw three-monomer lengths of the polymers formed by: Propene Solution: Polypropylene CH3 C H n 29

Drawing Polymers Draw three-monomer lengths of the polymers formed by: Styrene Solution: Polystyrene C H CH2 H2C n 30

Drawing Polymers Draw three-monomer lengths of the polymers formed by: Pent-2-ene Solution: Polypropylene CH2 CH3 C H n 31

Homework Research the economic importance of alkenes including: Manufacture of margarine Manufacture of ethanol Polymerisation 32

SL Organic Pathways M KETONE ALKANE HALOGENOALKANE ALCOHOL ALDEHYDE UV + Hal2 KETONE ALKANE Heat/UV M Free Rad Chain Rxn HALOGENOALKANE Warm OH‾(aq) Nucleophilic Substitution ALCOHOL Heat (Distill/Reflux) H+ K2Cr2O7/KMnO4 2º ALCOHOL Heat (Distillation) 1º ALCOHOL CARBOXYLIC ACID ALDEHYDE Heat (Reflux) K2Cr2O7 TRIHALOGENOALKANE TETRAHALOGENOALKANE ALKENE POLYALKANE Heat, Pressure, Catalyst H-Hal/Hal2 H2 + Hot Nickel H2SO4 H3PO4 CATALYST Al2O3 300ºC, 7 atm, H2O DIHALOGENOALKANE + Hal2 ALCOHOL ALKENE + H2O ALKENE ALCOHOL - H2O R-OH conc. H2SO4 ESTER 33

Homework Complete Kerboodle QUIZ on 10.2 Study for TEST 34