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Reactions of Organic Compounds

Reactions of Organic Compounds. The Alkanes. homologous series with the general formula C n H 2n+2 . have trends in physical properties e.g. density and m.p. and b.p. all increase with M r . all undergo similar chemical reactions.

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Reactions of Organic Compounds

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  1. Reactions of Organic Compounds

  2. The Alkanes • homologous series with the general formula CnH2n+2. • have trends in physical properties e.g. density and m.p. and b.p. all increase with Mr. • all undergo similar chemical reactions. • SATURATED HYDROCARBONS. i.e. they contain only single C to C bonds and are made up of C and H atoms only.

  3. Alkanes are obtained from crude oil by fractional distillation. • They are mainly used as fuels. • The large Mr alkanes do not ignite easily so there is little demand for them as fuels so they are CRACKED to make smaller more useful alkanes and alkenes. • Apart from combustion alkanes undergo few chemical reactions. This is for two main reasons: • The bonds in alkanes are relatively strong. • The bonds have a relatively low polarity as the electronegativity of C and H is similar. • As a consequence alkanes can be used as lubricating oils, although they do degrade over time.

  4. Chemical Properties of Alkanes • Similar for all alkanes • Not very reactive Reasons: • C-H and C-C covalent bonds essentially non-polar, unlikely to attract polar molecules or ions. • Strong C-C and C-H bonds require a lot of energy to break.

  5. The combustion of Alkanes • Smaller alkanes burn easily in air when ignited • If the combustion is complete, products formed are CO2 and H2O. CH4 + 2O2 CO2 + H2O H = -890kJ/mol • Reaction is exothermic - a result of the high relative strength of the C=O in CO2 and O-H in H2O molecules. The large amount of heat energy released in making these bonds means the reaction is strongly exothermic. Write equations for the complete combustion of butane and octane.

  6. If the combustion is incomplete, products formed are CO, C (soot) and H2O. 2C H4 + 3O2 2CO + 4H2O CH4 + O2 C + 2H2O What problems do the gases released on combustion of alkanes cause?

  7. Reactions of Alkanes with halogens – Substitution reactions • In the presence of ultraviolet light, methane can combine with chlorine to give a mixture of products (chloroalkanes) • Light energy is used to start the reaction (by providing the energy to break the covalent bond between the chlorine atoms in Cl2. • The chlorine atom(radical) produced then reacts with alkane by substituting the hydrogen atom.

  8. Cl Cl Free radical = species with an unpaired electron. Free radicals are formed by homolytic fission of bonds. In homolytic fission one electron from the shared pair goes to each atom. So

  9. Cl + Cl unpaired electron or Cl2 2Cl. Heterolytic fission of Cl – Cl would result in the formation of Cl+ and Cl-. There are three steps in the mechanism: initiation, propagation and termination

  10. Free radical substitution CH4 + Cl2 CH3Cl + HCl chlorination of methane i.e. homolytic breaking of covalent bonds Overall reaction equation Conditions ultra violet light (breaks weakest bond) excess methane to reduce further substitution

  11. Free radical substitution mechanism Cl2 Cl + Cl Cl + Cl Cl2 CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH3 + Cl CH3Cl CH3 + CH3 CH3CH3 UV Light initiation step propagation steps termination step Also get reverse of initiation step occurring as a termination step.

  12. Further free radical substitutions CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl ultra-violet light excess chlorine Overall reaction equations Conditions Details of free radical substitution Page 435

  13. The Alkenes • general formula CnH2n. • unsaturated hydrocarbons containing C = C. Much more reactive than alkanes. Industrial importance of alkenes: • Making polymers (plastics) • Hydrogenation of vegetable oils to make margarine • Hydration of ethene to make ethanol.

  14. When naming alkenes have to include position of double bond, for example: CH3CH=CHCH3 is but - 2 - ene and CH3CH2CH=CH2 is but -1- ene Alkenes undergo ADDITION reactions. Two substances combine to form one new substance. Unsaturated molecules are converted to saturated molecules.

  15. More than 1 C=C bond Page 440

  16. Chemical Properties of Alkenes • Similar for all alkenes • More reactive than alkanes Reasons: • C=C covalent bonds (4 electrons) represents a region of high electron density, therefore attracts electrophile ( + ion / + end of dipole ) • Ä bond in C=C can break easily – allow other atoms to join to the carbon atoms resulting in an addition reaction.

  17. The combustion of Alkenes • Alkenes burn in an excess air to form CO2 and H2O. C2H4 + 3O2 2CO2 +2H2O H = -1322kJ/mol • Reaction is exothermic. • If the combustion is incomplete, products formed are CO, C (soot) and H2O. C2H4 + O2 2C + 2H2O C2H4 + 2O2 2CO + 2H2O More soot (C) is produced compared to the corresponding alkane, due to higher percentage by mass of carbon. How can you tell whether the reaction is complete or incomplete?

  18. R R C =C R R Addition Reactions • R : alkyl group or H R R + X-Y R– C – C – R X Y

  19. H H H H C=C H – C – C – H + H2 H H H H Hydrogenation (with hydrogen) • Addition of hydrogen (hydrogenation) Alkenes react with hydrogen in the presence of a nickel catalyst at 150 °C to form an alkane. e.g. Ni / 1500C C2H4 + H2 C2H6 ethane ethene

  20. Hydrogenation • Hydrogenation is used in margarine industry to convert oils containing unsaturated hyrocarbon chains into saturated compounds with higher melting points. • Margarine is a solid at room temperature

  21. H H H H C =C + Cl2 H – C – C – H H H Cl Cl Halogenation (with halogens,X2) • Addition of halogens (halogenation) • Halogens react with alkenes at room temperature and pressure in a non-polar solvent to form a dihalogenoalkane. C2H4 + Cl2 C2H4Cl2 ethene 1,2-dichloroethane Write the equation when propene reacts with chlorine.

  22. H H C =C H H With hydrogen halides (H-X) • Alkenes react with hydrogen halides (HCl, HBr etc.) to solvent to form a dihalogenoalkane. • The reaction occurs at room temperature and pressure. e.g. H H + HBr H – C – C – H H Br C2H4 + HBr  CH3CH2Br bromoethane ethene

  23. H H H H C =C + H2O H – C – C – H H H OH H Hydration (with water) • Hydration reaction can be done in 2 ways: • addition of concentrated sulfuric acid at room temperature and warm with water to produce an alcohol. Conc. H2SO4 heat ethanol

  24. Bromine water is used as a test for unsaturation. In the presence of an alkene, bromine water turns from red brown to colourless. Alkanes do not react with bromine water.

  25. C =C + Br2(aq) – C – C – Br Br Test for Unsaturation – bromine water test for alkenes heat colorless orange colorless

  26. R R C =C R R Addition Polymerisation R R • monomer polymer • A large number of monomers are joined together into a polymer. –C – C– n R R n

  27. H H C =C H H Poly(ethene) H H • ethene poly(ethene) 2000C, 2000 atm O2 / peroxide –C – C– n H n H

  28. H H C =C H Cl Poly(vinylchloride) H H • ethene poly(chloroethene) –C – C– n repeating unit H Cl n

  29. H H C =C H CH3 Poly(propene) H H • ethene poly(ethene) C – C n H CH3 n repeating unit

  30. CH2=CHCH3 - CH2 – CH - CH3 - CH2 – CH - CH2=CHC6H5 C6H5

  31. poly(chloroethene) polyvinylchloride PVC CH2=CHCl - CH2 – CH - Cl CF2=CF2 - CF2 – CF2 - Poly (tetrafluoroethene) PTFE Non-stick coating (Teflon) Practice questions Page 446

  32. Uses of Alkenes • To make margarine • To make alcohols, used as antifreeze and solvents • To make plastic (polymers) like poly(ethene), PVC • Used in agriculture (in low concentration) to help hasten the ripening of fruits like bananas.

  33. The combustion of Alcohols 2CH3OH(l)+ 3O2(g)  2CO2(g) + 4H2O(l) ΔH = -726kJ/mol C2H5OH(l)+ 3O2(g)  2CO2(g) + 3H2O(l) ΔH = -1371kJ/mol • In countries such as Brazil, ethanol is mixed with petrol and used to power cars. Ethanol is less efficient as a fuel than petrol as it is already partially oxidised but does make the country less reliant on supply of petrol. As it can be produced by fermentation of sugar beet, many consider ethanol a carbon neutral fuel.

  34. O H C H H H H H C C C OH H H H The oxidation reactions of Alcohols • Primary alcohols are oxidised first to aldehydes. A suitable oxidising agent is acidified potassium dichromate(VI) Cr2O72-/H+ + H2O heat ethanol ethanal

  35. O O H H C C H H H OH C C H H An aldehyde still has one hydrogen atom attached to the carbonyl carbon, so it can be oxidised one step further to a carboxylic acid. Cr2O72-/H+ heat ethanal ethanoic acid Cr2O72-/H+ Cr2O72-/H+ 10 alcohol  aldehyde  carboxylic acid heat heat Inductive reasoning – applicable to another 10 alcohol Page 450

  36. In practice, a primary alcohol such as ethanol is dripped into a warm solution of acidified potassium dichromate(VI). • The aldehyde, ethanal, is formed and immediately distils off, thereby preventing further oxidation to ethanoic acid, because the boiling point of ethanal (23 °C) is much lower than that of either the original alcohol ethanol (78 °C) or of ethanoic acid (118 °C). Both the alcohol and the acid have higher boiling points because of hydrogen bonding. • If oxidation of ethanol to ethanoic acid is required, the reagents must be heated together under reflux to prevent escape of the aldehyde before it can be oxidised further.

  37. H H H H H H H C H H C C C C C O OH H H H H • Secondary alcohols are oxidised to ketones. These have no hydrogen atoms attached to the carbonyl carbon and so cannot easily be oxidised further. • Tertiary alcohols are resistent to oxidation Cr2O72-/H+ heat propan-2-ol propanone

  38. Distinguishing between 1°, 2° and 3° alcohols • When orange acidified potassium dichromate(VI) acts as an oxidising agent, it is reduced to green chromium(III) ions. • 1° and 2° alcohols both turn acidified dichromate(VI) solution from orange to green when they are oxidised, and this colour change can be used to distinguish them from 3° alcohols. • 3° alcohols are not oxidised by acidified dichromate(VI) ions, so they have no effect on its colour, which remains orange. Practice Page 452

  39. O O H H C C H H H OH C C H H Reactions of aldehydes and ketones An aldehyde can be oxidised by heating with acidified potassium dichromate(IV) to carboxylic acid. Cr2O72-/H+ heat ethanal ethanoic acid

  40. Nucleophilic substitution reactions The + carbon atom is susceptible to attack by NUCLEOPHILES. A nucleophile is a species with a lone pair of electrons. E.g. OH-, NH3, CN-. When attack by a nucleophile occurs, the carbon – halogen bond breaks releasing a halide ion. A suitable nucleophile for experimentation is OH- from an aqueous solution of an alkali such as sodium hydroxide.

  41. H H OH Br H H H H H H C C C C C C H H H H H H CH3CH2CH2X + OH-  CH3CH2CH2OH + X- X = Cl, Br or I. OH has replaced the X so overall we have NUCLEOPHILIC SUBSTITUTION NaOH(aq) heat Propan-1-ol 1-bromopropane

  42. A nucleophile is a molecule / negatively charged ion with a lone pair of electrons which is attacted to a more positively charged region in a molecule and donates a lone pair of electrons to form a covalent bond.

  43. Mechanisms for nucleophilic substitution SN1 = unimolecular nucleophilic substitution (only one species in the slow step of the mechanism, rate determining step) SN2 = bimolecular nucleophilic substitution (two species in the slow step of the mechanism, rate determining step) Use of curly arrows: Curly arrows are used in reaction mechanisms to show the movement of electron pairs.

  44. X TAILS come from eithera bond pairof electrons ora lone pairof electrons HEADS point eithernext to an atom orat an atom X to form a bond pair of electrons to forma lone pairof electrons

  45. H H H + + C C CH3 CH3 Br C CH3 H H H - Br H - OH C OH CH3 H Heterolytic fission of C – Br bond slow + Intermediate carbocation fast SN1 Page 461

  46. - H H HO Br C C CH3 Br H CH3 H - H Br CH3 C - HO OH H Transition state + SN2 Page 460

  47. Which is best? SN1 or SN2? For primary halogenoalkanes – SN2 For tertiary halogenoalkanes – SN1 3° halogenoalkanes cannot undergo the SN2 mechanism as 5 bulky groups would not fit around the C in the transition state - steric hindrance. 1° halogenoalknes are less likely to undergo SN1 as this would involve the formation of a primary carbocation as an intermediate. Alkyl groups push electron density to the C atom they are attached to (positive inductive effect) which stabilises the positive charge. More alkyl groups mean a more stable carbocation.

  48. 2° halogenoalkanes react via a mixture of SN1 and SN2. The mechanism predominating depends upon the nature of the alkyl groups and the nature of the solvent.

  49. Reaction pathways trihalogenoalkane tetrahalogenoalkane alkane dihalogenoalkane M1 halogenoalkane alkene poly(alkene) M2 alcohol aldehyde carboxylic acid M1 and M2 : You should know the mechanisms for these reactions ketone The flow chart above enables you to convert 2-bromobutane into butanone using a two-step synthetic route 2-bromobutane butan-2-ol butanone reflux with NaOH reflux with H+/Cr2O7 Page 482-486

  50. Example Devise two-step syntheses of the following products from the starting material. Include any experimental conditions. • Ethanoic acid from ethene • Butan-1-ol from butane • Propanal from 1-bromopropane

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