Presentation on theme: "Organic Reactions A detailed study of the following:"— Presentation transcript:
1 Organic Reactions A detailed study of the following: Dehydration SynthesisAdditionFree Radical ReactionsSubstitution (SN1 & SN2)Elimination (E1 & E2)
2 Dehydration Synthesis A reaction involving the formation of a single product through the formation & removal of water.These reactions usually involve reactions between an alcohol and something else.
3 What can be made using this process? Alcohol + alcohol Ether*Alcohol + acid Ester*Alcohol + ammonia AmineAlcohol + Acid Amide* These are discussed further
4 Dehydration of Alcohols to form Ethers Simple, symmetrical ethers can be formed from the intermolecular acid-catalyzed dehydration of 1° (or methyl) alcohols (a “substitution reaction”)2° and 3° alcohols can’t be used because they eliminate (intramolecular dehydration) to form alkenesUnsymmetrical ethers can’t be made this way because a mixture of products results:
5 Mechanism of Formation of Ethers from Alcohols First, an alcohol is protonated by H3O+Next, H2O is displaced by another alcohol (substitution)Finally, a proton is removed by H2O to form the product
6 Combustion of alkanesAlkanes are unreactive as a family because of the strong C–C and C–H bonds as well as them being nonpolar compounds. At room temperature alkanes do not react with acids, bases, or strong oxidizing agents.Alkanes do undergo combustion in air (making them good fuels):2C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(l) H = –2855 kJComplete combustion produced carbon dioxide and water while incomplete may produces a combination of carbon monoxide, carbon and water in addition to carbon dioxide. Carbon dioxide contributes to global warming while carbon monoxide is toxic; hemoglobin binds to carbon monoxide in preference to oxygen causing suffocation and even death.
7 Products of combustion Complete combustion produces:carbon dioxidewater vapourwhile incomplete may produces a combination of :carbon monoxidecarboncarbon dioxide.Carbon dioxide contributes to global warming.Carbon monoxide is toxic; hemoglobin binds to carbon monoxide in preference to oxygen causing suffocation and even death.
8 Alkane Substitution Reaction In the presence of light alkanes undergo substitution reaction with halogens.RH + Br2 RBr + HBrIn a substitution reaction, one atom of a molecule is removed and replaced or substituted by another atom or group of atoms.Mechanism of subtitution reaction involves free radicals.
9 Free Radical Substitution reaction 1-bromohexaneFor a reaction between an alkane and bromine to occur, C-H and Br-Br bonds must break.The C-H bond is stronger than Br-Br bondTherefore, the reaction proceeds by first the breakage of Br-Br bond, which is brought about by UV light.Br-Br bond can be broken in one of two ways..or
10 Free Radical Substitution reaction When the bond is broken, eitherthe bond pair can be equally shared between the two atoms producing two bromine atoms (called free radicals), orThe bond pair goes with one atom producing a positive and a negatively charged ions of bromine.The first type of bond breakage producing free radicals is referred to as a homolytic fission and the second heterolytic fission.Homolytic fission because the bond pairs are equally distributed, or particles that are the same in every way is produced.homolytic fission of the halogen takes place.In the next step, the free radical removes a hydrogen atom from the alkane forming hydrogen bromine and a free radical of the alkane.CH3CH2CH2CH2CH2CH2-H + Br• CH3CH2CH2CH2CH2CH2• + HBr
11 Free Radical Substitution reaction The free radical goes on to react with a molecule of chlorine and regenerate another chlorine free radical.CH3CH2CH2CH2CH2CH2• + Br2 CH3CH2CH2CH2CH2CH2Br + Br•And so on.Because this reaction, once initiated, can keep itself going is referred to as a chain reaction.The reaction can conducted with any halogen and the mechanism would be the same.Not only that, more than one hydrogen can be substituted.1,1 dibromohexane
12 Mechanism of chlorination of methane CHAIN REACTION“dissociation”R E P E A T I N GST E P S“hydrogen abstraction”
13 Mechanism of chlorination of methane 4. Termination Steps“recombinations”These steps stopthe chain reaction
14 Reactions of Alkenes: Addition Reactions Hydrogenation of Alkenes – addition of H-H (H2) to theπ-bond of alkenes to afford an alkane. The reaction must becatalyzed by metals such as Pd, Pt, Rh, and Ni.H°hydrogenation = -136 KJ/molC-C π-bond H-H C-H= 243 KJ/mol = 435 KJ/mol = 2 x -410 KJ/mol = -142 KJ/mol• The catalysts is not soluble in the reaction media, thus thisprocess is referred to as a heterogenous catalysis.• The catalyst assists in breaking the -bond of the alkene andthe H-H -bond.• The reaction takes places on the surface of the catalyst. Thus,the rate of the reaction is proportional to the surface areaof the catalyst.14
15 • Carbon-carbon -bond of alkenes and alkynes can be reduced to the corresponding saturated C-C bond. Other -bond bondsuch as C=O (carbonyl) and CN are not easily reduced bycatalytic hydrogenation. The C=C bonds of aryl rings are noteasily reduced.15
16 H°combustion : -2710 KJ/mol -2707 KJ/mol Heats of Hydrogenation -an be used to measure relative stability of isomeric alkenestrans isomer is ~3 KJ/molmore stable than thecis isomerH°combustion : KJ/mol KJ/molH°hydrogenation: -119 KJ/mol KJ/moltrans isomer is ~4 KJ/mol more stable than the cis isomerThe greater releaseof heat, the lessstable the reactant.16
18 Electrophilic Addition of Hydrogen Halides to Alkenes C-C -bond: H°= 368 KJ/molC-C -bond: H°= 243 KJ/mol-bond of an alkene canact as a nucleophile!!Electrophilic addition reactionBonds broken Bonds formedC=C -bond KJ/mol H3C-H2C–H KJ/molH–Br KJ/mol H3C-H2C–Br KJ/molcalc. H° = -84 KJ/molexpt. H°= -84 KJ/mol18
19 Reactivity of HX correlates with acidity: slowest HF << HCl < HBr < HI fastestRegioselectivity of Hydrogen Halide Addition:Markovnikov's RuleFor the electrophilic addition of HX across a C=C bond, the H (ofHX) will add to the carbon of the double bond with the most H’s(the least substitutent carbon) and the X will add to the carbon ofthe double bond that has the most alkyl groups.19
20 Mechanism of electrophilic addition of HX to alkenes Regioselectivity determined by Markovnikov’s rule – which can be explained by comparing the stability of the intermediate carbocations20
21 For the electrophilic addition of HX to an unsymmetrically substituted alkene:• The more highly substituted carbocation intermediate isformed.• More highly substituted carbocations are more stable thanless substituted carbocations. (hyperconjugation)• The more highly substituted carbocation is formed fasterthan the less substituted carbocation. Once formed, themore highly substituted carbocation goes on to the finalproduct more rapidly as well.21
22 Note that the shifting atom or group moves with its electron pair. Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes - In reactions involvingcarbocation intermediates, the carbocation may sometimes rearrange if amore stable carbocation can be formed by the rearrangement. These involve hydride andmethyl shifts.Note that the shifting atom or group moves with its electron pair.A MORE STABLE CARBOCATION IS FORMED.22
23 Free-radical Addition of HBr to Alkenes Polar mechanism(Markovnikov addition)Radical mechanism(Anti-Markovnikov addition)The regiochemistry ofHBr addition is reversedin the presence ofperoxides.Peroxides are radicalinitiators - change inmechanism23
24 The regiochemistry of free radical addition of H-Br to alkenes reflects the stability of the radical intermediate.
25 Acid-Catalyzed Hydration of Alkenes The addition of water (H-OH) across the -bond of an alkene to give an alcohol; opposite of dehydrationThis addition reaction follows Markovnikov’s rule The morehighly substituted alcohol is the product and is derived fromThe most stable carbocation intermediate.Reactions works best for the preparation of 3° alcohols
27 Mechanism for this reaction is the reverse of the acid-catalyzed dehydration of alcohols:
28 Bonds broken Bonds formed C=C -bond 243 KJ/mol H3C-H2C–H -410 KJ/mol 6.11: Thermodynamics of Addition-Elimination EqulibriaBonds broken Bonds formedC=C -bond KJ/mol H3C-H2C–H KJ/molH–OH 497 KJ/mol (H3C)3C–OH KJ/molcalc. H° = -50 KJ/molG° = -5.4 KJ/mol H° = KJ/mol S° = KJ/molHow is the position of the equilibrium controlled?Le Chatelier’s Principle - an equilibrium will adjusts to any stressThe hydration-dehydration equilibria is pushed toward hydration (alcohol) by adding waterand toward alkene (dehydration) by removing water.
29 The acid catalyzed hydration is not a good or general method for the hydration of an alkene.Oxymercuration: a general (2-step) method for the Markovnokovhydration of alkenesNaBH4 reduces the C-Hgbond to a C-H bond
30 Addition of Halogens to Alkenes X2 = Cl2 and Br2 (vicinal dihalide)Stereochemistry of Halogen Addition - 1,2-dibromide has the anti stereochemistry
31 Substitution Reaction with Halides (1)(2)bromomethanemethanolIf concentration of (1) is doubled, the rate of the reaction is doubled.If concentration of (1) and (2) is doubled, the rate of the reaction quadruples.If concentration of (2) is doubled, the rate of the reaction is doubled.
32 Substitution Reaction with Halides (1)(2)bromomethanemethanolRate law:rate = k [bromoethane][OH-]this reaction is an example of a SN2 reaction.S stands for substitutionN stands for nucleophilic2 stands for bimolecular
33 Mechanism of SN2 Reactions Alkyl halideRelative rate1200401≈ 0The rate of reaction depends on the concentrations of both reactants.When the hydrogens of bromomethane are replaced with methyl groups the reaction rate slow down.The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of only one stereoisomer
34 Mechanism of SN2 Reactions Hughes and Ingold proposed the following mechanism:Transition stateIncreasing the concentration of either of the reactant makes their collision more probable.
35 Mechanism of SN2 Reactions Steric effectactivationenergy: DG2Energyactivationenergy: DG1reaction coordinatereaction coordinateInversion of configuration(R)-2-bromobutane(S)-2-butanol
36 Factor Affecting SN2 Reactions The leaving grouprelative rates of reaction pKa HXHO- + RCH2I RCH2OH + IHO- + RCH2Br RCH2OH + BrHO- + RCH2Cl RCH2OH + ClHO- + RCH2F RCH2OH + FThe nucleophileIn general, for halogen substitution the strongest the base the better the nucleophile.pKaNuclephilicity
37 SN2 Reactions With Alkyl Halides an alcohola thiolan ethera thioetheran aminean alkynea nitrile
38 Substitution Reactions With Halides 1-bromo-1,1-dimethylethane1,1-dimethylethanolRate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a SN1 reaction.S stands for substitutionN stands for nucleophilic1 stands for unimolecularIf concentration of (1) is doubled, the rate of the reaction is doubled.If concentration of (2) is doubled, the rate of the reaction is not doubled.
39 Mechanism of SN1 Reactions Alkyl halideRelative rate≈ 0 *12The rate of reaction depends on the concentrations of the alkyl halide only.When the methyl groups of 1-bromo-1,1-dimethylethane are replaced with hydrogens the reaction rate slow down.The reaction of an alkyl halide in which the halogen is bonded to an asymetric center leads to the formation of two stereoisomers* a small rate is actually observed as a result of a SN2
40 Mechanism of SN1 Reactions nucleophile attacks the carbocationslowC-Br bond breaksfastProton dissociation
41 Mechanism of SN1 Reactions Rate determining stepCarbocation intermediateDGR++ X-+R-OH2R-OH
42 Mechanism of SN1 Reactions Inverted configuration relative the alkyl halideSame configuration as the alkyl halide
43 Factor Affecting SN1 reaction Two factors affect the rate of a SN1 reaction:The ease with which the leaving group dissociate from the carbonThe stability of the carbocationThe more the substituted the carbocation is, the more stable it is and therefore the easier it is to form.As in the case of SN2, the weaker base is the leaving group, the less tightly it is bonded to the carbon and the easier it is to break the bondThe reactivity of the nucleophile has no effect on the rate of a SN1 reaction
44 Comparison SN1 – SN2 SN1 SN2 A two-step mechanism A one-step mechanism A unimolecular rate-determining stepA bimolecular rate-determining stepProducts have both retained and inverted configuration relative to the reactantProduct has inverted configuration relative to the reactantReactivity order:3o > 2o > 1o > methylmethyl > 1o > 2o > 3o
45 Elimination Reactions 1-bromo-1,1-dimethylethane2-methylpropeneRate law:rate = k [1-bromo-1,1-dimethylethane][OH-]this reaction is an example of a E2 reaction.E stands for elimination2 stands for bimolecular
46 The mechanism shows that an E2 reaction is a one-step reaction The E2 ReactionA proton is removedBr- is eliminatedThe mechanism shows that an E2 reaction is a one-step reaction
47 Elimination Reactions 1-bromo-1,1-dimethylethane2-methylpropeneRate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a E1 reaction.E stands for elimination1 stands for unimolecularIf concentration of (1) is doubled, the rate of the reaction is doubled.If concentration of (2) is doubled, the rate of the reaction is not doubled.
48 The E1 Reaction The base removes a proton The alkyl halide dissociate, forming a carbocationThe mechanism shows that an E1 reaction is a two-step reaction
49 Products of Elimination Reaction 30%50%80%2-butene2-bromobutane20%1-buteneThe most stable alkene is the major product of the reaction for both E1 and E2 reactionThe greater the number of alkyl substituent the more stable is the alkeneFor both E1 and E2 reactions, tertiary alkyl halides are the most reactive and primary alkyl halides are the least reactive
50 Competition Between SN2/E2 and SN1/E1 rate = k1[alkyl halide] + k2[alkyl halide][nucleo.] + k3[alkyl halide] + k2[alkyl halide][base]SN2 and E2 are favoured by a high concentration of a good nucleophile/strong baseSN1 and E1 are favoured by a poor nucleophile/weak base, because a poor nucleophile/weak base disfavours SN2 and E2 reactions
51 Competition Between Substitution and Elimination SN2/E2 conditions:In a SN2 reaction: 1o > 2o > 3oIn a E2 reaction: 3o > 2o > 1o10%90%75%25%100%
52 Competition Between Substitution and Elimination SN1/E1 conditions:All alkyl halides that react under SN1/E1 conditions will give both substitution and elimination products (≈50%/50%)
53 Summary of Elimination & Substitution Reactions Alkyl halides undergo two kinds of nucleophilic subtitutions: SN1 and SN2, and two kinds of elimination: E1 and E2.SN2 and E2 are bimolecular one-step reactionsSN1 and E1 are unimolecular two step reactionsSN1 lead to a mixture of stereoisomersSN2 inverts the configuration od an asymmetric carbonThe major product of a elimination is the most stable alkeneSN2 are E2 are favoured by strong nucleophile/strong baseSN2 reactions are favoured by primary alkyl halidesE2 reactions are favoured by tertiary alkyl halides