aryl halide and vinyl halide

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reaction and mechanism for aryl halide and vinyl halide


  • Aryl halide and vinyl halide

  • Aryl HalidesAryl halides are halides in which the halogen is attached directly to an aromatic ring.

    Carbon-halogen bonds in aryl halides are shorter and stronger than carbon-halogen bonds in alkyl halides.

  • Whats the Difference Between Ar- and Ph-?

  • NomenclatureAryl halides are named by prefixing the name of the halogen to benzene. For example:

  • Numbering of the ring begins at the halogen-substituted carbon and proceeds in the direction of the next substituted carbon that possesses the lower number.

    Ortho, meta or para ? Mono-substituted aryl halides are characterised using the prefix ortho (o-), meta (m-) or para (p-) depending on the placement of the substituent from the halogen or the halogen from a higher priority functional group: 1,2-, 1,3- or 1,4- respectively.1-chloro-2-ethylbenzene or o-ethylchlorobenzene1-chloro-3-ethylbenzene or m-ethylchlorobenzene1-chloro-4-ethylbenzene or p-ethylchlorobenzene

  • Physical properties

    The physical properties of unsubstituted aryl halides are much like those of the corresponding alkyl halides.

    Thus, boiling points, melting points, and solubilities of aryl halides are very similar to those of alkyl halides containing the same number of carbon atoms.

    Boiling points

    Chlorobenzene, bromobenzene and iodobenzene are all oily liquids. The boiling points increase as thehalogen atom gets bigger.

  • The main attractions between the molecules will be van der Waals dispersion forces. These increase as thenumber of electrons in the molecule increases. This is the reason that the boiling points increase as thehalogen atom gets bigger.

    There will also be permanent dipole-dipole attractions involved in the chlorobenzene and bromobenzene, butvery little in the iodobenzene. Iodine has much the same electronegativity as carbon.

    Compoundsboiling point(C)C6H5Cl132C6H5Br156 C6H5I189

  • Solubility in waterThe aryl halides are insoluble in water. They are denser than water and form a separate lower layer.

    Themolecules are quite large compared with a water molecule. In order for chlorobenzene to dissolve it wouldhave to break lots of existing hydrogen bonds between water molecules and also have to break the quitestrong van der Waals dispersion forces between chlorobenzene molecules. Both of these cost energy.

    The only new forces between the chlorobenzene and the water would be van der Waals dispersion forces.

    These aren't as strong as hydrogen bonds (or the original dispersion forces in the chlorobenzene), and sowouldn't get much energy released when they form.

    It simply isn't energetically profitable for chlorobenzene (and the others) to dissolve in water.

  • Resonance PictureC-X bonds in aryl halides have more double bond character than C-X bonds in alkyl halides

  • Synthesis of Aryl HalidesThe two most common methods of preparing aryl halides are by direct halogenation of benzene andvia diazoniumsalt reactions.

    Preparing chlorobenzene and bromobenzene by reacting chlorine or bromine with benzene, and preparingiodobenzene from benzenediazonium chloride.

  • Preparation of Chlorobenzene

    Benzene reacts with chlorine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine atom.

    The reaction happens at room temperature. The catalyst is either aluminium chloride or iron.

    Strictly speaking iron isn't a catalyst, because it gets permanently changed during the reaction. It reacts with some of the chlorine to form iron(III) chloride, FeCl3.

  • The reaction between benzene and chlorine in the presence of either aluminium chloride or iron giveschlorobenzene.

  • Preparation of Bromobenzene

    The reaction between benzene and bromine in the presence of either aluminium bromide (rather than aluminium chloride) or iron gives bromobenzene.

    Iron is usually used because it is cheaper and more readily available. If we use iron, it is first converted into iron(III) bromide by the reaction between the iron and bromine.

  • Preparation of Iodobenzene

    Iodobenzene can be made from the reaction of benzene with iodine if they are heated under reflux in the presence of concentrated nitric acid, but it is normally made from benzenediazonium chloride solution. That's what we will concentrate on here.

    If you add cold potassium iodide solution to ice-cold benzenediazonium chloride solution, nitrogen gas is given off, and you get oily droplets of iodobenzene formed.

    There is a simple reaction between the diazonium ions present in the benzenediazonium chloride solution and the iodide ions from the potassium iodide solution.

  • o-toluidine

  • Sandmeyer reaction

    A second method for preparing aryl halides is the Sandmeyer reaction.

    During aSandmeyer reaction,a diazonium salt reacts with copper (I) bromide, copper (I) chloride, or potassium iodide to form the respective aryl halide.

    The diazonium salt is prepared from aniline by reaction with nitrous acid at cold temperatures.

  • An aromatic (or heterocyclic) amine quickly reacts with a nitrite to form an aryl diazonium salt, which decomposes in the presence ofcopper(1) salts, such ascopper (1)b chloride, to form the desired aryl halide.The reaction is aradical nucleophilic aromatic substitution

  • Reactions of alkyl halides SN2NRE2NR organo metallic compoundssimilarreductionsimilar

  • In aryl halides, the carbon to which the halogen is attached is sp2 hybrizided. The bond is stronger and shorter than the carbon-halogen bond in aliphatic compounds where the carbon is sp3 hybridized. Hence it is more difficult to break this bond and aryl halides resist the typical nucleophilic substitution reactions of alkyl halides. The same is true of vinyl halides where the carbon is also sp2 hybridized and not prone to nucleophilic substitution.In a manner analogous to the phenols & alcohols, we have the same functional group in the two families, aryl halides and alkyl halides, but very different chemistries.

  • Aryl halides, reactions:Formation of Grignard reagentEAS (Electrophilic aromatic substitution)Nucleophilic aromatic substitution (bimolecular displacement)(Ar must contain strongly electron withdrawing groups ortho and/or para to X)Nucleophilic aromatic substitution (elimination-addition)(Ring not activated to bimolecular displacement)

  • 1) Preparation of Grignard reagent

  • Overall an electrophilic aromatic susbtitution (EArS) can be represented as follows:2) EASThere are three fundamental components to an electrophilic aromatic substitution mechanism:

    formation of the new bond from aC=Cin the arene nucleophileremoval of the proton by breaking theC-H bondreforming theC=Cto restore the aromaticity

  • The mechanism is represented by the following series of events:

    1.Formation of the reactive electrophile,E+from the reagents. 2.Slowreaction of the areneC=Cwith theE+to give a resonance stabilisedcarbocation.3. Loss ofH+from the carbocation to restore theC=Cand the aromatic systemThe reaction of the electrophileE+with the arene is the slow step since it results in the loss of aromaticity even though the resulting cation is still resonance stablised. This carbocation is also described as thecyclohexdienyl cationorarenium ionor as asigma-complex.

  • Electrophilic Aromatic Substitution of Aryl Halides Aryl halidesare themselves reactive towardsEASbut they are less reactive than benzene.2. This is because halidesareweak deactivators3. Halides direct subsequent reactionsortho, para.4. This makes them a little unusual (activators are usuallyortho, para-directing, deactivatorsmeta-directing).5. The weak deactivation is due to the electronegativity of the halogen making the intermediate cations less stable than those produced when benzene undergoes substitution:

  • The directing effect is due to the resonance stabilisation of the cationic intermediates derived byorthoorparaattack but not bymetaattack. For example, the stabilisation duringorthoattack is shown below : However, aryl halides can undergo many of the same electrophilic aromatic substitution reactions that benzene can includingnitration,sulfonation, furtherhalogenation and Friedel-Craftsalkylationoracylationreactions.

  • The X group is electron-withdrawing and deactivating in EAS, but is an ortho/para director.

  • Aryl halides and vinylic halides are relatively unreactive toward nucleophilic substitution under conditions that give facile nucleophilic substitution with alkyl halides.Reason:

    (1) Phenyl cations are very unstable. (2) Halogen bonds of aryl (and vinylic) halides are shorter and stronger than those of alkyl, allylic, and benzylic halides because of the hybridized state and the resonance.But aryl halides can be remarkably reactive toward nucleophiles if they bear certain substituents or when we allow them to react under the proper conditions.Nucleophilic aromatic substitution (bimolecular displacement)

  • Chapter 21*Aryl Halides and Nucleophilic Aromatic SubstitutionSimple aryl and vinyl halides do not undergo nucleophilic substitution

    Back-side attack required for SN2 reaction is blocked in aryl halides

  • Chapter 21*SN2 reaction also doesnt occur in aryl (and vinyl halides) because the carbon-halide bond is shorter and stronger than in alkyl halidesBonds to sp2-hybridized carbons are shorter, and therefore stronger, than to sp3-hybridized carbonsResonance gives the carbon-halogen bond some double bond character

  • bimolecular displacement (nucleophilic aromatic substitution)mechanism:

  • NUCLEOPHILIC AROMATIC SUBSTITUTION BY ADDITION ELIMINATION: THE SNAr MECHANISMNucleophilic substitution can occur when strong electron-withdrawing groupsare ortho or para to the halogen atom.

  • But the meta-nitro group does not produce a similar activating effect.Mechanism:

    The delocalized carbanion is stabilized by electron-withdrawing groups in the positions ortho and para to the halogen atom.

  • Addition-Elimination MechanismTwo step mechanism:

    Step 1nucleophile attacks aryl halide and bonds to the carbon that bears the halogen (slow: aromaticity of ring lost in this step)

    Step 2intermediate formed in first step loses halide (fast: aromaticity of ring restored in this step)

  • Addition-Elimination Mechanism

  • Addition-Elimination MechanismStep 1 - Additionbimolecularconsistent with second-order kinetics;

    first order in aryl halide, first order in nucleophile

    Rate = k [CH3ONa] [arene]

  • Addition-Elimination MechanismStep 1 - Addition

  • intermediate is negatively charged

    formed faster when ring bears electron-withdrawing groups such as NO2 because negative charge is stabilized..Reaction Involves an Anionic Intermediate

  • Stabilization of Addition Product by Electron-Withdrawing Group

  • Rapid Collapse of CyclohexadienylAnion IntermediateStep 2 - Elimination

  • carbon-halogen bond breaking does not occur until after the rate-determining stepelectronegative F stabilizes negatively charged intermediateF > Cl > Br > I is unusual, but consistent with mechanismThe Role of Leaving Groups

  • The Role of Leaving Groups

  • *Nucleophilic Aromatic Substitution through an Elimination-Addition Mechanism: BenzyneUnder forcing conditions, chlorobenzene can undergo an apparent nucleophilic substitution with hydroxideBromobenzene can react with the powerful base amide

  • Chapter 21*The reaction proceeds by an elimination-addition mechanism through the intermediacy of a benzyne (benzene containing a triple bond)

  • Chapter 21*A calculated electrostatic potential map of benzyne shows added electron density at the site of the benzyne p bondThe additional p bond of benzyne is in the same plane as the ring

  • When chlorobenzene labeled at the carbon bearing chlorine reacts with potassium amide, the label is divided equally between the C-1 and C-2 positions of the productThis is strong evidence for an elimination-addition mechanism and against a straightforward SN2 mechanism

  • Chapter 21*Benzyne can be generated from anthranilic acid by diazotization The resulting compound spontaneously loses CO2 and N2 to yield benzyneThe benzyne can then be trapped in situ using a Diels-Alder reaction

  • Phenylation

    Acetoacetic esters and malonic esters can be phenylated by benzyne generated in situ from bromobenzene

  • 1. Preparation of vinyl halidesExample of a vinyl halide:Example of an aryl halide:a. Halogenation of alkynesb. Hydrohalogenation of alkynesc. Elimination in dihaloalkanesd. SEAr - halogenation2. Elimination in aryl and vinyl halidesThese reactions never proceed by E1 because of low stability of aryl and vinyl carbocations.Vinyl halides are much less reactive in E2, than alkyl halides because of a stronger C(sp2)-Cl bond .

  • 3. Substitution in vinyl halidesThis reaction proceeds through addition, followed by elimination. It never goes via SN2, because of the lack of stabilization of the transition state. (Same reason, why vinyl cations are less stable, than alkyl cations.)

  • Write a structural formula for each of the following:

    m-Chlorotoluene 1-Chloro-1-phenylethanep-Bromobenzyl chloride(d) 2-Chloronaphthalene(e) 1,8-Dichloronaphthalene(f) 2-Bromo-1-chloro-4-nitrobenzeneLatihan

  • Reactions Involving Aryl HalidesElectrophilic aromatic substitutionhalide substituents are ortho-para directing & deactivating

  • Reactions Involving Aryl HalidesElectrophilic aromatic substitution (Section 12.14)


  • Reactions Involving Aryl HalidesFormation of aryl Grignard reagents (Section 14.4)

  • Substitution Reactions Involving Aryl HalidesWe have not yet seen any nucleophilic substitution reactions of aryl halides.

    Nucleophilic substitution on chlorobenzene occurs so slowly that forcing conditions are required.

  • Nucleophilic Substitution of ChlorobenzeneThis reaction does not proceed via SN2..

  • the SN2 is not reasonable because the aromatic ring blocks back-side approach of the nucleophile. Inversion is not possible.Why is Chlorobenzene Unreactive?

  • SN1 Also Unlikely:Aryl Cations are Highly Unstable SN1 not reasonable because:

    CCl bond is strong; therefore, ionization to a carbocation is a high-energy process

    aryl cations are highly unstable

  • What is the Mechanism of This Reaction?

  • 23.5Nucleophilic Substitution inNitro-Substituted Aryl Halides

  • *6*6*6*9*2*13*13*When there isnt a good leaving group, this type on intermediate is stable and can be isolated and characterized - Meisenheimer complexes13*17*Restoration of Aromaticity13*18*18*6*6*6*6*1