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Hydrolysisis Water, Is Applied.The Term Staphylococcus Aureus Bacteria (Opaque and Grey), an Organism Hydrolysis Is Applied Organic Chemistry I Mohammad Jafarzadeh Faculty of Chemistry, Razi University Organic Chemistry, Structure and Function (7th edition) By P. Vollhardt and N. Schore, Elsevier, 2014 1 CHAPTER 7 Further Reactions of Haloalkanes Unimolecular Substitution and Pathways of Elimination N O N O :Base Br e have learned that the SN2 displacement process is an important reaction pathway for Whaloalkanes. But is it the only mechanism for displacement available? Or are there other, fundamen- tally different types of transformations that haloal- kanes undergo? In this chapter, we shall see that haloalkanes can indeed follow reaction pathways other than SN2 displacement, especially if the haloalkanes are tertiary or secondary. In fact, bimolecular substitu- tion is only one of four possible modes of reaction. The other three modes are unimolecular substitution and two types of elimination processes. The elimina- tion processes give rise to double bonds through loss of HX and serve as our introduction into the prepara- 7. Further Reactionstion of multiply bonded organicof compounds.Haloalkanes Medicinal chemists use many 7.1 SOLVOLYSIS OF TERTIARY7-1 SOLVOLYSISAND SECONDARY OF TERTIARYHALOALKANES AND SECONDARY HALOALKANES reactions to explore structure- activity relationships in physiologically active compounds. The rate of the SN2 Wereaction have learneddiminishes that the ratedrastically of the SN2 whenreactionthe diminishesreacting drasticallycenter whenchanges the reactingfrom Above, the bromocyclohexyl primary to secondarycenterto tertiary changes. from primary to secondary to tertiary. These observations, however, pertain substituent to a b-lactam is converted to a cyclohexenyl only to bimolecular substitution. Secondary and tertiary halides do undergo substitution, group by elimination of HBr. Secondary and tertiarybut byhalides another domechanism.undergo In fact,substitution, these substratesbut transformby another readily,mechanism even in the presence. In fact, b-Lactams are four-membered of weak nucleophiles, to give substitution products. ring amides featured in the these substrates transform readily, even in the presence of weak nucleophiles, to give structure of many antibiotics, such For example, when 2-bromo-2-methylpropane (tert-butyl bromide) is mixed with water, it as penicillin and cephalosporin, substitution productsis. rapidly converted into 2-methyl-2-propanol (tert-butyl alcohol) and hydrogen bromide. Water and their modi! cation is essential is the nucleophile here, even though it is poor in this capacity. Such a transformation, in which in combating drug resistance. Such a transformation, in which a substrate undergoes substitution by solvent molecules, is The photo shows Petri dish a substrate undergoes substitution by solvent molecules, is called solvolysis, such as methano- cultures of two strains of called solvolysis (e.lysis,g. methanolysis ethanolysis, and ,soethanolysis on. When the, solventetc.). isWhen water, thethe termsolvent hydrolysisis water, is applied.the term Staphylococcus aureus bacteria (opaque and grey), an organism hydrolysis is applied. An Example of Solvolysis: Hydrolysis that causes boils, abscesses, and urinary tract infections. At Poor nucleophile left, one bacterial strain shows yet fast reaction! CH3 CH3 sensitivity to penicillin (white A Relatively fast A pellet) as indicated by the clear CH3CBršš ϩ HOšOH CH3CšOH ϩ HBršš zone of inhibited growth around A š š Aš š it. At right, a second strain of CH3 CH3 bacteria shows resistance to the drug and its growth is not 2-Bromo-2-methylpropane 2-Methyl-2-propanol inhibited. (tert-Butyl bromide) (tert-Butyl alcohol) 2 248 CHAPTER 7 Further Reactions of Haloalkanes 248 CHAPTER 7 Further Reactions of Haloalkanes 2-Bromopropane is hydrolyzed similarly, albeit much more slowly, whereas 1-bromopropane, bromoethane,2-Bromopropane and bromomethane is hydrolyzed similarly,are unchanged albeit muchunder morethese slowly,conditions. whereas 1-bromopropane, 2-Bromopropane is hydrolyzedbromoethane,similarly, and bromomethanealbeit much are unchangedmore slowly, under thesewhereas conditions.1-bromopropane, bromoethane,ReactionR and bromomethane are unchanged under these conditions. ReactionR Hydrolysis of a Secondary Haloalkane Hydrolysis of a Secondary Haloalkane CH3 CH3 A CH3 Relatively slow A CH3 CH3CBršAš ϩ HOšOH Relatively slow CH3COHšA ϩ HBršš Reminder CHA3šCBršš ϩ HOš šOH CHA3šCOHš ϩ šHBršš Reminder Nucleophile: red H A š š H Aš š Electrophile:Nucleophile: blue red 2-BromopropaneH 2-PropanolH LeavingElectrophile: group: greenblue (Isopropyl2-Bromopropane bromide) (Isopropyl2-Propanol alcohol) Leaving group: green (Isopropyl bromide) (Isopropyl alcohol) Methyl and Primary Methyl and Primary Solvolysis also takes place in alcohol solvents. Haloalkanes:Solvolysis also takes placeSolvolysisin alcohol also takessolvents place .in alcohol solvents. UnreactiveHaloalkanes: in Solvolysis Unreactive in Solvolysis CH Br Solvolysis of 2-Chloro-2-methylpropane in Methanol 3 Solvolysis of 2-Chloro-2-methylpropane in Methanol CH3CHCH23BrBr Solvolysis—the CH CH Br solventSolvolysis—the is also CH3CH23CH22Br CH3 CH3 the solventnucleophile is also EssentiallyCH no 3reactionCH2CH with2Br H2O A CH3 A CH3 at room temperature the nucleophile Essentially no reaction with H2O CH3CClšš A ϩ CH3šOH CH3CšOCHA 3 ϩ HClšš at room temperature CHA3šCClšš ϩSolventCHš3šOH CHAš3CšOCH3 ϩ šHClšš CHA3š Solventš CHA3š š 2-Chloro-CH3 2-Methoxy-CH3 2-methylpropane2-Chloro- 2-methylpropane2-Methoxy- 2-methylpropane 2-methylpropane 3 Relative Reactiv- The relative rates of reaction of 2-bromopropane and 2-bromo-2-methylpropane with itiesRelative of Various Reactiv- water Theto give relative the correspondingrates of reaction alcohols of 2-bromopropane are shown in Tableand 2-bromo-2-methylpropane 7-1 and are compared with with Table 7-1 ities of Various Table 7-1 Bromoalkanes thewater corresponding to give the rates corresponding of hydrolysis alcohols of their unbranchedare shown in counterparts. Table 7-1 andAlthough are compared the process with withBromoalkanes Water givesthe thecorresponding products expected rates of hydrolysisfrom an S Nof2 theirreaction, unbranched the order counterparts. of reactivity Although is reversed the processfrom with Water gives the products expected from an S 2 reaction, the order of reactivity is reversed from Relative that found under typical SN2 conditions. Thus,N primary halides are very slow in their reac- Bromoalkane rateRelative tionsthat with found water, under secondary typical S halidesN2 conditions. are more Thus, reactive, primary and halides tertiary are halides very areslow about in their 1 mil- reac- Bromoalkane rate liontions times with as water,fast as secondary primary ones. halides are more reactive, and tertiary halides are about 1 mil- CH3Br 1 lionThese times observations as fast as primarysuggest ones.that the mechanism of solvolysis of secondary and, espe- CH3Br 1 CH3CH2Br 1 cially, Thesetertiary observations haloalkanes suggestmust be thatdifferent the mechanismfrom that of of bimolecular solvolysis substitution.of secondary To and, under- espe- in solvolysis CH3CH2Br 1 cially, tertiary haloalkanes must be different from that of bimolecular substitution. To under- (CH3)2CHBr 12 in solvolysis stand the details of this transformation, we shall use the same methods that we used to study (CH3)2CHBr 12 6 Increasing reactivity stand the details of this transformation, we shall use the same methods that we used to study (CH3)3CBr 1.2 3 10 the SN2 process: kinetics, stereochemistry, and the effect of substrate structure and solvent 6 Increasing reactivity (CH3)3CBr 1.2 3 10 onthe reaction SN2 process: rates. kinetics, stereochemistry, and the effect of substrate structure and solvent on reaction rates. H3CCH2Br H3C Br Exercise 7-1 A A A A H3CCH2Br H3C Br Exercise 7-1 A A A A Whereas compound A (shown in the margin) is completely stable in ethanol, B is rapidly converted intoWhereas another compoundcompound. A Explain. (shown in the margin) is completely stable in ethanol, B is rapidly converted A B into another compound. Explain. A B 7-2 UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION 7-2 UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION In this section, we shall learn about a new pathway for nucleophilic substitution. Recall that theIn S thisN2 reaction section, we shall learn about a new pathway for nucleophilic substitution. Recall that the SN2 reaction • Has second-order kinetics • Has second-order kinetics • Generates products stereospeci" cally with inversion of con" guration • Generates products stereospeci" cally with inversion of con" guration • Is fastest with halomethanes and successively slower with primary and secondary halides• Is fastest with halomethanes and successively slower with primary and secondary halides • Takes place only extremely slowly with tertiary substrates, if at all • Takes place only extremely slowly with tertiary substrates, if at all 248 CHAPTER 7 Further Reactions of Haloalkanes 2-Bromopropane is hydrolyzed similarly, albeit much more slowly, whereas 1-bromopropane, bromoethane, and bromomethane are unchanged under these conditions. ReactionR Hydrolysis of a Secondary Haloalkane CH3 CH3 A Relatively slow A CH3CBršš ϩ HOšOH CH3COHš ϩ HBršš Reminder A š š Aš š Nucleophile: red H H Electrophile: blue 2-Bromopropane 2-Propanol Leaving group: green (Isopropyl bromide) (Isopropyl alcohol) Methyl and Primary Solvolysis also takes place in alcohol solvents. Haloalkanes: Unreactive in Solvolysis Solvolysis of 2-Chloro-2-methylpropane
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