1-Pentene, bromine addition

It has been shown that in the case of bromine addition to 1-pentene in solvents of different polarity, the overall rate constant varies by a factor of 10 ( ) [81]. This dramatic solvent effect has been taken - together with other findings - as strong evidence for the so-called AdnCl-mechanism, which involves considerable charge separation in the activation step. It has also been demonstrated that protic solvents enhance this addition by a specific electrophilic solvation of the anionic part of the activated complex... 

Addition of bromine to the same allene in methanol led to the formation of 83% of the (S)-trans-3-bromo-4-methoxy-2-pentene (85), confirming the anti pathway for the addition of bromine under these conditions. Also formed was 17% of a s-3-bromo-4-methoxy-2-pentene (86), which was again presumed to have the R configuration. As shown in Figme 9.49, the same ratio of diaster-eomers was observed in the oxymercuration reaction. This result is consistent with an anti addition mechanism for bromine addition that is closely analogous to the mechanism of oxymercuration. 

Quaternary ammonium salts of pyrrolines 106) can be prepared only indirectly 197). Addition of bromine to l-dimethylamino-4-pentene followed by removal of hydrogen bromide afforded, depending upon the dehydrohalogenation conditions, quaternary bromides derived from either l,2-dimethyl-/f -pyrroline (107) or l-methyl-2-methylenepyrrolidine (108) (Scheme 7). 

A typical example of a stereospecific olefin addition reaction is the addition of bromine to olefins. If d.v-2-pentene is used as the substrate, only the 2R,3R and 2S,3S pair will be produced (they are enantiomers). 

Because the addition of bromine is stereospecifically trails or anti, one bromine atom adds to each face of the olefin and can go to either carbon. If traw.v-2-pcntcnc is used as the substrate, then only the 2R,3S and 2S,3R pair is produced (they are also enantiomers.). However, the pair from cw-2-pentene is diastereomeric with die pair from fraws-2-pentene. 

Quaternary ammonium salts of such bases could be prepared only indirectly addition of bromine to l-dimethylamino-4-pentene, followed by removal of hydrogen bromide, afforded—according to the dehydrohalogenation conditions—quaternary ammonium salts (53 or 54) derived from 1,2-dimethyl-d2-pyrroline or l-methyl-2-methy lenepyrrolidine.225... 

Fig. Reaction of 3-methyl-2-pentene with bromine and water. Addition of Water...

Problem 7.13 (a) Predict the products of addition of bromine to /raiis-2-pentene. 

When 1,4-pentadiene is treated with bromine under conditions (what are they ) that favor formation of the dihalide, there is obtained the expected product, 4,5-dibromo-l-pentene. Addition of more bromine yields the 1,2,4,5-tetrabromo-... 

A chemist requires a large amount of l-bromo-2-pentene as starting material for a synthesis and decides to carry out an NBS allylic bromination reaction. What is wrong with the following synthesis plan What side products would form in addition to the desired product ... 

Internal alkenes react less readily with RBeH. The reactivity of 1-pentene to EtHBe-n OEtj is >200 times that of 2-pentene the trialkylated ethene, 2-methylbut-2-ene, is inactive to addition of RBeH (R = Et, i-Bu) at 84°C and reacts only slowly with the more active, base-free i-BuBeH at 110°C (see Table 1) . Beryllium migrates to the end of the carbon chain as shown by the bromination of the products from 2-pentene and EtBeH 82% 1-bromopentane, 12% 2-bromopentane and 6% 3-bromopentane. ... 

Electrophilic additions also occur to cumulated dienes (allenes). Caserio and co-workers established that the mechanisms of both the oxy-mercuration reaction and the electrophilic addition of bromine to allenes are similar to the corresponding additions to olefins. The oxymercuration of ( )-(—)-2,3-pentadiene (82) in methanol produced 83% of (S)-traMS-3-acetoxy-mercuri-4-methoxy-2-pentene (83), confirming the anti pathway for the addition. Also formed was 17% of cis-3-acetoxymercuri-4-methoxy-2-pentene (84), which was presumed to have the R configuration. As shown in Figure 9.48, the product ratios can be explained on the basis of preequilibrium formation of mercurinium ions resulting from attachment of mercury to either the top or the bottom of one of the double bonds. Subsequent rate-limiting attack of methanol on the mercurinium ions is easier for the pathway... 

Now, let s draw the forward scheme. In the presence of peroxides, the reaction of 1-pentene with HBr produces 1-bromopentane (via anfe -Markovnikov addition). Subsequent reaction with acetylide [produced from ethylene as shown by bromination (Br2), double elimination and deprotonation (excess NaNH2)] provides 1-heptyne. Reduction to the alkene (H2 / Lindlar s catalyst) followed by anP-Markovnikov addition (HBr / peroxide) yields 1-bromoheptane. This primary alkyl bromide can then undergo an Sn2 reaction when treated with acetylide (prepared above), giving the... 

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