2- Butanol reaction with hydrogen bromide

Butanol and 2 butanol are converted to their corresponding bromides on being heated with hydrogen bromide Write a suitable mechanism for each reaction and assign each the appropriate symbol (SnI or Sn2)... 

When a reactant is chiral but optically inactive because it is racemic any products derived from its reactions with optically inactive reagents will be optically inactive For example 2 butanol is chiral and may be converted with hydrogen bromide to 2 bromo butane which is also chiral If racemic 2 butanol is used each enantiomer will react at the same rate with the achiral reagent Whatever happens to (/ ) (—) 2 butanol is mir rored m a corresponding reaction of (5) (+) 2 butanol and a racemic optically inactive product results... 

The mechanism of the reaction of 1-butanol with hydrogen bromide proceeds by displacement of water by bromide ion from the protonated form of the alcohol (the alkyloxonium ion). 

The reaction of 2-butanol with hydrogen bromide involves a carbocation intermediate. 

The slow step, dissociation of the oxonium ion, is unimolecular. The reaction of 2-butanol with hydrogen bromide follows the SN1 mechanism. 

Problem 28.13 Drawing structures like those in Fig. 28.5, show the stereochemical course of reaction of optically active erv//fi r -3-bromo-2-butanol with hydrogen bromide. 

Butanol is a primary alcohol 2-butanol is a secondary alcohol. A carbocation intermediate is possible in the reaction of 2-butanol with hydrogen bromide but not in the corresponding reaction of 1-butanol. 

Hydrogen bromide is eliminated from 10,11-dibromo-l 0,1 l-dihydrodibenz[7>,/]oxepin with potassium tert-butoxide at room temperature to give 10-bromodibenz[i,/]oxepin (17a).160161 When the elimination reaction was performed in boiling toy-butanol the yield increased from 58 to 92%.261 Dehydrohalogenation of 10-chloro-2,3-dimethoxy-10,ll-dihydrodi-benz[/),/]oxepin afforded 2,3-dimethoxydibenz[6,/]oxepin (17b) in 52% yield.162... 

Elimination of hydrogen bromide from 1,2-dibromo-4-butanol, BrCHjCHBrCHjCHjOH, is accomplished with powdered potassium hydroxide in dry ether. The resulting /3-bromotetrahydrofuran loses another molecule of hydrogen halide when heated with excess powdered base. The over-all yield of dihydrofuran is 62%. This elimination reaction has been extended to the preparation of /3-bromofurans and 2,5-dihydrofurans having two alkyl groups on one of the a-carbon atoms. ... 

Preparation (b). The reaction sequence is shown in the formulation. Treatment of paraldehyde and absolute ethanol with hydrogen chloride gas at —5° produces a-chloroethyl ethyl ether (1), which on bromination affords a,j8-dibromoethyl ethyl ether (2). Coupling with allylmagnesium bromide gives (3), which on reaction with zinc dust in n-butanol generates 1,4-pentadiene. Thus one double bond is that of allyl bromide, and the other is generated by elimination of BrOCjH, from the bromohydrin ethyl ether (3). 

These reagents are prepared by the reaction of 4-chloro-l-butanol with paraformaldehyde and hydrogen bromide or hydrogen chloride in about 50% yield. 

Scheme 1034. The synthesis of (l/ )-l-phenylpropylamine hydrochloride. Beginning with tert-butyl disnlfide, oxidation with hydrogen peroxide in the presence of VO(acac>2 and (5 )-2-(A -3,5-di-f-butylsaIicylidene)amino-3,3-dimethyl-l-butanol prodnces the chiral sulfinate (5 )-f-butyl-f-butanethiosulfinate. Then, addition of the sulfinate in THF (oxocyclopentane, THF) to a suspension of lithium amide (L1NH2) in Uquid ammonia (NHs )) generates K)-t-butanesulfinamide. The optically active sulfinamide reacts with propanal to form the corresponding sulfinimine. Reaction of the latter with phenylmagnesium bromide (CtllsMgBr) in ether yields A -(l-phenylpropyl)-f-butanesulhnamide and hydrolysis in methanolic HCI generates the corresponding (IR)-l-phenylpropylamine hydrochloride. The cartoon drawing of the presumed cyclic transition state is used to account for the stereochemical outcome. But see Hose, D. R. I Mahon, M. E Molloy, K. C. Raynham, T Wills, M. /. Chem. Soc. Perkin Trans. 7,1996, 7, 691.

Treatment of 2-propanol with concentrated hydrogen bromide gives 2-bromopropane, as expected. However, exposure of the more highly substituted secondary alcohol 3-methyl-2-butanol to the same reaction conditions produces a surprising result. The expected SnI product, 2-bromo-3-methylbutane, is only a minor component of the reaction mixture the major product is 2-bromo-2-methylbutane. 

For the conversion of (15)-(+)-3-carene approximately 0.045 mg mL ( 1.2 /tm) CPO was incubated in 100 mM citric acid buffer, pH 3.5 with 25 % (v/v) tert-butanol containing 10 him (15)-(+)-3-carene (final assay concentration) and 10 him sodium chloride, sodium bromide or sodium iodide (final assay concentrations) in a 50 mL vessel on a magnetic stirrer (300 rpm) at room temperature. Hydrogen peroxide was added to a total concentration of 10 ruM over a reaction time of 60 min at a rate of 165 /iM min (165 portions every minute). 

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