2-Bromobutane chlorination

Bromination of alkanes follows the same mechanism as chlorination. The only difference is the reactivity of the radical i.e., the chlorine radical is much more reactive than the bromine radical. Thus, the chlorine radical is much less selective than the bromine radical, and it is a useful reaction when there is only one kind of hydrogen in the molecule. If a radical substitution reaction yields a product with a chiral centre, the major product is a racemic mixture. For example, radical chlorination of n-butane produces a 71% racemic mixture of 2-chlorobutane, and bromination of n-butane produces a 98% racemic mixture of 2-bromobutane. 

SAMPLE SOLUTION (a) To test for chemical-shift equivalence, replace the protons at C-1, C-2, C-3, and C-4 of 1-bromobutane by some test group such as chlorine. Four constitutional isomers result ... 

Addition of halogens proceeds stepwise, sometimes accompanied by oxidation. Iodine forms 2,3-diiodo-2-butene-l,4-diol and, depending on conditions, bromine gives 2,3-dibromo-2-butene-l, 4-diol, 2,2,3,3-tetra-bromobutane-1, 4-diol, mucobromic acid, or 2-hydroxy-3,3,4,4-tetrabro-motetrahydrofuran. Addition of chlorine is attended by more oxidation, which can be lessened by esterification of the hydroxyl groups. 

Let s now examine chlorination of the chiral starting material (/ )-2-bromobutane at C2 and C3. 

Thus, four isomers are formed by chlorination of (R)-2-bromobutane at C2 and C3. Attack at the stereogenic center (C2) gives a product with one stereogenic center, resulting in a mixture of enantiomers. Attack at C3 forms a new stereogenic center, giving a mixture of diastereomers. 

In an earlier paper Barton and Onyon considered the unimolecular mechanism of dehydrochlorination to be of more universal application than the radical chain mechanism and postulated that a chloro-compound will decompose by a radical chain mechanism only so long as neither the compound itself nor the reaction products will be inhibitors for the chains . On the basis of this postulate the authors correctly predicted the mechanism of decomposition of a number of chlorine compounds. The postulate does not hold well for bromine compounds which show a greater tendency to decompose via radical chain mechanisms. However, from their early studies on 2-bromopropane 2-bromobutane, t-butyl bromide, and bromo-cyclohexane, Maccoll et a/.234,235,397,410,412 concluded that these compounds also decompose unimolecularly via a four-centre transition state similar to that proposed by Barton and Head. 

Noller et a/. 23-725,767,900,952 continued their investigations on the catalytic decomposition of chlorinated and brominated hydrocarbons on Ba, Ca, Mg, A1 oxides and salts studies on these and related surfaces have reported by other workers - - - - - . Decomposition of chlorinated methanes on titanium , on sodium , and on other metal surfaces have been reported by Anderson et al. similar studies on 2-bromobutane with nickel and platinum surfaces have been made by Burwell et Nutt and Carter carried out a molecular beam study of the decomposition of methyl iodide on a hot tungsten filament. 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

When butane reacts with Br2 in the presence of CI2, both brominated and chlorinated products are obtained. Under such conditions, the usual selectivity of bromination is not observed. In other words, the ratio of 2-bromobutane to 1-bromobutane is very similar to the ratio of 2-chlorobutane to 1-chlorobutane. Can you offer an explanation as to why we do not observe the normal selectivity expected for bromination ... 

For example, the radical bromination of butane gives a 98% yield of 2-bromobutane, compared with a 71% yield of 2-chlorobutane obtained when butane is chlorinated (Section 13.4). 

Two stereocenters can give four stereoisomers chlorination of 2-bromobutane at C3... 

Chlorinating racemic 2-bromobutane at C3 is stereochemically equivalent to pairing two different pairs ( racemates") of shoes four combinations. 

The presence of a stereocenter affects the outcome of the reaction chlorination of (S)-2-bromobutane... 

For example, consider the radical chlorination of the S enantiomer of 2-bromobutane. In this case, the chlorine atom has several options for attack the two terminal methyl groups, the single hydrogen at C2, and the two hydrogens on C3. Let us examine each of these reaction paths. 

What about halogenation at C2, the stereocenter The product from chlorination at C2 of (S)-2-bromobutane is 2-bromo-2-chlorobutane. Even though the substitution pattern at the stereocenter has changed, the molecule remains chiral. However, an attempt to measure the [a] value for the product would reveal the absence of optical activity Halogenation... 

The chlorination of (5)-2-bromobutane at C3 does not affect the existing chiral center. However, the formation of a second stereocenter gives rise to diastereomers. Specifically, attachment of chlorine to the left side of C3 in the drawing gives (2S,3S)-2-bromo-3-chlorobutane, whereas attachment to the right side gives its 25,3/ diastereomer. 

What are the conventions of writing chemical equations when racemates are involved Unless specifically indicated by the R/S notation, the sign of optical rotation, or some surrounding text, it is assumed that all ingredients in a reaction are racemic. To avoid the clutter of writing both enantiomers in such cases, only one is shown, the equimolar presence of the other being tacitly assumed. The chlorination of racemic 2-bromobutane at C3 is then written as follows ... 

A reaction that leads to the predominant (or exclusive) formation of one of several possible stereoisomeric products is stereoselective. For example, the chlorination of (5)-2-bromobutane at C3 is stereoselective, as a result of the chirality of the radical intermediate. The corresponding chlorination at C2, however, is not stereoselective The intermediate is achiral and a racemate is formed. 

The photochemical chlorination of (+)-(5)-2-bromobutane with r-butyl hypochlorite at -78°C gave the following, among other products ... 

Free radical chlorination of (5)-2-bromobutane yields a mixture of compounds with chlorine substituted at any of the four carbon atoms. Write the structure of the 2-bromo-l-chlorobutane formed. Determine the configuration (s) of the stereogenic center(s). Is the product optically active ... 

Free radical chlorination of (i)-2-bromobutane gives a mixture of compounds resulting from attack at any of the four nonequivalent carbon-hydrogen bonds. The products of reaction at C-1 and C-4 are both optically active. Explain why. 

Free radical chlorination of (5)-2-bromobutane at the C-2 atom gives an optically inactive product, but reaction at C-3 gives an optically active product. Explain why. 

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