Bromination of butane

When a bromine radical is the hydrogen-abstracting agent, the differences in reactivity are so great that the reactivity factor is vastly more important than the probability factor. For example, radical bromination of butane gives a 98% yield of... 

Consider the bromination of butane using sufficient bromine to cause dibromination. After the reaction is over, you separate all the dibromobutane isomers by gas chromatography or by fractional distillation. How many fractions would you obtain, and what compounds would the individual fractions contain Which if any of the fractions would be optically active ... 

When radical halogenation produces a chiral center or takes place at a hydrogen on an existing chiral center, the product is an equal mixture of R and S enantiomers. Consider, for example, radical bromination of butane, which produces 2-bromobutane. 

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). 

Contrary to currently held views, the variation in enol content as a function of structure in )9-diketones is due to variation in energy of the ketones, and not of the enols this situation is reversed, however, in )S-keto-esters. In the base-induced bromination of butan-2-one in aqueous solution, each hydrogen on the 1-carbon and 2-carbon is attacked equally fast, the resulting enolates rapidly giving bromoform and propionate and lactate salts, respectively (Scheme 92). 

How can a molecule exist as two nonsuperimposable mirror images Consider the radical bromination of butane. This reaction proceeds mainly at one of the secondary carbons to furnish 2-bromobutane. A molecular model of the starting material seems to show that either of the two hydrogens on that carbon may be replaced to give only one form of 2-bromobutane (Figure 5-3). Is this really true, however ... 

The radical mechanism explains why the bromination of butane results in a racemate... 

The radical bromination of butane at C2 creates a chiral molecule (Figure 5-3). This happens because one of the methylene hydrogens is replaced by a new group, furnishing a stereocenter—a carbon atom with four different substituents. 

Chlorination can occur from either side through enantiomeric transition states of equal energy, as in the bromination of butane (Figure 5-13), producing (5)- and (/ )-2-bromo-2-chlorobutane at equal rates and in equal amounts. The reaction is an example of a transformation in which an optically active compound leads to an optically inactive product (a racemate). 

Chlorination or bromination of methane, ethylene, etc Maleic anhydride (from butane)... 

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. 

Likewise, a study on the bromination of these compounds also indicated that the 1,3-addition was 100% syn stercospecific.10 Interestingly, 3-phenylbicyclo[1.1.0]butane-l-carbonitrile, from which a relatively stable benzylic cation can be formed, yielded a mixture of cis- and /ram-products.10 An electron-transfer mechanism has been proposed for these reactions.10 A recent investigation on perchloric acid catalyzed methanol addition to 3-methylbicyclo[1.1.0]butane-1-carbonitrile and methyl 3-mcthylbicyclo[1.1.0]butane-l-carboxylate, however, showed that mixtures of irons- and cA-cyclobutanes were generated, with the m-isomers predominating.11... 

When the sodium derivative of butane tetra-carboxylic ester is acted upon by bromine, the resulting product is tetramethylene-1 2-tetracarboxylic ester ... 

Most butynediol produced is consumed in the manufacture of butane-diol and butenediol. Butynediol is also used for conversion to ethers with ethylene oxide and in the manufacture of brominated derivatives that are useful as flame retardants. Butynediol was formerly used in a wild oat herbicide, Carbyne (Barban), 4-chloro-2-butynyl-A-(3-chlorophenyl)carba-mate (CnH9Cl2N02). 

Bromide ions have been demonstrated to be effective promoters of alkylaro-matic hydrocarbon oxidations with cobalt and/or manganese catalysts [101]. They have also been claimed to be beneficial in the oxidation of butane or cyclohexane [102, 103]. The effective mechanism is reported to be the generation of a bromine atom chain transfer agent [104] ... 

Brominations of polybutadienes with N-bromosuccinimide yield a-brominated poly butadienes [46,47] that may also contain butane diylidene units. The products act as typical alkyl halides and can undergo Grignard-Wurz reactions ... 

In the presence of catalysts, trichloroethylene is readily chlorinated to pentachloro- and hexachloroethane. Bromination yields l,2-dibromo-l,l,2-trichloroethane [13749-38-7]. The analogous iodine derivative has not been reported. Fluorination with hydrogen fluoride in the presence of antimony trifluoride produces 2-chloro-l,l,l-trifluoroethane [75-88-7] (8). Elemental fluorine gives a mixture of chlorofluoro derivatives of ethane, ethylene, and butane. 

Four-membered rings also exhibit angle strain, but much less, and are less easily opened. Cyclobutane is riiore resistant than cyclopropane to bromination, and though it can be hydrogenated to butane, more strenuous conditions are required. Nevertheless, pyrolysis at 420°C gives two molecules of ethylene. As mentioned earlier (page 177), cyclobutane is not planar. 

A base-induced bromination has been reported. 2-Methyl butane reacts with 50% aqueous NaOH and CBr4, in a phase-transfer catalyst, to give a modest yields of 2-bromo-2-methylbutane. ... 

Mercuric oxide, use in oxidation of hydrazones, 50, 28 with 3-chlorocyclobutanecar-boxylic acid and bromine to give l-bromo-3-chlorocyclo-butane, 51, 106 MERCURIC OXIDE-MODIFIED HUNS-DIECKER REACTION 1-BRQMO-... 

Dibromobutane (from butane-1,4-diol). Use 45 g (0.5 mol) of redistilled butane-1,4-diol, 6.84g (0.22mol) of purified red phosphorus and 80g (26 ml, 0.5 mol) of bromine. Heat the glycol-phosphorus mixture to 100-150 °C and add the bromine slowly continue heating at 100-150 °C for 1 hour after all the bromine has been introduced. Allow to cool, dilute with water, add 100 ml of ether and remove the excess of red phosphorus by filtration. Separate the ethereal solution of the dibromide, wash it successively with 10 per cent sodium thiosulphate solution and water, then dry over the anhydrous potassium carbonate. Remove the ether on a water bath and distil the residue under diminished pressure. Collect the 1,4-dibromobutane at 83-84 °C/ 12mmHg the yield is 73 g (67%). 

Peterson reactions. Reaction of Li in THF with 1 leads to the 1,4-dianion (2) of l,l,4,4-tetrakis(trimethylsilyl)butane in 96% yield. This dianion reacts with paraformaldehyde to form 1,5-diene 3, which can be converted to 4 by bromination-bromodesilylation. 

The recombination of bromine atoms can not occur except on collision with a third body but reaction between the more complicated ethyl radicals might be expected without the aid of walls or triple collisions. In any reaction involving the production of free ethyl radicals one might expect to find butane, ethane, etc., but actually very little of these products is found. Apparently in this case radicals of this type are not likely to combine with each other. 

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