Addition of bromine

The electrophilic addition of bromine produces 1,2-dibromide (or vicinal dibromide). The addition is stereo specific because of the formation of an intermediate bromonium ion. This ensures that the bromine atoms add to the opposite sides of the alkene in an anti- addition. 

The bromine molecule becomes polarised as it approaches the alkene. The bromine atom nearest the double bond becomes electrophilic (as the electrons in the Br-Br bond are repelled away from the electron-rich double bond) 

Overall anti- addition as the two bromine atoms end up on opposite sides of the planar alkene 

The anti- addition explains the formation of different diastereo-isomers when using E- or Z-alkenes. 

As BC can attack either carbon atom of the bromonium ion, reaction with Z-but-2-ene produces a 1 1 mixture of enantiomers (only the 2R,3R isomer is shown above). For -but-2-ene, the attack of Be at either carbon atom of the bromonium ion produces the same compound. This compound has a plane of symmetry and hence is an achiral meso compound. 

---

Eosin (Tetrabromofluorescein). Place 16 5 g. of powdered fluorescein and 80 ml. of rectified (or methylated) spirit in a 250 ml. flask. Support a small dropping funnel, containing 36 g. (12 ml.) of bromine, above the flask make sure that the stopcock of the funnel is well lubricated before charging the latter with bromine. Add the bromine diopwise during about 20 minutes. When half the bromine has been introduced, and the fluorescein has been converted into dibromofluor-escein, all the solid material disappears temporarily since the dibromo derivative is soluble in alcohol with further addition of bromine the tetrabromofluorescein (sparingly soluble in alcohol) separates out. Allow the reaction mixture to stand for 2 hours, filter ofiF the eosin at the pump, wash it with alcohol, and dry at 100°. The yield of eosin (orange-coloured powder) is 25 g. 

Apparatus 1-1 flask (see Fig. 1) for the addition of bromine 1-1 two-necked flask, provided with a dropping funnel and a 40-cm Vigreux column, for the dehydrohalo-genation the Vigreux column was connected to a condenser and receiver, cooled at -75°C. 

Apparatus 2-1 flask (Fig. 1) for the addition of bromine and the coupling with propargylmagnesium bromide 6-1 flask (Fig. 4) for the reaction in liquid ammonia. 

FIGURE 6 12 Mechanism of electrophilic addition of bromine to ethylene... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Figures 7 13 and 7 14 depict the stereochemical relationships associated with anti addition of bromine to (E) and (Z) 2 butene respectively The trans alkene (E) 2 butene yields only meso 2 3 dibromobutane but the cis alkene (Z) 2 butene gives a racemic mixture of 2R 3R) and 2S 3S) 2 3 dibromobutane...

Ha.loisoquinolines, The Sandmeyer reaction is commonly used to prepare chloroisoquinolines from the amino compound. The corresponding hydroxy compounds are also used by treatment with chlorides of phosphoms. The addition of bromine to a slurry of isoquinoline hydrochloride in nitrobenzene gives a 70—80% yield of 4-bromoisoquinoline [1532-97-4J. Heating 1-chloroisoquinoline [19493-44-8] with sodium iodide andhydriodic acid gives 1-iodoisoquinoline [19658-77-6] (179). 

Halogen-substituted succinimides are a class of products with important appHcations. /V-Bromosuccinimide [128-08-5] mp 176—177°C, is the most important product ia this group, and is prepared by addition of bromine to a cold aqueous solution of succinimide (110,111) or by reaction of succinimide with NaBr02 iu the presence of HBr (112). It is used as a bromination and oxidation agent ia the synthesis of cortisone and other hormones. By its use it is possible to obtain selective bromine substitution at methylene groups adjacent to double bonds without addition reactions to the double bond (113). 

Addition of bromine to 5-f-butyl-3,6-dimethoxy-4,5-dihydropyridazine produces 5-bromo-4-f-butyl-3-methoxy-4,5-dihydropyridazin-6(l//)-one. 

From acetone in water and concentrated hydrochloric acid by addition of bromine. Hughes, Watson, and Yates, J. Chem. Soc. 1931, 3322. 

Addition of bromine to the dienol acetate (49) gives the 6j5-bromo-A -3-ketone (50). Dehydrobromination of the crude bromo compound in DMF with lithium or calcium carbonate gives the title compound (51). ... 

During the preparation of the dihalo-(usually dibromo) 20-ketopregnanes, other reactive sites must be protected (e.g., addition of bromine to the A -double bond, ketal formation with a 3-ketone). An elegant method which avoids such problems has been devised by the Upjohn group in their studies on the conversion of 11-ketoprogesterone to hydrocortisone. The former is reacted with ethyl oxalate at C-2 and C-21, then addition of three moles of bromine gives a 2,21,21-tribromide. Alkaline rearrangement produces the side chain unsaturated acid, and the bromine at C-2 is subsequently removed with zinc. 

Preparation of 3a-Hydroxy-5) -pregn-17(20)-en-21-oic Acid . A solution of 15 g of 3a-acetoxy-5jS-pregnan-20-one in 290 ml of glacial acetic acid is treated with 13 g of bromine at room temperature. After complete addition of bromine the reaction mixture is heated at 40-50° for 30 min, and the product precipitated with water and filtered. The product is taken up in ethyl acetate (500-600 ml) and the resulting solution washed with dilute aqueous potassium bicarbonate. The solvent is concentrated in vacuo and the product crystallized from acetone to give 16g of dibromide mp, 173-175°. 

Additions of halogen fluorides to the more electrophilic perfluonnated olefins generally require different conditions Reactions of iodine fluoride, generated in situ from iodine and iodine pentafluoride [62 102 103, /05] or iodine, hydrogen fluoride, and parapeiiodic aud [104], with fluormated olefins (equations 8-10) are especially well studied because the perfluoroalkyl iodide products are useful precursors of surfactants and other fluorochemicals Somewhat higher temperatures are required compared with reactions with hydrocarbon olefins Additions of bromine fluoride, from bromine and bromine trifluonde, to perfluonnated olefins are also known [lOti]... 

Reactions of the halogen iluorides with other unsaturated functional groups are generally reported only with highly fluorinated substrates Hexafluorobenzene and denvatives [118] and octaflvoronaphthalene [119] react with mixtures of bromine and bromine trifluoride by 1,4 addition of fluorine followed by addition of bromine fluoride across a reniatning double bond (equation 22)... 

R = 4-C6H5-C6H4 Table 2. Stereochemistry of lonie Addition of Bromine [7] ... 

Generally, the addition of chlonne or bromine to tnfluoroacetonitrile leads to a mixture of partially halogenated imines, amines and azo alkanes [270, 271] In special cases, such as the HgF2-mediated addition of bromine, N,N dihaloper-fluoro-2-alkylamines can be obtained in good yields [272]... 

---