Ethene bromination

Benzyl trimethyl ammonium chioride Bismuth, subsalicylate trans-1,2-Bis (n-propylsulfonyl) ethene Bromine p-Chioroaniiine 4-Chloronitrobenzene Chlorophene Climbazole Dibutyl adipate 4,6-Dinitro-o-s-butylphenol ammonium salt 1,4-Dioxane Fenarimol Glyceryl-o-monochlorohydrin ... 

BrCHi CHjBr. A colourless liquid with a sweet odour, m.p. 10°C, b.p. 132°C. Manufactured by passing ethene through bromine or bromine and water at about 20 C. Chemical properties similar to those of 1,2-dichloroethane when heated with alkali hydroxides, vinyl bromide is formed. Used extensively in petrols to combine with the lead formed by the decomposition of lead tetraethyl, as a fumigant for stored products and as a nematocide. 

Still another species that adds readily to ethene is elementary bromine (Figure 22.14) ... 

FIGURE 18.11 As a bromine molecule approaches a double bond in an alkene, the atom closer to the ethene molecule acquires a partial positive charge (the blue region). The computation that produced this image was carried out for the point at which the bromine molecule is so close to the double bond that a carbon-bromine bond is starting to form. 

Compare the reaction enthalpies for the halogenation of ethene by chlorine, bromine, and iodine. What trend, if any, exists in these numbers Use bond enthalpies to estimate the enthalpies of reaction. 

Classify each of the following reactions as addition or substitution and write its chemical equation (a) chlorine reacts with methane when exposed to light (b) bromine reacts with ethene in the absence of light. 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

Particularly the chlorinated compounds have enjoyed range of applications vinyl chloride (chloro-ethene) as monomer for the production of PVC, tetra- and trichloroethenes as solvents for degreasing, and the insecticides l,l,l-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and isomers of hexachlorocyclohexane (HCH) (benzene hexachloride). The biodegradation of fluorinated aliphatic compounds is generally different from the outlines that have emerged from investigations on their chlorinated, brominated, and even iodinated analogues. They are therefore treated separately in Part 4 of this chapter. 

It must be emphasised that only a slightly unsymmetrical distribution of electrons is required for a reaction s course to be dominated the presence of a full-blown charge on a reactant certainly helps, but is far from being essential. Indeed the requisite unsymmetrical charge distribution may be induced by the mutual polarisation of reagent and substrate on their close approach, as when bromine adds to ethene (p. 180). 

In the bromonium ion of ethene (I), the bromine atom contributes substantial electron density. 

In a rather remarkable reaction, methylene groups activated by two electron-withdrawing substituents react with non-activated alkenes under soliddiquid phase-transfer conditions in the presence of a molar equivalent of iodine to yield cyclopropane derivatives (Scheme 6.29) [62, 63], The reaction fails, when the catalyst is omitted or if iodine is replaced by bromine or chlorine. The intermediate iodomethylene systems are unstable in the absence of the reactive alkene and dimerize to produce, for example, ethane-1,1,2,2-tetracarboxylie esters and ethene-1,1,2,2-tetracarboxy lie esters. 

The mechanism for the halogen-alkene addition reaction is a two-step process and is illustrated below using bromine and ethene. 

In this first step, the bromine molecule is acting as an electrophile and the ethene is acting as a nucleophile. 

Perfluoroalkyl)ethenes generally react with halogens in the presence of mcrcury(II) salts. Among this type of alkene. 3,3,3-trifluoroprop-l-ene is oxidized to the bromohydrin derivative at room temperature with 62% yield (Table 6).100 Bromohydrin acetates 64 are also formed by the oxidation of the vinyl group in (pcrfluoroalkyl)ethenes with bromine in acetic acid.101... 

We clearly can characterize C2H4 as reactive and C2H6 as unreactive toward bromine. The early organic chemists also used the terms unsaturated and saturated for this behavior, and these terms are still in wide use today. But we need to distinguish between unsaturated and reactive, and between saturated and unreactive, because these pairs of terms are not synonymous. The equations for the reactions of ethene and ethane with... 

Addition of bromine to ethene subsequently was formulated as breaking one of the carbon-carbon bonds of the double bond and attaching bromine to these valences. Substitution was written similarly but here bromine and a C-H bond are involved ... 

Exercise 1-8 There are a large number of known isomers of C5H10, and some of these are typically unsaturated, like ethene, while others are saturated, like ethane. One of the saturated isomers on bromine substitution gives only one compound of formula C5H9Br. Work out a structure for this isomer of C5H10 and its monobromo substitution product. 

Exercise 2-4 How many grams of bromine will react by addition with (a) 20 g of ethene (b) 20 g of ethyne ... 

The substantial difference in the heats of reaction of ethane, ethene, and ethyne with bromine is reflected in a very important practical consideration in handling ethyne (acetylene), namely its thermodynamic stability relative to solid carbon and hydrogen gas. Unlike ethane, both ethene and ethyne can be shown from bond energies to be unstable with respect to formation of solid carbon and gaseous hydrogen ... 

Figure 10-7 Representation of a one-step suprafacial mechanism for addition of bromine to ethene. Gas-phase additions appear to proceed in this manner.

Figure 10-8 Antarafacial addition of bromine to ethene usually observed in solution...

A somewhat oversimplified two-step mechanism that accounts for most of the foregoing facts is illustrated for the addition of bromine to ethene. [In the formulation shown below, the curved arrows are not considered to have real mechanistic significance, but are used primarily to show which atoms can be regarded as nucleophilic (donate electrons) and which as electrophilic (accept electrons). The arrowheads always should be drawn to point to the atoms that are formulated as accepting a pair of electrons.]... 

Reactivity Benzene does not undergo reactions typical of compounds with double bonds. For example, when ethene is bubbled through bromine water (bromine dissolved in water), the solution is decolorized as the Br2 molecules attack the double bond, attach to the molecule, and form dibromoethane (Fig. 2.7). Benzene does not decolorize bromine water. 

The addition reaction between bromine dissolved in an organic solvent, or water, and alkenes is used as a chemical test for the presence of a double bond between two carbon atoms. When a few drops of this bromine solution are shaken with the hydrocarbon, if it is an alkene, such as ethene, a reaction takes place in which bromine joins to the alkene double bond. This results in the bromine solution losing its red/brown colour. If an alkane, such as hexane, is shaken with a bromine solution of this type, no colour change takes place (Figure 14.13). This is because there are no double bonds between the carbon atoms of alkanes. 

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