Halogens reaction with alkenes

Alkenes are similar to alkanes in structure and also undergo combustion. However, they are much more likely to undergo addition reactions with halogens, rather than substitution. Addition reactions occur with a wide variety of reagents. 

Alkynes undergo addition reactions with halogens (CI2, Br2) to produce tetrahalo alkanes. To saturate each allqme molecule, two halogen molecules are needed. Alkynes decolorize aqueous Br2 solution, as alkenes do. 

Hydroalumination of alkenes. Hydroalumination of alkynes is a well-known reaction, but hydroalumination of alkenes has been achieved only recently under catalysis by TiCU or ZrCU, (8, 288). As expected hydroalumination affords a convenient, high-yield route to primary alkanes (by hydrolysis), terminal primary alcohols (by oxygenation), and primary alkyl halides (reaction with halogens, N-halosuccinimides, or CuXa). ... 

Write equations for two reactions in which alkenes undergo addition reactions with halogens. Give the lUPAC names for all compounds. 

Addition reactions with halogens, hydrogen halides hydrogenation to yield alkenes and alkanes... 

Some alkenes react with halogens to give substitution rather than addition. For example, with 1,1-diphenylethene, substitution is the main reaction at low bromine concentration. Substitution occurs when loss of a proton is faster than capture by bromide. 

Alkenes can undergo free radical substitution reactions with halogens. Which of the following best represents a chain propagation step during the free radical chlorination of methane ... 

Dihalogenation of alkynes gives a dihalogenated alkene, which is also susceptible to reaction with bromine, chlorine, or iodine. Tetrahalo derivatives are available from dihalogenated alkenes (vinyl dihalides). When 1-pentyne reacts with one molar equivalent of diatomic bromine. 111 is the product. Because alkenes are also subject to reaction with halogens. 111 can react with a second molar equivalent of bromine to give 1,1,2,2-tetrabromopentane, 112. 

When bromine (Br2) or iodine (I2) is added to benzene (CeHg), there is an instantaneous color change, ft has been argued that this is due to the formation of some sort of 71-complex [see Equation 6.93, where for bromine (Br2), E+ = Br" and the counter ion is Brj, and similarly for iodine (I2)]. However, unlike the situation for alkenes and alkynes, addition across the double bond does not occur. Indeed, benzene (CgHg) and iodine (E) or bromine (Br2) can be separated and the starting materials recovered unchanged. However, reaction with halogens does occur in the presence of a catalyst as shown in the third example in Table 6.12. 

As a center of high electron density, the triple bond is readily attacked by electrophiles. This section describes the resnlts of three such processes addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov rule is followed in transformations of terminal alkynes The electrophile adds to the terminal (less snbstituted) carbon atom. 

Additions of nucleophiles N (N=H, OEt, SR, C CH, etc.) to the cationic butyne complex [Fp( -MeC=CMe)]+ afford the alkenyl complexes FpC(Me)= C(N)Me from which the alkenes MeC(X)=C(N)Me are liberated upon reaction with halogens X2 (X = Br and I). Ethylation of the alkenyl complex [CpFe(CO)-(PPhs)CH=CHCOMe] with [EtsO]+ followed by reaction of the cationic allylic product with primary and secondary amines (RNHRO affords iminium... 

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... 

This method is also used with alcohols of the stmcture Cl(CH2) OH (114). HaloaLkyl chlorosulfates are likewise obtained from the reaction of halogenated alkanes with sulfur trioxide or from the chlorination of cycHc sulfites (115,116). Chlorosilanes form chlorosulfate esters when treated with sulfur trioxide or chlorosulfuric acid (117). Another approach to halosulfates is based on the addition of chlorosulfuric or fluorosulfuric acid to alkenes in nonpolar solvents (118). 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

The necessary vicinal dihalides are themselves readily available by addition of Br2 or Cl2 to alkenes. Thus, the overall halogenation/dehvdrohalogenation sequence makes it possible to go from an alkene to an alkyne. for example, diphenylethylene is converted into diphenylacetylene by reaction with Br2 and subsequent base treatment. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

---