Alkenes halogen atoms

The efficiency of the halomethane addition process depends on the relative rate of halogen-atom abstraction versus that of addition to the alkene ... 

Another route commonly used in the laboratory to produce alkenes is the dehydrohalogenation of haloalkanes, the removal of a hydrogen atom and a halogen atom from neighboring carbon atoms ... 

The most characteristic chemical reaction of an alkene is an addition reaction, in which atoms supplied by the reactant form o-bonds to the two atoms originally joined by the double bond (Fig. 18.9). In the process, the 7r-bond is lost. An example is halogenation, the addition of two halogen atoms at a double bond, as in the formation of 1,2-dichloroethane ... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

As we have seen already (p. 104) secondary carbocations are more stable than primary, and in so far as this also applies to the transition states that precede them, (24) will be formed in preference to (23). In fact it appears to be formed exclusively, as the only addition product obtained is 2-bromopropane (25). Addition, as here, in which halogen (or the more negative moiety of any other unsymmetrical adduct) becomes attached to the more highly substituted of the two alkene carbon atoms is known as Markownikov addition. 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

In this section, you reviewed how to name and draw alkanes, alkenes, and alkynes. You also learned how to name aromatic hydrocarbons. The names of all the other organic compounds you will encounter in this unit are based on the names of hydrocarbons. In the next section, you will learn about organic compounds that have single bonds to halogen atoms, oxygen atoms, and nitrogen atoms. 

When a haloalkane with p-hydrogen atom Is heated with alcoholic solution of potassium hydroxide, there Is elimination of hydrogen atom from p-carbon and a halogen atom from the a-carbon atom. As a result, an alkene is formed as a product. Since p-hydrogen atom is involved in elimination. It Is often called p-elimination. 

Alcohols may be prepared (1) by hydration of alkenes (1) in presence of an acid and (11) by hydroboratlon-oxidatlon reaction (2) from carbonyl compounds by (1) catalytic reduction and (11) the action of Grignard reagents. Phenols may be prepared by (1) substitution of (1) halogen atom In haloarenes and (11) sulphonic acid group In aiyl sulphonic acids, by -OH group (2) by hydrolysis of diazonium salts and (3) industrially from cumene. 

Halogen atoms. The introduction of side-chains on 9-trifluoromethyl-paullone 409 can be accomplished applying a Stille coupling (Scheme 86, Section 5.2.1.1 (2005EJM655)). Similarly, a Heck reaction of iodo 409 with terminal alkenes under standard conditions affords 2-substituted paullones 413 exclusively as E-isomers. The reaction of terminal alkynes with 409 in the presence of cuprous iodide and a palladium catalyst in triethylamine furnishes the 2-alkynyl-paullones 412 (2000BMCL567). 

Phenyl-substituted alkenes with halogen atoms at the double bond react with xenon difluoride in dichloromethane in the presence of hydrogen fluoride forming vicinal difluorides in 50% yield.35... 

Highly halogenated alkenes, e.g. 6, react with sulfur tetrafluoride at elevated temperatures in the presence of Lewis acid type fluorides to afford halofluoroalkanes, which result from both addition of fluorine to the C = C bond and substitution of halogen atoms.191... 

It is significant that no other hydrogen halides add to alkenes, contrary to the Markovnikov rule, even in the presence of free radicals. This appears to be due to the difficulty with which other halogen atoms, particularly fluorine and chlorine, are produced by reaction of the hydrogen halides with radicals ... 

Cyclic alkenes with alkali alkoxides give almost exclusively products of nucleophilic substitution of one or more vinylic or allylic fluorine or halogen atoms. The preference of the attack is expressed by the sequence fluorine > chlorine > bromine.68... 

---