Alkenes with hydrogen halides

Alkenes undergo electrophilic addition reactions with hydrogen halides, to form alkyl halides. 

In the process, the hydrogen halide attacks the double bond in the alkene, and the pi electrons in the double bond are transferred to the electrophile, resulting in a carbocation intermediate. This is followed by the formation of the alkyl halide. 

An example of a Markovnikov addition is shown below. Wateh where the hydrogen and the bromine are added. 

H goes to the double-bonded carbon that has the greatest number of hydrogens 

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Increasing rate of reaction of alkene with hydrogen halides H2C=CH2 H2C=CHCH3 H2C=C(CH3)2... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

The third-order process presumably involves reaction of a complex formed between the alkene and hydrogen halide with the second hydrogen halide molecule, since there is little likelihood of productive termolecular collisions. 

Cleavage of epoxides with hydrogen halides Addition of hypohalous acids to alkenes... 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

An incorrect statement to the effect that there is no evidence for the complexing of double bonds with hydrogen halides must be corrected. This writer himself has quoted several times the demonstration by O. Maas and his collaborators that the HX (X = Cl, Br) do form complexes with alkenes [A, B],... 

Alkenes undergo addition reactions with hydrogen to form alkanes, with halogens to form dihaloalkanes, with hydrogen halides to form monohaloalkanes and with water to form alcohols. For example ... 

When an alkene is used in the alkylation of arenes, metal halides with hydrogen halide or water as cocatalyst, protic acids, and acidic oxides can be used as catalysts. Both linear and cyclic alkenes are used in alkylations. Alkylation with alkenes is usually preferred in industry because the processes are simpler and olefins are readily and cheaply available in pure form from petroleum refining processes. 

Alkynes. Because of their less nucleophilic character, alkynes react less readily with hydrogen halides than do alkenes and often require the use a metal halide catalyst. Vinyl halides are formed in the reaction with one equivalent of HHlg. They may react further in an excess of the reagent to yield geminal dihalides. High yields of these compounds can be achieved. The addition of HC1 to acetylene was studied in detail because of the practical importance of the product vinyl chloride (see Section 6.2.4). 

Alcohols and phenols are also weak bases. They can be protonated on the oxygen by strong acids. This reaction is the first step in the acid-catalyzed dehydration of alcohols to alkenes and in the conversion of alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols by reaction with thionyl chloride or phosphorus halides. 

Alkenes react with hydrogen halides (HC1, HBr, and HI) to produce an alkyl halide. The hydrogen halide molecule gets split... 

The course of reaction of alkenes with hydrogen fluoride occasionally differs from that with other hydrogen halides because of the strong bond between hydrogen and fluoride. The reaction products are monofluorides with Markovnikov orientation or polymers. ... 

In view of the very low reactivity of simple alkenes towards hydra-zoic acid, 2ind since these electron-withdrawing groups render the homopolar double bond less basic, it is unlikely that the reactions are Adfi in type. By comparison with hydrogen halide additions to such... 

Among the cases in which this type of kinetics has been observed are the addition of HCl to 2-methyl-1-butene, 2-methyl-2-butene, 1-methylcyclopentene," and cyclo-hexene. The addition of HBr to cyclopentene also follows a third-order rate expression. The TS associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second, and is an example of general mechanism D (Ad S). Reaction occurs through a complex formed by the alkene and hydrogen halide with the second hydrogen halide molecule. 

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