Alkenes, addition reactions Hydrogenation

The mechanism for the hydrogen halide-alkene addition reaction is again a two-step process and is Illustrated below using hydrogen bromide and propene. 

In summary, perfluoroalkyl radicals exhibit extraordinary reactivity in both their alkene addition reactions and their hydrogen-abstraction processes, relative to their hydrocarbon counterparts. This reactivity can be attributed partially to the increased exothermicity of such reactions when compared to the analogous reactions of hydrocarbon radicals, and partially also to the fact that perfluoro-n-alkyl radicals are a-radicals. However the major source of the reactivity of 1°, 2°, 3° perfluoroalkyl radicals must be their high electronegativity, which gives rise to stabilizing polarization of the transition states of these radicals addition and hydrogen-abstraction processes. 

Hydrogenation reactions are exothermic because the bonds in the product are stronger than the bonds in the starting materials, making them similar to other alkene addition reactions. The AH° for hydrogenation, called the heat of hydrogenation, can be used as a measure of the relative stability of two different alkenes that are hydrogenated to the same alkane. 

Reactions of alkenes involve the carbon-carbon double bond. The key reaction of the double bond is the addition reaction. This involves the addition of two atoms or groups of atoms to a double bond. The major alkene addition reactions include addition of hydrogen (Hj), halogens (Clj or Brj), water (HOH), or hydrogen halides (HBr or HCl). A generalized addition reaction is ... 

We have already studied one addition reaction of alkenes—hydrogenation—in which a hydrogen atom is added at each end of a double (or triple) bond. In this chapter we shall study other alkene addition reactions that do not involve the same mechanism as hydrogenation. We can depict this type of reaction generally, using E for an electrophilic portion of a reagent and Nu for a nucleophilic portion, as follows. 

Vladimir Markovnikov observed this regioselectivity and made the generalization, known as Markovnikov s rule, that, in the addition of HX to an alkene, hydrogen adds to the doubly bonded carbon that has the greater number of hydrogens already bonded to it. Although Markovnikov s rule provides a way to predict the product of many alkene addition reactions, it does not explain why one product predominates over other possible products. 

Electrophilic aromatic substitution represents the second instance in which we have encountered a C = C double bond attacking an electrophile. The first instance was in our discussion of alkene addition reactions in Section 5.3. Notice the similarities in the first step where a C = C double bond attacks an electrophilic atom (H or E ). In Step 2, however, alkene addition results in the attack of a nucleophile on the carbocation, while EAS results in abstraction of a hydrogen by base. In one reaction, the C = C double bond is destroyed, while in the other, the C = C double bond is regenerated. 

Second, despite the generalization above, specihc conditions required for reduction may well vary from one alkene to the next. It is particularly worthy to note that most arenes (in accord with their extraordinary stability and as thus expected) can only be reduced with difficulty. In contrast to simple alkenes, which can often be reduced at or near 1 atm of hydrogen in the presence of a Pt or Pd catalyst, benzene requires high pressures and temperatures (e.g., powdered Ni, 40 atm H2, 200°C) (Equation 3.3). Further, the reduction of alkynes to (Z)-alkenes (addition of hydrogen [H2] being suprafacial) frequently cannot be stopped at the alkene stage and complete reduction to alkane occurs. Special poisoned catalysts have been developed, which frequently succeed in stopping the reaction. (Equation 3.4). 

Classify each organic reaction as combustion, alkane substitution, alkene addition or hydrogenation, aromatic substitution, or alcohol substitution, elimination, or oxidation. 

In Summary Alkynes are very similar in reactivity to alkenes, except that they have two TT bonds, both of which may be saturated by addition reactions. Hydrogenation of the first TT bond, which gives cis alkenes, is best achieved by using the Lindlar catalyst. Alkynes are converted into trans alkenes by treatment with sodium in liquid ammonia, a process that inclndes two snccessive one-electron reductions. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

The relationship between reactants and products m addition reactions can be illustrated by the hydrogenation of alkenes to yield alkanes Hydrogenation is the addition of H2 to a multiple bond An example is the reaction of hydrogen with ethylene to form ethane... 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

The bond highlighted m yellow is the peptide bond ) Pencyclic reaction (Section 10 12) A reaction that proceeds through a cyclic transition state Period (Section 1 1) A honzontal row of the penodic table Peroxide (Section 6 8) A compound of the type ROOR Peroxide effect (Section 6 8) Reversal of regioselectivity oh served m the addition of hydrogen bromide to alkenes brought about by the presence of peroxides m the reaction mixture... 

The propylene double bond consists of a (7-bond formed by two ovedapping orbitals, and a 7t-bond formed above and below the plane by the side overlap of two p orbitals. The 7t-bond is responsible for many of the reactions that ate characteristic of alkenes. It serves as a source of electrons for electrophilic reactions such as addition reactions. Simple examples are the addition of hydrogen or a halogen, eg, chlorine ... 

Free-Radical-Initiated Synthesis. Free-radical-initiated reactions of hydrogen sulfide to alkenes are commonly utilized to prepare primary thiols. These reactions, where uv light is used to initiate the formation of hydrosulfuryl (HS) radicals, are utilized to prepare thousands of metric tons of thiols per year. The same reaction can be performed using a radical initiator, but is not as readily controlled as the uv-initiated reaction. These types of reactions are considered to be anti-Markownikoff addition reactions. 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

The addition of hydrogen halides to alkenes has been studied from a mechanistic point of view over a period of many years. One of the first aspects of the mechanism to be established was its regioselectivity, that is, the direction of addition. A reaction is described as regioselective if an unsymmetrical alkene gives a predominance of one of the two possible addition products the term regiospecific is used if one product is formed... 

The order of reactivity of the hydrogen halides is HI > HBr > HCl, and reactions of simple alkenes with HCl are quite slow. The studies that have been applied to determining mechanistic details of hydrogen halide addition to alkenes have focused on the kinetics and stereochemistry of the reaction and on the effect of added nucleophiles. The kinetic studies often reveal complex rate expressions which demonstrate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or Itydrogen... 

The anti-Markownikoff addition of hydrogen bromide to alkenes was one of the earliest free-radical reactions to be put on a firm mechanistic basis. In the presence of a suitable initiator, such as a peroxide, a radical-chain mechanism becomes competitive with the ionic mechanism for addition of hydrogen bromide ... 

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