Alkene reaction with hydrogen

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

Monohydride (MH) catalysts, such as [RhH(CO)(PPh3)3], react with substrates such as alkenes, according to Scheme 1.1, yielding rhodium-alkyl intermediates which, by subsequent reaction with hydrogen, regenerate the initial monohydride catalyst. This mechanism is usually adopted by hydrogenation catalysts which contain an M-H bond. 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

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 ... 

The rate of hydroformylation was proportional to the concentration of the acyl complex. The apparent activation parameters were Ai-T = 49.3 kj mol" and AS = 121 J moT K". Both the activation parameters and the reaction order are consistent with the hydrogenolysis reaction being rate determining. The low order of 0.1 in alkene suggests that the rate-determining step is not purely the reaction with hydrogen and that either a pre-equilibrium also contributes or one of the earlier steps in the cycle is also somewhat slower. 

The 5-phenyldibenzophosphole (25) complex [RhH(DBP)4] catalyzes the hydrogenation of terminal alkenes, 1,5-hexadiene and some substituted alkenes. With 1-hexene it was far more active than [RhCl(PPh3)3] or [RhH(PPh3)4]. The mechanism is again thought to involve dissociation of DBP followed by alkene insertion into the rhodium hydride bond and subsequent reaction with hydrogen.123... 

A-8. What two alkenes give 2-chloro-2-methylbutane on reaction with hydrogen chloride ... 

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 can be converted to alkanes by their reaction with hydrogen over a finely divided metal catalyst such as palladium, nickel, or platinum (Following fig.). This is an addition reaction, as it involves the addition of hydrogen atoms to each end of the double bond. It is also called a catalytic hydrogenation or a reduction reaction. 

There are two possible ways to approach the M(H)n(alkene) intermediate in a hydrogenation reaction the hydride route <2), implies an initial reaction with hydrogen followed by coordination of the substrate, while the unsaturated... 

The predominant chain termination mechanisms are (i) /3-hydride elimination see Polymerization), giving a chain with a terminal alkene (ii) reaction with hydrogen, frequently used to control chain length (iii) chain transfer to monomer by <7-bond metathesis see a-Bond Metathesis)-, and (iv) chain transfer to cocatalyst (equation 4). 

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