The Addition of Hydrogen to an Alkene

Hydrogen (H2) adds to the double bond of an alkene, in the presence of a metal catalyst, to form an alkane. The most common metal catalyst is palladium, which is used as a powder adsorbed on charcoal to maximize its surface area it is referred to as palladium on carbon and is abbreviated as Pd/C. The metal catalyst is required to weaken the very strong H—H bond (Table 5.1). 

The addition of hydrogen to a compound is a reduction reaction. A reduction reaction increases the number of C—H bonds and/or decreases the number of C—O, C—N, or C — X bonds (where X is a halogen). 

The addition of hydrogen is called hydrogenation. Because hydrogenation reactions require a catalyst, they are called catalytic hydrogenations. 

We can think of catalytic hydrogenation as occurring in the following way both the H—H bond of H2 and the ir bond of the alkene break, and then the resulting hydrogen radicals add to the resulting carbon radicals. 

The relative energies (stabilities) of three alkenes that can be catalytically hydrogenated to 2-methylbutane, The most stable alkene has the smallest heat of hydrogenation. 

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The addition of hydrogen to an alkene is called hydrogenation and the product is an alkane. Hydrogenation is carried out in the presence of a catalyst (nickel, palladium or other platinum metals) and at a high pressure of hydrogen ... 

Although the addition of hydrogen to an alkene is exothermic, reduction is immeasurably slow in the absence of a catalyst. Commonly used transition metal catalysts include platinum, palladium, ruthenium, and nickel. Because the conversion of an alkene to an alkane involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or, alternatively, catalytic hydrogenation. 

The addition of hydrogen to an alkene involving a transition metal catalyst, (a) Hydrogen and the alkene are adsorbed on the metal surface, and (b) one hydrogen atom is transferred to the alkene forming a new C—H bond. The other carbon remains adsorbed on the metal surface, (c) A second C—H bond is formed, and the alkane is desorbed. 

The addition of hydrogen to an alkene (hydrogenation, Sections 4.16A and 7.13) is an exothermic reaction the enthalpy change involved is called the heat of reaction or, in this specific case, the heat of hydrogenation. 

The type of reaction that takes place in these examples is an addition reaction. The product that results from the addition of hydrogen to an alkene is an alkane. Alkanes have only... 

The uncatalyzed addition of hydrogen to an alkene although exothermic is very slow The rate of hydrogenation increases dramatically however m the presence of cer tain finely divided metal catalysts Platinum is the hydrogenation catalyst most often used although palladium nickel and rhodium are also effective Metal catalyzed addi tion of hydrogen is normally rapid at room temperature and the alkane is produced m high yield usually as the only product... 

Markovnikov s rule (Section 6.8) A guide for determining the regiochemistry (orientation) of electrophilic addition reactions. In the addition of HX to an alkene, the hydrogen atom bonds to the alkene carbon thal has fewer alkyl substituents. 

If an unsymmetrical alkene combines with a hydrogen halide, the halide ion adds to the carbon atom with fewer hydrogen atoms (The addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double that already has the greater number of hydrogen atoms). 

This addition of hydrogen to an alkene, often called hydrogenation, is used commercially to convert unsaturated vegetable oils to the saturated fats used in margarine and cooking fats. 

This trend is now called the Markovnikov s Rule and it states that in the addition of HX to an alkene, hydrogen adds to the carbon with the most hydrogen atoms. This rule is a very useful aid for predicting experimental results. 

Electrophilic addition to alkenes, such as the addition of hydrogen bromide under ionic conditions, follows the Markownikoff rule which states that in the addition of HX to an alkene, the hydrogen atom (the electrophile) becomes attached to the less-substituted carbon atom. The addition of halogens such as bromine proceeds via a halonium ion, and takes place with an overall trans stereochemistry. 

Q4. In the presence of a catalyst such as Pt, addition of hydrogen to an alkene brings about reduction to an alkane. Hydrogen is adsorbed on to the surface of the catalyst, and is then added syn to the alkene in a reaction that proceeds via a four-centre transition slate 68 (cf. hydroboration, Section 4.8.2). What are the products of reduction of compounds 69 and 70 with H-,/Pt and are the products racemic or meso1 Also, what is the product from reduction of I with D-j/Pt, and is this compound racemic or mesa What is the configuration of C(2) in the product ... 

An atom or a molecule does not have to be positively charged to be an electrophile. Borane (BH3), a neutral molecule, is an electrophile because boron has only six shared electrons in its valence shell. Boron, therefore, readily accepts a pair of electrons in order to complete its octet. Thus, alkenes undergo electrophilic addition reactions with borane serving as the electrophile. When the addition reaction is over, an aqueous solution of sodium hydroxide and hydrogen peroxide is added to the reaction mixture, and the resulting product is an alcohol. The addition of borane to an alkene, followed by reaction with hydroxide ion and hydrogen peroxide, is called hydroboration-oxidation. The overall reaction was first reported by H. C. Brown in 1959. 

In a catalytic hydrogenation, the alkene sits on the surface of the metal catalyst and both hydrogen atoms add to the same side of the double bond (Section 4.11). Therefore, the addition of H2 to an alkene is a syn addition reaction. 

If addition of hydrogen to an alkene forms a product with two asymmetric carbons, only two of the four possible stereoisomers are obtained because only syn addition can occur. (The other two stereoisomers would have to come from anti addition.) One stereoisomer results from addition of both hydrogens from above the plane of the double bond, and the other stereoisomer results from addition of both hydrogens from below the plane. The particular pair of stereoisomers that is formed depends on whether the reactant is a c/5-alkene or a fran -alkene. Syn addition of H2 to a cis-alkene forms only the erythro enantiomers. (In Section 5.8, we saw that the erythro enantiomers are the ones with identical groups on the same side of the carbon chain in the eclisped conformers.)... 

The mechanism of the reaction is the same as that for the addition of HBr to an alkene in the presence of a peroxide. That is, the peroxide is a radical initiator and creates a bromine radical (an electrophile). If the alkyne is a terminal alkyne, the bromine radical adds to the sp carbon bonded to the hydrogen if it is an internal alkyne, the bromide radical can add with equal ease to either of the sp carbons. The resulting vinylic radical abstracts a hydrogen atom from HBr and regenerates the bromine radical. Any two radicals can combine in a termination step. 

This section is concerned with reactions in which the addition of hydrogen cyanide (HCN) to multiple bonds results in the formation of a nitrile, either directly or on subsequent reaction or rearrangement. While the addition of HCN to an alkene or alkyne remains an important industrial process, there have been few innovations of value to laboratory scale nitrile nthesis. 

Chemists account for the addition of HX to an alkene by a twonstep mechanism, which we illustrate by the reaction of 2-butene with hydrogen chloride to give 2step mechanism in general and then go back and study each step in detail. 

Markovnikov s rule In the addition of H—to an alkene, the hydrogen becomes attached to the carbon atom that is already bonded to more hydrogens. 

As pointed out in Chapter 3 the exact details of reduction of alkenes in the presence of catalysts are unclear. Further, just as oxidation can be considered as addition of oxygen to the double bond so reduction can be thought of as addition of hydrogen to an alkene. Overall, of course, the process is one that converts an unsaturated to a saturated system (with one equivalent of H2 being consumed for every site of unsaturation—double bond or ring). 

The uncatalyzed addition of hydrogen to an alkene, although exothermic, is very slow, but its rate is increased in the presence of certain finely divided metal catalysts, such as platinum, palladium, nickel, and rhodium. The reaction is normally rapid at room temperature, and the alkane is the only product. 

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