Half-hydrogenated states

Catalysts have a profound effect on the extent of double-bond migration. The influence is a property of the metal itself and its structure and is little altered by the support(7 7,7 ). It is related to the relative tendencies of the half-hydrogenated states to reform an unadsorbed olefin. A decreasing ordering of metals for double-bond migration (46) is Pd > Ni Rh Ru Os > Ir - Pt. 

FIGURE 1.10 Various possible surface species on a Pt or Pd (111) surface. A and B represent possible locations of 1,2-di-a-Cj 2-cyclohexane, and C, D, and E represent possible locations of Jt-complexed Jt-C -cyclohexene. Full complements of hydrogens are assumed at each angle and terminal that is not either a- or Jt-bonded to a surface site as indicated by a small circle. Half-hydrogenated states, which are mono-a-C -adsorbed species (where n is the number of the carbon attached to the surface), would be represented by one small circle at the carbon bonded to a surface site. F, G, and I represent possible locations of Jt-C -cyclohexene. F shows the three carbons of the Jt-allyl moiety adsorbed in three adjacent three-point hollow sites and G shows it over one three-point hollow site, whereas I shows adsorption over the approximate tops of three adjacent atoms. (Note Label H is not used to avoid confusion with hydrogen, which is not shown.)... 

Before we examine the hydrogenation of each type of unsaturation, let us first take a look at the basic mechanism assumed to be operating on metal catalytic surfaces. This mechanism is variously referred to as the classic mechanism, the Horiuti-Polanyi mechanism, or the half-hydrogenated state mechanism. It certainly fits the classic definition, since it was first proposed by Horiuti and Polanyi in 193412 and is still used today. Its important surface species is a half-hydrogenated state. This mechanism was shown in Chapter 1 (Scheme 1.2) as an example of how surface reactions are sometimes written. It is shown in slightly different form in Fig. 2.1. Basically, an unsaturated molecule is pictured as adsorbing with its Tt-bond parallel to the plane of the surface atoms of the catalyst. In the original Horiuti-Polanyi formulation, the 7t-bond ruptures... 

FIGURE 2.1 Classical Horiuti-Polanyi half-hydrogenated state mechanism for hydrogenation, double bond migration, cis-trans isomerization, and deuterium exchange. 

Cis-trans isomerization occurs either by formation of a half-hydrogenated state (Horiuti-Polanyi mechanism) followed by rotation around the newly formed single bond and abstraction of an appropriate hydrogen onto the surface or by double bond migration (either Horiuti-Polanyi or Jt-allyl) from a cis (trans) position to an adjacent trans (cis) position (deuterium exchange studies favor the rotation mechanism). 

The remaining steps consist in the union of the adsorbed olefin and its derivative, the half-hydrogenated state, with an adsorbed hydrogen atom, reactions which occur with retention of configuration of the substituted carbon atom, reactions (3) and (4),... 

The Reduction of the " Half-Hydrogenated State" is Product Controlling... 

The assumption that the rate-limiting surface reaction is the formation of the half-hydrogenated state [reaction (3)] provides the condition that 1 and, consequently, leads to the approximate expres-... 

The fact that compounds with an 8,14 double bond (VIII) cannot be hydrogenated implies that the isomerization cannot proceed via a half-hydrogenated species, an essentially saturated structure. To avoid the excessive compression between the angular methyl groups at C-10 and C-13 which is enforced by the required geometry of the transition to the half-hydrogenated state (IX) the isomerization proceeds via an allylic intermediate (X) which permits the carbon atom at C-8 to retain its hybridization (Fig. 13). 

D. The Geometry of the Transition State for the Formation of THE Half-Hydrogenated State ... 

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. 

Fig. 17. Preferred conformations of the transition state which yields the half-hydrogenated state from a 4-substituted methylcyclohexene and a methylenecyclohexane.

The relation between the constants of the above mechanism and the apparent rate constants measured when 2-butyne or 1,3-butadiene are examined separately depends upon whether the most stable species on the surface is the 7r-complexed butyne (or 1,3-butadiene) or the respective half-hydrogenated states. If the former situation prevails, the apparent rate constants are for 2-butyne and for 1,3-butadiene if the latter, the respective constants are k and A,. 

Therefore, the measurement of the relative reactivities in separate and in competitive experiments will permit the evaluation of either K jK or KiK IK K depending upon whether the principal surface species are the TT-complexed multiply unsaturated hydrocarbons or the respective half-hydrogenated states. If the former situation exists, the evaluated ratios might be expected to correlate with the association constants of the hydrocarbons with silver ion (78), but not if the main surface species are the half-hydrogenated states. Apparently, it is the latter condition which prevails. 

The majority of observations concerning the isomerization of alkenes are in harmony with the Horiuti-Polanyi associative mechanism,170 171 which involves the reversible formation of the 24 half-hydrogenated state (Scheme 4.11) (see in more detail in Section 11.1.2). 

While the last step [Eq. (11.4)] is virtually irreversible under hydrogenation conditions, both the adsorption of alkene [Eq. (11.2)] and the formation of alkyl intermediate (half-hydrogenated state) [Eq. (11.3)] are reversible. The reversibility of these steps accounts for the isomerization of alkenes accompanying hydrogenation (see Section 4.3.2). Isomerizations, either double-bond migration or cis-trans isomerization, may not be observable, unless the isomer is less reactive, or the isomerization results in other structural changes in the molecule, such as racemization. 

According to the Horiuti-Polanyi mechanism, isomerization requires the participation of hydrogen. The first addition step, formation of the half-hydrogenated state [Eq. (11.3)], cannot take place without hydrogen. Numerous investigations have supported the role of hydrogen in these so-called hydroisomerizations. 

In accordance with this observation, the fraction of the cis isomer increases with increasing hydrogen pressure. Since an increase in the hydrogen partial pressure affects step 3 [Eq. (11.3)] in the Horiuti-Polanyi mechanism by shifting the equilibrium to the formation of the half-hydrogenated state, isomerization is suppressed. Palladium, in turn, which exhibits the highest tendency to isomerization among platinum metals, may yield the trans isomer as the major product under certain conditions. 

It has been observed [23,91,92] that when an unsaturated hydrocarbon is reacted with (a) equilibrated and (b) non-equilibrated hydrogen—deuterium mixtures, the deuteroalkane distributions are identical. Such observations indicate that the direct addition of a hydrogen molecule across the olefinic bond does not occur, and provides strong evidence for the formation of a half-hydrogenated state , that is, an adsorbed alkyl radical, first suggested by Horiuti and Polanyi [81], as a relatively stable reaction intermediate. The process of hydrogenation may thus be represented as... 

The processes of olefin exchange, double bond migration and cis—trans isomerisation, observed to occur concomitant with hydrogenation, may be accounted for by considering that the formation of the half-hydrogenated state is reversible. For olefin exchange we can write... 

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