Other reactions at surfaces

Hydrogenation of pyridines and their benzoderivatives can be carried out with various catalysts. Pyridines are readily hydrogenated to piperidines over Raney nickel at 120°C. The optimal pressure for 

Reductions with noble metal catalysts proceed smoothly (at 20°C) when the bases are in the form of hydrochlorides the free bases tend to poison the catalyst. A pyridine ring is reduced more easily than a benzene ring thus, 2-phenylpyridine gives 2-phenylpiperidine (384), quinoline gives 1,2,3,4-tetrahydroquinoline (385) and acridine gives 9,10-dihydroacridine (386). 

Pyridinium and pyrylium cations, pyridones and pyrones are all readily hydrogenated e.g. flavylium ion (220) and coumarin yield (387) and (388), respectively. 

Palladium on charcoal (Pd/C) is commonly used in the catalytic hydrogenation of pyrimidines in acidic media to form 1,2,4,5-tetrahydro derivatives which are stabilized as amidinium salts (62JOC2170, 65JCS1406). Platinum effects hydrogenation of the 5,6-double bond of uracils, for example, in the addition of deuterium to produce [5,6-2H2]5,6-dihydrouracil. The use of rhodium-on-charcoal and Raney nickel also gives good results. The addition of hydrogen to the 5,6-bond of thymidine and other 5-substituted uridines is stereospecific with rhodium-on-alumina as catalyst. 

Transition metals also catalyze isotopic exchange reactions. Platinum is the most active catalyst for most heterocycles. The mechanism may involve metallation, addition, o--addition and ir-complex formation. a-Hydrogen exchange in pyridine is favored over 3- and 7-positions, particularly by a cobalt catalyst whereas platinum is much less selective. In isoquinoline both the 1- and 3-position protons are exchanged at almost the same rates with very little exchange at any other position. In 3-substituted pyridines exchange is preferred at the 6-position, the more so as the size of the 3-substituent increases (73AHC(15)140). 

---

Other reactions at surfaces (heterogeneous catalysis and reduction reactions)... 

Other reactions at surfaces (catalytic hydrogenation and reduction by dissolving metals) 535... 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

The neutral rings react readily with radicals and other electron-deficient species, and a variety of reactions at surfaces are known. 

Several other discussions of the significance of the activation energy in heterogeneous reactions have been given (34, 37,53, 119,134,135). Garn (32) has concluded that values of E measured for solid phase decomposition reactions have very questionable meanings and aspects of his theoretical analyses are equally applicable to other reactions at the surfaces of solids. 

On the other hand, the rate constant of the catalytic reaction at surface kc increases exponentially with increasing temperature as shown in Eq. 8. Thus in Eq. 9, kc < /cd at low temperatures and ku high temperatures. Therefore, rj increases with an increase in temperature at an activation energy Ec at lower temperatures, and gradually approaches an almost constant value at higher temperatures. The value of rx is obtained by measuring CO2 which is one of the final products in the catalytic oxidation of combustible gas containing carbon. 

For the coherent control of reactions at surfaces, the manipulation of adsorbate motion is essential. Cs/Pt(lll) is a suitable system which provides us with a good opportunity to test whether or not we can excite preferentially one of the two modes whose frequencies are very close to each other by using tailored laser pulses. We have demonstrated the mode-selective excitation of coherent surface phonon modes on Cs/Pt(lll) by synthesized femtosecond pulse trains. 

The O2/H2O system is very slow so that the exchange current a I equilibrium is extremely low (10 /10 A cm 2) as a consequence, any other reaction at the electrode will hamper its study and that could be the reaction of impurities or other redox reactions involving the electrode itself. The so-called noble metals are not really inert and do interact with oxygen a platinum surface in contact with an O saturated solution adsorbs oxygeti as an electronically conducting monolayer but can be further oxidized to PIO, PtO . A detailed analysis of these phenomena, which falls outside the scope of the present review, can be found elsewhere [311. A platinum electrode, when a complete electronically conducting monolayer of I l—O is formed at the surface of the metal, behaves as an ideally inert electrode in such conditions, rest potentials dependent on pO2 and pH can be measured during a few hours, close to... 

Sodium cyanide does not dissolve m butyl bromide The two reactants contact each other only at the surface of the solid sodium cyanide and the rate of reaction under these con ditions IS too slow to be of synthetic value Dissolving the sodium cyanide m water is of little help because butyl bromide is not soluble m water and reaction can occur only at the interface between the two phases Adding a small amount of benzyltrimethyl ammonium chlonde however causes pentanemtnle to form rapidly even at room temper ature The quaternary ammonium salt is acting as a catalyst it increases the reaction rate How7... 

Nonfaradaic Currents Faradaic currents result from a redox reaction at the electrode surface. Other currents may also exist in an electrochemical cell that are unrelated to any redox reaction. These currents are called nonfaradaic currents and must be accounted for if the faradaic component of the measured current is to be determined. 

---