Lead atoms, reaction

The species Pbl2( r/) in Reaction 6-13 is dissolved Pbl2, containing two iodine atoms bound to a lead atom. Reaction 6-13 is not the reverse of Reaction 6-11, in which the species is solid Pbl2. 

There are a couple of reasons why collision between reactant molecules does not always lead to reaction. For one thing, the molecules have to be properly oriented with respect to one another when they collide. Suppose, for example, that the carbon atom of a CO molecule... 

FIGURE 13.28 Whether a reaction takes place when two species collide in the gas phase depends on their relative orientations. In the reaction between a Cl atom and an HI molecule, for example, only those collisions in which the Cl atom approaches the HI molecule from a direction that lies inside the cone indicated here lead to reaction, even though the energy of collisions in other orientations may exceed the activation energy. 

The reaction between a K atom and a CH I molecule takes place by a different mechanism. A collision leads to reaction only if the two reactants approach each other very closely. In this mechanism, the K atom effectively bumps into a brick wall, and the KI product bounces out in the backward direction. 

Not only must the OH be near the hydrogen, but the hydrogen mu.st be oriented anti to the chlorine atom. When the two reacting molecules collide, if the OH should be near the chlorine atom or near R or R, no reaction can take place. In order for a reaction to occur, the molecules must surrender the freedom they normally have to assume many possible arrangements in space and adopt only that one that leads to reaction. Thus, a considerable loss in entropy is involved, that is, A5, is negative. 

A modification of the rf discharge method, in which the BCI3 source is held at -78°C, yields B4CI4 containing a tetrahedron of linked B atoms. Reaction of B4CI4 with diborane leads to chlorohexaborane and chlorodecaborane clusters". 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

The ions having five tin or lead atoms are prepared by the reaction of a solution containing sodium and the cryptand reacting with alloys of sodium and tin or lead, respectively. It should also be mentioned that numerous derivatives of these materials have been prepared that contain alkyl and other groups. 

Bis(l-methylboratabenzene)lead 42 is synthesized in 63% yield by the reaction of LifCsHsBMe) with lead(n) chloride. The X-ray structure analysis of 42 reveals some remarkably short intermolecular contacts from the lead atoms to the ortho-carbon atoms of neighboring molecules which create a tetrameric packing unit. Compound 42 forms weak adducts such as 43 with nitrogen Lewis bases like TMEDA or 4,4 -dimethyl-2,2 -bipyridine.71... 

The analogous reactions of PbCl2 with lithium l-(dimethylamino)-3,5-dimethylboratabenzene or l-[bis(trimethyl-silyl)amino]-3,5-dimethylboratabenzene yield the corresponding sandwich compounds 44 and 45. The lead atom of 45 displays a rather asymmetric bonding to the boratabenzene rings as evidenced by the fact that the lead-to-carbon/ boron distances span the large range from 2.519 to 3.033 A.7... 

The reaction of the diaminocarbene 49 with the diaminoplumbylene 50 furnishes dark red crystals of the zwitterionic adduct 51 in 71% yield. Compound 51 reveals a very long Pb-C bond length of 258.6(7) pm. While the C atom of the Pb-C bond is in an almost trigonal-planar environment (sum of angles 350.3°), the lead atom has a pyramidal environment with a sum of angles of 274.5°. This can be attributed to the presence of a stereochemically active lone pair at the lead atom (Equation (18)).74... 

Figure 11. Mechanism for F-atom reaction with a silicon film leading to the products Sip2 and Sip4.

The observation of the induced emission, its time behavior and threshold conditions allow to study with new techniques details of chemical reactions which lead to specific states of the molecular or atomic reaction product A quantitative study of such laser systems will also yield information about collisional deactivation rates of the excited states (see also Section 111.4). 

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