Lewis oxygen

Instead of direct proton transfer to a basic Lewis oxygen atom, coadsorbed H2O will more readily accept the proton and then transfer one of its other hydrogen atoms as a proton to the accepting oxygen atom. This two-step path can also be carried out simultaneously, thus leading to more direct hydrogen shuttling path. 

Figure B3.3.14. Template molecule in a zeolite cage. The CFIA stmcture (periodic in the calculation but only a fragment shown here) is drawn by omitting the oxygens which are positioned approximately halfway along the lines shown coimecting the tetrahedral silicon atoms. The molecule shown is 4-piperidinopiperidine, which was generated from the dicyclohexane motif suggested by computer. Thanks are due to D W Lewis and C R A Catlow for this figure. For fiirther details see [225].

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

The most effective Lewis-acid catalysts for the Diels-Alder reaction are hard cations. Not surprisingly, they coordinate to hard nuclei on the reacting system, typically oxygen atoms. Consequently, hard solvents are likely to affect these interactions significantly. Table 1.4 shows a selection of some solvents ranked according to their softness. Note that water is one of the hardest... 

A regioselective aldol condensation described by Biichi succeeds for sterical reasons (G. Biichi, 1968). If one treats the diaidehyde given below with acid, both possible enols are probably formed in a reversible reaaion. Only compound A, however, is found as a product, since in B the interaction between the enol and ester groups which are in the same plane hinders the cyclization. BOchi used acid catalysis instead of the usual base catalysis. This is often advisable, when sterical hindrance may be important. It works, because the addition of a proton or a Lewis acid to a carbonyl oxygen acidifies the neighbouring CH-bonds. 

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

Three equally stable Lewis structures are possible for nitrate ion The negative charge in nitrate is shared equally by all three oxygens... 

The most stable Lewis structure for cyanate ion is F because the negative charge is on its oxygen... 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

What IS the formal charge of oxygen in each of the following Lewis structures ... 

Ozone (O3) IS the triatomic form of oxygen It is a neutral but polar molecule that can be represented as a hybrid of its two most stable Lewis structures... 

Their polar carbon-oxygen bonds and the presence of unshared electron pairs at oxygen contribute to the ability of ethers to form Lewis acid Lewis base complexes with metal ions... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

The resonance effect of the carbonyl group Electron delocalization expressed by resonance between the following Lewis structures causes the negative charge in acetate to be shared equally by both oxygens Electron delocalization of this type IS not available to ethoxide ion... 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Stereoselective All lations. Ben2ene is stereoselectively alkylated with chiral 4-valerolactone in the presence of aluminum chloride with 50% net inversion of configuration (32). The stereoselectivity is explained by the coordination of the Lewis acid with the carbonyl oxygen of the lactone, resulting in the typ displacement at the C—O bond. Partial racemi2ation of the substrate (incomplete inversion of configuration) results by internal... 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

Methylarsine, trifluoromethylarsine, and bis(trifluoromethyl)arsine [371-74-4] C2HAsF, are gases at room temperature all other primary and secondary arsines are liquids or solids. These compounds are extremely sensitive to oxygen, and ia some cases are spontaneously inflammable ia air (45). They readily undergo addition reactions with alkenes (51), alkynes (52), aldehydes (qv) (53), ketones (qv) (54), isocyanates (55), and a2o compounds (56). They also react with diborane (43) and a variety of other Lewis acids. Alkyl haUdes react with primary and secondary arsiaes to yield quaternary arsenic compounds (57). 

Boric oxide is an excellent Lewis acid. It coordinates even weak bases to form four-coordinate borate species. Reaction with sulfuric acid produces H[B(HSO 4] (18). At high (>1000° C) temperatures molten boric oxide dissolves most metal oxides and is thus very corrosive to metals in the presence of oxygen. 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

Typical Lewis acids like BF3 and SbCls coordinate with oxirane oxygen to give (presumably) a cyclic oxonium ion (41) which reacts further (Scheme 28) (64HC 19-1)446, B-67MI50505). 

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