Hydrogen atom deficient molecules

The metal halide thus functions in similar manner to the proton and may be considered to be an acidic catalyst (cf. Luder and Zuffanti, 19). The catalyst-olefin complex differs in one significant respect from the product formed by the addition of the proton (or the corresponding acid) to the olefin the halide catalyst is a neutral but electronically deficient molecule and combines with the pi electrons of the double bond to form a coordinate bond between the carbon atom and the aluminum or boron. On the other hand, the addition of the positive proton to the double bond results in the formation of a true (covalent) link between carbon and hydrogen. In other words, the complex, while it contains an electron-deficient (hence, positive) carbon atom, is in itself electronically neutral the product of the addition of a proton to the alkene contains a similar carbon atom but is itself electrically positive. It has been suggested (Whitmore and Meunier, 20) that this difference is related to the fact that metal halide catalysts tend to yield much higher polymers than do the acid (proton) catalysts. 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

An alternative possible route for the formation of carbamoyl radicals may involve a hydrogen atom abstraction from formamide by the excited acetone molecule in its triplet, state (45, 57). Ketonic compounds at this state of excitation mainly if the excitation is of an n - -7r transition, are known to be hydrogen atom abstraction agents, due to the electron deficient oxygen in the excited state. Thus, the formation of carbamoyl radicals through the second route may be summarized by the following scheme ... 

The oxyanion produced in the first step can help stabilize the electron-deficient BH3 molecule by adding to its empty p orbital. Now we have a tetravalent boron anion again, which could transfer a second hydrogen atom (with its pair of electrons) to another molecule of aldehyde. 

A number of options are provided for the user to adjust the constitutional formulas being output to his special desires. Atom numbers or atomic symbols, or both, can be put onto the nodes of the constitutional graph. Further, the entire constitutional formula with all hydrogen atoms, or only the skeleton of the molecule, can be drawn. In a few cases of molecules with complex polycyclic ring structures only partially complete formulas are output. Then, messages pointing to the deficiences are given which usually allow the completion of the constitutional formulas by hand. 

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