Rings of hydrogen atoms

In this chapter we examine some results for four model systems consisting of rings of H atoms. These calculations show how the number of atoms in a complex reaction may influence rates of reaction, particularly through the activation energy. The systems are as follows. 

Since the geometries of these systems are in most regions not regular polygons, we will symbolize them as (H2) , emphasizing the number of H2 molecules rather than the total number of atoms. 

For any of these, iiRA = 0.7 A and RB is quite large, the rings represent 2-5 normal H2 molecules well separated from one another. If the roles of RA and RB are reversed, the H2 molecules have executed a metathesis in whieh the molecules transform into an equivalent set. 

These are, without doubt, somewhat artifieial systems. For real systems, one could not tell if anything happened, unless isotopie labeling could be arranged. An even greater problem would oeeur in the gas phase, sinee the entropy penalty required for these peeuliar geometries would be expected to make them very improbable. 

Nevertheless, the results have eonsiderable interest, bearing, as they do, on the same sort of eonsiderations as the Woodward-Hoffinan rules [5 8], 

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As measured on Corey-Pauling-Koltun molecular models the smaller value is for the ring of hydrogen atoms bonded to C-5, and the larger value is for the ring of hydrogen atoms bonded to C-3. The depth of the cyclodextrin cavity is 790-800 pm. 

Functional Group — an atom or group of atoms, other than hydrogen, bonded to the chain or ring of carbon atoms (e.g., the -OH group of alcohols, the -COOH... 

The framework for the tropylium cation, C7H7f, is a seven-membered ring of carbon atoms with a hydrogen atom... 

A line structure (introduced in Section C) represents a chain of carbon atoms as a zigzag line. The end of each short line in the zigzag represents a carbon atom. Because carbon nearly always has a valence of 4 in organic compounds, we do not need to show the C—H bonds. We just fill in the correct number of hydrogen atoms mentally, as we see for methylbutane (6), isoprene (7), and propyne (8). As explained in Section 2.7, a benzene ring is represented by a circle inside a hexagon, and we need to remember that one hydrogen atom is attached to each carbon atom. 

Fig. 18. The hydrogen-bonded sheet structure of l,l,5,5-tetrakis(hydroxydimethylsi-loxy)-3,3,7,7-tetraphenylcyclotetrasiloxane. Note the crown-shaped eight-membered rings of oxygen atoms at the junctions where four of the molecules meet. All hydrogen and carbon atoms have been omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

The aromatic hydrocarbons are characterized by molecules containing six-membered rings of carbon atoms with each carbon atom attached to a maximum of one hydrogen atom. The simplest member of the series is benzene, ChH6. Using the total bond order rules discussed above, the structural formula of benzene can be written as follows ... 

Because of its great stability, the six-membered ring of carbon atoms persists in most reactions. For simplicity, the ring is sometimes represented as a hexagon, each corner of which is assumed to be occupied by a carbon atom with a hydrogen atom attached (unless some other atom is explicitly indicated at that point). The delocalized electrons are indicated by a circle within the hexagon. The following representations illustrate these rules ... 

Equatorial hydrogens the hydrogen atoms lie around the perimeter of the ring of carbon atoms. 

The photochemical reduction of Barton ester 40 is depicted in Scheme 12. A series of hydrogen atom donors were screened. A stoichiometric amount of benzenethiol at - 78 °C provided the product in 86% ee (entry 3). This implies that, in the presence of an efficient hydrogen atom donor, radical trapping is competitive with the ring/radical inversion, generating an enantiomeri-... 

Stepwise Cg dehydrocyclization (aromatization) involving the gradual loss of hydrogen atoms from an alkane followed by a triene - cyclo-hexadiene ring closure step 20, 21). This can be ... 

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