Hydrogen electron attachment

Consider first the electronic effect of alkyl groups versus hydrogen atoms attached to C=0 Recall from Section 17 2 that alkyl substituents stabilize C=0 making a ketone carbonyl more stable than an aldehyde carbonyl As with all equilibria factors... 

A hydrogen atom and a proton serve as the starting point of the calculation. With the nuclei a distance rAB apart, and with the electron attached to the nucleus A to form a hydrogen atom, the zero h order eigenfunction is... 

All of the delocalization discussed so far involves n electrons. Another type, called hyperconjugation, involves <7 electrons. When a carbon attached to at least one hydrogen is attached to an unsaturated atom or one with an unshared orbital, canonical forms such as 110 can be drawn. In such canonical forms, there is no bond at all between the carbon and hydrogen. The effect of 110 on the actual molecule is that the electrons in the C—H bond are closer to the carbon than they would be if 110 did not contribute at all. 

Figure 3 The numbers at each site in the top half (above the dotted line connecting the extreme atoms to the left and right of the diagram) are the numbers of classical structures which can be constructed with hydrogen (muonium) attached to the position indicated and the unpaired electron at the indicated site. The corresponding numbers in the bottom half are the spin densities in atomic units from UHFAA calculations on the fully optimised geometry of CeoMu using an ST0-3G basis set within the ROHF method.

The reactivity of hydrogens in norbomane towards abstraction is of interest since the difference between two hydrogen atoms attached to the same carbon atom of position 2 can well be explained. The frontier electron density values 105> are in accord with the reactive exo hydrogen (Fig. 7.20). 

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 ... 

Toluene is chemically related to benzene, but a methyl (CH3—) grouping has replaced one of the hydrogen atoms attached to the ring of six carbon atoms. The circle drawn inside the six-carbon ring represents a set of six electrons that comprises the chemical bonds involved in the so-called aromatic ring. This collection of six carbon... 

Figure 2.30 Molecular models depicting 4,4-dimethylcyclohexanecarboxylic acid (a) framework (b) ball-and-stick (c) space-filling. Note that the size of atoms reflects the electronic charge associated with the atom. Therefore, as seen in models (b) and (c), a hydrogen atom attached to electronegative oxygen appears smaller than a hydrogen atom attached to carbon...

We have seen above that acidity is influenced by the hybridization of the atom to which the acidic hydrogen is attached. The acidity of a C-H bond was found to increase as the s character of the bond increased. The more s character in the orbital, the closer the electrons are held to the nucleus. Similar reasoning may be applied to basicity. If the lone pair is in an sp or sp orbital, it is held closer to the nucleus and is more difficult to protonate than if it is in an sp orbital. 

This property of the — SiMes group has also been quite clearly demonstrated in an extremely elegant manner by Bedford et al. (77). It has been amply demonstrated that in an electron spin resonance spectrum the isotropic hyperfine coupling constant, an, of a hydrogen atom attached to an sp2 hybridised carbon atom having an unpaired electron in the 2p—orbital is given approximately by an Equation (3) due to McConnel (18)... 

The first electron attachment energies of the first 36 elements are plotted in Figure 1.5 and show the values for H and He followed by a characteristic pattern, the second repetition of which is split by the values for the 10 transition elements. The value for hydrogen is -72.8 kJ mol, which is very different from the Is orbital energy of -1312 kJ mol-1 because of the interelectronic repulsion term amounting to -72.8 -... 

It also includes the enthalpy of ionization of the hydrogen atom (equal to, but opposite in sign to, the electron attachment enthalpy of the gaseous proton), the enthalpy of atomization of dihydrogen and the enthalpy of hydration of the proton. The enthalpy of formation of the cation is estimated by use of the equation ... 

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