Hydrogen bond directional nature

A similar kind of orientation has been well known for many years in some naturally occurring fibrous proteins such as silks (Geddes et al. 1968). The perpendicular orientation of chain axes with respect to the fibre axis, described as cross-p, occurs at low draw ratio and has been explained by the fact that in these soft silks the small crystallites are elongated along the hydrogen bond directions, which run perpendicular to chain axes (Geddes et al. 1968). The cross-p orientation in iPP may be attributed to the simultaneous occurrence of two kinds of shp processes at low... 

Ricci et al studied a series of thiourea catalysts for the Friedel-Crafts alkylation of aromatic and heteroaromatic compounds with nitroalkenes. They have succeeded in developing the Friedel-Crafts alkylation of indoles with nitroalkenes for the first time by means of a novel thiourea catalyst (13) (Scheme 2.48) [101]. The authors proposed the bifunctional nature of the thiourea catalyst, where thiourea activates the nitro group and at the same time the free alcoholic function interacts with the indole proton through a weak hydrogen bond, directing the attack of the incoming nucleophile on the Si face of the nitroalkene (Figure 2.18). 

Figure 4 shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the Hquid state. The O—H bond length within the H2O molecule is 0.96 x 10 ° m. Dipole—dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 4 (not to the same scale) shows the measured H-bond distance of 2.76 x 10 ° m or 0.276 nm. 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Ihe boiling points of different molecular substances are directly related to the strength of the intermolecular forces involved. The stronger the intermolecular forces, the higher the boiling point of the substance. In the remainder of this section, we examine the nature of the three different types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. 

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

Water is a special liquid that forms unique bonds involving protons between the oxygen atoms of neighboring molecules, the so-called hydrogen bond. The solvation forces are then due not simply to molecular size effects, but also and most importantly to the directional nature of the bond. They can be attractive or hydrophobic (hydration forces between two hydrophobic surfaces) and repulsive or hydrophilic (between two hydrophilic surfaces). These forces arise from the disruption or modification of the hydrogen-bonding network of water by the surfaces. These forces are also found to decay exponentially with distance [6]. 

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