how OH react with hydrogen and forming waterby crossing methods
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The goal of every chemist, no matter what types of chemical compounds he or she works with, is to understand how and why chemicals react and change. Yet this is the most difficult task of all. It is not enough to know the structures of all the reactants and products, although such knowledge is a vital starting point. We must also know how the molecules approach one another and with what energies and with what orientations they interact. The concepts of energy and entropy are important in understanding chemical reactions. In this chapter we shall look at some of the problems that face us because we cannot examine individual molecular events. We shall examine two theories for predicting the rates of such simple reactions and compare their success or lack of it. We shall look at the two factors that often make reactions slow - energy and entropy - and see how catalysts can overcome these factors and accelerate chemical changes. Although we cannot present a complete theory of chemical reaction (no one can do this yet), we can outline the foundations on which this theory will someday be constructed.
What Happens When Molecules React?
Mechanisms of Reaction
Let’s suppose that we can watch what happens when two molecules react. We can take as an example the reaction of a molecule of thioacetamide, CH3—CS—NH2, with water to yield acetamide, CH3—CO—NH2, and H2S (Figure 22-1).
thioacetamide, with water to make acetamide, and H2S, (a) The thioacetamide molecule has S, C and N in one plane w around a central C. The orbitals of the double bond are distorted toward S to represent its greater elecironegativity. Orbitals that play no part in the reaction are not drawn.]]
state, with partial bonds from C to both S and O. The former 0—H bonding electron pairs are becoming lone pairs. (c) Products of the reacuon acetamide and H2S. The tetrahedral geometry of the transition state has reverted to trigonal planar geometry as the S atom leaves. The bond angle in H2S is 92° in contrast to the 105° in (a)]]In the original thioacetamide molecule, the central carbon atom is bound to carbon and to nitrogen by σ bonds, and by a σ and a π double bond to sulfur (Figure 22-1a). Since sulfur and nitrogen are more electronegative than carbon, electrons are more attracted to these atoms. Thus the electron pairs of the carbon-sulfur and carbon-nitrogen bonds their bonds are slightly displaced toward sulfur and nitrogen, causing these two atoms to bear a small negative charge and the central carbon to bear a small positive charge.
The most favorable direction of approach of a water molecule is perpendicularly from either side of the plane of the four heavy atoms. The most favorable orientation of the incoming water molecule is as shown in Figure 22-1a. Here a lone electron pair from the water is attracted to the positive charge on the central carbon. As the water molecule approaches this carbon atom, the lone-pair electrons are drawn to it and begin to form a partial bond. This partial bond formation has two effects: It weakens the bond between carbon and sulfur by repelling the electrons even more toward sulfur, and it simultaneously weakens the O—H bonds in water by pulling electrons from these bonds toward oxygen as the oxygen lone-pair electrons are attracted toward carbon. This intermediate state appears in Figure 22-1b. The central carbon atom now has two single bonds to carbon and nitrogen, and two partial bonds to sulfur and oxygen.
This intermediate state is not stable. If the water molecule falls away again and the situation reverts to that of Figure 22-1a (and there is no reason why this could not happen), then we see no net reaction. The water molecule rebounds from a collision with thioacetamide and goes its separate way. It also could happen that the sulfur atom falls away, as in Figure 22-1c. In this reaction, the two protons released by oxygen as it makes a double bond with carbon are attracted by the sulfur atom with four electron pairs, and a molecule of H2S results. The reaction
is complete. The reverse reaction also can occur; a molecule of H2S can collide with one of acetamide and produce water and thioacetamide. We would be less likely to see such an event if we could watch reactions at the molecular level, simply because there are very few H2S molecules in comparison to the number of water molecules.
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