Forward reactions

Although the previous two sections of this chapter emphasized hydrolytic processes, two mechanisms that led to O- or N-acylation were considered. In the discussion of acid-catalyzed ester hydrolysis, it was pointed out that this reaction is reversible (p. 475). Thus, it is possible to acylate alcohols by reaction with a carboxyhc acid. To drive the reaction forward, the alcohol is usually used in large excess, and it may also be necessary to remove water as it is formed. This can be done by azeotropic distillation in some cases. 

In the kidney and in muscle tissues, fructose is readily phosphorylated by hexokinase, which, as pointed out above, can utilize several different hexose substrates. The free energy of hydrolysis of ATP drives the reaction forward ... 

FIGURE 25.2 (a) The acetyl-CoA carboxylase reaction produces malonyl-CoA for fatty acid synthesis, (b) A mechanism for the acetyl-CoA carboxylase reaction. Bicarbonate is activated for carboxylation reactions by formation of N-carboxybiotin. ATP drives the reaction forward, with transient formation of a carbonylphosphate intermediate (Step 1). In a typical biotin-dependent reaction, nncleophilic attack by the acetyl-CoA carbanion on the carboxyl carbon of N-carboxybiotin—a transcarboxylation—yields the carboxylated product (Step 2). 

Phosphatidylethanolamine synthesis begins with phosphorylation of ethanol-amine to form phosphoethanolamine (Figure 25.19). The next reaction involves transfer of a cytidylyl group from CTP to form CDP-ethanolamine and pyrophosphate. As always, PP, hydrolysis drives this reaction forward. A specific phosphoethanolamine transferase then links phosphoethanolamine to the diacylglycerol backbone. Biosynthesis of phosphatidylcholine is entirely analogous because animals synthesize it directly. All of the choline utilized in this pathway must be acquired from the diet. Yeast, certain bacteria, and animal livers, however, can convert phosphatidylethanolamine to phosphatidylcholine by methylation reactions involving S-adenosylmethionine (see Chapter 26). 

Ethylene is produced as the small molecule, which upon removal drives the reaction forward. 

In this case, the increase in entropy of the surroundings drives the reaction forward, but it is opposed by the decrease in entropy of the system. 

These early observations have evolved into the branch of chemistry called electrochemistry. This subject deals not only with the use of spontaneous chemical reactions to produce electricity but also with the use of electricity to drive non-spontaneous reactions forward. Electrochemistry also provides techniques for monitoring chemical reactions and measuring properties of solutions such as the pK, of an acid. Electrochemistry even allows us to monitor the activity of our brain and heart (perhaps while we are trying to master chemistry), the pH of our blood, and the presence of pollutants in our water supply. 

This step restores the H+(aq) ions needed for reaction E, and so hydrogen ions function as a catalyst. The removal of Fe3+ ions from solution drives the reaction forward. The overall process is the sum of reactions F, F, and G ... 

In addition to finding the minima for each intermediate in a synthesis, it is also necessary to be able to analyse reactivity. This is a more difficult problem than conformation searching, because it is now possible for bonds to break. The range of movements available to the molecule is far larger, and it is also necessary to consider which bond will break most easily, and what factors are present which will drive the reaction forwards. If there are many competing reactions, then these calculations may have to be very precise in order to distinguish between them. 

A further application of this technology will certainly be the fabrication of membranes of these materials. Membrane reactors have shown great utility in many systems, where one component of a reaction mixture can be separated by permeation through a membrane, thus driving a reaction forwards, by continuous separation. Such continuous processes can themselves save a great deal of waste. 

This is an equilibrium process and two techniques are used to drive the reaction to completion. One is to use a large excess of the alcohol, which is feasible for simple and inexpensive alcohols. The second method is to drive the reaction forward by irreversible removal of water, and azeotropic distillation is one way to accomplish this. Entries 1 to 4 in Scheme 3.5 are examples of acid-catalyzed esterifications. Entry 5 is the preparation of a diester starting with an anhydride. The initial opening of the anhydride ring is followed by an acid-catalyzed esterification. 

This reaction, while still not an elementary reaction, more closely describes how the oxidation proceeds on a molecular level. Even where Reaction 31.4 operates, however, our model may still represent the overall process occurring in nature, since 02 is needed to produce the Fe+++ that drives the reaction forward and is, therefore, the ultimate oxidant in the system. 

Changing the temperature really changes the amount of heat in the system and is similar to a concentration effect. In order to treat it in this fashion, one must know which reaction, forward or reverse, is exothermic (releasing heat). 

The subsequent steam reforming section is operated at very high temperatures 850-900 °C. The SMR catalysts themselves are already active below 400 °C, but high temperatures are necessary to drive the strongly endothermic reaction forward [8]. In industry, nickel catalysts are used in high-alloy reaction tubes, which are heated by external burners. This design is expensive and leads to heat losses, although much of the heat is recuperated. Noble metal catalysts such as sup-... 

Final deprotonation of the carboxylic acid drives the reaction forward. Pathway B ... 

The purified DNAs are used as templates for in vitro transcription reactions. Forward primer (P-40, same sequence as primer used during SELEX process) and reverse primer (p-22 pGEM). 

The reaction is done in water with an excess of OH to drive the reaction forward by the mass action effect. There is another reason too. Think which species will actually be formed in aqueous alkali. 

For example, for formaldehyde (R = H) at neutral pH, the pseudo-first-order rate constant for the hydration reaction (forward reaction), k =k [H20], is about 10 s 1 and the first-order rate constant for dehydration, k2, is about 5x 10-3 s-1. In Chapter 20 we will use this example to show that the reactivity of compounds can influence the kinetics of air/water exchange if both processes (reaction and exchange) occur on similar time scales. 

Why is the ortho isomer 30 times more acidic than the para isomer Any effect that increases the stability of the product of a reaction drives the reaction forward. In the ortho isomer, the product of the acid dissociation reaction can form a strong, internal hydrogen bond. 

Overpotential, ohmic potential, and concentration polarization make electrolysis more difficult. They drive the cell voltage more negative, requiring more voltage from the power supply in Figure 17-1 to drive the reaction forward. 

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