Hydrogen single-displacement reactions

Single displacement reaction where magnesium is displacing hydrogen in the water molecule. 

Alternatives to the oxocarbenium ion mechanism are the direct attack of water on the substrate, and the nucleophilic attack of Asp-52 on the C-1 carbon to give an ester intermediate. The single-displacement reaction has been ruled out by showing that the reaction proceeds with retention of configuration (Chapter 7, section C3 Chapter 8, section C2).212-214 The oxocarbenium ion or SN1 mechanism has been substantiated by secondary isotope effects, using substrates containing either deuterium or tritium instead of hydrogen attached to the C-1 carbon 215 216 For example, kK/kD for structures 16.42 is 1.11,... 

Through experimentation, chemists have ranked the relative reactivity of the metals, including hydrogen (in acids and in water), in an activity series. The reactive metals, such as potassium, are at the top of the activity series. The unreactive metals, such as gold, are at the bottom. In Investigation 4-A, you will develop an activity series using single displacement reactions. 

The element displaced from the compound is always the more metallic element—the one nearer the bottom left of the Periodic Table. The displaced element need not always be a metal, however. Consider a common type of single-displacement reaction, the displacement of hydrogen from water or from acids by metals. 

The displacement of hydrogen from water or acids is just one type of single-displacement reaction. Other elements can also be displaced from their compounds. For example, copper metal reduces aqueous solutions of ionic silver compounds, such as silver nitrate, to deposit silver metal. The copper is oxidized. 

Many combination, decomposition, and single-displacement reactions involve oxidation-reduction. Let s examine the combustion of hydrogen and oxygen from this point of view ... 

In solution, single-displacement reactions occur when an atom of one element displaces the ion of another if the displacement involves metals, the atom reduces the ion if it involves nonmetals (specifically halogens), the atom oxidizes the ion. In two activity series—one for metals and one for halogens—the elements are ranked in order of their ability to displace hydrogen (for metals) and one another. 

Single Displacement Reaction in this single-displacement reaction, the hydrogen in hydrochloric acid, HCI, is replaced by magnesium, Mg. 

Some acids react with active metals and release hydrogen gas, H2. Recall that metals can he ordered in terms of an activity series. Metals above hydrogen in the series undergo single-displacement reactions with certain acids. Hydrogen gas is formed as a product, as shown by the reaction of barium with sulfuric acid. 

One example of a single displacement reaction is when magnesinm replaces hydrogen in water to make magnesium hydroxide and hydrogen gas ... 

This reaction could be viewed in terms of a double displacement reaction One of the oxygen atoms and one of the hydrogen atoms have displaced each other. One of the most famous examples of this type of exchange reaction, a single exchange, occurs between iron oxide and aluminum ... 

To predict which single substitution reactions will occur, we need to know a little about the relative reactivities of some of the important metals and nonmetals. Some metals and a few nonmetals are listed in Table 8.2 in order of decreasing reactivity. A more complete list is given in Section 17.2. Hydrogen is included in the list of metals because it can be displaced from aqueous acids by reactive metals (Figure 8.5) and can displace less active metals from their compounds ... 

In this displacement reaction, a single element reacts with a compound and displaces another element from the compound. The products are a different element and a different compound than the reactants are. In general, a metal may displace another metal (or hydrogen), while a non-metal may displace only another nonmetal. 

The nature of the action of the mutarotase from P. notatum has been investigated extensively by Bentley and Bhate,140 and compared with the acid-, base-, and solvent-catalyzed reactions (see also, p. 31). Through use of lsO on C-l, it was shown that dehydrogenations do not occur on C-l. In addition, no dehydrogenation occurred at carbon-bound hydrogen atoms a single-displacement mechanism was thus eliminated. The enzyme probably transfers a proton, in a process similar to that usually involved in nonenzymically catalyzed muta-rotations. 

DL-Valiolamine (205) was synthesized from the exo-alkene (247) derived from 51 with silver fluoride in pyridine. Compound 247 was treated with a peroxy acid, to give a single spiro epoxide (248, 89%) which was cleaved by way of anchimeric reaction in the presence of acetate ion to give, after acetylation, the tetraacetate 249. The bromo group was directly displaced with azide ion, the product was hydrogenated, and the amine acety-lated, to give the penta-A, 0-acetyl derivative (250,50%). On the other hand. 

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