Zinc Couples

It is possible to provide cathodic protection to base plate up to 1 720 MNm yield strength, by coupling to mild steel or possibly to zinc , but zinc and metals more active than zinc tend to induce hydrogen embrittlement. Welds up to 1 380 MNm may be cathodically protected by zinc, but at impressed potentials of — 1-25V (S.C.E.) both 1240 and 1380 MNm welds fail rapidly due to hydrogen embrittlement. Neither mild steel nor zinc couples protect AISI 4340 steel . 

Lack of cooling during preparation of the Simmons-Smith organozinc reagent caused the reaction to erupt. The possibly pyrophoric nature of organozinc compounds and the presence of ether presents a severe fire hazard [1]. An alternative, safer method of activating the zinc for the reaction involves use of ultrasonic irradiation rather than the copper-zinc couple [2]. 

The direct reaction of zinc metal with organic iodides dates back to the work of Frankland(67). Several modifications have been suggested since that time to increase the reactivity of the metal. The majority of these modifications have employed zinc-copper couples(68-72), sodium-zinc alloys(73), or zinc-silver couples(77). Some recent work has indicated that certain zinc-copper couples will react with alkyl bromides to give modest yields of dialkylzinc compounds(74,73). However, all attempts to react zinc with aryl iodides or bromides have met with failure. The primary use of zinc couples has been in the Simmons-Smith reaction. This reaction has been primarily used with diiodomethane as 1,1-dibromides or longer chain diiodides have proven to be too unneactive even with the most reactive zinc couples. 

Worked Example 7.4 We construct a cell with the copper(II) copper and zinc(II) zinc redox couples, the copper couple being more positive than the zinc couple. What is the cell schematic ... 

According to G. Pellagri,41 potassium chlorate inaq. soln. is reduced to the chloride by shaking it with iron filings and H. Eccles noted that it is readily reduced by boiling it with a copper-zinc couple—potassium perchlorate is not reduced under similar conditions. Hence, the amount of the one salt can be readily determined when in the presence of the other. On the other hand, J. G. Williams found the perchlorates are reduced by titanium trichloride, while the chlorates are not affected. Here again an analytical process is available. 

The detection and determination ot the perchlorates.—The perchlorates give no precipitates with silver nitrate or barium chloride soln. cone. soln. give a white crystalline precipitate with potassium chloride. Unlike all the other oxy-acids of chlorine, a soln. of indigo is not decolorized by perchloric acid, even after the addition of hydrochloric acid and they do not give the explosive chlorine dioxide when warmed with sulphuric acid unlike the chlorates, the perchlorates are not reduced by the copper-zinc couple, or sulphur dioxide. Perchloric acid can be titrated with —iV-alkali, using phenolphthalein as indicator. The perchlorates can be converted into chlorides by heat and the chlorides determined volumetrically or gravimetrically they can be reduced to chloride by titanous sulphate 28 and titration of the excess of titanous sulphate with standard permanganate they can be fused with zinc chloride and the amount of chlorine liberated can be measured in terms of the iodine set free from a soln. of potassium iodide and they can be... 

According to E. Laurent,58 a soln. of sodium nitrate is decomposed in sunlight with the evolution of oxygen, while in darkness it is stable. Zinc dust reduces a soln. of potassium nitrate to the nitrite and hydroxide with the evolution of some oxygen above 60°, only a little nitrite but much nitrogen and ammonia are given off.59 The copper-zinc couple also reduces soln. of the nitrate, first to nitrite, and then to ammonia. Potassium amalgam, stannous chloride, etc., also reduce the nitrates in a similar way. 

The standard potentials for (Zn2+ Zn) and (Ag+ Ag) are —0.763 V and 0.799 V from Table 19-1. The standard potential of the cell is the difference between these two numbers, 0.779 V —(—0.763) = 1.542 V. The silver potential is higher, and this means that the silver ion is the oxidizing agent. The zinc couple provides the reducing agent and is the negative electrode. The equation for the cell is... 

The example of the iron/zinc couple refers to an acid solution, if a neutral or alkaline (basic) solution is considered the cathodic reaction would be ... 

Table 4.82 Galvanic corrosion rate of zinc coupled to other common commercial metals in different atmospheres (gm/yr)95...

As mentioned in Section 4.2, zinc couples such as zinc-copper and zinc-silver couples have frequently been used in organic synthesis due to their increased reactivity. Their use in the preparation of dialkyl zinc compounds has already been pointed out. To try to summarize all the reactions in which the use of zinc couples has been reported, even for just the past few years, would be much to ambitious for this review. Therefore, we shall just present some representative examples and key papers reporting the latest advances in new types of couples. 

---