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魔酸(超强酸)- 世界上最强的酸

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发表于 2016-3-16 14:43:31 | 显示全部楼层 |阅读模式
魔酸(超强酸)- 世界上最强的酸
发表时间:2007-5-5 15:52:57
      在很长的一段时间内,人们认为王水就是酸中之王,是最强的酸了,因为即使是黄金,遇到王水也会像“泥牛入海”一样很快变的无影无踪。#此前在首页部分显示#
      直到有一天奥莱教授和他的学生偶然发现了一种奇特的溶液,人们才知道其实王水并不是最强的酸,还有比它强的酸,这就是超酸,又叫超强酸,超酸是指酸性比普通无机酸强10^6~10^10倍的酸。它的发现非常有戏剧性:1966年圣诞前,奥莱教授的学生偶然将一支圣诞蜡烛放入到他们配置的混合酸液中,竟然惊奇的发现蜡烛溶解了,然后立即做出了酸性等一系列相关测试,发现蜡烛居然已经分解,溶液中没有任何蜡烛成分,这如同将铁丢入酸中产生了氢气和酸化铁一般的化学反应,因此也发现了它们的酸性强的令人难以置信。
      从成分上看,超强酸是由两种或两种以上的含氟化合物组成的溶液。比如氢氟酸和五氟化锑的混合等, 这些混合酸的均是比硫酸、盐酸;硝酸酸性强几百万倍,甚至几十亿倍的超强酸。
      下面就以氢氟酸(HF)和五氟化锑(sbF5)的混合酸为实例,介绍一下超强酸。
      氢氟酸(HF)和五氟化锑(sbF5)的混合酸也就是人们说的魔酸(magic acid)或魔术酸 ,魔酸(HSO3F-SbF5)是目前世界上已知最强的超酸,许多物质(如H2SO-4)在魔酸中可获得质子(即质子化)。当它们按1 :0.3(摩尔比)混合时,它的酸性是浓硫酸的 1亿倍;按1: 1混合时,它的酸性是浓硫酸的10亿倍,而以0.2:1的摩尔比混合时酸度更能达到100%纯硫酸的10^9倍以上, 随着SbF5的比例增加酸度还能增强 。它能轻易溶解不溶于王水的高级烷烃蜡烛。所以王水在它们面前只能是“小巫见大巫”。
      魔酸目前在市场上也可以购买,但是它只是五氟化锑和氢氟酸按体积比1:1(注意:不是按照摩尔比)混合制成的混酸,其酸度只是无水硫酸的100倍,它的盛放只用聚四氟乙烯制的容器盛放,因为即使是玻璃也会被它溶解。
      目前,超强酸在化学和化学工业上,极有应用价值,它既是无机及有机的质子化试剂,又是活性极高的催化剂。过去很多在普通环境下极难实现或根本无法实现的化学反应在超强酸环境中。却能异常顺利地完成。而由于超强酸的酸性和腐蚀性强的出奇,所以过去一些极难或根本无法实现的化学反应,在超强酸的条件下便能顺利进行。比如正丁烷,在超强酸的作用下,可以发生碳氢键的断裂,生成氢气,也可以发生碳碳键的断裂,生成甲烷,还可以发生异构化生成异丁烷,这些都是普通酸做不到的。
      可以预料,随着这些具有超常酸性和腐蚀性超强酸的相继问世,化学和化学工业将会迅速走进新时代。
      2004年12月9日,据中国科技信息网报道,美国加利福尼亚大学的科学家在实验室制造出一种世界上最酸的物质——碳硼烷酸。这种化合物酸性强度是浓硫酸的100万倍,令人惊奇的是,尽管酸性很强,但其腐蚀性却很低,可以在玻璃容器中保存。
  说到世界最酸的物质,可能很多人要流口水,不过这个酸并非是尝起来“酸”。如果你还记得初中的化学老师,应该就记得化学里“酸”指的是氢离子浓度。
  人们对酸的认识也是逐渐加深的。硫酸、盐酸、硝酸,被称为三大强酸,它们可以腐蚀我们日常生活中见到的很多物质。而王水则是由一份浓硝酸和三份浓盐酸混合而成,即使是“总要闪光”的金子在王水里也要“香消玉殒”。所以在很长的一段时间内,人们认为王水就是酸中之王,是最强的酸了。
  直到有一天,奥莱教授和他的学生偶然发现了一种奇特的溶液,它能溶解不溶于王水的高级烷烃蜡烛,人们才知道其实王水并不是最强的酸,还有比它强的酸,这就是魔酸,又叫超强酸。自从奥莱教授和他的学生发现超强酸以后,人们又开始研究起强酸,相继找到了多种新的超强酸。
  在碳硼烷酸出现之前,最强的酸性物质为氟乙酰氨硫酸,这种酸腐蚀性极强,可以轻易穿透玻璃器皿,而碳硼烷酸则是已知超强酸中第一个可以在玻璃器皿中保存的超强酸性物质。
  碳硼烷酸具备令人吃惊的释放氢离子的能力,酸性是水的100万亿倍,但由于碳硼烷酸中碳硼烷的结构十分稳定,释放氢离子后,由11个硼原子和一个碳原子排列而成的20面体的结构没有发生任何变化,不发生进一步的化学反应,因此腐蚀性很低。
  碳硼烷酸的应用十分广泛,可以用来制造“酸化”的有机分子,研究这些在自然界中短暂存在的有机分子有助于科学家了解物质发生变化的深层次机理,而目前科学家希望用碳硼烷酸酸化惰性气体氙,确定该气体的惰性强度。
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Superacid - The world's strongest acid uncovered Researchers at the University of California, Riverside have discovered the world's strongest acid. Remarkably it is also the gentlest acid. This non-toxic and non-corrosive acid may have a role in processes such as improving the quality of gasoline, developing polymers and synthesising pharmaceuticals.
How can an acid be both strong and gentle? The answer lies in the way chemists define the strength of an acid. Acid strength is the ability of an acid to add a hydrogen ion (H+) to basic molecules. On the other hand, corrosiveness has a lot to do with the nature of the negatively charged part of an acid that always accompanies H+.
For example, hydrofluoric acid (HF) dissolves glass because the fluoride (F-) part of the acid attacks the silicon atom in silica glass at the same time that H+ attacks an oxygen atom, according to UC Riverside Distinguished Professor of Chemistry, Christopher Reed, one of the authors of the findings.
Another example is the choice of an acid to clean lime deposits from inside a copper kettle, he pointed out. The wise homeowner chooses hydrochloric acid not nitric acid because the chlorine part of hydrochloric acid does not attack copper whereas the nitrate part of nitric acid would dissolve the kettle in a mess of toxic brown fumes.
The findings were published in Angewandte Chemie in a paper titled 'The Strongest Isolable Acid', co-authored with Reed, UCR Colleagues Mark Juhasz, Stephan Hoffmann and Kee-Chan Kim, and Evgenii Stoyanov of the Boreskov Institute of Catalysis in Novosibirsk, Russia.
The new 'strong-yet-gentle' acids are called carborane acids. The secret to their strength is twofold. Most importantly, the carborane part of the acid is an extremely weak base (ie, weakly alkaline), and carboranes have extraordinary chemical stability.
They have an icosahedral arrangement of eleven boron atoms plus one carbon atom, which is probably the most chemically stable cluster of atoms in all of chemistry, according to Reed. This means that the carborane part of the acid cannot participate in the chemistry of corrosion and decomposition that fluoride and nitrate show in hydrofluoric acid and nitric acid. As a result, carborane acids can add hydrogen ions to weakly basic molecules without destroying the often delicate positively charged molecules that are formed. This is the essence of their strong-yet-gentle qualities, Reed added.
Examples of molecules that add a hydrogen ion and are stabilised with a carborane as the negatively charged part of the product include benzene to give benzenium ion, C60 to give 'protonated buckyball,' and alkenes to give carbocations.
None of these positively charged molecules had been 'put in a bottle' at room temperature before because the acids used previously would decompose them. The strong-yet-gentle carborane acids overcome this difficulty, allowing chemists to take a closer look at important molecules whose existence was typically fleeting, Reed said. Acidified molecules are important short-lived intermediates in a huge variety of acid-catalysed chemical transformations including the digestion of food, gasoline improvement, polymer formation and the synthesis of pharmaceuticals.
How strong are carborane acids? The strongest one is at least a million times stronger than concentrated sulfuric acid (H2SO4) and hundreds of times stronger than the previous record holder, fluorosulfuric acid (HFSO3). Concentrated sulfuric acid is already more than a billion times (1012) stronger than dilute swimming pool acid or the acid in one's stomach. Acidic media having or exceeding the acidity of carborane acids had been achieved previously by adding antimony pentafluoride (SbF5) to fluorosulfuric acid but these mixtures are very corrosive and have other limitations. Acids that are this strong are called superacids and they react with hydrocarbons from oil in a process called hydrocarbon cracking. This is an important process for raising the octane levels of gasoline. The new acids could become very important for understanding and improving this process, Reed said. The 1994 Nobel Prize in Chemistry was awarded to George Olah at USC for his pioneering studies in this field. Carborane acids have advanced this field even further. There are many other molecules whose reactions with traditional acids are messy and therefore not very useful. Carborane acids deliver very clean acidity without ferocity. Thus, cleaner acid-catalysis of reactions important to the manufacture of pharmaceutical drugs and petroleum products should be possible. Also, there are atoms such as the element Xenon (Xe), which have so far resisted reaction with acid. Reed and his research group want to add hydrogen ions to Xe atoms "because it's never been done before".
Reed says, "Our research is driven by making molecules that have never been made before. Carborane acids are allowing us to do this. That is the true value of this research. Science gets advanced, and at the same time, students are experiencing the thrill of discovery as they become scientists."
For further information contact University of California, Riverside
900 University Avenue, Riverside, Ca 92521, USA

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