Week 1-Lecture 3 : Reppe Reactions ( Part-1)

Welcome to this course on Transition Metal Organometallics in Catalysis and Biology We have been discussing a type of very interesting reactions, which is Reppe chemistry With regard to this particular topic, now, Reppe chemistry stands very relevant in today’s context, and we are going to be looking at the development of Reppe’s chemistry in the overall scheme of the developments that happened in transition metal organometallic chemistry In this context, in today’s lecture, we are going to take a look at the chronological sequence, in which the Reppe’s chemistry evolved under conditions and requirements that was the need for the day Now, what we had seen in the earlier class is the fact that this Reppe’s chemistry allows access to large number of functionalized chemical feed stocks, or compounds, all originating from acetylene chemistry It was as if, the acetylene serves as feed stock for various different functional chemistry And today we are going to see this development in light of the overall development of transition metal organometallics in the overall scheme of things, we are going to see how the need of the day resulted in the development of Reppe chemistry Much of the research that goes on in the present day as well as in earlier times, depends on the need of the day and is guided by the economics and the need of the time And hence, in current context, for example, the much of the research is about development renewable source of energy, and this involved looking into options like solar, wind, water, biomass so and hence so forth The reason for more need for the development of renewable source of energy arises from the fact that these are clean energy, that mean no carbon foot print or no CO2 and so on and hence so forth These are the reasons, which allows us to focus more on renewable energy The other reason to focus on renewable energy arises from the fact that depletion of the carbon foot print from the face of the earth in terms of the so called non-renewable energy like coal and petroleum of fossil fuels So this way, the focus has been on renewable energy in the current century The things were, however, different if we go back just about a 100 years or so For example, around 1800 to around 1900 and till around 1950’s, the focus of energy source had been primarily on non-renewable energy These involved fossil fuels, which consists of sources like coal, natural gas, crude oil Now, chronologically, if one were to look at that this emphasis on the energy demand from the sources had been in, the order is shown here

Petroleum as well as natural gas crude oil sources were probed in mainly 1950’s whereas those of coal or coal driven energy sources were more popular even before that, in 1800’s and 1900 Now, to see the connection between, with respect to acetylene from coal is the major source and the production of acetylene from natural gas or from petrochemical industry what people get is ethylene or propylene Now the way, in current times, the focus had been on generating energy from renewable sources, but about 100 years or so before, that the focus had been on gathering energy from non-renewable sources, which were, for example, from that of coal, or from that of acetylene, or to be more accurate that in early 1900 or so, the focus had been on getting it from acetylene which is a product from the coal Whereas may be about three or four decades later in 1950’s, the focus shifted from acetylene to more economically feasible source like ethylene and propane Now, the whole gamut of Reppe chemistry starts from here, that we talk is about utilizing acetylene Reppe chemistry as we had seen in our previous discussion, that acetylene could be converted to a large number of functionalized products, and that had solely been because of the efforts of Walter Reppe, who found out how to deal with acetylene To elaborate further on that, for example, one can convert acetylene to acetaldehyde, which is a useful intermediate, an aldehyde is a useful intermediate to other functionalized chemicals, functionalized feedstocks The conversion from acetylene can be achieved by treatment with water in presence of sulfuric acid and mercuric sulphate, and this has been one of the major exploits of acetylene so that one can see the need for developing acetylene as a feedstock for carrying our chemical reactions This had been the reason that had led to the development of Reppe chemistry However, with time the focus shifted from acetylene to ethylene, which was more easier and more cheaply obtained from natural gas and crude oil, and then, that had led to exploration of the possibility of using ethylene for making acetaldehyde, an useful intermediate for accessing other functionalized feedstocks Even this, resulted in very important process, which is called Wacker oxidation Wacker oxidation involves reaction of ethylene in presence of PdCl2 catalyst and CuCl2 catalyst, which results in formation of acetylene This process for obtaining acetaldehyde from ethylene became more prevalent in the 1950’s, and over took production from that from acetylene, which were more prevalent in 1920’s So what we see is that the shift of technology for obtaining functionalized feedstocks moving from acetylene to ethylene Now, this shift was made about 50 or 60 years back, whereas now, we move on to more unconventional,

but to a more permanent solution, that is exploring energy from a renewable sources, like solar, wind, water, biomass so on and so forth The primary reason is that at some point or other, the whole of these non-renewable sources would depleted from face of the earth, and then our energy requirement would have to be made up from the sources available like, the renewable energy Now, even under the current scenario, there is a larger argument still prevalent in favor of developing this Reppe chemistry, which sort of makes comparative estimates of how long are the fossil fuels like coal or natural gas or crude oil would last on earth Based on the estimate, it is still suggested that probably the natural reserve of coal will outlive that of petroleum products, and hence, one could still focus on getting some amount of energy using the Reppe chemistry And, this brings us to the relevance of Reppe chemistry in today’s context Even though both coal and natural gas, crude petroleum, at some point or other would get exhausted, but coal reserve would outlast or outlive the natural gas and the crude oil reserve Hence, the energy demend arising from coal should be exploited, even in today’s context when the focus is shifting towards more renewable forms of energy Another point, mentioned over here, is that in today’s world, where everything is yet to standardized, or people does not have a standardized opinion on several issues, there are countries pursuing technologies depending on their natural reserves For example, China has a large coal reserve, and so, China still explores the potential of acetylene as a chemical feedstock Now, this being the scenario, there is a lot of thrust in looking into acetylene chemistry As far as the chemical nature, the chemical intuition is concerned, acetylene has a higher reactivity than ethylene Hence, would require less steps in reactions to reach the intermediates then what would one would require from that of acetylene Last but not the least, but an important argument, in favor of non-renewable energy, is that unlike in renewable energy, for example, like solar or wind or water, there is a huge fluctuation in the power that is generated arising out of different sources of renewable energies Whereas in contrast to that, energy from non-renewable sources like coal provides a steady supply of energy in terms of the electrical power that is generated Hence, there is still a demand as well as argument in favor of exploring the non-renewable energy sources as an option for making our energy needs Now with these background, I think, now, we would have an understanding of the reasons, as to why Reppe chemistry, which was developed so early on in 1900s is still relevant in today’s context, and that those reactions as lead to so many different products, because of its various chemical exploits, and that have been carried out by very dedicated scientists

For example, Reppe developed this chemistry and the name suggests so Now, with that, I think I have provided you a picture how relevant is acetylene and conversion of acetylene to other chemical feedstocks, and the challenges involved in this, as dictated by the need of the day in early 1900, when the energy options from non-renewable sources were explored Even now, about 100 or more years later, the Reppe chemistry is still of relevance because of reasons just mentioned now Now with these, we come back to the core of the Reppe reaction, which is acetylene, and then we are going to sort of look into the utility of acetylene apart from what I have been talking about Reppe kind of conversion in our daily need Now acetylene, as I mentioned apart from the Reppe chemistry, has been long used for welding purposes The first use of acetylene in welding was reported as early as 1906, and the reason being the applications of acetylene in welding is because this is the strongest, safest and simplest to use as fuel gas Now, why it is so useful is because it gives a temperature of around 3000 oC, when mixed with oxygen in 1 is to 1.1 ratio These by and large is an important aspect that you reach a temperature of about 3000 oC Now if you compare this temperature, this is almost about half of the temperature of sun, that may be 6000 or 7000 oC This is very high temperature, in which most of the metals would melt So, applications wise this temperature is by far the highest or the hottest of all fuel gases, and hence, it can easily melt all of them Hence, it is not a surprise that acetylene can find extensive applications in welding purpose Now, it has a specific gravity of about 0.9 relative to 1.0 for air, so that means acetylene is lighter, and hence, would move up if unused So, it is not going to, sort of, you know sink, or stay low, if there is unused acetylene still in there So, in a way, it is kind of very safe to use, and also the oxygen ratio that it requires is very less, which is about 1 : 1 Many other fuel gases, for example, ethylene or propylene, require more amount of oxygen to burn than what acetylene requires Because of this reason, because of it is light nature, because of its low oxygen consumption ratio, and because of very high temperature that it can attain when burning with oxygen,

acetylene is the best, safest, strongest and simplest to use fuel gas It has found applications in various welding type applications Apart from conversion of acetylene to chemical feedstocks, acetylene also has tremendous applications in welding industry Lastly, it also has a smell of garlic, so if there is any leak or anything like that, it can be easily deducted by its garlic like odor All of these provided an overview of as to why acetylene is important in context of its applications, as early as the beginning of 1900, and why so much research activities were centered around acetylene, which has led to the development of Reppe chemistry that we are talking about With this, you know, we are going to be looking into some more reactions, some of the reactions relevant to Reppe chemistry as we go on For example, the reaction of acetylene with alcohols results in the formation of vinyl ether This is the reaction, when vinyl functional group is formed, and is a part of the reaction called vinylation Similarly, another example of vinylation involves reaction with cyanides in the presence of a catalyst giving acrylonitrile Now, these kind of intermediates are very important intermediates, they are monomers for various polymerization reactions including, polymerization to produce functionalized polyethylenes, in which they have this functionalized cyanide moiety attach to the polyolefins These intermediates have a lot of applications as monomers for various polymerization reactions With these, we come to the end of today’s lecture on Reppe reactions, in which we have looked into the prospects of the development of the Reppe reactions in the context of the energy need of the time and also as a function of time What we had seen is that with the passage of time from 1800 to 1900 to the present day, the demand for energy have changed from non-renewable sources to that of the renewable or more technologically cleaner sources This has led to a shift in the energy sources on moving from coal to natural gas and crude oil in 1950’s, which was that time sought because of the discovery of olefin polymerization, and because of the development of the petrochemical industry at that point of time, to the modern day scenario of clean technology involving solar, wind, biomass, water, so on and so forth

Acetylene has been produced largely from coal, and hence, the conversion of acetylene to other chemical feedstocks has been explored, and successfully demonstrated by Reppe through an wonderful set of reactions achieved by the development of the expertise of being able to handle acetylene in higher pressures Unfortunately, in the 1950’s, with economic considerations taking over, the ethylene and propylene, were obtained from crude oil and natural oil, and were much cheaper Here, is the development of ethylene chemistry, particularly from ethylene to the that of acetaldehyde using Wacker oxidation, that took over as opposed to the conversion of acetaldehyde using mercury and in sulphuric acid in water The latter was sort of taken over by Wacker oxidation, where one could convert ethylene to acetaldehyde, and which could then finally be used for synthesizing or accessing other functionalized chemical feedstocks However, given the fact that the coal reserve on fossil fuel is going to outlast the oil reserve of natural oil and crude oil, the Reppe chemistry is still relevant Still even after 100 years, there are important arguments in favor of developing acetylene chemistry that was initiated by Reppe In this context, we have also looked at two vinylation reactions, particularly, the reaction of alcohol with acetylene in potassium hydroxide and that of hydrogen cyanide with acetylene presence of catalyst giving acrylonitrile With these, we come to the end of today’s class, and we are going to look into some more reactions of Reppe chemistry in subsequent lectures that are going to be taken up in the next few classes, so till then, good bye and thank you!