These days most of the excitement in organic chemistry is in synthesizing bigger and more complex molecules. Yet most of the economic value is on the other extreme: over 51 billion pounds of ethylene, the most important olefin used in petrochemicals, are produced every year in the USA (the next three, in case you're interested, are propylene, butadiene, and isoprene). That enormous scale means that exotic reagents and complex synthetic schemes are out of the question.
All petrochemicals start out as reduced hydrocarbons, mostly saturated alkanes and aromatics. Regardless of whether their destination is chemicals or fuel, the alkanes are subjected to cracking (adding a double bond) or reforming (aromatization, isomerization, hydrocracking, or hydroalkylation) to improve their octane rating or make them chemically reactive. Both cracking and reforming are catalyzed reactions. This means the sulfur and metal contaminants have to be removed to avoid poisoning the catalyst.
There are enormous engineering challenges involved in handling such large volumes of chemicals. What might need a paper towel when a reaction goes out of control in the lab may require a fleet of ambulances in a petrochemical plant. Here there are no photos but many tables and flow diagrams that describe the processes.
To get a sense of the scale and complexity of the oil refining industry, I recommend Meyers' Handbook of Petroleum Refining Processes (4th ed. due out this July) instead. But you cannot understand those processes without knowing the chemistry, which tells you why those magnificent refineries were built in the first place.
Most of the reactions here are simple carbocation or free radical reactions.
But even so there are surprises. One is the metathesis of olefins, where
propylene, 2-butene, and ethylene can be interconverted at will:
CH3CH=CHCH3 + CH2=CH2⇌ 2CH3CH=CH2. A similar reaction is disproportionation of toluene, where toluene at 450–530°C and 20 atmospheres in the presence of CoO-MoO3 on ZSM-5 zeolite catalyst is in equilibrium with benzene and xylenes.
This book ain't Shakespeare, and in some places the list of chemicals and reactions starts to read like an encyclopedia. But ... oh, who am I kidding, this book is dull. Even so, after reading it you'll know what we do with all those 27.5 billion pounds of propylene that we synthesize each year. You might not impress many chicks with that bit of knowledge, but it's important.
feb 27, 2016