Isoprene from plants

Trees and shrubs contain a group of fragrant compounds called terpenes. The simplest terpene is isoprene. All other terpenes are built around carbon skeletons constructed from one or more isoprene units. Plants emit terpenes into the atmosphere, as anyone who has walked in a pine or eucalyptus forest will have noticed. The possible effect of terpenes on the concentration of ozone in the troposphere has been the subject of much debate and has led to careful measurements of rates of reaction with ozone. 

Biological emissions of particles may occur from plants and trees additionally, volatile organics such as isoprene and terpenes can react in the air to form small particles. 

Extractive Distillation Recovery of Isoprene. A typical flowsketch and material balance of distillation and solvent recovery towers for extracting isoprene from a mixture of cracked products with aqueous acetonitrile appears in Figure 13.26. A description of the flowsheet of a complete plant is given in Example 2.10. In spite of the fact that several trays for washing by reflux are provided, some volatilization of solvent still occurs so that the complete plant... 

Isoprene (melting point -146°C, boiling point 34°C, density 0.6810) may be produced by the dehydrogenation of iso-pentane in the same plant used for the production of butadiene. However, the presence of 1,3-pentadiene (for which there is very little market) requires a purification step. One method produces isoprene from propylene. Thus, dimerization of propylene to 2-methyl-1-pentene is followed by isomerization of the 2-methyl-1-pentene to 2-methyl-2-pentene, which upon pyrolysis gives isoprene and methane. 

Terpenes are a diverse family of compounds with carbon skeletons composed of five-carbon isopentyl (isoprene) units. Terpenes are commonly isolated from the essential oils of plants the fragrant oils that are concentrated from plant material by steam distillation. The term essential oils literally means oils derived from the essence of plants. They often have pleasant tastes or aromas, and they are widely used as flavorings, deodorants, and medicines. Figure 25-12 shows the structures of four terpenes that are isolated from essential oils. 

The primary source of isoprene today is as a by-product in the production of ethylene via naphtha cracking. A solvent extraction process is employed. Much less isoprene is produced in the crackers than butadiene, so the availability of isoprene is much more limited. Isoprene also may be produced by the catalytic dehydrogenation of amylenes, which are available in C-5 refinery streams. It also can be produced from propylene by a dimerization process, followed by isomerization and steam cracking. A third route involves the use of acetone and acetylene, produced from coal via calcium carbide. The resulting 3-methyl-butyne-3-ol is hydrogenated to methyl butanol and subsequently dehydrogenated to give isoprene. The plants that were built on these last two processes have been shut down, evidently because of the relatively low cost of the extraction route. 

Sharkey, T.D. and Yeh, S. (2001) Isoprene emission from plants. Annu Rev Plant Physiol Plant Mol. Biol., 52, 407-36. 

Silver, G.M. and Fall, R. (1991) Enzymatic synthesis of isoprene from dimethylallyl diphosphate in aspen leaf extracts. Plant Physiol, 97,1588-91. 

There definitely are very large emissions of higher hydrocarbons from plants, including ethylene. This has been definitely discovered above the tropical forests in Brazil. The most active emissions are actually isoprene and the terpenes, but there are many other hydrocarbons which are chemically very reactive. 

The isoprene unit is one of nature s favorite building blocks. It occurs not only in rubber, but in a wide variety of compounds isolated from plant and animal sources. For example, nearly all the terpenes (found in the essential oils of many plants) have carbon skeletons made up of isoprene units joined in a regular, head-to-tail way. Recognition of this fact—the so-called isoprene rule—has been of great help in working out structures of terpenes. 

Terpenoids form a large and the most structurally diverse family of natural products derived from repetitive fusion of branched C5 isoprene units. The name terpenoid, or terpene, derives from the fact that the first members of the class were isolated from turpentine ( terpentin in German). They play numerous essential roles in both plant and animal life. Those from plant origin are essential for several plant processes, including respiration, photosynthesis, growth,... 

Harley P.C, R.K. Monson and M.T. Lerdau Eeological and evolutionary aspects of isoprene emission from plants, Oecologia 118 (1999) 109-123. 

Many other terpenes are plant products familiar to us because of their distinctive aromas. Geraniol, the familiar scent of geraniums, is a molecule made up of two isoprene units. Purified from plant sources, geraniol is the active ingredient in several natural insect repellants. These can be applied directly to the skin to provide four hours of protection against a variety of insects, including mosquitoes, ticks, and fire ants. 

Sesquiterpenes (sesquiterpenoids). A structurally highly diverse class of terpenoids with 15 carbon atoms skeleton derived biosynthetically from famesyl pyrophosphate (FPP) ( famesol, isoprene rule, ter-penes). More than 70 different ring systems are formed by enzyme-catalyzed cyclization of the linear parent structure these cyclic structures can be further modified by 1,2- and 1,3-hydride shifts, renewed cycliza-tions, hydroxylations, and other subsequent reactions. S. are widely distributed in plants, fiingi, and animals but are less common in bacteria. Specific biosynthetic routes are often characteristic for certain organisms. Thus, basidiomycetes preferentially use humulene as the basis for the syntheses of protoilludanes, illu-danes, lactaranes, hirsutanes, and related S. skeletons. Individual S. systems are also known for liverworts and marine organisms. In addition, liverworts often contain the optical antipodes of S. known from plants. 

The tenn terpenes originates from turpentine lat. balsamum terebinthinae). Turpentine, the so-called "resin of pine trees", is the viseous pleasantly smelling balsam which flows upon cutting or carving the bark and the new wood of several pine tree species (Pinaceae). Turpentine contains the "resin acids" and some hydrocarbons, which were originally referred to as terpenes. Traditionally, all natural compounds built up from isoprene subunits and for the most part originating from plants are denoted as terpenes (section 1.2). 

Sometimes called isoprenoids, terpenoids are a large and diverse class of naturally occurring organic chemicals similar to terpenes, derived from five-carbon isoprene units. Plant terpenoids are used extensively for their aromatic qualities. Just like terpenes, the terpenoids can be classified according to the number of isoprene units. They play an important role in traditional herbal remedies and are under investigation for antibacterial, anti-neoplastic, and other pharmaceutical functions. ... 

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