Mechanisms[ edit ] The aldol reaction may proceed by two fundamentally different mechanisms. Carbonyl compounds, such as aldehydes and ketones, can be converted to enols or enol ethers.
Upsidedowncake Rubber is an elastomer—that is, a polymer that has the ability to regain its original shape after being deformed. Rubber is also tough and resistant to weathering and chemical attack. Elastomers can be naturally occurring polymers, such as natural rubber, or they can be synthetically produced substances, such as butyl rubber, Thiokol, or neoprene.
For a substance to be a useful elastomer it must possess a high molecular weight and a flexible polymer chain. Natural Rubber Natural rubber is one of nature's unique materials.
The Native Americans of tropical South America's Amazon basin knew of rubber and its uses long before Christopher Columbus's explorations brought it to the attention of Europeans. The Indians made balls of rubber by smoking the milky, white latex of trees of the genus Hevea that had been placed on a wooden paddle, to promote water evaporation and to cure the substance.
In a Spanish writer enumerated the practical uses of rubber. He reported that the Indians waterproofed their cloaks by brushing them with this latex and made waterproof shoes by coating earthen molds with it and allowing these coatings to dry.
In interest was revived in this unusual substance when French mathematical geographer and explorer Charles-Marie de La Condamine — sent several rolls of crude rubber to France with an accompanying description of products made from it by the South American natives.
Although it met with some use in waterproofing boots, shoes, and garments, it largely remained a museum curiosity. Crude rubber possessed the valuable properties of elasticity, plasticity, strength, durability, electrical nonconductivity, and resistance to water; however, products made from it hardened in winter, softened and became sticky in summer, were attacked by solvents, and smelled bad.
Early Uses Rubber, sometimes called "gum-elastic," was known to the Indians by the Reaction paper in natural science of caoutchouc from caa, "wood," and o-chu, "to flow or to weep".
In English chemist and Unitarian clergyman Joseph Priestley —the discoverer of oxygen, proposed the name "rubber" for the substance because it could be used to erase pencil marks by its rubbing on paper in lieu of previously used bread crumbs. In rubber was first used commercially when English manufacturer Samuel Peal patented a method for waterproofing cloth by treating it with a solution of rubber in turpentine.
In the modern rubber industry began when English coachmaker and inventor Thomas Hancock — established the first rubber factory.
He was the first to compound rubber with other materials to be shaped into molds, a common modern industrial practice. In Scottish chemist and inventor Charles Macintosh — began to manufacture double-textured rainproof garments known as "mackintoshes.
Composition and Structure Crude rubber is primarily hydrocarbon in nature. In English chemist Michael Faraday — analyzed natural rubber and found it to have the empirical simplest formula C5H8, along with 2 to 4 percent protein and 1 to 4 percent acetone-soluble materials resins, fatty acids, and sterols.
In English chemist Charles Hanson Greville Williams — confirmed Faraday's analysis and in distilled natural rubber to obtain the pure monomer, which he named isoprene.
He determined isoprene's vapor density and molecular formula, and he showed that it polymerizes to a rubbery product—an observation that led to the notion that rubber is a linear polymer of isoprene, proposed in by English chemist Samuel Shrowder Pickles — The molecular weights of rubber molecules range from 50, to 3, Sixty percent of the molecules have molecular weights of greater than 1, The repeating unit in natural rubber has the cis configuration with chain extensions on the same side of the ethylene double bondwhich is essential for elasticity.
If the configuration is trans with chain extensions on opposite sides of the ethylene double bondthe polymer is either a hard plastic naturally occurring gutta-percha, obtained from the leaves of Palaquium, a species of sapotaceous Malaysian and East Indies trees that was used for wire and cable coating during the nineteenth century; or a substance like gutta-percha balata, obtained from Mimusops globosa, trees native to Panama and South Americaused for modern golf ball covers.
Vulcanization Because there are few if any cross-links in the chains of rubber molecules, natural rubber is thermoplastic; that is, it becomes soft and sticky in summer and hard and brittle in winter. It is also malodorous and softened or dissolved by various solvents, as noted.
These undesirable properties of natural rubber were not overcome untilwhen American inventor Charles Goodyear —at the end of five years of constant experimentation, accidentally placed a sample of rubber mixed with sulfur and litharge lead oxide, PbO on a hot stove in Woburn, Massachusetts.
The operation converted rubber into a heavily cross-linked, and therefore insoluble and infusible, thermosetting polymer or "thermoset. Goodyear later used the term, but only reluctantly. In practice, vulcanization was so simple that many persons used it without paying royalties, and Goodyear spent much of his time contesting approximately sixty infringements of his patent.
His name lives on in Goodyear tires and Goodyear blimps. Paradoxically, neither Goodyear nor any of his family members or descendents were involved with the Goodyear Tire and Rubber Company, whose founder, Frank A.
Seiberling, named it to honor one of America's most famous inventors and the founder of an industry that is indispensable to modern life. In Goodyear's brother Nelson used sulfur to convert natural rubber into ebonite, the first thermosetting plastic. The Modern Rubber Industry Vulcanization marked the birth of the modern rubber industry, and although later discoveries have somewhat modified Goodyear's original procedure, today it remains essentially the same as his process of Vulcanization is still an imperfectly understood chemical reaction between rubber and sulfur.
It results in cross-linking between linear chains of rubber molecules and prevents slippage of the chains as the material retains the desired elasticity. Accelerators permit the reaction to occur at lower temperatures and in less time, and antioxidants prolong the life of rubber products by reducing the deterioration that is caused by atmospheric oxygen or ozonewhich breaks covalent bonds and lowers the molecular weight.
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Nowadays, many scientists still use his theory in science research, like the scientists who find. The aldol reaction is a means of forming carbon–carbon bonds in organic chemistry. Discovered independently by the Russian chemist Alexander Borodin in and by the French chemist Charles-Adolphe Wurtz in , the reaction combines two carbonyl compounds (the original experiments used aldehydes) to form a new β-hydroxy carbonyl caninariojana.com products are known as aldols, from .
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