Ethyne, acidity water

CH2 CH C CH. Colourless gas with a sweet odour b.p. 5°C. Manufactured by the controlled low-temperature telomerization of ethyne in the presence of an aqueous solution of CuCI and NH Cl. Reduced by hydrogen to butadiene and, finally, butane. Reacts with water in the presence of HgSO to give methyl vinyl ketone. Forms salts. Forms 2-chloro-butadiene (chloroprene) with hydrochloric acid and certain metallic chlorides. 

Nonmetals form covalent molecular hydrides, which consist of discrete molecules. These compounds are volatile and many are Bronstcd acids. Some are gases— for example, ammonia, the hydrogen halides (HF, HC1, HBr, HI), and the lighter hydrocarbons such as methane, ethane, ethene, and ethyne. Liquid molecular hydrides include water and hydrocarbons such as octane and benzene. 

Alkynes, unlike alkenes, are not hydrated readily in aqueous acid unless a mercuric salt is present as a catalyst. Also, the products that are isolated are either aldehydes or ketones instead of alcohols. Even though the addition of one molecule of water to ethyne probably gives ethenol (vinyl alcohol) initially, this compound is unstable relative to its structural isomer (ethanal) and rapidly rearranges ... 

Water is too weak a base to accept protons from alkynes consequently no measurable concentration of H30 is expected from the ionization of alkynes in dilute aqueous solutions. Therefore we have no quantitative measure of 1-alkyne acidity in aqueous solution other than that it probably is about 1010 times less acidic than water, as judged from measurements in other solvents to be discussed shortly. In the gas phase, however, the situation is reversed, and ethyne is a stronger acid than water ... 

This reaction shows that the methide ion is a very strong Bronstcd base. The species C22- is the acetylide ion, and the carbides that contain it are called acetylides. The acetylide ion is also a strong Bronstcd base, and acetylides react with water to produce ethyne (acetylene, the conjugate acid of the acetylide ion) and the corresponding hydroxide (Section 14.12). Calcium carbide, CaC2, is the most common saline carbide. 

Methane Ethene Carbon (Ethylene) dioxide Ethyne (Acetylene) Ammonia Nitrous acid Hydrogen cyanide Water Formaldehyde... 

Draw the enol and ketone product formed when cyclopentyl-ethyne is treated with mercuric sulfate and mercuric acetate in water containing phosphoric acid. 

Allynes can be hydrated in the presence of acid and HgS04 by electrophilic addition of a molecule of water to the triple bond. The reaction proceeds by way of a carbocation intermediate. Hydration of acetylene (ethyne) produces acetaldehyde (ethanal). Outline the steps that occur in this transformation. 

Hydrocarbons are the weakest acids (and thus, their conjugate bases are the strongest bases). The best estimates for the pKa s of saturated hydrocarbons, that is, R3C-H RsC" + H (relative to water) are of the order of =50. A proton on the carbon-carbon double bond of an alkene is some 4-5 pKa units more acidic (and that is about the same for unsubstituted arenes, i.e., benzene itself has a reported pKa of =37). Alkynes are considerably more acidic. The pKa of ethyne (acetylene HC CH]) is about 23. 

The acid-catalyzed hydration of alkynes (Table 6.7, example 2) is commonly carried out using mercury (11) salts, such as mercuric sulfate (HgS04), as catalysts. The addition (Scheme 6.67) appears to involve a bridged mercurinium ion, which, for unsymmetrical cases such as 1-alkynes other than ethyne (acetylene [HC CH]), is subsequently attacked by water (FI2O) at the carbon that best supports a positive charge. The regiochemistry of Markownikoff addition, seen with alkenes, is followed. 

Ethyne has a p value of 25, water has a pK value of 15.7, and ammonia (NH3) has a p a value of 36. Draw the equation, showing equilibrium arrows that indicate whether reactants or products are favored, for the acid-base reaction of ethyne with a. HO. b. NH2. 

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