Acetylene conjugate base

Amide ion is a much stronger base than acetylide ion and converts acetylene to its conjugate base quantitatively... 

Alkylation of acetylene involves a sequence of two separate operations In the first one acetylene is converted to its conjugate base by treatment with sodium amide... 

The acidity of acetylene and terminal alkynes permits them to be converted to their conjugate bases on treatment with sodium amide These anions are good nucleophiles and react with methyl and primary alkyl halides to form carbon-carbon bonds Secondary and tertiary alkyl halides cannot be used because they yield only elimination products under these conditions... 

With a p fa of 16, cyclopentadiene is only a slightly weaker acid than water (pA a = 15.7). It is much more acidic than other hydrocarbons—its for ionization is 10 ° times greater than acetylene, for example—because its conjugate base is aromatic and stabilized by electron delocalization. 

Where do hydrocarbons lie on the acidity scale As the data in Table 8.1 show, both methane (pKa 60) and ethylene (plC, = 44) are very weak acids and thus do not react with any of the common bases. Acetylene, however, has piCa = 25 and can be deprotonated by the conjugate base of any acid whose pKa is greater than 25. Amide ion (NH2-), for example, the conjugate base of ammonia (pKa - 35), is often used to aeprotonate terminal aikynes. 

Thus, in liqnid ammonia, the amide ion may be used to convert an acetylene to its acetylide ion conjugate base (see Section 6.3.4). 

A simple example follows from the reactions considered in Section 4.11.1. We saw that we needed to use a strong base such as the amide ion to form the conjugate base of an acetylene. This reaction was favoured, in that the products were the weaker base acetylide and the weaker acid ammonia. 

The organometallic chemistry of alkynylcyclopropanes involves primarily the formation and reactions of carbon-metal er-bonds. Metals come essentially from the main group elements, with lithium playing a major role. The two metallation sites are the cyclopropyl and the acetylenic positions, which are expected to differ considerably in their acidity values (t-butylacetylene, pKa = 25230, cyclopropane, pKa = 46183) but less in the reactivity of their metal conjugated bases towards electrophiles. 

A subtle, but important, point must be made before we can extend our understanding of acid-base chemistry to the reaction between a Grignard or alkyllithium reagent and a carbonyl group. The data in the table of Br /nsted acids and their conjugate bases reflect the strengths of common acids and bases when they act as Brif/nstedacids or bases. These data predict that methyllithium should react with acetylene to form methane and an acetylide ion, for example. 

Bauerle and coworkers have synthesized a re-conjugated catenane, utilizing Cu(I) templation and a newly developed method for acetylenic homocoupling based on the reductive elimination of platinum to generate the desired C-C bond formation (Scheme 6.16) [65,66]. The C-shaped 57 was first preorganized around Cu(I), resulting in the homoleptic bis-phenanthroline complex 58. After removal... 

Compounds 155 were apparently obtained from initial addition of triphenylphosphine as a good nucleophile to acetylenic ester as a Michael acceptor and concomitant protonation of the intermediate 152 by the NH acid 151. Then the positively charged ion 153 is attacked by the nitrogen of the conjugated base of the NH acid 154 to form phosphoranes 155 containing several functional groups (Scheme 32) <2006ARK55>. 

Compare any two entries in Table 2.1, such as ethanol (CH3CH2OH p/ a =16) and acetylene (HC=CH p Ka = 25), and their conjugate bases, ethoxide (CH3CH2O") and acetyUde (HC=C ). Ethanol is a stronger acid than acetylene, so acetylide is a stronger base than ethoxide. 

The conjugate bases formed by removing a proton from ethane, ethylene, and acetylene are each carbanions—species with a negative charge on carbon. 

Stable phosphorus ylides (50) and (51) have been prepared from the reaction of electron-deficient acetylenic esters, such as dialkyl acetylenedicarboxylates or alkyl propiolates and triphenylphosphine in the presence of 3-chlorotetrahyd-rofuran-2,4-dione (Scheme 10). These reactions are thought to proceed via vinylphosphonium salt intermediates which undergo Michael addition with the conjugate base of the CH-acid. Similar methodology has been used to prepare phosphonium ylide (52) from triphenylphosphine, isatin (indoline-2,3-dione) and dimethyl acetylenedicarboxylate. " ... 

Typical functions of substituted poly acetylenes are based on their (i) high gas permeability and (ii) electronic and photonic properties. The former originates from the rigid main chain and bulky substituents. Though electrical insulators, substituted polyacetylenes are more or less conjugated polymers, and this feature has been utilized to develop their electronic and photonic functions such as photoconductivity, electrochromism, optical nonlinearity and ferromagnetism. 

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