Acetylenic hydrogen atom

Replacement, of acetylenic hydrogen atom by bromine, 46, 86 of acyl chlorine atom by fluoride atom using hydrogen fluoride, 46, 3... 

Acetylene as well as its compounds have an acetylenic hydrogen atom with an acid character that can be substituted by a chlorine atom. Thus,with acetylene, calcium hypochlorite and sodium hypochlorite in an acid medium form (this can be formed by calcium carbide hydrolysis) chloroacetylene and dichloro-acetylene, which combust spontaneously in air. 

The dangerous reactions of halogenation, which affect either the acetylenic hydrogen atom or the triple bond have just been described. There are also reactions which lead to accidents and affect one of the acetylenic sites. 

Replacement of the Acetyleneic Hydrogen Atom of Terminal Alkynes... 

Replacement, of acetylenic hydrogen atom by bromine, 45, 86 of acyl chlorine atom by fluorine atom using hydrogen fluoride, 46, 3 of amino hydrogen atoms by thio-carbonyl group, 46, 19 of aromatic bromine atom by potassium /-butoxide, 46, 89 of a-chloro atom in ester using azide ion, 46, 47... 

Substitution of the acetylenic hydrogen atom in 1-ethynylsilatrane by a phenyl group slightly increases the toxicity. The presence of bi- or tricyclic hydrocarbon radicals at the silicon atom leads to a greater increase in toxicity (the LDS0 value for compounds (30) and (41) is 850 and 80 mg/kg, respectively). 

Comparison of the electronic structures of acetonitrile and propyne (methylacetylene). In both compounds, the atoms at the ends of the triple bonds are sp hybridized, and the bond angles are 180°. In place of the acetylenic hydrogen atom, the nitrile has a lone pair of electrons in the sp orbital of nitrogen,... 

Here, the phenyl radical once again attacks the unsamrated bond. However, the steric effect and larger cone of acceptance (the methyl group screens the p carbon atom and makes it less accessible to addition) direct the addition process of the radical center of the phenyl radical to the a carbon atoms of methylacetylene and propylene (the carbon atom holding the acetylenic hydrogen atom). Consequently, crossed beam reactions with complex hydrocarbon molecules can be conducted and valuable information on the reaction pathways can be derived if (partially) deuterated reactions are utilized. 

Acetylenes condense with diethyl carbonate in the presence of sodium ethoxide to substitute a carbethoxyl group in place of the acetylenic hydrogen atom. The condensation is followed, however, by the addition of alcohol to the triple bond. ... 

Interestingly, the yield of the hydrogenated products is substantially higher than the yield of hydrogen evolved in the absence of the unsaturated compounds. This was attributed to the function of surface-activated ethylene or acetylene in scavenging metal-associated H atoms In the absence of the unsaturated substrates, surface-associated H atoms recombine with valence-band holes. This recombination process competes with H-atom dimerization. In the presence of ethylene or acetylene, hydrogen atoms are trapped by the hydrogenation pathway, and consequently, the recombination process is diminished. 

The dipropargyl ether of bisphenol A undergoes oxidative polymerization in which the two terminal acetylenic hydrogen atoms react to form a high molecular weight linear polymer with two conjugated triple bonds in their main chain. 

Apart from the shielding effects mentioned above, the external field of the instrument can induce electron currents which strongly shield acetylenic hydrogen atoms (8 2.3) whereas aromatic protons are strongly deshielded (8 7 to 8). 

Monosubstituted acetylenes or 1-alkynes are called terminal alkynes, and the hydrogen attached to the carbon of the triple bond is called the acetylenic hydrogen atom ... 

The hydrogen bonded to the carbon of a terminal alkyne, called an acetylenic hydrogen atom, is considerably more acidic than those bonded to carbons of an alkene or alkane (see Section 3.8A). The values for ethyne, ethene, and ethane illustrate this point ... 

Acetylenic hydrogen atom (Sections 4.6, and 7.9) A hydrogen atom attached to a carbon atom that is bonded to another carbon atom by a triple bond. 

Acetylenes can react with metal complexes in a number of ways. Internal acetylenes usually coordinate to metal centers in a q -fashion, in whieh the C=C triple bond donates electrons to the metal center, and both carbon atoms form metal-carbon bonds. Terminal acetylenes can also coordinate to metal centers in this fashion, but ean react in other ways as well. In particular, because the acetylenic hydrogen atom is relatively acidic for a hydrocarbon, the C—H bond of terminal acetylenes ean also add oxidatively to metal centers to form metal acetyhde complexes. In some cases, however, a third kind of reaction is seen, in which the terminal aeetylene rearranges to a vinylidene complex, M=C=CHR. 

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