Triple-bond-containing ‘starting materials

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Acetylene.. H—C= C—H, the simplest alkyne, was once widely used in industry as the starting material for the preparation of acetaldehyde, acetic acid, vinyl chloride, and other high-volume chemicals, but more efficient routes to these substances using ethylene as starting material are now available. Acetylene is still used in the preparation of acrylic polymers but is probably best known as the gas burned in high-temperature oxy-acetylene welding torches. 

Analogously, the trienynoate 92 reacted in a 1,10-addition to give the 3,5,7,8-tetra-enoate 93 and the even higher unsaturated allene 95 was obtained from the Michael acceptor 94 containing four double bonds between the triple bond and the acceptor substituent (Scheme 2.33). In the latter case, however, the yield was only 26% this is presumably due to the reduced thermal stability of the starting material and/or the addition product (the 1,12-adduct 95 was the only isolable reaction product, apart from polymeric compounds) [57]. 

Similarly, allenes [32] and alkynols [33] were used as starting materials and their carbonylation provides /3,y-unsaturated acids and unsaturated diacids, respectively. The specific reactivity of alkynols is explained by three formal steps during nickel catalysis (i) carbonylation of the triple bond leading to an acid containing an allylic alcohol moiety (ii) second carbonylation of the double bond to provide a hydroxydiacid and (iii) a dehydration step giving the corresponding unsaturated diacid (Scheme 2). 

Acetylene is listed as one of the starting materials, and we have good methods (Section 9-7) for making carbon-carbon bonds next to triple bonds, by using acetylide ions as nucleophiles. We can break the target structure into three pieces, each containing no more than four carbon atoms. 

Nitriles can also be used as starting materials for the synthesis of ketones. Discussed in Chapter 21, nitriles are compounds containing the cyano (—C=N) functional group. Since nitrogen is more electronegative than carbon, the —C=N triple bond is polarized like the C=O bond of the carbonyl group. Nucleophiles can add to the — C = N triple bond by attacking the electrophilic carbon atom. 

Vinyl bromides can themselves be made by elimination reactions of 1,2-dibromoalkanes. Watch what happens when 1,2-dibromopropane is treated with three equivalents of K NLi first, elimination to the vinyl halide then, elimination of the vinyl halide to the alkyne. The terminal alkyne is amply acidic enough to be deprotonated by R2NU, and this is the role of the third equivalent. Overall, the reaction makes a lithiated alkyne (ready for further reactions) from a fully saturated starting material. This may well be the first reaction you have met that makes an alkyne from a starling material that doesn t already contain a triple bond, making an alkyne from 1,2-dibromopropane... 

Alkynes and functionally substituted alkynes may be synthesized either from starting materials that do not already contain the triple bond by elimination reactions, or the ethynyl and alkynyl groups may be introduced onto substrates by nucleophilic or electrophilic substitution reactions. 

Plasma-induced polymerization is essentially conventional (molecular) polymerization that is triggered by a reactive species created in an electric discharge. In order for one to form polymers by plasma-induced polymerization, the starting material must contain polymerizable structures, such as olefinic double bonds, triple bonds, or cyclic structures. 

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