Alkynes acidic hydrogen

First, attempt to identify the most acidic hydrogen in the starting material, based on hybridization or on the nature of neighboring atoms. Explain your rationale. Next, examine the electrostatic potential map for starting material alkyne). Which hydrogen appears to be most electron poor Is this the one that you predicted What makes this hydrogen more electron poor than the others ... 

The acidic hydrogen in terminal alkynes can readily be replaced by silver, in a diagnostic test. [(Me3P)Ag(C=CPh)] has a polymeric structure while [(Ph3P)Ag(C=CPh)]4 is made of [Ag(PPh3)2]+ and [Ag(C=CPh)2] fragments linked so that the silver atoms form a square [147]. 

The position of the triple bond can alter the reactivity of the alkynes. Acidic alkynes react with certain heavy metal ions, e.g. Ag+ and Cu+, to form precipitation. Addition of an alkyne to a solution of AgNOs in alcohol forms a precipitate, which is an indication of hydrogen attached to the triple bonded carbon. Thus, this reaction can be used to differentiate terminal alkynes from internal alkynes. 

Terminal alkynes by virtue of the presence of an acidic hydrogen atom can be converted into the corresponding alkynylsodium or alkynylmagnesium halide (see Section 5.4.2, p. 531) which may then be alkylated to give a homologous alkyne. 

Carbon nucleophiles are very useful species because their reactions with carbon electrophiles result in the formation of carbon—carbon bonds. Section 10.8 introduced acetylide anions as nucleophiles that could be used in Sm2 reactions. These nucleophiles are prepared by reacting 1-alkynes with a strong base such as sodium amide. The relatively acidic hydrogen on the. vp-hybridized carbon is removed in this acid-base reaction ... 

Utimoto et al. [145] reported a procedure for the synthesis of macrocyclic ynones by intramolecular acylation of )-(trimethylsilyl)ethynylalkanoyl chlorides in the presence of Lewis acid. For example, in the synthesis of f — )-muscone 216), cyclization of alkyne acid chloride 238 gave the macrocycle 239 in 52% yield, which was hydrogenated to f — )-muscone (Scheme 80). 

The most common mode of reactivity of Y—H with unsaturated hydrocarbons in the absence of acidic hydrogens is 1,2-addition as shown in Scheme 5. The hydrides react with alkenes to form alkyl complexes, with 1,2-propadiene to form allyl systems, and with alkynes to form cis-alkenyl complexes (45). Crystallographic confirmation of the 1,2-addition mode of Y—H with unsaturated hydrocarbons was obtained using terf-butylnitrile as a substrate. The structure of the alkylideneamido product, [(C5Hs)2Y(/Li-N==CHCMe3)]2, was determined by X-ray crystallography. 

The interaction of a lanthanide metal with a substrate such as 3-hexyne could occur in several ways 60) by it complex formation, by oxidative addition into a C—H bond, or by reduction involving radical species. Subsequent lanthanide metal vapor studies were designed to test some of these possibilities. Co-condensation of lanthanide metal vapor with reagents containing acidic hydrogen atoms, e.g., terminal alkynes, demonstrated that oxidative addition of C—H was a viable reaction [Eqs. (32) and (33)] 51). These reactions also provided access to a new class of... 

Terminal alkynes have weakly acidic hydrogens that can be abstracted by strong bases such as sodium amide. 

Terminal alkynes with tetrahydroaluminates lead to substitution of the acidic hydrogen (see 5.3.3.5.2). However, in the presence of a small amount of i-Bu2AlH more than 90 % hydroalumination product is obtained when NaAlH and phenyl ethyne react in diglyme . This is a useful method of trans-hydroalumination of acetylenes, mainly... 

Solutions of Ca, Sr and Ba metal in liq NHj react with acidic hydrocarbons. Thus the complexes (C5H5)jCa n NH3 and (C9H7)2Ca-n NH3 are formed from cyclopenta-diene or indene with Ca metal. Analogously the acidic hydrogen atoms of alkynes are substituted by alkaline-earth metals ... 

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