Acetylide ions reactions

Because acetylene is a far weaker acid than water and alcohols these substances are not suitable solvents for reactions involving acetylide ions Acetylide is instantly converted to acetylene by proton transfer from compounds that contain —OH groups... 

The properties of organometallic compounds are much different from those of the other classes we have studied to this point Most important many organometallic com pounds are powerful sources of nucleophilic carbon something that makes them espe cially valuable to the synthetic organic chemist For example the preparation of alkynes by the reaction of sodium acetylide with alkyl halides (Section 9 6) depends on the presence of a negatively charged nucleophilic carbon m acetylide ion... 

We won t study the details of this substitution reaction until Chapter 11 but for now can picture it as happening by the pathway shown in Figure 8.6. The nucleophilic acetylide ion uses an electron pair to form a bond to the positively polarized, electrophilic carbon atom of bromomethane. As the new C-C bond forms, Br- departs, taking with it the electron pair from the former C-Br bond and yielding propyne as product. We call such a reaction an alkylation because a new alkyl group has become attached to the starting alkyne. 

The alkylation reaction is limited to the use of primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane. 

Ketones react with acetylide ion (Section 8.7) to give alcohols. For example, the reaction of sodium acetylide with 2-butauone yields 3-methy -l-pentyn-3-ol ... 

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

Organic compounds can be metalated at suitably acidic positions by active metals and by strong bases.The reaction has been used to study the acidities of very weak acids (see p. 228). The conversion of terminal alkynes to acetylid ions is one... 

The acetylide ion, C/, is isoelectronic with N2, CO, and CN. The reaction producing acetylene from calcium carbide can be shown as... 

Ab initio calculations at the HF/6-31G level have been used to explore energy changes, structural variation, and electron density shifts during jr-face selective addition of substituted acetylide ions to cyclohexanone and cyclohexanethione. Charge polarization of the jr-bond on approach of the nucleophile is such that the carbonyl carbon becomes considerably electron deficient for most of the reaction path (and may... 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetylide ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, halide ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4,-chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4,-sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

Some carhon-carbon bond forming reactions with carbon nucleophiles cyanide ion, acetylide ion and Grignard reagents. 

Acetylide ions have also been reported as unreactive in SrnI reactions with aryl45 and heteroaryl134 halides. 

In these alkylation reactions primary alkyl halides (the bromide for preference) should be used as the alkylating agents, since secondary and tertiary halides undergo extensive olefin-forming elimination reactions in the presence of the strongly basic acetylide ion. A typical synthesis is that of hex-l-yne (Expt 5.26). 

B-6. Which of the following produces a significant amount of acetylide ion on reaction with acetylene ... 

The best route to this alcohol is through reaction of an acetylide ion with ethylene oxide. 

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. 

Using retrosynthetic analysis, we recognize that the c/.v-epoxide can be prepared from the c/s-alkene. The m-alkene can be prepared by catalytic hydrogenation of an alkyne. Finally, substituted alkynes can be prepared by nucleophilic substitution reactions using acetylide ion nucleophiles (see Section 10.8). On the basis of this analysis, the synthesis reported in the literature was accomplished as shown in Figure 23.3. 

Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

Acetylide ions are strong nucleophiles. In fact, one of the best methods for synthesizing substituted alkynes is a nucleophilic attack by an acetylide ion on an unhindered alkyl halide. We consider this displacement reaction in detail in Section 9-7A. 

Two different approaches are commonly used for the synthesis of alkynes. In the first, an appropriate electrophile undergoes nucleophilic attack by an acetylide ion. The electrophile may be an unhindered primary alkyl halide (undergoes Sn2), or it may be a carbonyl compound (undergoes addition to give an alcohol). Either reaction joins two fragments and gives a product with a lengthened carbon skeleton. This approach is used in many laboratory syntheses of alkynes. 

If this Sn2 reaction is to produce a good yield, the alkyl halide must be an excellent SN2 substrate It must be methyl or primary, with no bulky substituents or branches close to the reaction center. In the following examples, acetylide ions displace primary halides to form elongated alkynes. 

We have already discussed some of the most important reactions of alkynes. The nucleophilic attack of acetylide ions on electrophiles, for example, is one of the best methods for making more complicated alkynes (Section 9-7). Now we consider reactions that involve transformations of the carbon-carbon triple bond itself. 

Acetylenic alcohols result when acetylides add to ketones and aldehydes (Section 9-7B). Reaction of the acetylide ion with 2-methylpropanal gives one of the groups needed on the triple bond. 

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