Sodium acetylide reaction with, alkyl halides

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... 

Lithium acetylide is best prepared from acetylene and LiNH in liquid ammonia. Lithium acetylides are more soluble in liquid ammonia than sodium acetylides and therefore give higher yields (50-80%) on reaction with higher halides. trans-2-Alken-4-ynols have been generally obtained on alkylation of sodium acetylides with... 

Reaction of sodium acetylides with alkyl halides permits conversion of smaller alkynes into larger ones. Practically, the reaction is limited to the use of primary halides because of the great tendency for secondary and tertiary halides to undergo a side reaction, elimination this point will be discussed further (Sec. 8.12) after we have learned something about the nature of acetylides. 

Reaction of sodium acetylides with alkyl halides. Substitution vs, elimination... 

Sodium acetylide can react with alkyl halides in a substitution reaction to produce higher alkynes. The reaction involves substitution of acetylide ion for halide ion as shown ... 

Sodium acetylides react with primary alkyl halides in a substitution reaction to give alkynes of carbon skeleton greater than two. Hence, if methyl iodide is added to sodium acetylide, propyne results ... 

Even if not impaired by side reactions, the condensation of lithium, sodium, or potassium acetylides with alkyl halides is only moderately fast in ethereal media unless alkyl iodides or sulfonates are applied. Good results can be obtained in liquid ammonia. To speed up the reaction with primary alkyl bromides in THF, they may be performed in the presence of 10% of sodium iodide, in other words, under halogen/halogen exchange conditions. Polar solvents such as dimethylformamide (DMF) are mandatory with alkyl bromides and, a fortiori, chlorides. ... 

Reactions in liquid ammonia (cf. Chapter 3, Section III) require a certain amount of care, since the solvent is low boiling (—33 ) and its fumes are noxious. Nevertheless, with reasonable caution, the preparation of an ammonia solution of sodium acetylide can be carried out as described. The reagent so prepared can then be directly used for displacements on alkyl halides or for additions to suitable carbonyl compounds. Examples of both reactions are given. 

The acetylide ion is a strongly basic and nucleophilic species which can induce nucleophilic substitution at positive carbon centres. Acetylene is readily converted by sodium amide in liquid ammonia to sodium acetylide. In the past alkylations were predominantly carried out in liquid ammonia. The alkylation of alkylacetylenes and arylacetylenes is carried out in similar fashion to that of acetylene. Nucleophilic substitution reactions of the alkali metal acetylides are limited to primary halides which are not branched in the -position. Primary halides branched in the P-position as well as secondary and tertiary halides undergo elimination to olefins by the NaNH2. The rate of reaction with halides is in the order I > Br > Cl, but bromides are generally preferred. In the case of a,o)-chloroiodoalkanes and a,to-bromoiodoalkanes. 

The sodium acetylide solution thus prepared may be used for a variety of organic syntheses by the addition of alkyl halides, sulfates, sulfonates, ketones, aldehydes, and esters. Where a fine suspension of the dry acetylide is desired in an inert solvent such as ether or a hydrocarbon, the solvent is added to the ammonia solution and the mixture is stirred whde the ammonia is evaporated. Extra solvent must be used to replace that entrained by the ammonia, the last traces of which are removed by a period of refluxing. Such a suspension gives better yields of, for example, propiolic acid (by the reaction with carbon dioxide) than sodium acetylide prepared in any other way. 

Sodium acetylide (prepared from sodium amide) is useful for the condensations with primary alkyl halides. However, secondary, tertiary, and primary halides branched at the second carbon atom are dehydrohalogenated to olefins by the reagent. Iodides react at a faster rate than bromides and the latter faster than chlorides. Chlorides are rarely used. The bromides are more common for preparative reactions. Sodium acetylide can also react with carbonyl compounds to yield acetylenic carbinols. 

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