The Preparation of Acetylene

Introduction. Acetylene has the empirical formula C2Hs. It shows a greater unsaturation than ethylene, in that it can add four univalent groups to each carbon atom. This xmsaturation is indicated by a triple bond between the two carbon atoms HC=CH. Acetylene is produced in the thermal decomposition of many hydrocarbons. Of theoretical interest is its formation from hydrogen and carbon in the electric arc. It is prepared industrially by the pyrolysis of methane, and by the action of water on calcium carbide. The last method is employed for preparation of small quantities in the laboratory  

Although acetylene adds four monovalent atoms while ethylene adds only two, it does not follow that the greater unsaturation is accompanied by a more rapid rate of addition. As a rule, the activity of the triple bond, measured by the rate of addition, is lower than the activity of the double bond. Acetylene forms explosive mixtures with air, and care should be exercised by the student to keep the generator away from all flames. 

Wash each precipitate once with water by decantation. Filter a small portion, and place a pinch of the solid acetylide on the end of a wire or spatula and heat it in the flame. Punch a hole in the filter paper, and wash the remaining metallic acetylide into a test tube. Flush the filter paper with tap water before throwing it into the residue jar. Silver and copper acetylides are very explosive when dry. The suspension of acetylides is allowed to settle and the water is decanted. Add 5 ml of dilute nitric acid in each tube. Warm gently and note the ga.s evolved. Replace the tube in the rack until all the acetylide is decomposed. 

The ammoniacal silver nitrate is prepared by the addition of ammonium hydroxide to 2 ml of 0.1 N silver nitrate until the precipitate which forms just redissolves. Sufficient water is added 

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To a mixture of O.BB mol of anhydrous lithium chloride and 100 ml of OMSO was added a solution of 0.40 mol of the acetylenic tosylate (for a general procedure concerning the preparation of acetylenic tosylates, see Chapter VllI-3, Exp. 3) in IBO ml of DMSO. The flask was equipped for vacuum distillation (see Fig. 5). Between the receiver, which was cooled at -75°C, and the water-pump was placed a tube filled with KOH pellets. The apparatus was evacuated (10-20 mmHg) and the flask gradually heated until DMSO began to reflux in the column. The contents of... 

Equations 37 and 38 show a convenient procedure for the preparation of acetylenes and polyenes which utilizes an excess of f-BuOK as a base in THF or THF-f-BuOH. 

The N-aminoaziridine version7 of the a,/3-epoxyketone->alkynone fragmentation is a possible alternative in situations where the simple tosylhydrazone version6-9 fails. The tosylhydrazone method often gives good yields at low reaction temperatures, but it tends to be unsuccessful with the epoxides of enones that are not cyclic or are not fully substituted at the /5-carbon atom. For example, it has been reported9 that 2,3-epoxycyclohexanone docs not produce 5-hexynal by the tosylhydrazone route. The A-aminoaziridine method can also be recommended for the preparation of acetylenic aldehydes as well as ketones. 

It occurs with the alkyls, aryls or acetylides of metals more electropositive than magnesium, but including Grignard reagents, and is often carried out by adding a solution of the organometallic compound in an inert solvent to a large excess of powdered, solid C02 it is a particularly useful method for the preparation of acetylenic acids. The Kolbe-Schmidt reaction (p. 291) is another example of carbanion carbonation. 

The preparation of acetylenic ketones from soluble silver acetylenides and acid chlorides was reported in 1956 (Scheme 108).512 Analogously to that report, the reaction of silver acetylenides with acylpyridinium salts was reported to proceed effectively.513 This chemistry was applied to the synthesis of cotarnine derivatives, as exemplified in Scheme lQ9.514 514a... 

J. Klein, "Propargylic Metallahon" D.A. Ben Efraim, "The Preparation of Acetylenes and their Protection" P.F. Huldrik, A.M. Huldrik, "Applications of Acetylenes in Organic Synthesis" F. Thdron, M. Veraey, R. Vessiere, "Rearrangements Involving Acetylenes" W.D. Huntsman, "Synthetic Acyclic... 

Another method for the preparation of acetylenic carboxylates involves the reaction of 1-alkenylboranes with CO and alcohols (equation 135). The alkenyl boranes are readily prepared from alkynes by addition of boranes. PdCl2 was again the catalyst, with NaOAc as base and benzoquinone as oxidant.541... 

Having established that the end game of the proposed synthesis of cylindrospermopsin (1) from bromoketone 4 was viable, we turned our attention to the preparation of acetylene 5 from 4-methoxy-3-methylpyridine (7). 4-Methoxy-3-methylpyridine (7) was prepared by modifications of the literature procedure.17,18 3-Methyl-4-nitropyridine A-oxide (27),19 was treated with K2CO3 in methanol at 70 °C to displace the nitro group to... 

Bowden, K., I. M. Heilbron, E. R. H. Jones, and B. C. L. Weedon (1946) Researches on acetylenic compounds. Part I. The preparation of acetylenic ketones by oxidation of acetylenic carbinols and glycols. Journal of the Chemical Society (London), 39-45. 

How to Use the Book to Locate Examples of the Preparation or Protection of Monofunctional Compounds. Examples of the preparation of one functional group from another are located via the monofunctional index on p. xiii, which lists the corresponding section and page. Thus Section 1 contains examples of the preparation of acetylenes from other acetylenes Section 2, acetylenes from carboxylic acids and so forth. 

Examples of alkylation, dealkylation, homologation, isomerization, and transposition are found in Sections 1, 17, 33, and so on, lying close to a diagonal of the index. These sections correspond to such topics as the preparation of acetylenes from acetylenes carboxylic acids from carboxylic acids and alcohols, thiols, and phenols from alcohols, thiols, and phenols. Alkylations that involve conjugate additions across a double bond are found in Section 74 (alkyls, methylenes, and aryls from olefins). 

Another procedure for the preparation of acetylenes 8 and 9 is the cleavage of substituted... 

A by-product from the reaction of acetone and sodium acetylide is the acetylenic diol, (CHj)2C(OH)C= C(OH)(CH,)j, formed by condensation of two molecules of acetone with one molecule of sodium acetylide. A general method for the preparation of acetylenic diols of this type is from calcium carbide, potassium hydroxide, and ketones. Ciethynyl glycols in which the triple bonds are separated by two or four carbon atoms are made from sodium acetylide and a- or /S-diketones. ... 

Many more dehydrohalogenations are, however, being carried out in aqueous or alcoholic solution under reflux. Under these conditions the reaction is particularly suitable for the preparation of acetylenes which will not prototropically isomerize, such as arylacetylenes. The following examples illustrate the large number of structural types which can be converted to acetylenes while surviving the strong reaction conditions (equations 9, 29-35). A facile conversion of aryl ketones and... 

R = Me) in 30-48% yields. With KF in DMSO even higher yields are obtained. Thus acetylenes 15 are obtained in 50-94% yields from 14 at 80-120 °C, whereas allenes 17 (R = Me) are obtained from 15 (R = Me) in 70-93% yields at 100 °C. Use of KF in the presence of a crown ether increases the rate of elimination. The above results make the fluoride ion an effective elimination agent for the preparation of acetylenes from vinyl halides in which the hydrogen and halogen are trans-related . 

The preparation of acetylenes and their protection 3. Elimination of sulphonic acids °... 

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