The Structure of Alkynes

All hydrocarbons—alkanes, alkenes, and alkynes—have similar physical properties. They are all insoluble in water but soluble in nonpolar solvents (Section 3.9). They are less dense than water and, like other homologous series, have boiling points that increase with increasing molecular weight (Table 7.2). Alkynes are more linear than alkenes, and a triple bond is more polarizable than a double bond (Section 3.9). These two features cause an alkyne to have stronger van der Waals interactions and, therefore, a higher boiling point than an alkene with the same number of carbons. 

In Table 7.2, what is the smallest alkane, the smallest terminal alkene, and the smallest terminal alkyne that are liquids at room temperature, which is generally taken to be 20 °C to 25 °C  

Why does c/s-2-butene have a higher boiling point than frans-2-butene  

The structure of ethyne was discussed in Section 1.9, where we saw that each carbon is sp hybridized. As a result, each carbon has two sp orbitals and two p orbitals. One sp orbital overlaps the s orbital of a hydrogen, and the other overlaps an sp orbital of the other carbon. (The small lobes of the sp orbitals are not shown.) Because the sp orbitals are oriented as far from each other as possible to minimize electron repulsion, ethyne is a linear molecule with bond angles of 180°. 

Also recall that a carbon-carbon triple bond is shorter and stronger than a carbon-carbon double bond, which in turn, is shorter and stronger than a carbon-carbon single bond, and that a IT bond is weaker than a a bond (Section 1.15). 

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Carbon-13 NMR spectroscopy also is useful in deducing the structure of alkynes. For example, the triple-bonded carbons in alkyl-substituted alkynes resonate in the range of 6 = 65-95 ppm, quite separate from the chemical shifts of analogous alkane (5 = 5 5 ppm) and alkene (S = 100-150 ppm) carbon atoms (Table 10-6). 

Detailed study of the outcome of alkynes hydrophosphorylation reaction have revealed rather complicated picture with several by-products depending on the structure of alkyne, H-phosphonate, catalytic system and reaction conditions (Scheme 8.13) [76]. 

Because of the regioselectivity of alkyne hydration acetylene is the only alkyne structurally capable of yielding an aldehyde under these conditions... 

Ozonolysis is sometimes used as a tool m structure determination By identifying the carboxylic acids produced we can deduce the structure of the alkyne As with many... 

The alkynylation of estrone methyl ether with the lithium, sodium and potassium derivatives of propargyl alcohol, 3-butyn-l-ol, and propargyl aldehyde diethyl acetal in pyridine and dioxane has been studied by Miller. Every combination of alkali metal and alkyne tried, but one, gives the 17a-alkylated products (65a), (65c) and (65d). The exception is alkynylation with the potassium derivative of propargyl aldehyde diethyl acetal in pyridine at room temperature, which produces a mixture of epimeric 17-(3, 3 -diethoxy-T-propynyl) derivatives. The rate of alkynylation of estrone methyl ether depends on the structure of the alkyne and proceeds in the order propar-gylaldehyde diethyl acetal > 3-butyn-l-ol > propargyl alcohol. The reactivity of the alkali metal salts is in the order potassium > sodium > lithium. 

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

It has been shown how alkenylcarbene complexes participate in nickel(0)-me-diated [3C+2S+2S] cycloaddition reactions to give cycloheptatriene derivatives (see Sect. 3.3). However, the analogous reaction performed with alkyl- or aryl-carbene complexes leads to similar cycloheptatriene derivatives, but in this case the process can be considered a [2S+2S+2S+1C] cycloaddition reaction as three molecules of the alkyne and one molecule of the carbene complex are incorporated into the structure of the final product [125] (Scheme 82). The mechanism of this transformation is similar to that described in Scheme 77 for the [3C+2S+2S] cycloaddition reactions. 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

Carbon forms a huge number of binary compounds with hydrogen. Three major categories of these compounds are alkanes, alkenes, and alkynes. An alkane has only single bonds between carbon atoms. The four simplest alkanes, which are shown in Figure 3-7. are methane, ethane, propane, and butane. An alkene, on the other hand, contains one or more double bonds between carbons, and an alkyne has one or more triple bonds between carbon atoms. Figure shows the structures of ethylene, the simplest alkene, and acetylene, the simplest alkyne. 

Hong and coworkers have investigated the cycloaddition chemistry of fulvenes with a wide variety of alkenes and alkynes in great detail [191]. As one example, the reaction of 6,6-dimethylfulvene with benzoquinone is shown in Scheme 6.92. Under microwave conditions in dimethyl sulfoxide (DMSO) at 120 °C, an unusual hetero-[2+3] adduct was formed in 60% yield, the structure of which was determined by X-ray crystallography. The adduct is a structural analogue of the natural products aplysin and pannellin and differs completely from the reported thermal (benzene, 80 °C) Diels-Alder cycloaddition product of the fulvene and benzoquinone (Scheme 6.92) [191]. 

It is interesting to compare the structure of the species (1 ) (X-ray diffraction study for PR3 = PMePh2) with those of the isolobal bridged alkyne complexes (1 ) (L = PR3 or L2 = cyclo-... 

Within group 8, a bis-dinitrogen complex of an iron(O) tridentate pyridinediimine structure has also recently been shown to catalyze the hydrosilylation of alkyne.60 This discovery is a new example of the utility of low-valent iron in catalysis.61... 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

The variety of these products and the complexity of the structural forms observed emphasize the difficulties in elucidating the structures of these compounds by methods other than X-ray analysis. The bonding of the alkyne fragment in these complexes involves major changes in the carbon-carbon distance, as well as deviation of the molecule from linearity. Table VIII (57,100,113,118-128) includes some of the C- C distances for various modes of bonding. 

Further reaction occurs with excess alkyne to yield products which may be formally considered as disubstituted adducts, Ru4(CO)u-(RC2R1)(PhC2R2) (where R = R = Ph R2 = Me R = R = Ph, R2 = Et R = Ph, R = R1 = Me). The structures of these products have not been... 

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