Linear molecule alkynes

Now consider the alkynes, hydrocarbons with carbon-carbon triple bonds. The Lewis structure of the linear molecule ethyne (acetylene) is H—O C- H. To describe the bonding in a linear molecule, we need a hybridization scheme that produces two equivalent orbitals at 180° from each other this is sp hybridization. Each C atom has one electron in each of its two sp hybrid orbitals and one electron in each of its two perpendicular unhybridized 2p-orbitals (43). The electrons in the sp hybrid orbitals on the two carbon atoms pair and form a carbon—carbon tr-bond. The electrons in the remaining sp hybrid orbitals pair with hydrogen Ls-elec-trons to form two carbon—hydrogen o-bonds. The electrons in the two perpendicular sets of 2/z-orbitals pair with a side-by-side overlap, forming two ir-honds at 90° to each other. As in the N2 molecule, the electron density in the o-bonds forms a cylinder about the C—C bond axis. The resulting bonding pattern is shown in Fig. 3.23. 

The expansions, Eqs. (5.36) and (5.37), of IT(p) can be used to reduce dimensionality by focusing on the radial coefficients [157]. Thakkar and coworkers [157] formulated empirical mles to help understand the smaU-p behavior of II( p) for linear molecules using only the first four n p) terms. This technique was subsequently used to analyze bonding in 14-electron diatomics [337], strong hydrogen bonding [338], substiment effects in alkynes and cyanides [339], and bonding in alkaline-earth oxides [340,341]. 

Further bonding is possible because each carbon has half-filled p orbitals. Hence, the 2py and 2pz orbitals of each atom can overlap side-on to form two n bonds (Following fig.). The n bond formed by the overlap of the 2py orbitals is represented in dark gray. The n bond resulting from the overlap of the 2pz orbitals is represented in light gray. Alkynes are linear molecules and are reactive due to the relatively weak n bonds. 

The alkynes are a family of hydrocarbons that contain a carbon-carbon triple bond, with the general formula C H2 2- The first member of the family is ethyne (old name acetylene). Ethyne is a linear molecule ... 

The functional group of an alkyne is a carbon-carbon triple bond. The simplest alkyne is ethyne, C2H2. Ethyne is a linear molecule all of its bond angles are 180° (Figure 1.10). 

The geometry about each carbon atom in ethyne is linear, making ethyne a linear molecule. Eth3me (or acetylene) is commonly used as fuel for welding torches. The names and sfructures of several other alkynes are shown in Table 18.6. Like alkenes, the alkynes do not have familiar uses ofher than their presence as minority components of gasoline. 

Nitriles are linear molecules with short carbon-nitrogen bond distances. The sp hybridization of the carbon and nitrogen atoms ensures this. Figure 18.19 es orbital pictures of both the a and n systems, which closely resemble those of the alkynes (p. 125). 

Alkynes are unsaturated hydrocarbons containing a carbon—carbon triple bond. The general formula is C H2 2. The simplest alkyne is acetylene (ethyne), a linear molecule. 

The most important alkyne by far is the first member of the series, commonly called acetylene. Recall from Chapter 7 that the C2H2 molecule is linear, with 180° bond angles. The triple bond consists of a sigma bond and two pi bonds each carbon atom is sp-hybridized. The geometries of acetylene and the next member of the series, C3H4, are shown in Figure 22.7. 

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. 

This makes the acetylene molecule linear, i.e. bond angles of 180°, and there are two n bonds with electron density either side of this axis. The properties of an alkyne, like acetylene, are also special in that the Jt bonds are again much more reactive than the a bond. 

Salts of the linear [S=N=S]" cation (isoelectronic with CS2) are readily prepared. This cation undergoes quantitative, 1,3-dipolar cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles, and also with NS" , to give a variety of ring compounds (Scheme 2.8). The... 

A molecule making use of the two sp orbitals for bonding will be linear in shape. There are two common functional groups where such bonding occurs alkynes and nitriles. 

The excellent coordination properties of alkynes with transition metals led to their use as partners for the coupling with a large variety of unsaturated molecules. Two partners such as alkynes and alkenes can produce various modes of C-C bond formation. Linear or cyclic couplings can occur via different pathways, similar to those reported for two C=C bonds couplings (Scheme 1). 

A symmetry-based molecular orbital description of the unusual four-coordinate C3v W(RC=CR)3(CO) series of molecules was presented by King in 1968 (32). The tt orbitals of the three alkynes yield linear combinations of A2 and E symmetry. Since there is no metal orbital of A2 symmetry only the degenerate E combination of n orbitals finds metal orbital mates for bonding and antibonding combinations. The three alkyne 7T orbitals serve as er donors [(Ax + E) symmetry] as does the fourth ligand (Al symmetry). Thus the total metal electron count adheres to the effective atomic number rule [W(0)(6) + 37T j(6) + 2ir (4) + lo-(2) = 18 electrons]. 

The first step is the formation of a symmetrical allyl cation, which then initiates the cyclization. The next double bond is disubstituted so that it has no built-in regioselectivity but prefers to form a six-member ed rather than a five-member ed ring B. The next double bond is trisubstituted and directs the formation of a six-membered ring C. The alkyne, being linear, can reach only through its inner end and so a five-membered ring D is formed. The resulting linear vinyl cation picks up a molecule of water to... 

As you might expect, the presence of a triple bond in alkynes makes their physical and chemical properties different from those of alkanes and alkenes. A structure with a triple bond must be linear around the bond. (See Figure 13.24.) This means that the shapes of alkynes are different from the shapes of alkanes and alkenes. As well, the triple bond makes the molecule much more reactive—even more so than the double bond. 

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