Ethyne, bonding

HCNO appears to take part in more than four covalent bonds an almost triple bond between C and N ( j/3 and /6 account for one of the components of this bond, which would be of n symmetry in linear HCNO), a o bond between N and O, and a highly polar bond between N and O (of n symmetry in linear HCNO, described here by /i and /5). The remaining two orbitals, j/2 and /4, form the ethyne bond (of k symmetry in linear C2H2) that is broken during the reaction. 

Triple bonds are formed by the sharing of three pairs of electrons to form a a and two n bonds. Spatially these three bonds behave as a single bond. Consequently acetylene (ethyne) C2H2 has the linear configuration often represented as H—C=C—H. 

VViberg and Rablen found that the charges obtained with the atoms in molecules method were relatively invariant to the basis set. The charges from this method were also consistent v it i the experimentally determined C-H bond dipoles in methane (in which the carbon is p isitive) and ethyne (in which the carbon is negative), unlike most of the other methods they examined. 

In naming alkynes the usual lUPAC rules for hydrocarbons are followed and the suffix ane is replaced by yne Both acetylene and ethyne are acceptable lUPAC names for HC=CH The position of the triple bond along the chain is specified by number m a manner analogous to alkene nomenclature... 

Bond Distances, Bond Angles, and Bond Energies in Ethane, Ethene, and Ethyne (Table 9 1, p 342) Stmctures of a-Ammo Acids (Table 27 1, pp 1054-1055)... 

Fig. 1. Some molecules with different C-C bonds. Al, ethane, CjH, (sp ) A2, ethene, C2H4 (sp ) A3, ethyne, (sp ) Bl, benzene, (aromatic) B2, coronene,...

In the third type of hybridisation of the valence electrons of carbon, two linear 2sp orbitals are formed leaving two unhybridised 2p orbitals. Linear a bonds are formed by overlap of the sp hybrid orbitals with orbitals of neighbouring atoms, as in the molecule ethyne (acetylene) C2H2, Fig. 1, A3. The unhybridised p orbitals of the carbon atoms overlap to form two n bonds the bonds formed between two C atoms in this way are represented as Csp Csp, or simply as C C. 

Another analogous series of unsaturated hydrocarbons that contain just one multiple bond, but, instead of being a double bond, it is a triple bond is the alkynes. The names of all the compounds end in -yne. The only compound m this series that is at all common happens to be an extremely hazardous material. It is a highly unstable (to heat, shock, and pressure), highly flammable gas that is the first compound in the series. This two-carbon unsaturated hydrocarbon with a triple bond between its two carbon atoms is called ethyne, and indeed this is its proper name. It is, however, known by its common name, acetylene. 

Measure and record the carbon-carbon bond lengths in ethane, ethene and ethyne. These will serve as standards for single, double and triple bonds, respectively. 

A single shared pair of electrons is called a single bond. Two electron pairs shared between two atoms constitute a double bond, and three shared electron pairs constitute a triple bond. A double bond, such as C 0, is written C=0 in a Lewis structure. Similarly, a triple bond, such as C C, is written G C. Double and triple bonds are collectively called multiple bonds. The bond order is the number of bonds that link a specific pair of atoms. The bond order in H, is 1 in the group C=0, it is 2 and, for O C in a molecule such as ethyne, C2H2, the bond order is 3. 

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 alkynes are hydrocarbons that have at least one carbon-carbon triple bond. The simplest is ethyne, FIO CH, which is commonly called acetylene (20). Alkynes are named like the alkenes but with the suffix -yne. 

Jeong, H. Y., Han, Y., Comment on A Computational Study of the Structures of Van der Waals and Hydrogen Bonded Complexes of Ethene and Ethyne Chem. Phys. Lett., 263, 345. 

Ethyne has two jt bonding orbitals at right angles to each other and a resultant jt electron density that is cylindrically symmetric with respect to the internuclear axis. Complexes of ethyne with HF [133], HC1 [134], HBr [135], C1F [66], CI2 [47], BrCl [50], Br2 [92] and IC1 [95] have been characterised by... 

The lower signal is more complicated, and before we can interpret it exactly we need some background information. The magnitude of one-bond C-C coupling constants depends on bond hybridization (ethane 35, ethene 68, benzene 56, ethyne 172 Hz), while two- and three-bond C-C couplings are very small, often around 2-5 Hz. The second thing we have to remember, and this is a new concept, is that the lines in the multiplets from INADEQUATE spectra often come from different spin systems ... 

A well-known example of the effect of bond order on bond length is provided by the bonds in ethane, ethene, and ethyne, which have the lengths of 154, 134, and 120 pm, respectively. Covalent radii for doubly and triply bonded atoms can be obtained from double and triple bond lengths in the same way as for single bonds. Some values are given in Table 2.2. 

Figure 3.18 Conventional representations of ir orbitals, (a) The n orbital in the ethene molecule, (b) The two 7r orbitals in the ethyne molecule, (c) A dot density diagram of the electron density in a plane perpendicular to the bond axis in ethyne. (Reproduced with permission from M. J. Winter, Chemical Bonding, 1994, Oxford University Press, Oxford.)...

Figure 4.18 Model of ethyne showing that only the electrons of the C—H bonds are localized into pairs. The remaining six electrons are evenly distributed in a ring around the C—C axis.

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