Molecular structures ethene

This polymer has one of the simplest molecular structures ([CH2CH2— ] ) and is at present the largest toimage plastic material, having first been produced commercially in 1939 for use in electrical insulation. There is a difficulty over the nomenclature of this polymer. The lUPAC recommended name for the monomer is ethene, rather than the older ethylene. Hence the lUPAC name for the polymer is poly (ethene). However, this name is almost never used by chemists working with the material throughout this book, therefore, this polymer will be referred to by its more widespread name, poly(ethylene). 

In molecular structures, planes of symmetry can cut through atoms or through bonds between atoms, (n) CHj has no center but has two planes, as shown in Fig. 5-I(a). Each plane cuts the C and a pair of H s. while bisecting the other H—C—H bond angle, (b) CHCI, has no center and has one plane that cuts the C. H. and one of the Cl s while bisecting the Cl—C—Cl bond angle, as shown in Fig. 5-l(ft). (c) In ethene all six atoms lie in one plane, which, in Fig. 5-l(c). is the plane of the paper. It has a center of symmetry between the C s. It also has three symmetry planes, one of which is the aforementioned plane of the paper. Another plane bisects the C=C bond and is perpendicular to the plane of the paper. The third plane cuts each C while bisecting each H—C—H bond. 

Compound 3a was found to react with excess water to produce a mixture of organic products including butane, ethene and ethane.17 However, when trace amounts of water were introduced into a hexane solution of 3b, the oxo complex 12 is formed. The molecular structure of 12 shows that it is formed by reaction of 2 equivalents of 3b and 1 equivalent of water ... 

The main conclusions are that (i) Fivefold coordinated Cr3+ sites exposed on the (0112) faces, although reactive in the formation of weak molecular Cr3+-ethene complexes, are not active in catalytic polymerization, and (ii) polymerization (and oligomerization) activity is attributed only to Cr2+ centers, located at structural defects (such as edges, steps, and corners). This last conclusion strongly suggests that a highly coordinatively unsaturated state is a necessary prerequisite for the polymerization activity of Crx+ centers. 

The crystal and molecular structure of quinazoline was solved in 1976 by X-ray diffraction. Bond lengths and interbond angles of quinazoline (1) [with two crystallographically independent molecules (A and B) in the unit cell], 2-methylquinazolin-4(3/f)-one (2a), ° 2-phenylquinazolin-4(32f)-one (2b)," 2-acetylquinazolin-4(3//)-one (2c), " quinazoline-2,4-diamine monohydrate (3), 5-chloroquinazoline-2,4,6-triamine (4), 2-phenylquinazoline 1,3-dioxide (S). and ( )- , 2-bis(2-methylquinazolin-4-yl)ethene (6) obtained by X-ray crystallography are collected in Table 1. 

Draw out the molecular structure of ethene, and in particular show the electron orbitals that are associated with the carbon/carbon double bond. Also describe the geometry of this molecule. 

A systematic structural study of the DPX Pacman framework has been provided by a homologous series of dinuclear zinc, copper, and nickel derivatives. The molecular structures of the homobimetallic zincfll), copper(II), and nickel(II) complexes of the DPX construct (5-7) are shown in Fig. 3. Trends in bond lengths and angles of macrocyclic core structures and side chains agree well with those observed in related systems including H4(DPA), H4(DPB), and l,2-bis[5-(2,3,7,8-12,13,17,18-octaethyIporphyrinato)]-cw-ethene porphyrins (80-83). 

The hybridization concept indicates some additional aspects of molecular stmcture. The tetrahedral, trigonal, and digonal natures of sp, sp, and sp carbon atoms provide an approximation of bond angles. The idea that tt bonds are formed by the overlap of p orbitals puts some geometrical constraints on structure. Ethene, for example, is planar to maximize p-orbital overlap. Allene, on the other hand, must have the terminal CH2 groups rotated by 90° to accommodate two tt bonds at the central sp carbon. 

Pauling s prediction that the use of the molecular structure are in better agreement with the bent bond description than with the a,n description. Figure 1.35(a) shows calculated contour lines of a carbon orbital in a plane that is perpendicular to the molecular plane of ethene, and Figure 1.35(b) shows contour lines for an orbital in a plane containing the carbon atoms of cyclopropane. Clearly, much of the orbital lies outside the intemuclear bond line in each case. It is generally agreed that the bonds in cyclopropane are bent the picture from this theoretical calculation reinforces Ae view that they are bent in ethene also. 

The term hydrocarbon covers compounds formed from hydrogen and carbon atoms only. In addition to ethane, there are two more hydrocarbons containing of two carbon atoms, C2H4 (ethene) and C2H2 (ethyne). The molecular structures of these compounds are shown in Fig. 14.1. 

Fig. 14.1. Above the molecular structure of ethane side and end views. Below the molecular structures of ethene and ethyne.

The molecular structures of ethene analogues of the heavier Group 14 elements... 

The molecular structures of several disilenes in the solid state have been determined by X-ray crystallography. All of them are characterized by very bulky alkyl or aryl substituents, R. The structures of (tert-butyl)(Mes)SiSi(fert-butyl)(Mes), Mes = phenyl-2,4,6-trimethyl (A), and (Mes)2SiSi(Mes)2 (B) are shown in Eig. 14.7. The central C2SiSiC2 framework of the former is planar as expected for an ethene analogue. The SiSi bond distance is 214 pm, about 9% shorter than the single bond distance in Me3SiSiMe3, 234 pm. (By comparison the CC double bond in ethene is 13% shorter than the single bond in ethane.)... 

An alternative but equivalent model for describing benzene (and other resonance-stabilized structures) is molecular orbital theory. We have already seen how this theory can explain the formation of molecular structures such as methane, ethene and others. In localized molecules like ethene, C2H4, two unhybridized p orbitals overlap to form a Jt molecular orbital in which a pair of electrons is shared between tbe nuclei of two carbon atoms. In molecular orbital theory, resonance-stabilized structures are described in terms of delocalized Jt orbitals where the Jt electron clouds extend over three or more atoms. 

On the last two pages you met the alkanes. There is another family of hydrocarbons called the alkenes. The first two alkenes are ethene and propene. Their molecular structures are shown below. Compare them with the corresponding alkanes ... 

Figure 8.15 Molecular structures of naphthalene-substituted ethenes 32-35.

Molecular structures of pyrene- or fluoranthene-substituted ethenes 55... 

Covalently bonded substances with a simple molecular structure, for example water and ammonia, are usually liquids or gases. This is because the forces between the molecules are weak. It does not take much energy to overcome these intermolecular forces, so these substances have low melting points, low boiling points and low enthalpy changes of vaporisation compared with ionic compounds. Some substances that have covalently bonded molecules maybe solids at room temperature, for example iodine and poly(ethene). These are usually molecules where the van der Waals forces are considerable. However, the melting points of these substances are still fairly low compared with ionic compounds or most metals. 

The aminoborane molecule, H2NBH2, is isoelectronic with ethene. The molecular structure determined by microwave spectroscopy is ethene-like, planar, and with an N-B bond distance of 139 pm, 5 pm longer than in ethane (Fig. 8). The planar structure and the short N-B bond suggest the presence of jt-bonding. 

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