Hybridisation

The carbon atom can mix the 2s and 2p atomic orbitals to form new hybrid orbitals in a process known as hybridisation. 

The shape of organic molecules is therefore determined by the hybridisation of the atoms. 

Functional groups (see Section 2.1) which contain re-bonds are generally more reactive as the re-bond is weaker than the 0-bond. The re-bond in an alkene or alkyne is around 210-230 kJ mol-1, while the 0-bond is around 350kJmor . 

The shorter the bond length, the stronger the bond. For C-H bonds, the greater the s character of the carbon orbitals, the shorter the bond length, because the electrons are held closer to the nucleus. 

A single C-C o-bond can undergo free rotation at room temperature, but a Jt-bond prevents free rotation around a C=C bond. For the maximum orbital overlap in a Jt-bond, the two p-orbitals need to be parallel to one another. Any rotation around the C=C bond will break the Jt-bond. 

AUcenes have a C=C bond containing one strong a-bond and one weaker Ji-bond (Section 6.1) 

All carbonyl compounds have a C=0 bond, which contains one strong r-bond and one weaker Jl-bond (Section 8.1) 

For a single C C or C—O bond, the atoms are sp hybridised and the carbon atom(s) is tetrahedral. 

CO C l + 0 V2-2Py + 2Pz Cr 11 sp3 hybrid (large lobe in front of the plane of the page, and small lobe behind) 

The conventional representations of hybrid orbitals used in Fig. 1.18 are just as misleading as the conventional representations of the p orbitals from which they are derived. A more accurate picture of the sp3 hybrid is given by the contours of the wave function in Fig. 1.19. Because of the presence of the inner sphere in the 2s orbital (Fig. 1.11a), the nucleus is actually inside the back lobe, and a small proportion of the front lobe reaches behind the nucleus. This follows from the way a hybrid is constructed by adding one-quarter of the wave function of the s orbital (Fig. 1.11a) and three-quarters in total of the wave functions of the p orbitals (Fig. 1.12a). As usual, we draw the conventional hybrids relatively thin, and make the mental reservation that they are fatter than they are usually drawn. 

The interaction of the Is orbital of a hydrogen atom with an sp3 hybrid on carbon can be used in the usual way to create a Tch bonding orbital and a t Ch antibonding orbital (Fig. 1.20). Four of the bonding orbitals, each with two electrons in it, one from each of the four hybrids, point towards the corners of a regular tetrahedron, and give rise to the familiar picture for the bonds in methane shown in Fig. 1.21a. 

For other purposes, however, it is undoubtedly helpful to take advantage of the simple picture provided by the hybridisation model, even though hybridisation is an extra concept to leam. It immediately reveals, for example, that all four bonds are equal. It can be used whenever it offers a simplification to an argument as we shall find later in this book, but it is good practice to avoid it wherever possible. In particular, the common 

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When elements in Period 2 form covalent bonds, the 2s and 2p orbitals can be mixed or hybridised to form new, hybrid orbitals each of which has. effectively, a single-pear shape, well suited for overlap with the orbital of another atom. Taking carbon as an example the four orbitals 2s.2p.2p.2p can all be mixed to form four new hybrid orbitals (called sp because they are formed from one s and three p) these new orbitals appear as in Figure 2.9. i.e. they... 

In ethene the situation is rather different here, each carbon atom has one 2s and two 2p orbitals hybridised to form three sp single-pear orbitals which are trigonal planar (shown shaded in each half of Figure 2. JO). The remaining 2p orbital is not hybridised,... 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

There are a number of different ways that the molecular graph can be conununicated between the computer and the end-user. One common representation is the connection table, of which there are various flavours, but most provide information about the atoms present in the molecule and their connectivity. The most basic connection tables simply indicate the atomic number of each atom and which atoms form each bond others may include information about the atom hybridisation state and the bond order. Hydrogens may be included or they may be imphed. In addition, information about the atomic coordinates (for the standard two-dimensional chemical drawing or for the three-dimensional conformation) can be included. The connection table for acetic acid in one of the most popular formats, the Molecular Design mol format [Dalby et al. 1992], is shown in Figure 12.3. 

As with the molecular connectivity indices, higher-order shape indices have also been defined. The kappa indices themselves do not include any information about the identity of the atoms. This is the role of the kappa-alpha indices. Tlie alpha value for each atom is a measure of its size relative to some standard (chosen to be the sp -hybridised carbon) ... 

M.o. theory and the transition state treatment In 1942 Wheland proposed a simple model for the transition state of electrophilic substitution in which a pair of electrons is localised at the site of substitution, and the carbon atom at that site has changed from the sp to the sp state of hybridisation. Such a structure, originally proposed as a model for the transition state is now known to describe the (T-complexes which are intermediates in electrophilic substitutions... 

Moreover, disposable electrochemical sensors for the detection of a specific sequence of DNA were realised by immobilising synthetic single-stranded oligonucleotides onto a graphite or a gold screen-printed electrode. Tire probes became hybridised with different concentrations of complementary sequences present in the sample. 

ELECTROCHEMILUMINESCENT HYBRIDISATION CHIP USING ELECTRIC FIELD AIDED HYBRIDISATION AND IMMOBILISATION... 

Table 2.6. Structural features (carbon hybridisation, electronegativity, ring size) and typical one-bond CH coupling constants Jch (Hz) ...

The C NMR spectrum illustrates the connection between earbon hybridisation and C shift on the one hand and Jqh coupling eonstants on the other. 

Here, the bonding between carbon atoms is briefly reviewed fuller accounts can be found in many standard chemistry textbooks, e.g., [1]. The carbon atom [ground state electronic configuration (ls )(2s 2px2py)] can form sp sp and sp hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, CjH, where a Csp -Csp (or C-C) a bond is formed between two C atoms by overlap of sp orbitals, and three Csp -Hls a bonds are formed on each C atom. Fig. 1, Al. 

A second type of hybridisation of the valence electrons in the carbon atom can occur to form three 2sp hybrid orbitals leaving one unhybridised 2p orbital. 

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. 

Carbon has six electrons around the atomic core as shown in Fig. 2. Among them two electrons are in the K-shell being the closest position from the centre of atom, and the residual four electrons in the L-shell. TTie former is the Is state and the latter are divided into two states, 2s and 2p. The chemical bonding between neighbouring carbon atoms is undertaken by the L-shell electrons. Three types of chemical bonds in carbon are single bond contributed from one 2s electron and three 2p electrons to be cited as sp bonding, double bond as sp and triple bond as sp from the hybridised atomic-orbital model. 

Carbon with its wide range of sp bond hybridisation appears as the key element of a future nanotechnology. However, so far there is almost no control over the formation processes, and the structures of interest cannot be built at will. Tubes, for example, are produced under the very virulent conditions of a plasma discharge and one would like to have more elegant tools to manipulate the carbon structures, a task which remains a challenge for the future. 

A small isotope effect has been observed in nitration of benzene by nitronium borofluoride in tetramethylene sulphone at 30 °C (kH/kD = 0.86) and this has been attributed to a secondary effect of the change in hybridisation from sp2 to sp3 of the ring carbon during the course of the reaction109. However, naphthalene gives an isotope effect of 1.15 under the same conditions, and anthracene a value of 2.6115. It does not seem at all clear why these relatively unhindered and normally more reactive molecules should give rise to an isotope effect when benzene does not. 

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