G.N Lewis was first to suggest that atoms can be combine with one another by sharing of electrons in their valence shell so that the combining atoms attain the nearest noble gas configuration.
This type of linkage is called as covalent linkage or covalent bond and the compound involve in such type of bond formation are known as covalent compounds.
For example, two hydrogen approaches to each other, each atom contributed one electron and the pair of electrons is shared by both of atoms to form a molecule of hydrogen.
Due to the formation of hydrogen molecule, each attains the nearest noble gas configuration that is of He, 1s2
In covalent bond, the binding atoms may share one or more than pair of electrons which depends upon the requirement of completing the octet configuration. For example, in oxygen molecule, each oxygen atom required two electrons for attaining the noble gas configuration, hence it will contribute two electrons in sharing with other oxygen atom and form oxygen molecule, which contains double bond or two shared electron pair between both atoms.
Hence the number of electrons which an atom contributes for sharing in a covalent bond is called as its covalency of valency.
For example, Nitrogen required three electrons for getting octet configuration, so covalency will be three. In the same way some atoms, their valence electrons and their covalency is as follows.
||Number of Valence
Atom like carbon atom (atomic number = 6) has a total of six electrons with electronic configuration of; 1s2, 2s2, 2p2.
Hence the first two shells the K-shell has two electrons and the L-shell has four electrons. It means in outer shell there is one completely filled ’s’ orbital and two half-filled 'p' orbitals. For attaining noble gas configuration, it can accept four electrons to form C4- anion or it can loos these four valence shell electrons to form C4+ cation. But both of these paths for attaining the octet configuration take carbon far away from achieving stability by the octet. Hence to overcome this problem carbon forms bonding by sharing its valence electrons and form covalent bonds.
In other words, each carbon atom can form four covalent bonds with one, two, three or four carbon atoms or atoms of other elements or groups of atoms. Just like carbon, hydrogen also wants to achieve nearest noble gas configuration that is of helium. Hydrogen has a tendency to share its one electron and form one covalent bond. Due to this bonding electrons pair, hydrogen gets the helium gas configuration that is 1s2
In methane molecule there are four carbons –hydrogen bond which arrange in a tetrahedral manner. The bond angle of carbon –hydrogen in methane molecule is 109°28’ and bond length is 1.09Å. All carbon-hydrogen bonds in methane molecule is single sigma bond formed by one pair of electrons. If one carbon formed multiple bonds with other carbon atom, the bond angle as well as bond length will vary.
For example, in ethene molecule, both carbon atoms are bonded by two pairs of electrons, also called as double covalent bond. Since there are two pair of electrons in ethene compare to methane which has only single bonds, bonded atoms
will be little close to each other, hence the bond length will decreases. The bond length of carbon-carbon double bond length is 133.9 pm and bond angle is 120°. But the carbon-hydrogen bond length is almost same as methane that is 108.7pm or 1.087Å.
In saturated and unsaturated hydrocarbons, it’s difficult to cleave the carbon - hydrogen bond due to its un-reactive nature. Those reactions which involve the cleavage of carbon-hydrogen bond under drastic conditions are called as
Generally these reactions proceed in the presence of Organometallic complexes in which hydrocarbon coordinated with inner-sphere of metal. The reaction completed through the formation of either an intermediate “alkane or arene complex” or a transition state.
Because of very less reactivity of carbon-hydrogen bond, it can only be cleave by coordination. Instead of all these limitation, a lot of effort has been devoted to synthesize of new reagents and catalysts that can affect Carbon-Hydrogen activation is enable the conversion of cheap and abundant alkanes into valuable fictionalized organic compounds.
The formation of 1-Iodopenatne from pentane in the presence of a tungsten complex is an example of carbon-hydrogen activation. Due to less reactivity most Carbon-Hydrogen activations competed under rather strong reaction conditions like high temperature, strongly acidic or basic condition and strong oxidant. Some other examples of carbon-hydrogen activation are oxidative addition, sigma-bond metathesis, metalloradical activation, 1,2-addition and electrophilic activation.
The contribution of structures III, IV and V imparts a partial double bond character to the carbon-chlorine bond. The shortening of bond length imparts stability to aryl halides and as a result, the bond cleavage becomes rather difficult. The aryl halides are, therefore, less reactive than alkyl halides.
The overall combination of all canonical forms is known as resonance hybrid. This form is more stable than all canonical forms and shows all the characteristics of the molecule. For example, the response hybrid of benzene is as follow.
Carbon dioxide is one of the examples of carbon resonance. Other example of carbon resonance is carbonate ion (CO32-)
In the same way, benzene (C6H6) also shows resonance with two conical forms. In benzene all single and double bonds
are interchanged in ring. The carbon-carbon bond length is intermediate between single bond and double bond that is 139 pm. This intermediate value of bond length (single bond and double bond) is the main feature of all molecules in which bonds have a different bond order in different contributing structures.
The sp2 hybridized carbon atom in the C-X bond in haloarene molecule is more electronegative than the sp3 hybrid carbon atom in alkyl halide.This carbon has less tendency to release electrons to the chlorine atom and so the C-Cl bond in aryl halides is less polar than in alkyl halides. This is supported by the fact that the dipole moment of chlorobenzene is 1.73 D while the dipole moment of chloroethane is 2.05 D. Lesser the polarity of C-Cl bond, lesser is the reactivity.
|More topics in Carbon Bonding|
|Werner Heisenberg Atomic Theory||Valence Bond Theory|
|Electron Dot Formula||Lewis Dot Structures|
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