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Carbon Bonding

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

 

Carbon Hydrogen Bond

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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.

Atom
Number of Valence
Shell Electrons
Covalency
Carbon(C) 4 4
Oxygen(O) 6 2
Nitrogen(N) 5 3
Phosphorous(P) 5
3
Sodium(Na) 1 1
Potassium(K) 1 1
Chlorine(Cl) 7 1
Bromine 7 1
Hydrogen 1 1
Fluorine 7 1
Boron 2 3
Magnesium 2 2
Calcium 2 2
Beryllium 2 2

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

  • So the covalency of hydrogen is one and for carbon it is four. One carbon atom can form covalent bond with four hydrogen atoms. For example, methane molecule (CH4).
  • The four valence electrons from carbon form covalent bond with four hydrogen atoms. Each hydrogen atom contributes one electron in one covalent bond with carbon atom.
  • The covalent bonding can be easily explained by using hybridization concept from valence bond theory.
  • An electron located in 2s-orbital of the carbon atom get excited to the empty 2pz orbital, hence now carbon atom having four orbitals each containing one electron : 2s, 2px, 2py and 2pz.
  • These four orbitals are mixed up to form four new degenerate orbitals with one electron in each orbital. These orbital are called as sp3 hybrid orbitals.
  • Each sp3 orbital overlapped with a 1s-orbital of a hydrogen atom and form four covalent bonds sigma-bonds. Since these bonds are equivalent, they arranged in a regular tetrahedron.

Carbon Hydrogen Bond Length

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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.


C-H Activation

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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
carbon-hydrogen activation.

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.

Carbon Oxygen Bond

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The  Below Points Describe about Carbon Oxygen Bond :-
  • The carbon dioxide molecule (CO2) is the best example of carbon-oxygen bond.
  • The carbon atom in carbon dioxide molecule has four valence electrons, hence required four shared electrons for bond formation.
  • While each oxygen atom with six valence electrons required two electrons for completing the octet configuration. In other word the covalency of carbon is four and for oxygen it is two.
  • So in carbon dioxide molecule; two electron pairs shared between carbon and one of the oxygen atom and a double bond, C=O is formed. There are two such C=O bonds in CO2 molecule.
According to valence bond theory, each carbon involve in sp hybridization to form two bonds with oxygen atoms. sp hybrid orbitals arranged in a linear arrangement with 180° bond angle.

Carbon Chlorine Bond

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The electronic configuration of carbon atom is 1s2, 2s2, 2Px1, 2Py1. So for the formation of four single bonds, the 2s2 electron will excite to 2pz and these four unpaired electron form four covalent single bonds with chlorine atoms and forms carbon tetrachloride. Carbon tetrachloride shows sp3 Hybridization with 3p orbitals of four chlorine atoms.

The 2s orbital orbital will hybridize with the rest of three 2P orbitals to form four hybridized orbitals to make four hybridize orbital. These hybridized orbitals form covalent bond with 3p orbital of chlorine atom by overlapping. Just like methane, all four carbons-chlorine bond arranged in a tetrahedral manner with bond angle 109°5’ and the Carbon-Chlorine bond length is approximately 174 picometers.

Carbon Fluorine Bond

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Just like carbon-chlorine bond, carbon-fluorine bond is also formed by the overlapping of carbon orbitals and 2p orbitals of fluorine.
Carbon difluoride dichloride contains two carbon-chlorine and two carbon-fluorine bonds. As the electronegativity of halogens decreases form fluorine to iodine, the bond length of carbon-halogen bond increases. This is due to less attraction force between halogen and carbon. The bond length of carbon-fluorine bond is 134 pm while carbon-chlorine is 176 pm, for Carbon-bromine is 193 pm and for C-I, it is 213pm.

Resonance Effect

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When a molecule cannot be completely represented by a single structure but its chemical and physical properties can be described by two or more different structure, then the true structure is said to be resonance hybrid and the phenomenon is called resonance effect. All contributing structures are known as canonical forms or resonance.

For example, carbon resonance can be possible in carbon dioxide and benzene.
  1. There are certain rules for the selection of resonance structure;
  2. The conical forms should have same atomic position with different position of electrons.
  3. The conical forms must have same number of unpaired electrons. They should not be differing much in energy.
  4. In conical forms, the negative charge must reside on electronegative atom and positive charge must be on electropositive atom.
Resonance energy is the difference between the actual energy of the molecule and that of the most stable form of the canonical structures. 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.
Resonance Hybrid Chlorobenzene Structures
The halogen atom in haloalkane is very reactive and can be easily replaced by nucleophiles whereas the halogen atom in haloarenes is strongly held to the nucleus and cannot be replaced easily with nucleophiles. The differences between haloalkanes and haloarenes are due to the following reasons. There is delocalization of electrons in haloarenes due to resonance.

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.

Resonance Hybrid

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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.

Resonance Hybrid of Benzene

Carbon Resonance

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Carbon dioxide is one of the examples of carbon resonance. Other example of carbon resonance is carbonate ion (CO32-)
Resonance of Carbon Ion
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.

Hybridization States of Carbon Atom

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Below are Some Points about Hybridization States of Carbon atom:-
  • The carbon atom of the C-X bond in haloalkanes, is sp3 hybridized while in haloarenes the carbon atom is sp2 hybridized.
  • The sp2 hybridized carbon atom with a greater 's' character is more electronegative.
  • It can hold the electron pair of the bond more tightly than the sp3 hybridized carbon atom in alkyl halides. This has less tendency to release electrons to the halogen.
  • As a result, the bond cleavage in aryl halides is somewhat more difficult than in alkyl halides.
Hybridization States of Carbon Atom

Polarity of Carbon Halogen Bond

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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.

Thus, haloarenes are less reactive towards the substitution reactions than haloalkanes. However, under drastic conditions, aryl halides undergo substitution reactions.
More topics in Carbon Bonding
Werner Heisenberg Atomic Theory Valence Bond Theory
Resonance Hybridization Chemistry
Electron Dot Formula Lewis Dot Structures
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