Chapter 1:- Structure and Reactivity

B.sc 1st year Book
Organic Chemistry
(Page 12)

Covalent Bond

Lowis-Langmuir Concept (1916−19): Lewis proposed that an atom attain inert gas configuration (1s2 or ns2np6) by sharing one or more electron pairs by the combination of similar or dissimilar atoms. Langmuir called these shared electron pairs between two or more atoms as ‘Lewis electron-pair bond’ or ‘covalent bond’ or sometimes as ‘non-polar’ bond because it does not acquire polarity. Thus, the concept of a covalent bond is also known as the Lewis-Langmuir concept and the compounds formed by the sharing of electrons of their atoms are called ‘Covalent compounds‘.
According to Lewis’s concept, the number of electrons that an atom contributes for sharing in a covalent bond is called ‘covalency’. Thus, the covalency of H, O, and N In H2O2, and N2 are 1,2 and 3 as they contribute 1,2 and 3 electrons respectively in sharing. Similarly, the covalency of C, P, and S in CH4, PCl5, and SF6 are 4.5, and 6, since they share 4,5, and 6 electrons with louring Hfive Cl, and six F – atoms respectively.

Types of covalent bonds :

Covalent bonds may be either single, double, or triple depending upon the number of shared electron pairs available between the atoms in the molecules. double and triple covalent bonds are called ‘multiple covalent bonds.
Single covalent bonds are formed when two similar or different atoms share one electron each, whereas double and triple bonds are formed when they contribute 2 and 3 electrons each in sharing e.g.
covalent-bond, type of covalent bonds

 

If sharing of electrons occurs between two like atoms, non-polar covalent bonds are formed. whereas if dissimilar atoms share their electrons the covalent bond formed is called a polar covalent bond. e.g.

 

HCl molecules hydrogen bonding

Polar and non-polar covalent bonds :

A covalent bond is formed by equal sharing of electrons between two like atoms or atoms having nearly equal electronegativity value and has no ionic character, such a covalent bond is known as a true covalent bond’ or apolar bond. Such covalent molecule is called ‘nonpolar‘ or ‘apolar’ molecule, examples are ,H2,Cl2,Br2,O2, N2 etc. But in the case of covalent molecules formed between two dissimilar atoms having different electronegativity, the magnitude of sharing electrons is not equal in both atoms. One of them has a strong attractive force for shared electron pairs due to which it develops a partial negative charge (−δ) on it.
       As the result of the development of a partial negative charge, an equal partial positive charge (+δ) will develop on the other atom. It is noticed that these fractional charges developed on both atoms should not be considered unit charges. Molecules having partial charges are called ‘polar molecules’ such as HFHCl, NH3, H2O, etc, and bond is called a polar covalent bond. It is interesting to know that all the non-polar covalent molecules show approximately 2% ionic character. This arises due to small contributions of the ionic resonance structure of the H2 molecule. Similarly, the extreme case of ionic compounds like LiCl, NaCl, and CsCl has a small contribution to-wards covalent character.
dipole of HCl
The degree of polarity is measured in terms of dipole moment (μ)
i.e. μ=e×d debye
wheree= Partial charge on the atom ‘X‘ of the order of 10−power10 e.s.u. and d= bond length measured in A.

Characteristics of Covalent compounds :

1. The covalent compounds are formed by sharing of electrons hence their crystal lattice of molecules is held together by weak Van der Waal forces.
2. Covalent compounds are generally insoluble in water but soluble in organic solvents like ether CCl4, alcohol, acetone, etc.
3. The covalent compounds may be solids, liquids, or gases whereas electrovalent compounds are solids.
4. They show low values of melting and boiling points as compared to ionic solids.
5. Covalent compounds are generally soft, volatile, and fusible in nature,
6. The bonds formed in covalent compounds are directional due to which they show a different type of isomerism but bonds in ionic compounds are non-directional.
7. Covalent compounds do not dissociate or conduct electricity.
8. The reaction rates of covalent compounds are much slower as compared to ionic compounds.

The breaking and making of covalent bonds

Organic reactions involve the breaking and making of new covalent bonds. Any reaction is said to be successful if the bonds formed in the products are more stronger than the bonds broken in the reactants. Depending upon the relative electronegativities of the two concerned atoms two alternative ways are possible by which the covalent bonds are breaked in organic compounds. When a bond breaks symmetrically it is called “homolysis” and when it dissymmetrically it is called “heterolysis“.
homolysis with free radicals
In homolytic bond fission, one electron of the bonding pair goes with each of the departing atoms or groups and two electrically neutral fragments are formed, generally called ‘free radicals‘. e.g.
Homolysis, free radicals
In heterolytic bond fission, the electrons pair forming the covalent bond goes to a single atom or group, and thus electrically charged fragments are formed. The reaction involving heterolytic fission are called ionic reactions and are said to proceed via an ionic mechanism. This type of bond fission can occur in either of the following ways-
1- When the electrons pair forming the C−X bond leaves the organic group and remains with the departing substituent X and thus the latter attains a negative charge and the former attains a positive charge due to the gain and loss of electrons respectively.
Such organic species having a positive charge is commonly known as “carbocation. In short, it is represented as R+.
2- When the electrons pair forming the C−X bond moves towards the organic group and remains with it consequently organic group carries a negative charge and substituent X carries a positive charge due to the transfer of electrons. Such organic species is called as_ “carbanion” and is represented as R.
heterolysis and homolysis formula image
Homolytic bond fission requires much less energy than heterolytic bond fission. For example, the C−Br bond fission in CH3CH2−Br into CH3CH2 
and Br requires energy equal to 67.2 kcal mol−1 while the same band fission into CH3CH2 and Br requires 183.0 kcal mol−1.
Organic fragments such as free radical, carbocation, and carbanion formed in the above reactions are collectively known as “reaction intermediates” which are very reactive. These intermediates differ from each other as follows-
Free radicals Carbocations Carbanions
 1. A free radical may be defined as “an atom or group of atoms having a single, odd or unpaired electron”. 1. A Carbocation may be defined as “an ion containing a positively charged carbon”. 1. Structure of an alkyl carbocation Carbanions containing a negatively charged carbon”.
 2. The free radicals are symbolized by putting a dot (⋅) against the atom or group of atoms. 2. They are generally symbolized as R+ 2. They are symbolized as R.

3. They are very reactive, electrically neutral, and paramagnetic in nature. 3. They are very reactive, electrically positive, and diamagnetic in nature. 3- They are very reactive intermediates with diamagnetic nature.
4. The carbon atom of alkyl free radicals, which is bonded to only three atoms or groups of atoms, is sp2 hybridized. 4. The positively charged carbon of carbocations has six electrons in three pairs and is sp2 hybridized with an empty p-orbital. 4- The negatively charged carbon of carbanion has eight electrons (three bond pairs ÷ one lone pair) and is sp3 hybridized.
5. Free radicals have a shallow pyramidal structure with the odd electron situated in the unused p-orbital as shown below-

structure of an alkyl free radical

5. Carbocations have planar structures with an empty p-orbital as shown below-

 

Structure of an alkyl carbocation

5. A carbanion has a pyramidal structure as shown in the figure given below –
structure of an alkyl carbanion

 

Free Radicals - Carbocation-Carbanion,

Carbenes (>: ) :

A carbene is a highly reactive neutral species that contains a carbon atom with only six valence electrons. Examples are :
carbenes
They are usually formed from precursors by the loss of small, stable molecules through one of the following ways :
  1. α-elimination

(Elimination in which both the proton and the leaving group are located on the same atom) follows a mechanism akin to an E1cB.

 

 2. β-Elimination

a strong base removes an acidic proton adjacent to an electron-withdrawing group to give a carbanion. Loss of a leaving group from the carbanion creates a carbene. One of the best-known elimination reactions occurs when chloroform is treated with base, forming a dichlorocarbene.
  1. Thermal decomposition via hydrazone compounds:

Carbene can be made from diazoalkanes if the diazoalkane is just an intermediate in the reaction and not the starting material. Good starting materials for these reactions are tosylhydrazones (here for simplicity mesylhydrazone is used as an example) which produce transient diazo compounds by base-catalysed elimination of toluenesulphinate(methyisulphinate). The diazo compound is not normally isolated and decomposes to the carbene on heating.

Methylene (:CH2): 

A most simple carbene is methylene. It is the first member of the alkenes which has a very shorf life. It is formed by the photolysis(photochemical decomposition) or pyrolysis of diazomethane or keten.
Methylene undergoes two types of reactions, insertion and addition .

Insertion Reactions :

Insertion reactions occur mainly in the C−H bond, but can also occur in O−H and C−Cl bonds. e.g.

Insertion Reactions

Addition Reactions : 

CH2 adds across double bonds to form cyclopropane, at the same time insertion reactions also occur. Skell et al(1956, 1959) showed that the addition of to cis and trans-but-2-ene is a cis addition. Thus, the addition is spectrospecific isomer forms one product and the configurations of the two products are different. i.e. each geometrical isomer forms one product and the configurations of the two products are different.

addition reaction

Anet et all-1960) however,showed that this spectrospecific addition is lost when the reaction is carried out in the presence of an inert gas(e.g. N2 gas) i.e. each substrate now gives a mixtion is carried trans-products. On the other hand, Duncan et at(1962) showed gives a mixture of the cis and keten added to the above substrates in a non stereospecific mantered motheris of indication that in an inert gas, CH2 becomes less reactive, it also follows that less reactive than that from diazomethane.
Herzberg et al(1961) showed, from stereospecific evidence that CH2 was initially formed in the singlet (excited) state and then rapidly changes to the triplet (ground) state.
Stereospecific addition reaction
Figure 1.22 : (a) The stereospecific addition involves sp2 singlet CH2. (b) The non a cyclic transition state is formed. In this case, a cyclic transition state is involves sp2 triplet CH2.

Stereospecific addition reaction, non stereospecific addition

Dichloromethylene (CCl2)  

Hine(1950, 1954) showed that chloroform and other haloforms undergo alkaline hydrolysis to produce the formate and carbon monoxide by what Hine calls the α-elimination mechanism. In α-elimination, the reaction involves the removal of H+ and Clion from the carbon atom.

Mechanism :

dichloromethylene, CCl2

Nitrenes (−N:) :

A nitrene is the nitrogen analog of a carbene. In nitrene, the nitrogen atom has only six valence electrons. Some examples of nitrenes are-
nitrenes
Like methylene, nitrene can exist in the singlet and triplet states. Its triplet state is the most stable(ground state). reactions.

Imidogen (H−N¨¨) :

The parent nitrene is HN: also known as imidogen, azene, or imene, and is formed when hydraulic acid is irradiated with UV light. In the presence of ethylene, nitrene is trapped to form ethylenimine.
Imidogen reaction

aziridine is a 3-membered heterocyclic compound. It is a syrupy liquid, bp 56∘C. It combines with sulphurus acid to form taurine(2-aminoethanesulphonic acid) which occurs in human bile.

Alkylnitrenes( R−N¨¨ ) :

It may be prepared by the photolysis of isocyanate.
alkyl nitrenes
Spread The Love