Bonding-Functional Groups-Nomenclature of Organic Compounds

INTRODUCTION:

Organic Chemistry is an extremely organized discipline. It is the study of the connection between the structures of molecules or their reactions. We will start our study by the kind of bonding and structural aspects of the molecules. We are previously well-known with the detail that compounds can be largely separated into 2 classes, covalent and ionic. Ionic compounds are composed of positively and negatively charged ions that are held mutually via electrostatic forces. Because ions can be regarded as spheres having balanced distribution of charge, no particular path can be allocated to this kind of bonding. For instance, in NaCl lattice, Na+ and Cl^- ions are held mutually via electrostatic forces; no Na⁺ ion can be regarded as bonded to a particular Cl^- ion. In other terminology, there is no these entity that can be termed as NaCl molecule. Indeed, the electrostatic forces activate between a particular ion (Na⁺) and all its neighboring ions (Cl^-) of reverse charge. On the other hand, in covalent compounds, molecules are the structural chapter. In difference to the ionic compounds, in covalent compounds, the molecules are formed through the sharing of electron pair(s) between the constituent atoms. The bonds formed via sharing of pair(s) of electrons are termed covalent bonds. Because in organic compounds, the bonds formed with carbon atom are covalent in nature, we will study several traits of the covalent bonding in fact and then illustrate forms of molecules using the idea of hybridization. We shall as well study several kinds of functional groups present in organic compounds and organize such compounds into several divisions on the origin of the functional groups.

The covalent bond:

The sharing of electrons to shape a covalent bond leads to an amplify in electron density in between the nuclei. In these an arrangement, the arrangement, the forces holding the atoms together are as well electrostatic in nature; but this  time the forces function between the electrons of 1 atom and the nucleus of the other. These a system has lower energy and is more constant as compared to the energy of isolated atoms. It is so since each and every electron is now attracted via 2 nuclei. Consequently, the formation of the bond is accompanied via the liberate of the energy. The similar amount of energy has to be supplied to split that particular bond. The amount of energy needed to break a particular bond ('represented in terms of kJ mol^-1') is termed its bond dissociation energy. The Bond dissociation energy with another term bond energy that is an average charge for a particular bond. The difference in such 2 energies can be demonstrated via taking the instance of methane, CH₄. If the C - H bonds are consecutively broken as following below, then the bond dissociation energy for each step is as specified on the right hand side.

Bond dissociation energy:

CH₄ → H. + CH₃       427 kJ mol^-1

CH₃ →H. +:CH₂       460 kJ mol^-1

CH₂ →H. +: CH        435 kJ mol^-1

 CH →H. +:   C.        339 kJ mol^-1

We can see from such values that the dissociation energies are different for each C-H bond breakage. On the other hand, bond energy is a single average value that can be attained as, Bond energy of the C-H bond =   427 + 460 + 435 + 339   kJ mol^-1 / 4

                                      = 1661/4    kJ mol^-1

                                      = 415.25     kJ mol^-l

Hence, the C-H bond energy in methane is ¼ of the energy wanted for the given transform.

CH₄   →   C + 4H.

Evidently, if the molecule is diatomic, then bond dissociation energy and bond energy are the similar. Usually, bond dissociation energy values are more positive.

Table: Lists the bond energies and bond dissociation energies for several bonds in kJ mol^-l (at 298 K and 1 atm, pressure).

There are 2 more parameters related by a covalent bond that resolve the shape of a molecule and are recognized as bond length and bond angle. Bond length can be described as the average distance between the nuclei of the atoms that are covalently bound mutually. Bond angle can be identified as the angle between the atoms, forming the bonds to similar atom. Table shows the bond lengths for several of the bonds.

Table: Bond lengths for some of the bonds

From these values of bond lengths, we can conclude that:  

(i)  Bond length reduces by the increase in multiplicity of the bond. Therefore, the decreasing - order for bond lengths for carbon - carbon bonds is 

 C-C > C = C > C≡C.

(ii) Bond lengths amplify by the increasing size of the bonded atoms, for that, the increasing order of bond lengths is C-H < C-F < C-Cl < C-Br < C-l.

Structural formulas:

The structural formula of a compound is its Lewis structure that illustrates how diverse atoms are associated to each other. We are already familiar through Lewis structures of several of the compounds. Various instances are:

To save space and time, such structures are symbolized via condensed procedures that don't explain the bonds. For instance, the condensed formula for ethane can be written as CH₃CH₃.

Similarly, we can write condensed structural formula for 

N.B. Remember that all the representations of the formulas are in 2 dimensions but in fact molecules are 3-dimensional in nature.

Repeating units such as (-CH₂ -) in the structural formula can be enclosed in brackets and hence hexane.

Condensed formulas for compounds having multiple bounds can be written as show below:

H₂C=CH₂                 HC = CH

Ethylene                  Acetylene

For simple compounds, it is easy to write the condensed formulas. But, when the molecules are complex, these formulas look rather awkward and can be further abbreviated. These representations are called line or skeletal structures. Here, the Hydrogens are not shown and each end and bends represents the carbon atoms as shown below for some cases

     Compound                                      Line structure

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