study of organic reaction mechanism

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Substrate: The reactant molecule undergoing attack is referred to as the substrate. Reagent: The general term used to describe the attacking species is the reagent. The substrate and the reagent interact to yield the products of the reaction. SUBSTRATE + REAGENT PRODUCTS

Most of the attacking reagents carry either a positive or a negative charge. The positively charged reagents attack the regions of high electron density in the substrate molecule.

On the other hand, the negatively charged reagent will attack the regions of low electron density in the substrate molecule. The carbon bonds in the substrate molecule (organic) are broken (or cleaved) to give fragments which are very reactive and constitute transitory intermediates. At once they may react with other species present in the environment to form new bonds to give the products.

The steps of an organic reaction showing the breaking and making of new bonds of carbon atoms in the substrate leading to the formation of the final products through transitory intermediates, are often referred to as its mechanism.




Factors which influence a reaction A reaction may occur or may not occur depending upon the density of electrons at the site of reaction in the substrate. The factors which influence the electron density in the substrate are : - Inductive Effect - Mesomeric Effect - Electromeric Effect

Inductive effect The inductive effect (I Effect) refers to the polarity produced in a molecule as a result of higher electronegativity of one atom compared to another. (sigma) electrons. The (sigma) electrons It involves which form a covalent bond are seldom shared equally between the two atoms. This is because different atoms have different electronegativity values, i.e. different powers of attracting the electrons in the bond. Consequently, electrons are displaced towards the more electronegative atom. This introduces a certain degree of polarity in the bond. The more electronegative atom acquires a small negative charge (H-). The less electronegative atom acquires a small positive charge (H+).

Consider the carbon-chlorine bond. As chlorine is more electronegative, it becomes negatively charged with respect to the carbon atom.

Structure I indicates the relative charges on the two atoms. In structure II, the arrow head placed in the middle of the bond indicates the direction in which the electrons are drawn. In structure III, the more heavily shaded part shows the region in which the electron density is greatest.

For measurement of relative inductive effects, hydrogen is chosen as reference in the molecule CR3 H as standard. If, when the H atom in this molecule is replaced by Z (atom or group), the electron density in the CR3 part of the molecule is less in this part than in CR3 H, then Z is said to have a I effect (electron-attracting or electron-withdrawing). If the electron density in the CR3 part is greater than in CR3 H, then Z is said to have a +I Effect (electron-repelling or electron-releasing) e.g., Br is I; C2H5 is +I.

Some common atoms or groups which cause +I or -I effects are shown below: -I effect groups (Electron-attracting): NO2, F, Cl, Br, I, OH, C6H5+I effect groups (Electron-releasing): (CH3)3C-, (CH3)2CH-, CH3CH2-, CH3Tertiary alkyl groups exert greater +I effect than secondary which in turn exert a greater effect than primary.

An inductive effect is not confined to the polarization of one bond. It is transmitted along a chain of carbon atoms, although it tends to be insignificant beyond the second carbon. C 3 C 2 C 1 Cl -I

The inductive effect of C1 upon C2 is significantly less than the effect of the chlorine atom on C1. The inductive effect results in a permanent state of the molecule and can be observed practically in the form of dipole moments. The effect does not depend upon the presence of a reagent.

Mesomeric or resonance effect It involves T (pi) electrons of double and triple bonds. The mesomeric effect (M Effect) refers to the polarity produced in a molecule as a result of interaction between two T bonds or a T bond and lone pair of electrons. The effect is transmitted along a chain in a similar way as are inductive effects. The mesomeric effect is of great importance in conjugated compounds. In such systems, the T electrons get delocalized as a consequence of mesomeric effect, giving a number of resonance structures of the molecule.

Consider a carbonyl group (>C=0). The oxygen atom is more electronegative than the carbon atom. As a result, electrons of the carbon-oxygen double bond get displaced toward the oxygen atom. This gives the following resonance structures:

The mesomeric effect is represented by a curved arrow. The head of the arrow indicates the movement of a pair of T electrons. If the carbonyl group is conjugated with a carboncarbon double bond, the above polarization will be transmitted further via the T electrons.

The mesomeric effect like the inductive effect may be positive or negative. Atoms which lose electrons toward a carbon atom are said to have a +M Effect. +M effect groups: Cl, Br, I, NH2, OH, OCH3

Those atoms or groups which draw electrons away from a carbon atom are said to have a -M Effect. -M effect groups: NO2, CN, >C=O

The mesomeric effect like the inductive effect results in a permanent state of the molecule. It does not depend upon the presence of a reagent. The inductive and mesomeric effects indicate the charge distribution in a molecule. Thus, they provide an effective way of determining the point of attach of electrophile and nucleophiles on the molecule.


Electromeric effect Like the mesomeric effect, it also involves the T (pi) electrons. The electromeric effect (E Effect) refers to the polarity produced in a multiple bonded compound in the presence of a reagent. When a double or a triple bond is exposed to an attack by an electrophile E+ (a reagent), the two T electrons which form the bond are completely transferred to one atom or the other. The electromeric effect is represented as:

The curved arrow shows the displacement of the electron pair. The atom A has lost its share in the electron pair and B has gained this share. As result, A acquires a positive charge and B a negative charge. Notice that the arrow points away from the centre of the bond and towards the atom that gains the electron pair. Consider the example where an electrophile (E+) attacks a carbon-carbon double bond in the ethylene molecule. We know that the double bond is made up of one bond and one T bond. The electrons in the T bond are quite exposed. Under the influence of the electric field of the positively charged electrophile, the symmetry of the molecular orbital is disturbed entirely in favor of one carbon atom.

This gives a negative charge to the carbon atom to which the T electron-pair migrates, while the other atom acquires a positive charge.

The electromeric effect is a temporary effect. It takes place only in the presence of a reagent.

Homolytic and heterolytic fission Every reaction of organic compounds involves the breaking (fission) of at least one bond and the making of another bond. To break a bond, in fact, we are breaking down a molecular orbital to give atomic orbitals. We known that molecular orbitals are at a lower energy (more stable) than the atomic orbitals. Therefore, energy has to be supplied to break a bond. Assuming that sufficient energy is available, a covalent bond ( bond) can undergo fission in two ways: - Homolytic Fission or Homolysis - Heterolytic Fission or Heterolysis

Homolytic fission In this process each of the atoms acquires one of the bonding electrons. A B or A:B Ay + yB

The products, Ay and yB, are called free radicals. They are electrically neutral and have one unpaired (odd) electron associated with them. Free radicals are extremely reactive because of the tendency of this electron to become paired at the earliest opportunity. Homolytic fission is the most common mode of fission in the vapor phase. Homolytic reactions are usually initiated by heat, light or organic peroxides.

Heterolytic fission In this process one of the atoms acquires both of the bonding electrons when the bond is broken. A A B B + A :A + + :B + B (1) (2)

In example 1, B is more electronegative than A which thereby acquires both the bonding electrons and becomes negatively charged. The arrow indicates that the sigma ( ) electrons that form the A B bond are leaving A and becoming the exclusive property of B. In example 2, A is shown to be more electronegative than B.

The products of heterolytic fission are ions. Reactions which involve heterolytic fission take place at measurable rates. Heterolytic fission occurs most readily with polar compounds in polar solvents. Reaction intermediates Heterolytic and homolytic bond fissions result in the formation of short-lived fragments called reaction intermediates. Among the important reaction intermediates are: - Carbonium ions - Carbanions - Carbon free radicals - Carbenes

Carbonium ions (carbocations) Organic ions which contain a positively charged carbon atom are called carbonium ions or carbocations. They are formed by heterolytic bond fission. - where Z is more electronegative than carbon - Carbon bearing 6 electrons - are lewis acid

The positively charged carbon atom in a carbonium ion uses sp2 hybrid orbitals to form three bonds. An empty p orbital extends above and below the plane of the bonds. This empty p orbital makes the carbon atom electron-deficient and gives it a positive charge. Thus a carbonium ion will combine with any substance (e.g., nucleophiles) which can donate a pair of electrons.

Structure of carbonium