The phenomenon in which two or more structures can be written for a compound which involve identical positions of atoms, is called resonance.
A number of organic compounds cannot be accurately represented by one structure; the actual structure is a hybrid of the various possible structures.
All the structures which contribute to the properties of a molecule are called as contributing or canonical or resonating structures.
For example, benzene is ordinarily represented as:
This structure has three carbon–carbon single bonds and three carbon-carbon double bonds, However, it has been determined experimentally that all carbon-carbon bonds in benzene are identical and have the same bond length (1.39A). Furthermore, the carbon– carbon bond length of (1.39 A) is intermediate between the normal carbon-carbon double bond length (1-34 A) and the normal carbon-carbon single-bond length (1.54 A). Actually two alternative structures (1 and 2) can be written for benzene.
Resonance involves the delocalization of electrons in the molecules so as to give a new stable form which explains its complete behavior.
Conditions for resonance:
- The relative positions of the nuclei should be same in all contributing structures. In other words, the carbon –carbon skeleton should be same.
- All the contributing structures should have nearly same energy.
- The total number of paired and lone pair of electrons should be same.
Consequences of resonance:
- Resonance energy: The resonance hybrid is more stable than anyone of the various resonance structures. The difference in energy between the hybrid and the most stable resonance structure” is known as the Resonance energy or delocalization energy or stabilization energy. The resonance energy of benzene is 38 Kcal/ mole. The benzene is said to be “stabi1ised” by resonance energy of 38 Kcal/mole.
- Stability: The structure which contributes most in the hybrid is the most stable structure.
- The more the resonance structures that are possible to draw for a particular molecule or ion, the more stable is the molecule or ion.
- The more the covalent bonds the structure has, the more is the stability. Example: 1, 3-butadiene.
+CH2-CH=CH -CH2 – CH2= CH -CH= CH2 –CH2-CH=CH-CH2+
(I) ( II) (III)
Where, the most stable structure is (II).
Most of the organic reactions take place via certain short lived (10-6 sec to a few seconds) chemical species. These short lived highly reactive chemical species involved in large number of organic reaction called reactive intermediates.
They are formed by the cleavage of a covalent bond in the following two ways:
- Homolytic bond fission: When a covalent bond breaks (in the presence of a reagent) such that shared electron goes to the two fragments, the fission is known as homolytic bond fission,
For example, in the presence of uv light, the covalent bond of chlorine molecule breaks homolytically, chlorine free radicals are formed.
The products, Clx and Clx are called Free Radical. 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. Consequently, reactions which proceed via the intermediate formation of free radicals often take place very rapidly.
Homolytic fission is the most common mode of fission in the vapour phase.
Homolytic reactions are usually initiated by heat; light or organic peroxides.
- Heterolytic bond fission: When a covalent bond breaks (in the presence of a reagent) such that one of the two fragment acquires both the shared electron the fission is known as heterolytic bond fission,
Naturally, the ions are formed in heterolytic fission. For example, chlorine molecule breaks heterolytically in the presence of a Lewis acid (AICl3) to form carbocation (cation) and chloride ion (anion).
Organic reagents fall into two main groups:
1) Electrophiles or Electrophilic Reagents
2) Nucleophiles or Nucleophilic Reagents
Electrophile:A reagent which can accept an electron pair in a reaction is called an electrophile. The name electrophile means “electron-loving” and indicates that it attacks regions of high electron density (negative centres) in the substrate molecule. Electrophiles are electron-deficient. They may be positive ions (including carbonium ions) or neutral molecules with electron-deficient centres. Examples are,
H+, CI+, Br+, I+, NO2+, R3C+, RN2+, +SO3H, AlCI3, BF3
electrophile ‘can be represented by the general symbol E+.
Nucleophiles. A reagent which can donate an electron pair in a reaction is called a nucleophile. The name nucleophile means “nucleus-loving” and indicates that it attacks regions of low eJectron density (positive centres) in the substrate molecule. Nucleophiles are electron-rich. They may be negative ions (including carbanions) or neutral molecules with free electron pairs. Examples are,
Cl–, Br–, I–, CN–, OR–, R-CH2–
NH2, RNH2, H2O, ROH
A nucleophile can be represented by a general symbol Nu:–
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:
(1) Inductive Effect
(2) Mesomeric Effect
(3) Electromeric Effect
It. involves electrons which form a covalent bond, which are seldom shared equally between the two atoms. This is because different atoms have different electro negativity 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 (-). The less electronegative atom acquires a small positive charge (_+).
Suppose – X is electron attracting (-Cl) as shown in case I. Then the electrons will be shifted towards X
Consider the carbon-chlorine bond. As chlorine is more electronegative, it will become negatively charged with respect to the carbon atom.
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.
This type of permanent effect, whereby, polarity is induced on the carbon atom and the group attached to it due to minor displacement of bonding electron pair caused by their different electro negativities, is known as inductive effect.
Atoms or groups which lose electrons toward a carbon atom are said to have a +I Effect. Such groups are referred as electron-releasing. Those atoms or groups which withdraw electrons away from a carbon atom are said to have a -I Effect. Such groups will be referred to as electron-attracting.
+ I effect groups: With their order
(CH3)3C-> (CH3)CH-> CH3CH2-> -CH3
– I effect groups: -NO2 >-F>-COOH>-Cl >-Br>-I>-OH>-C6H5
It involves π- electrons of double and triple bonds i.e conjugate system.
The mesomeric effect (M effect) refer to the polarity produced in a molecule as a result of interaction between two π bonds or a π 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. (Conjugated compounds are those in which the carbon atoms are linked alternately by single and double bonds). In such systems, the π electrons get delocalized as a consequence of mesomeric effect, giving a number of resonance structures of the molecule.
When an electron pumping or electron withdrawing group is conjugated with a π -bond or a set of alternately arranged σ and π -bonds the electron displacement is transmitted through π electrons in the chain.
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. Those atoms or groups which draw electrons away from a carbon atom are said to have a -M effect .
Some common atoms or groups which cause +M: or –M effects are listed below:
(a) +M Effect Groups: -Cl, -Br, -I, -NH2, -NR2, -OR, -OCH3
(b) -M Effect Groups: -NO2, -CN, -CO-,-CHO
The +M effect of the bromine atom is shown below:
The – M effect of the nitro group is shown below:
The electromeric effect (E Effect) refers to the polarity produced in a multiple bonded compound as it is approached by a reagent.
This effect is a temporary effect and is operative only at the requirement of reagent in a particular reaction,’
Consider a compound having double bond between the two atoms A and B, A – B. If a reagent requires a positively charged atom, then the electron pair of weak π-bond will go to the more electronegative atom making atom A with single positive charge.
Thus, the effect which causes a temporary polarization in the substrate molecule at the seat of a multiple bond by shift of an electron pair to the more electronegative atom under the influence of a reagent is called electromeric effect. Like inductive effect there are two types of electromeric effect:
- +E effect: When the electron transfer takes place towards the atom of the double bond to which the attacking reagent gets finalfy attached, it is called +E effect.
- -E effect: When the electron transfer takes place away from the atom of the double bond to which the attacking gets finally attached, it is called -E effect.
The electromeric effect can well be seen operative in the nucleophilic addition reactions of carbonyl compounds, in which π -electron pair shift to the oxygen atom creating positive charge on the carbon atom as per requirement of reagent.
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, and carbenes.
Carbonium ions :
Organic ions which contain a positively charged carbon atom are called carbonium ions or carbocations. They are formed by heterolytic bond fission. Due to electron deficient species, it has a positive charge and acts as an electrophile
where Z is more electronegative than carbon.
The positively charged carbon atom in a carbonium ion uses Sp2 hybrid orbitals to form three σ bonds. An empty p orbital extends above and below the plane of the σ 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. Carbonium ions are named after the parent alkyl group and adding the word carbonium ion.
The carbocation may be primary, secondary or tertiary in nature.
Stability: For the stability of carbocation, the dispersal of positive charge is necessary. That is why the order of stability among the alkyl radicals is,
The order of reactivity is reverse of the order of stability
(CH3)3C+ < (CH3)2CH + <CH3CH2+ < +CH3
This is also known as Baker-Nathan effect. The hyperconjugation is also known as no bond resonance. This is due to the participation of a α C-H bond in resonance. A situation comes when there is no bond between
The relative stability of various classes of carbonium ions may be_ explained by the number of no-bond resonance structures that can be written for them, Such structures are arrived at by shifting the bonding electrons from an adjacent C-H bond to the electron-deficient carbon. In this way, the positive charge originally on carbon is dispersed to the hydrogen. This manner of electron release by assuming no-bond character in the adjacent C-H bond is called Hyperconjugation or No-Bond Resonance.’
The more hyperconjugation structures (no-bond ,resonance structures) that can be written for a species, the more stable is the species ,for example,
(1) Ethyl carbonium ion is stabilized by three hyperconjugation structures:
(2) Isopropyl carbonium ion is stabilized by six hyper conjugation structures.
(3) t-Butyl carbonium ion is stabilized by nine hyper conjugation structures.
Thus, the following order of stability holds:
(CH3)3C+ > (CH3)2CH + > +CH3CH2+ > +CH3