Because the positive charge of each atom’s nucleus is balanced by the negative charge of its outer electrons, a molecule is neutral. The movement of electrons from one region of the molecule to another causes the separation of positive and negative charges. The substrate molecule is activated in various ways as a result of such charge isolation or polarization of the covalent bond. Depending on the nature of the effect, the substrate molecule is activated for interaction with an electrophile, a nucleophile, a free radical, and so on. Electronic effects are the effects that occur in a covalent molecule as a result of electron displacement. Different types of electronic effects are:
- Inductive effects
- Electromeric effect
- Mesomeric effect
- Hyperconjugation effect
- Steric effect
The inductive effect refers to the displacement of a bonding electron pair in an organic molecule caused by a difference in electronegativity between two covalently bonded atoms or groups (one of which is always the carbon atom) and resulting in the polarization of the bond. It is symbolized by the letter I. It is a permanent effect that works via sigma bonds. The polarization produced is not limited to a single bond, but is spread throughout the carbon chain. However, the intensity of the inductive effect decreases as the bond’s distance from the source increases.
It is important to note that although the bonding electron pair is permanently dis[placed it practically remains in the same valence shell.
Based on the nature of the group attached there are two types of inductive effects:
a. Positive inductive effect (+ I effect)
b. Negative inductive effect (-I effect)
Positive inductive effect (+ I effect)
If a group bonded to a carbon atom in the organic molecule is less electronegative than carbon then the bonding electrons are displaced towards the carbon atom or the carbon atom becomes negative. A positive inductive effect (+I effect) occurs as a result of the presence of a less electronegative group attached to the carbon.
+I effect of some of the groups is in the following order
H- < -D< -CH3 < -CH2R < CHR2 < -CR3 < -COO
Negative inductive effect (-I effect)
If the atom or group of atoms bonded to a carbon atom in an organic molecule is more electronegative than carbon the bonding electron pair is displaced towards the more electronegative group as a result of which carbon atoms become more positive. This type of inductive effect is called the negative inductive effect. Such types of more electronegative groups are called electron-withdrawing groups e.g., -NR3, -SR2, -NO2, -CN, -COOH, -F, Cl, -Br, -I etc.
The increasing order of -I effect is
-H < -C6H5 < -OCH3 < -OH < -I < -Br < -Cl < -F < -COOH < -CN < -NO2 < -N+ (CH3)3
Applications of inductive effect
1. Different bond length
With the increase of the -I effect caused by more electron-withdrawing groups the bond length decreases which in turn increase in ionic character. Inductive effect of halogens decreases in the order of F > Cl > Br > I and their alkyl derivatives would be having increases bond length in that order.
|Methyl Flouride (CH3 -F)
|Methyl chloride (CH3 -Cl)
|Methyl bromide (CH3 -Br)
|Methyl iodide (CH3 -I)
2. Dipole moment
The inductive effect can cause a covalent bond to become polar. Because of the symmetrical sharing of electrons pair between carbon and hydrogen atoms in alkane, this bond lacks polarity. If one of an alkane’s hydrogen atoms is replaced by a more electronegative atom, such as halogen, the electron pair of a covalent bond between the C-X is shifted slightly towards the halogen, giving halogen a partial negative charge and carbon a partial positive charge. As a result, the C-X bond becomes polar.
CH3 – I (1.64 D), CH3 – Br (1.79 D), CH3 – Cl (1.83 D)
increase in -I effect →
3. Strength of base
The basicity increases with + I effect. Thus, amines are stronger bases than ammonia because the +I effect of the alkyl group increases the electron density at the nitrogen atom and makes it more basic.
(CH3 )2NH > CH3NH2 > NH3
4. Strength of acid
The inductive effect influences the acidity of various carboxylic acids. When electron-drawing groups are attached to an acid molecule, the acidity increases, whereas when electron-releasing groups are attached, the acidity decreases.
ClCH2COOH > HCOOH > CH3COOH
When the electron withdrawing group become more distant from carboxyl group, the acid strength decreases.
CH3CH2CHClCOOH > CH3CHClCH2COOH > CH2ClCH2CH2COOH
In the case of compounds having double or triple bonds, the pair of pi-electros get completely transferred to the more electronegative element under the influence of the attacking reagent. For example,
An electromeric effect is the complete transfer of a shared pair of electrons from multiple bonds to one of the bonded atoms under the influence of an attacking reagent. It is represented by the symbol E. It is only a temporary effect and is not reflected in the compound’s physical properties. The effect involves complete electron transfer, which results in the development of full positive and negative charges. The atom that acquires the electron pair is negatively charged, whereas the other atom becomes positively charged. The electromeric shift can occur in any direction when there is a multiple bond between similar atoms. In ethylene, for example, the electromeric shift in both conditions results in a similar structure
If multiple bonds are present between two different atoms, the electromeric shift will occur in the direction of a more electronegative element. For example, when the charged reagent approaches the carbonyl group, the pi-electron pair is completely transferred to the more electronegative oxygen atom.
Based on the nature of the group attached there are two types of electromeric effects:
1. + E effect:
The electron pair of Pi bond movement towards the attacking agent is called +E effect. When the attacking reagent is an electrophile, the pi electrons are transferred toward the positively charged atom, resulting in the +E effect. In other words, the presence of an electron-releasing group like the methyl group (-CH3) in the molecule displaces the electrons away from it, called the + E effect.
2. – E effect
The electron pair of Pi bond movement away from the attacking agent is called -E effect. When the attacking reagent is a nucleophile, the pi electrons are transferred to the adjacent atom with which the attacking reagent will not form a bond. In other words, the presence of an electron-attracting group or atom-like chlorine displaces the pi – electrons towards itself, and the electromeric effect is known as -E effect.
Mesomeric effect or resonance effect
The polarity developed in a molecule as a result of interaction between two pi bonds or a pi bond and a lone pair of electrons is known as the mesomeric effect or resonance effect.
If the compound has a conjugated system of double bonds, the mesomeric effect is transmitted throughout the conjugated system. As a result, the effect is more commonly known as a conjugative effect. The conjugative effect aids in the redistribution of electrons with permanent polarization of an entity’s ground state in unsaturated and, in particular, conjugated systems via pi orbitals. The conjugative effect increases the degree of delocalization, resulting in a more stable structure.
1. Positive mesomeric effect or resonance effect (+M, or +R)
When the groups release electrons to the rest of the molecule by delocalization, they have a positive mesomeric effect. These groups are denoted by the symbols +M or +R. The electron density of the rest of the molecular entity increases as a result of this effect. For example, -OH, -OR, -SH, -SR, -NH2, -Cl, -Br and so on.
2. Negative mesomeric effect or resonance effect (-M, or -R)
The groups that withdraw electrons from the rest of the molecule by delocalization show a negative mesomeric effect. These groups are denoted by the symbols -M or -R. The electron density of the rest of the molecular entity decrease as a result of this effect. For example, Carbonyl group (C=O), -NO2, -CN, -COOH, -SO3H, and so on.
Delocalization of sigma electrons from an alkyl group C-H bond directly attached to an unsaturated system atom or an atom with an unshared p orbital is known as hyperconjugation. It is also called a no-bond resonance. The idea of hyperconjugation was put forward by Baker and Nathan in 1935 and is thought to be a special type of resonance and mesomeric effect.
In another word, the excessive i.e., extended conjugation involving orbitals of the C – H or C -X or C -C bond is called hyperconjugation. For its operation, hyperconjugation requires a carbon-hydrogen bond at the alpha position of the double bond.
E.g., ethyl radical. The contributing structures can be written as follows:
Hyperconjugation is a permanent effect and occurs even in the ground state of the molecule. It also affects the compounds’ physical properties, namely stability, bond lengths, the heat of hydrogenation, dipole moment, etc. Generally, there are two types of hyperconjugation effects they are:
- C -H Hyperconjugation in which a carbon – hydrogen bond has been involved
- C – C Hyperconjugation in which a carbon – carbon bond has been involved
The presence of the bulky group around the functional group protects the functional group from the attacking group. It is known as a steric effect. A steric effect is an effect on relative rates caused by the space-filling properties of those parts of a molecule attached at or near the reacting sites
The presence of a bulky group near the reaction site causes the steric effect in a molecule. It causes a permanent deformation in the molecule’s structure, which raises its energy when compared to the structure that is not deformed. Steric strain, steric acceleration, and steric retardation are the three main steric effects.
Forces of steric repulsion are said to be acting when repulsion forces exist between non-bonded atoms in a molecule as a result of their proximity. When the internal forces produced by the interaction of the constituent parts reduce the stability of the molecule, the molecule is said to be under strain.
Steric strain is responsible for the greater stability of trans alkane in comparison to a cis alkane, as well as the higher stability of equatorially substituted cyclohexanes.
Strained chemical species try to avoid the steric strain and thus react quickly to produce the less strained species. When a steric strain speeds up of reaction, then that reaction is said to be subject to steric acceleration. Since the steric strain has assisted the reaction to occur, It is called steric assistance.
A sheer physical blockage owing to the greater bulk of the groups and atoms on the reactive part of the substrate hinders some reactions to occur or slows down the rate and the phenomenon is called steric retardation or steric hindrance or steric inhibition.
- Smith M. & March J. (2001). March’s advanced organic chemistry : reactions mechanisms and structure (5th ed.). Wiley.
- Morrison R. T. & Boyd R. N. (1983). Organic chemistry (4th ed.). Allyn and Bacon.