fundamentals-of-organic-chemistry-9

[4] Hyper conjugation or no bond resonance or Baker-Nathan Effect

Stability order of alkenes

Prob. Write down the reactivity order towards electrophilic substitution reaction (good Puzzle)

Toluene is maximum reactive due to maximum hyperconjugation which develop maximum negative charge on the benzene ring and accelerates attack on benzene nucleus.

In elimination reactions like dehydration and dehydrohalogenation product are formed according to saytzeff's rule.

Effects of hyperconjugation

(a) Bond Length : Like resonance, hyperconjugation also affects bond lengths because during the process the single bond in compound acquires some double bonded character and vice-versa.
e.g. C — C bond length in propene is as compared to in Ehtylene.

(b) Dipole moment : Since hyperconjugation causes these charge developments, it also affects the dipole moment of the molecule.

(c) Stability of carbonium ions is Tertiary > Secondary > Primary

Above order of stability can be explained by hyperconjugation. In general greater the number of hydrogen atoms attached to , the more hyperconjugative forms are formed and thus greater is the stability of carbonium ions.

(d) Stability of Free radicals : Stability of Free radicals can also be explained as that of carbonium ion

(e) Orientation influence of methyl group : The o,p–directing influence of the methyl group in methyl benzene is attributed partly to inductive and partly of hyperconjugation effect.

The role of hyperconjugation in o,p–directing influence of methyl group is evidenced by the part that nitration of p-iso propyl toluene and p-tert-butyl toluene form the product in which —NO₂ group is introduced in the ortho position with respect to methyl group and not to isopropyl or t- butyl group although the latter groups are more electron donating than methyl groups.

i.e., The substitution takes place contrary to inductive effect. Actually this constitutes an example where hyperconjugation over powers inductive effect.

[5] Electromeric effect

It is the complete transfer of of a multiple bond towards one of the bonded atoms at the demand of an attacking reagent. The transfer of electrons takes place towards the more electronegative of the two bonded atoms.
For example, when an addition reaction takes place at a carbonyl group (>C = O), the of the double bond are shifted at O-atom because it is more electronegative than carbon :

In Ethylene (ethene) molecule, shifting of p electrons may take place at any of the doubly bonded C-atoms since both the atoms are identical :

However, in Propene the shifting of electrons takes place at carbon atom no.1 because of the +I effect of methyl group :

Electromeric effect (shifting of p electrons) is a temporary effect and takes place only at the demand of an attacking reagent during the course of a chemical reaction. In short it is termed as the E effect.
If the I-effect and E - effect oppose each other then usually the E - effect predominates, i.e.,

Application of electromeric effect : The mechanisms of several organic reactions particularly the addition reactions, are explained by the help of electromeric effect.

[6] Steric effects

Whenever a chemical reaction between two compounds takes place, directly or indirectly it results in the formation of bond between the atoms of these two compounds. The bonding atoms of the two reacting compounds are, in fact, the active centres. These will react (form the bond) with one another only if they come with in the range of each other, i.e., within the attraction of each other. If we surround one of them with such mass of other atoms or groups that the other reacting atom is unable to force its way in, the reaction may not at all take place or may take place only slowly. Such a hindrance due to spatial crowding (crowding a space) is called steric hindrance. However, the spatial crowding may not always hinder a reaction, sometimes it may facilitate a reaction. Therefore, the term steric effect is better than the term steric hindrance.The phenomenon of steric effect was first identified by Hofmann in 1872. It may have sufficient influence on the physical and chemical properties of a molecule and may be defined as modification in molecular properties resulting from a spatial crowding of a reacting atom in a molecule. A good example of steric effect is discussed here:
The tertiary amine with the name Tri methylamine reacts with methyl iodide to form a quaternary salt tetra methyl ammonium iodide, but if the alkyl groups in tertiary amine are large, it does not react with methyliodide and so, does not form the quaternary ammonium salt :

Another good example is that of esterification between a carboxylic acid and an alcohol. Bulkier the alkyl group in acid or alcohol, slower is esterification.

[7] Reactivity of various chemical reactions

1. Electrophilic addition reaction

Alkene is more reactive than alkyne towards addition reaction because of thick in case of alkyne.
There is no addition reaction in case of benzene because resonance occurs in the molecule which stabilises the cloud

2. Nucleophilic addition reaction

As the - I effect or - M effect increases, + ve charge on > C = O group increases so that speed of primary attack of nucleophile on carbon also increases.

3. Nucleophilic Substitution reaction

“Weaker the base better the leaving group” and conjugate base of strong acid always weak.

Resonance or Back bonding of electrons in vacant orbital always creates double bond character in between C – X due to this, chemical reactivity decreases.

C₆H₅ClCH₂ = CH–Cl,CH₃–CH=CH–Cl and CH₃–CCl=CH₂ these all are less reactive halide due to resonance.
While C₆H₅CH₂Cl and Cl – CH₂ – CH = CH₂ are very reactive due to absence of resonance and high stability of intermediate carbocation.

As the +ve charge on carbon attached with halide, increases speed of attacking nature of nucleophile also increases.

4. Electrophilic substitution reaction

Chemical reactivity of aromatic compounds is decided by activating or deactivating series of group. Activating groups are more reactive towards electrophilic substitution reaction because they develop –ve charge on the benzene ring. These are o + p directing in nature. While deactivating groups are less reactive because they develop +ve charge on benzene nucleus and these are meta directing in nature.

“Deactivating groups are always less reactive than activating groups.”