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Electronic Displacement in Covalent Bonds

The following four types of electronic effects operates in covalent bonds

  1. Inductive effect

  2. Mesomeric and Resonance effect

  3. Electronic effects

  4. Hyperconjugation 

  • Inductive Effect

In a covalent bond between the two dissimilar atoms, the electron pair forming the bond is never shared absolutely equally between the two atoms but is attracted a little more towards the more electronegative atom of the two, eg. The electron pair forming the C–X bond is somewhat more attracted towards the atom X with the result – it attains a partial negative charge (–\delta) while the carbon atoms attain a partial positive  charge (+\delta)

On the other hand, in compounds like C–Y, where Y in an electropositive element or group i.e., C is more  electronegative than Y, the electron pair forming the C–Y bond is somewhat displaced towards the carbon atom and thus C and Y attain partial negative and partial positively charges respectively.

According to Ingold sign convention, the former is called as (–I) effect and the later is called as (+I) effect.

The inductive effect causes certain degree of polarity in the bond which in term renders the bond much more liable to be attacked by other charged atoms or group.

Thus, inductive effect may be defined as the permanent displacement of electron forming a covalent bond towards the more electronegative element or group.

The inductive effect is represented by the symbol → , the  arrow pointing towards the more electronegative element or group of elements eg. N – butyl chloride .

The extent of positive charge keeps on decreasing away from Cl atom and at third and fourth carbon it is almost zero for all practical purposes.

Examples of  Inductive effect: 

 (–I) effect group (electron  attracting)

 (+I) effect – group (electron – repelling)

  • Applications of Inductive effect :

Effect on Bond lengths: Since the inductive effect leads to ionic character in the bond, the increase in –I effect usually decreases the bond length.

Dipole moment : Since, inductive effect leads to a dipolar character in the molecule, it develops some dipole moment in the molecule, which increases with the increase in the inductive effect.

CH3 – I, CH3 — Br,     CH3 — Cl

 Increasing dipole moment

Reactivity of alkyl halides: Alkyl halides are more reactive than the corresponding alkanes due to presence of C––X bond which is polar due to I effect, furthermore reactivity increases with increase of branching.

Strength of Carboxylic Acids: Strength of an acid depends upon the ease with which an acid ionises to give proton. A molecule of carboxylic acid can be represented as a resonance hybrid of the following structures.

In the II structure, the oxygen atom of the hydroxyl group has a positive charge due to which it has a tendency to attract electron pair (inductive effect) of the O—H bond towards itself, which results in the removal of hydrogen atom as proton and hence carboxylic acids behave as acids.

Once, the carboxylate anion is formed, it is stabilised more easily by resonance than undissociated acid.

Thus, the acidity of carboxylic acid is due to inductive effect and resonance stabilisation of the carboxylate anion. Thus  any group or atom, which is highly electronegative help in removing the hydrogen atom as proton and the group or atom which is less electronegative than C makes the removal  of proton difficult.

Hence (–I) effect group increases acidic strength and (+I) effect groups decreases the acidic strength of carboxylic acid.

Basic strength of Amines : The basic character of amines is due to presence of unshared electron pair on nitrogen atom which accepts proton; the readiness with which the lone pair of electrons available for protonotion determines the relative strength of amines.

Due to +I effect of alkyl group, the nitrogen atom becomes rich in electrons with the result the lone pair of electron on nitrogen atom in amines is more easily  available than in ammonia and hence generally, amines are stronger bases than ammonia. On the other (–I) groups or electron groups attached to nitrogen atom makes it difficult for protonation.   

Note:   Relative basic strength of amines is not in total accordance with the inductive effect, other factors like steric effect and stabilisation of cation by hydration also play important role to determine the basic strength of amines.

Due to electron – releasing group electron density is increased, hence basic nature is also increased and naturally acidic nature is decreased, thus

 

-I effect

+I effect

Acidic Nature

Basic Nature

  • Resonance & Mesomeric Effect:

There are many organic molecules which can not be represented by a single lewis structure. In turn, they are assigned more than one structure called canonical forms or contributing of resonating structures. The phenomenon exhibited by such compounds is called resonance. For example, 1, 3 – butadiene has following resonance structure.

 

and canonical forms of vinyl chloride are

 

While drawing these canonical forms, the prime thing that has to be kept in mind is that the relative position of any of the atom should not change while we are allowed to change the relative positions of p - bonded electron pair or distribution of charge to other atoms. Also remember that it is not the case that some molecules have one canonical form and some have another form. All the molecules of the substance have the same structure. That structure is always the same all the time and is a weighted average of all the canonical forms. In real sense, these canonical forms have no expect in our imaginations. Now we are in a position to discuss about the conditions necessary for a compound to show resonance. The two essential conditions are

  1. There must be conjugation in the molecule. Conjugation is defined as the presence of alternate double and single bonds in the compound like 

  2. The part of the molecules having conjugation must be essentially planar or nearly planar. The first condition of conjugation is not only confined to the one mentioned above but some other systems are also categorized under conjugation. These are

(i)

(ii)

(iii)

(iv)

(v)

 

 

So, any molecules satisfying both the conditions will show resonance. For example, we consider phenol. The structure of phenol is

By looking at the structure, it must be clear to you that the compound possesses conjugation of the type

As well as the category (iv) because the lone pairs on oxygen are in conjugation with unsaturated (sp2 hybridised) carbon of the ring. Since, oxygen atom is sp3 hybridized in phenol.


The lone pairs on oxygen are nearly planar with respect to the PZ orbital of carbon linked to oxygen. Thus, both the conditions are fulfilled by phenol, therefore it does show resonance and its resonance structures are represented as

This has to be borne in mind that resonance always results in different distribution of electron density than would be the case if there were no resonance. It is a permanent effect, also referred as mesomeric effect.

Note:The acidity of phenol can be explained by resonance        

The above structure shows that the phenoxide ion formed is more resonance stabilised than phenol. Hence, the acidity of phenol is explained.

Similarly basicity of aniline can be explained.

The above structure shows that the lone pair present on N – atom undergoes into resonance and is not available for donation. Hence, the basicity of aniline decreases and is less than aliphatic amine.

Resonance (mesomeric) effect is of two types.

(i) If the atom or group of atoms is giving electrons through resonance, it is called +R or +M effect. For example,

(+M effect of -NH2 group)

Other groups that shows +M effect are -NHR, -NR2, -OH, -OR, -NHCOR, -Cl, -Br,-I etc.

(ii) If the atom or group of atoms is withdrawing electrons through resonance, it is called ¾R or ¾M effect. For example,

(-M effect of -NO2 group)

Other groups showing -M effect are -CN, -CHO, -COR, -CO2H, -CO2R, -CONH2, -SO3H, -COCl etc.

Now, let us consider resonance in nitrobenzene and its various canonical structures are

The -NO2 group in nitrobenzene has -M effect. In general, if any atom (of the group) attached to the carbon of benzene ring bears atleast one lone pair, then the group shows +M effect while if the atom (of the group) linked to the benzene carbon bears either a partial or full positive charge, then the group exhibits -M effect.

In drawing the canonical forms and deciding about their relative stabilities, following rules are give for your guidance.

  1. All the canonical forms must be bonafide lewis structures for example, none of them may have a carbon with five bonds.

  2. All atoms taking part in the resonance must lie in a plane or nearly so. The reason for planarity is to have maximum overlap of the p – orbitals.

  3. All canonical forms must have the same number of unpaired electrons. Thus CH2 - CH = CH -CH2 is not a valid canonical form for 1, 3 - butadien.

  4. The energy of the hybrid (actual) molecule is lower than that of any canonical form. Obviously then, delocalization is a stabilizing phenomenon. The difference in energy between the hybrid and the most stable canonical structure is called resonance energy.

  5. All canonical forms do not contribute equally to the actual molecule. Each form contributes in proportion to its stability, the most stable form contributing the most.

  6. Structures with more covalent bonds are generally more stable than those with fewer covalent bonds.

  7. Structure with formal charges is less stable than uncharged structures. For charged structure, the stability is decreased by an increase in charge separation and the structure with two like charges on adjacent atoms are highly unfavourable.

  8. Structures that carry a negative charge on a more electronegative atom are more stable than those in which the charge is on a less electronegative atom. For example,
    Structure (II) is more stable than (I). Similarly positive charges are best occupied on atoms of low electronegativity

  9. Those structures in which octet of every atom (expect for hydrogen which have douplet) is complete are more stable than the others with non complete octets. For example,        
    Structure (IV) is more stable than (III).

Resonance Energy: The difference in energy between the hybrid and the most stable canonical structure is  called as resonance energy

      

  • Effect of Resonance

Dipole moment: Dipole moment of certain compounds can be explained by resonance eg. Vinylchloride 

Bond length: the phenomenon of resonance explain the abnormal bond length between C—C, C = C, C = O, etc in compounds exhibiting resonance e.g. in benzene C—C bond length acquires a value which lies between C—C single bond length (1.54Å) and C = C (double bond) length (1.33Å)

Strength of acids and bases: The concept of resonance explain clearly the acidic character of acids and basic character of bases eg. resonance explains why the alcohols are neutral and carboxylic acids are strong acids.


But after loss of  H+ carboxylate ion R-COO- undergo  resonance and stabilised, hence it will favour the loss of H+ ions 

Stability of free radicals and carbonium ions :

Since number of resonating structures increases from I to II, hence stability also increases in the same order.

Similarly we can explain the stability of carbonium ions.

  • Electromeric Effect

It is a temporary effect in which a shared pair of electron (p - electron pair) is completely transferred from a double bond or triple bond to one of the atoms joined by the bond at the requirement of attacking reagent.

Case 1:     When multiple bond is present between two similar atom (symmetric alkenes or alkynes) electronic shift can take place in any direction 

CH_{2}=CH_{2} \overset{HBr}{\rightarrow}CH_{2}^{-}=CH_{2}^{+}

Note 1:      If two carbon atoms are different (asymmetric alkenes or alkynes) then the direction of electronic shift is determined by the direction of the inductive effect of the group present at doubly or triply bonded atom e.g.,.

Note 2: When inductive and electromeric effects oppose each other, in such cases, electromeric effect usually overcome inductive effect eg.

  • Hyperconjugation:

it is the delocalisation of sigma electron. Also known as  sigma-pi  conjugation or no bond resonance

Occurrence:  Alkene, alkynes. Free radicals (saturated type)  carbonium ions (saturated type)

Condition: Presence of a–H with respect to double bond, triple bond carbon containing positive charge (in carbonium ion) 

Example 

Note:   Number of hyperconjugative structures = number of a-Hydrogen hence, in above examples structures ii,iii,iv are hyperconjugative structures (H-structures).

Hyperconjugation is a permanent  effect

Effects of hyperconjugation

Bond Length: Like resonance, hyperconjugation also affects bond lengths because during the process the single bond in compound acquires some double bond character and vice-versa. E.g. C—C bond length in propene is 1.488 Å as compared to 1.334Å in ethylene .

Dipole moment : Since hyperconjugation causes the development of charges, it also affects the dipole moment of the molecule.

Stability of carbonium Ions: The order of stability of carbonium ions is as follows. Tertiary > Secondary > Primary

above order of stability can be explained by hyperconjugation. In general greater the number of hydrogen atoms attached to a-carbon atoms, the more hyperconjugative forms can be written and thus greater will be the stability of carbonium ions.

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

Orientation influence of methyl group: The o,p-directing influence of the methyl group in methyl benzenes is attributed partly to inductive and party of hyperconjugation effect.

(orientation influence of the methyl group due to  +I effect )

The role of hyperconjugation in o,p,-directing influence of methyl group is evidenced by the fact that nitration of p-isopropyl toluene and p-tert-butyl toluene from 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 overpowers inductive effect.

  • Mechanism of Organic Reactions:

A chemical equation is only a symbolic representation of chemical reaction which indicates the initial reactants and final products involved in a chemical change. Reactants generally consist of two species.

  1. Substrate: One which is being attacked in a chemical reaction

  2. Reagents: The species which attack the substrate molecule

      Substrate + Reagent →  Products

      

It is important to know not only what happens in a chemical reaction but also how it happens. Most of the reactions are complex and take place via reactive intermediates which may be or may not be isolated. The reaction intermediates are generally very reactive which readily react with other species present in the environment to form the products. The detailed step by step description of chemical reaction is called its mechanism. Mechanism is only a hypothesis to explain various facts regarding a chemical reaction.

Substrate → Reactive intermediates →  Products

By knowing the mechanism we can predict the product of a chemical reaction, adjust the experimental conditions to improve the yield of the products or even alter the course of reaction to get the different products.

Most of the attacking reagents carry either positive charge (an electron deficient species) or a negative charge (electron rich species). The positively charged reagents attack the substrate at points of high electron density while (-vely) charged reagents attack the point of low electron density. The organic reactions essentially involve changes in the existing covalent bonds present in the molecules. These changes may involve electronic displacements in covalent bonds breaking of some of the existing bonds (bond fission), formation of new bonds as well as energy change accompanying the bond fission and bond cleavage.

We can understand the mechanism of various organic reactions in terms of following well established basic concepts.

  1. Electronic displacement in covalent bond

  2. Fission (cleavage) of covalent bonds

  3. Nature of attacking reagents

  • Types of Organic Reactions:

All organic reactions can be broadly classified into four categories.

  1. Substitution reactions              

  2. Addition reactions

  3. Elimination reactions and          

  4. Rearrangement reactions

  1.  Substitution Reactions

In these an atom or a group of atoms in an organic molecule is replaced by another atom or group of atoms without any change in the remaining part of the molecule. These reactions may be initiated by free radical, electrophile or nucleophile.

(i) Free radical substitution reaction:

This substitution reaction is brought about by free radicals. For example chlorination of methane in presence of diffused sunlight. The mechanism of the reaction is as follows.

Cl : Cl →  2Cl·Chain initiation

(ii)  Nucleophilic substitution reactions:

These reactions are brought about by nucleophile. The reaction can proceed either via SN1 or SN2 mechanism.

SN1 mechanism:

Rate determining step involves only the species. For example the reaction.

takes place as follows

1st step: (CH3)3CBr 

2nd step: Attack by nucleophile                                    

The stability of carbocation is the controlling factor for this mechanism the formation of 3° carbocation as an intermediate proceeds via this mechanism. In an optically active compound substitution at chiral centre through SN1 mechanism produces recemic mixture (No 100% recemization is observed?).

SN2 mechanism: Rate determining step involves two species and reaction proceeds through transition state.

                  

Since 1° carbocation is less stable than the transition state formed above, the reaction involving 1° alkyl halides proceed via SN2 mechanism. During reaction configuration of carbon is inverted which is referred to as Walden inversion.

Points to Remember

  • The higher the polarity of solvent greater the tendency for SN1 reaction.

  • High concentration of the nucleophile favours SN2 reaction while low concentration favours SN1 reaction.

  • Rearrangement of the carbocation (formed in SN1 reaction) leading to more stable carbocation is observed in SN1 reaction (discussed latter).

  • In general SN2 mechanism is strongly inhibited by increasing steric bulk of the reagents. In such case SN1 mechanism is favoured.

(iii)   Electrophilic substitution reactions:

The reaction initiated by an electrophile is known as electrophilic substitution reaction. Aromatic substitution reactions are the examples of this type of reaction.

C6H6 + Cl2 C6H5Cl + HCl

The mechanism of this reaction as follows.

Formation of electrophile: Cl : Cl + AlCl3 →  Cl

Electrophile attack:

 

Elimination of proton:

            

  1.  Addition Reactions

Reactions which involve combination between two molecules to give a single molecule of the product are called addition reactions. Such reactions are typical of compounds containing multiple (double or tripe) bonds. Depending upon the nature of the attacking species (electrophiles, nucleophiles or free radicals) addition reactions are of the following types.

(i) Electrophilic addition reaction:

These reactions are brought about by electrophiles and are typical reactions of alkenes and alkynes.

(ii) Nucleophilic addition reactions:

These reactions are brought about by nucleophiles. The characteristics reaction of aldehyde and ketone are nucleophilic addition reaction i.e., base catalysed addition of HCN to aldehydes or ketones.

HO + HCN →  H2O + CN

(iii)   Free radical addition reactions:

Addition reactions brought about by free radicals are called free radical addition reactions for example addition of HBr to alkenes in presence of peroxides.

CH3—CH=CH2+HBr  

The reaction proceeds through following mechanism.

2RO°

RO· + HBr →  ROH + Br·

CH3—CH=CH2 + Br CH3·CH—CH2—Br

CH3·CH—CH2—BrCH3—CH2CH2—Br + Br

Br· + Br·→  Br2

  1.   Elimination Reactions

An elimination reaction is one which involves the loss of two atoms or groups of atoms from the same or adjacent atoms of a substrate molecule leading to formation of multiple (double or triple) bond. These are of two types.

(i) b - elimination reactions: In these reactions, loss of two atoms or groups occurs from the adjacent atoms of the substrate molecule e.g., acid catalysed dehydration of alcohol and base catalysed dehydrogenation of alkyl halides.

E1 mechanism

E2 Mechanism

E1 — CB mechanism

(ii) a - Elimination:  In these reactions loss of two atoms or groups occurs from the same atom of the substrate molecule. E.g., base catalysed dehydrohalogenation of chloroform to form dichlorocarbene.

Dichlorocarbene is the reactive intermediate involved in carbylamine reaction and 
Reimer – Tieman reaction.

  1.  Rearrangement Reaction

These reactions involve the migration of an atom or a group of atoms from one atom to another within the same molecule.

Some reactions involving rearrangement.

  

  

1, 2 Hydride shift

 

1, 2 Methyl shift

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