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Fundamental Concepts in Organic Reaction Mechanism

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  • Last Updated : 28 Mar, 2022

Organic chemistry is the chemistry of carbon compounds except for oxides of carbon and metal carbonates. Carbon has the uncommon characteristic of forming strong bonds with many other elements, particularly with other carbon atoms, to form chains and rings, giving rise to millions of organic molecules. Carbon compounds are essential for the survival of life on Earth because of this distinguishing property. Proteins, DNA (deoxyribonucleic acid), and other complex organic substances provide chemical, structural, or genetic functions in living beings.

A chemical equation only shows the starting and final products of a reaction; it rarely shows how the process proceeds. Some reactions occur through intermediates, which may or may not be separated depending on their stability.

Mechanism is the complete step-by-step description of the order in which bonds break and bonds form, to give the observed products.

Electron Movement in Organic reactions

Curved arrows are used to depict electron transport in chemical processes. The curving arrow is a handy indication for showing bonding changes caused by electronic redistribution during the reaction. A covalent connection is created when two atoms share a pair of electrons. The many methods in which the electron-pair travels are depicted below using curved arrows. The arrow begins at the point when the electron-pair is displaced and ends at the point where the electron-pair travels.

  • Shifting of electron-pair from π-bond to adjacent bond position.

  • Shifting of electron-pair from π-bond to adjacent atom.

  • Shifting of electron-pair from atom to adjacent bond position.

For Example: In the case of ethane when reacting in presence of UV light, due to the electron shifting, two methane molecules are obtained as shown below,

Bond Fission

In any chemical reaction, when a reactant is converted into products one or more bonds in the reactant are broken and new bonds are formed. The process of breaking or cleavage of a covalent bond is known as bond fission.

Now, the bond fission takes place in two ways as mentioned below,

Homolytic fission

The symmetrical breaking of a covalent bond between two atoms such that each atom acquires one electron of the shared pair is called homolytic fission or homolysis.

Such fission takes place in the presence of ultraviolet light or at high temperatures. The cleavage of a bond results in the formation of free radicals. A free radical is a neutral species (atom or group) that contains an unpaired electron.  Homolytic fission can be shown as, 

The movement of a single electron is shown by a half-headed curved arrow (Fishhook). Free radicals have transitory existence i.e. they are short-lived and are highly reactive. They are paramagnetic.

Generally, a covalent bond between two atoms of the same element or two atoms having nearly the same electronegativity breaks in this manner. For example, Organic reactions which proceed by homolysis are called free radical or homopolar or non-polar reactions as these reactions take place in a non-polar solvent. Homolysis generally occurs in the gaseous phase in presence of sunlight or ultra-violet light or in the presence of catalysts such as hydrogen peroxide.

Heterolytic fission

The unsymmetrical breaking of a covalent bond between two atoms such that the more electronegative atom acquires both the electrons of the shared pair is called heterolytic fission or heterolysis.

Such fission takes place in the presence of a polar solvent. The cleavage of a bond results in the formation of ions. One of the ions has a sextet electronic structure and a positive charge called a cation and the other ion has a valence octet with at least one lone pair and a negative charge called an anion. 

Heterolytic fission can be shown as,

  • where B is more electronegative than A. 

If A is more electronegative than B, then the fission will be shown as,

The ions formed are unstable and reactive. An example of heterolysis is,

The species in which a carbon atom possesses a sextet of electrons and a positive charge is called carbocation or carbonium ion. A carbocation is electron deficient. In the C-Br bond, the bromine atom is more electronegative than the carbon atom and hence the electron pair is retained by the bromine atom on fission.

But consider the reaction:

In the C-H bond, the carbon atom is more electronegative than the hydrogen atom and hence the electron pair is retained by the carbon atom on fission. The species in which a carbon atom possesses an octet of electrons and a negative charge is called carbanion. Carbanions are reactive as they are unstable. Organic reactions which proceed by heterolytic are called ionic or heteropolar or simply polar reactions as these reactions take place in a polar solvent. Heterolysis is uncommon in a gaseous state.

What are Free Radicals?

An uncharged species which is electrically neutral and contains a single electron is called free radical.

Free radical is highly reactive and therefore has a transitory existence i.e. it is short-lived. Consider methyl radical in which carbon is sp3  hybridized and has a planar structure. H – C – H bond angle is 120 degrees The odd electron is in the p orbital which P is perpendicular to the plane of three C – H bonds (refer to the image shown below). 

Carbon is electron deficient as it has only seven electrons in the valence shell.

The structure of the methyl radical is like that of the methyl cation, except there is an additional electron.

What are Reagents?

The reagent reacts with the substrate to give products. The reagent may be an electron-rich or electron-deficient chemical species that attacks the substrate during a chemical reaction. 

The following are two types of important reagents.

  1. Electrophilic reagents or electrophiles: Electrophiles are electron-deficient species. They are either positively charged species like H, NO₂, etc. or molecules containing the central atoms having incomplete octet of electrons in their outermost orbit like BF, AICI, ZnCl₂, etc. Since electrophiles are electron-deficient, they accept a pair of electrons from donor atoms and thus they are electron loving reagents. All electrophiles are basically Lewis acids.
  2. Nucleophilic reagents or nucleophiles: Nucleophiles are electron-rich species. They are either negatively charged species like OH, CN, CT, Br etc. or molecules containing at least one lone pair of electrons on the central atom-like H₂O, NH₂, H₂S, R OH, R-NH₂, R-OR, etc. Since nucleophiles are electron-rich, they donate a pair of electrons to acceptor atoms and thus they are nucleus loving reagents. All nucleophiles are Lewis bases.

Electromeric Effect: Electronic Displacements in a Covalent Bond

The displacement of electrons in a molecule’s covalent bond occurs either as a result of the presence of a suitable attacking reagent or as a result of the impact of an atom or a substituent group in the ground state.

Temporary electron displacements are observed when a reagent approaches the molecule, and this form of electron displacement is known as the electromeric effect. Electron displacements caused by an atom or a substituent group in the molecule create permanent polarisation of the bond. This form of electron displacement is exemplified by the inductive effect.

Electronic displacements in a covalent bond can cause bond fission under certain conditions. For instance, homolytic and heterolytic fission.

Inductive effect

Lets first understand the two important types of covalent bonds as,

  • Non-polar Bonds: When a covalent bond is established between two atoms of the same element or two atoms with the same electronegativity, the bonding pair of electrons is shared equally by the two atoms. This type of bond is non-polar in nature. Examples of such bonds are, H – H, Cl – Cl, O = O, etc.
  • Polar bonds: When a covalent bond is established between atoms of different elements with different electronegativity values, the electron density in the bond shifts towards the more electronegative atom. A polar covalent bond is formed when the electron density shifts. Examples are, H – CI, H – OH, H3C – Cl, etc.

Consider chloroethane, which has the formula CH3 – CH2 – Cl. It is polarised such that carbon number one receives a positive charge (+δ) and chlorine gains a negative charge (-δ). An arrow pointing from +δ to -δ of the polar bond depicts the shift in electron density.

C1, which has generated a positive charge (+δ), attracts electron density from the nearby C – C bond. As a result, some positive charge (+δ1) occurs on C2, where +δ1 represents a significantly lower positive charge than that on C1. In other words, the polar C – Cl bond causes the neighbouring bonds to become polar. Such polarization of sigma (σ) bond caused by the polarization of adjacent sigma bond is referred to as the inductive effect.

It is a long-term consequence. This impact is carried on to succeeding bonds as well, although it diminishes fast as the length of the carbon chain rises. After bonding, the effect is very negligible.

As a result, the electron pairs, although being permanently relocated, remain in identical valence shells.

The capacity of substituents to remove or give electron density to the connected carbon atom is related to the inductive effect. Based on this capacity, substituents can be classed as: 

  • -I Effect (Negative Inductive Effect) – Atoms or groups of atoms that are highly electronegative or carry positive charge are electron-withdrawing groups and such groups are said to have (-I) effect. For example, -F, -Cl, -Br, -I, -NO2, -CN, -COOH, -COOR, -SO3H, etc are the electron-withdrawing groups. The higher the electronegativity of an atom, the greater is the -I effect e.g. the -I effect decreases in the order F > Cl > Br > I. Positively charged atoms or groups have greater, -I effect than neutral atoms or groups e.g. -N+O2 has more -I effect than -NH2
  • +I Effect (Positive Inductive Effect) – Atoms or groups of atoms that are electropositive or carry negative charge are electron-donating groups and such groups are said to have (+I ) effect. Metals like Na, K, Mg, Zn, etc., and alkyl groups such as -CH3, – CH2CH3, -CH(CH3)2, etc. are electron-donating groups. The negatively charged groups such as CH3O-, C2H5O-, etc. show a strong +I effect. Less electronegative elements have a greater +I effect e.g. Be > B > C. Similarly, negatively charged atoms or groups have a greater +I effect than neutral atoms e.g. CH3 – CH2 > CH3 – CH3.

Electromagnetic Effect

Certain chemicals produce polarity in non-polar covalent bonds or improve polarity in polar covalent bonds. This is known as the electromeric effect.

The electromagnetic effect is a temporary effect, but it aids in increasing the reactivity of the molecule by inducing or boosting the polarity of the substrate with numerous bonds.

Resonance

When a carboxylic acid loses a proton, the electron density is shared by both oxygen atoms – the electrons are delocalized. Delocalized electrons are not bound to a single atom or a link between two atoms. A compound having delocalized electrons is said to have resonance.

The two structures that use localized electrons are known as resonance contributors, resonance structures, resonance forms, or contributing resonance structures. Neither of these resonance forms is the right structure for a carboxylate ion. The real structure, which is a hybrid of the two structures, is known as a resonance hybrid, and it is depicted with dotted lines to demonstrate that electrons are delocalized. Resonance forms are represented by a double-headed arrow.

Resonance Form

Resonance Hybrid

The negative charge (electrons) is distributed across both oxygen atoms. Each oxygen atom bears half of the negative charge, which stabilizes the ion. The carbon-oxygen bonds have a bond order of 11/2, which means they are halfway between a single bond and a double bond. Electron delocalization happens only when all of the atoms that share the delocalized electrons are in or near the same plane, allowing their p orbitals to overlap efficiently.

Note: The only difference between the two resonance forms of carboxylate ion is the placement of their π electrons and lone-pairs; all of the atoms remain in the same spot.

In short, resonance, also called mesomerism, refers to the phenomena in which compounds exist in a state that is a mixture of two or more electronic structures, each of which appears equally capable of expressing most of the attributes of the compound but none of which describes all of the qualities.

Resonance Stabilization: The resonance hybrid structure has lower energy than any of the contributing resonance structures. The energy difference between the real structure (resonance hybrid) and the lowest energy resonance structure is referred to as resonance stabilization energy, or simply resonance energy. The resonance energy increases as the number of key contributing resonance structures increases.

Resonance in Benzene

Structure of Benzene

  • In the below two images, each benzene resonance form clearly demonstrates that the ring has six π electrons. The resonance shapes are only a handy technique to portray the π electrons; they do not represent any actual electron distribution. In benzene, for example, the link between C-1 and C-2 is neither a double bond as indicated in figure1 nor a single bond as shown in figure 2.

  • It falls somewhere in the middle of the two resonance forms. The resonance hybrid, which is the average of the two resonance forms, is the true structure of benzene.

Resonance in Nitroethane

Resonance Form

  • In the above image, the double bond in the 1st structure is the single bond and in the 2nd structure, it’s vice versa.

Resonance Hybrid

  • The resonance hybrid shows that the two nitrogen-oxygen bonds are identical and the negative charge is shared by both oxygens. The p orbital of nitrogen overlaps the p orbital of each oxygen. In other words, the two electrons are shared by three atoms.

Rules for drawing resonance structure

  1. Only the electrons move; the nuclei of the atoms never move, therefore the bond angles must stay constant.
  2. The only electrons that can travel are π electrons and lone pairs.
  3. The number of unpaired electrons, if any, must stay constant. Most stable compounds have no unpaired electrons, and all electrons must stay coupled in all resonance structures.
  4. The resonance contributor with the least energy is the most important. Good contributions often have all octets completed, as many bonds as feasible, and as minimal charge separation as possible. Negative charges are more stable on more electronegative elements such as O, N, and S.
  5. Resonance stabilization is particularly essential when it serves to delocalize a charge across two or more atoms.

Electrons can be moved in one of the following ways –

  • Move π electrons towards a positive charge or towards a π bond:

(movement of π electron toward a positive charge)

(movement of π electron toward a π bond)

  • Move a lone pair of electrons towards an π bond:

  • Move single non-bonding electron towards π bond:

On the basis of electron transfer the Resonance Effect is classified in two as:

  • Positive resonance (+R) effect: The positive resonance effect occurs when electrons are transferred away from an atom or substituent group connected to the conjugated system. For example, the +R effect in aniline

Because of the electron transmission across the chain, specific sites in the molecule have high electron concentrations, which explains the reactivity at these places. The groups that reflect the +R electron displacement effect include halogen, -OH, -OR, -NH2, -NHR, -NR2, -NHCOR, OCOR, and so on.

  • Negative resonance (-R) effect:  The negative resonance effect occurs when electrons are transferred towards the atom or substituent group connected to the conjugated system. For example, -R effect in nitrobenzene

Some of the groups that exhibit the -R electron displacement effect are -COOH, -CHO, -CN, and so on.

Hyperconjugation

Hyperconjugation is the delocalization of electrons caused by the overlap of a p-orbital and a sigma (σ) bond (α C-H).

Only when the σ bond and the vacant p-orbital are properly oriented does hyperconjugation occur. Sigma bond electrons form a partial conjugation with an unshared p-orbital or the connected unsaturated system. It is a long-term impact with a stabilizing effect.

  • Consider the ethyl cation CH3CH2. The positively charged carbon atom possesses six electrons, is hybridized sp2, and has an unfilled p orbital. One of the nearby methyl group’s C – H bonds is aligned with the plane of the vacant p-orbital. The electrons of the σ bond (this C-H bond) delocalize into the vacant p orbital, stabilizing the cation.

  • Because of the overlap, the positive charge is diffused by the electron density of the surrounding σ bond, which stabilizes the cation. Hyperconjugation can be depicted as,

  • The more alkyl groups connected to a positively charged carbon atom, the stronger the hyperconjugation connection and the more stable the cation. As a result, the relative stability of the following cations diminishes with increasing order.

(CH3)3 C+  >  (CH3)2 CH  >  CH3 CH2  >  CH3 

  • This is due to the fact that tert-butyl cation has nine hyperconjugation structures, isopropyl cation has six, and ethyl cation has three. Because the vacant p orbital in C+H3 is perpendicular to the plane in which the C-H bonds are located, the overlap is not conceivable. As a result, C+H3 lacks hyper-conjugative stability. 
  • Electron delocalization through hyperconjugation is also feasible in alkenes and aromatic compounds such as alkyl arenes. Hyperconjugation in propene, for example, is seen in the below figure.

Sample Questions

Question 1: Explain the term inductive effect?

Answer: 

Polarization of sigma bond caused by the polarization of  adjacent sigma bond is called Inductive effect.

The inductive effect is related to the ability of substituents to either withdraw or donate electron density to attached carbon atom and hence classified as,

  • Electron withdrawing group
  • Electron donating group

Question 2: In which C – C bond of C4H9 – Br, the inductive effect is expected to be the least?

Answer: 

We know that magnitude of inductive effect decreases as the number of intervening bonds increases.

So, consider, C4H3 – C3H2 – C2H2 – C1H2 – Br

The inductive effect is least in C3 – C4 bond.

Question 3: Write a note on resonance, also draw the structure of resonance and resonance hybrid?

Answer:

Resonance refers to the phenomena in which compounds exist in a state that is a mixture of two or more electronic structures, each of which appears equally capable of expressing most of the attributes of the compound but none of which describes all of the qualities. It is also called as mesomerism.

  • A compound with delocalized electron is said to have resonance
  • The actual structure which is composite of two structure is called resonance hybrid and is drawn by using dotted lines to show that electrons are delocalized
  • Resonance forms are shown using double headed arrow between them

Question 4: Draw all the possible resonance structures of C6H5OH? 

Answer: 

The given compound, C6H5OH, is phenol. The possible resonance structures are,

Question 5: Differentiate between Electrophiles and Nucleophiles

Answer:

Electrophiles

Nucleophiles

Electrophiles are electron-deficient species.Nucleophiles are electron-rich species.
They are attracted towards negative charge.They are attracted towards positive charge.
They attack electron-rich centre of the substrate.They attack electron-deficient centre of the substrate.
These are Lewis Acid.These are Lewis Base.
THese are also called electron-pain acceptors.These are also called electron-pair donor.
These are cations or molecules having electron deficient atoms.These are anions or molecules containing atoms with atleast one lone pair of  electrons.

Question 6: Describe briefly the distinction between a nucleophile and a base.

Answer:

A nucleophile is the term used when an electron-pair is donated to a species other than H*. for e.g. when electron pair is donated to the carbocation.

If an electron-pair is donated H+, it is termed as a base.

NH4+, Na+, K+, etc. are even though positively charged species but are not electrophiles as they do not have empty orbitals.


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