Organic Chemistry

Table of Contents:

What is Organic Chemistry?

Organic chemistry is the study of the compounds formed by carbon. The name ‘organic’ was coined because, earlier scientists used to believe that these compounds could only be derived from creatures that were living or once living, i.e. dead. This was attributed to some ‘vital force’ that was present in organic substances as they had something that inanimate substances lacked, life.

The above theory was debunked when Urey Miller synthesised urea from inorganic substances, but the classification is still in use.

Organic chemistry is possible due to one main property exhibited extensively by the element carbon, catenation. It is the ability of an element to form bonds with an atom of the same kind. One can say that organic chemistry’s vastness can be attributed to the same.

Organic chemistry’s importance in the present age is as immense as it had been since its inception. Pharmaceuticals, polymer industries, paint industries, food industries, thrive because of the advancements in organic chemistry. It would be hard to name one field where organic chemistry doesn’t play an important role!

Cleavage of bonds

Organic reactions occur by the breaking and making of bonds. Bonds can cleave in either of two ways.

Homolytic Cleavage

If the covalent bonds between two elements break in such a way, that each of the elements gets their own electrons, it is called homolytic cleavage. That is, each element gets an electron. Homolytic cleavage results in the formation of free radicals.

Note that we use an arrow to show the movement of electrons, in this case, the arrow used is called the fish-hook arrow, as it signifies that there is a movement of only one electron.

Heterolytic cleavage

If the covalent bonds between two elements break heterolytically, i.e., unequally, it results in the formation of charged species. This type of bond breaking, where the electrons are unevenly distributed is called heterolytic cleavage.

Note here that we use arrows to signify the movement of electrons, a regular arrow signifies that two electrons are being moved.

Types of Intermediates formed during a Reaction

Intermediates can be understood as the first product of a consecutive reaction, for example, if A→B, and if B→C, B can be said to be the intermediate for the reaction A→C. In organic reactions, they occur via the formation of these intermediates.

Carbenes (H2C)

They are neutral, reactive species that have six electrons in the outer shell of carbon, making them electron deficient. Since carbenes are species having two odd electrons, we can classify them based on their spin states. Types of Carbenes.

Singlet carbene

The electrons are present in different orbitals with opposite spin. The electrons are paired in sp2 hybridised orbitals and behave as paired electrons.

Spin state= (2S + 1)

S for singlet carbene is zero as the electrons are antiparallel,

Therefore,

Spin state= (2 x 0 + 1)

Spin state= 1

Hybridisation: They are sp2 hybridised, with a bent shape. They have a bond angle of 103°, and bond length of 112 pm.

Triplet Carbene

Both the electrons are present in different orbitals and they possess the same spin.

Spin state = (2S + 1)

S for triplet carbene is 1 as both the electrons have the same spin.

Therefore, Spin state= (2×1 +1)

Spin state= 3

Hybridisation: They possess sp hybrid orbital with a linear shape. They have a bond angle of 180° and a bond length of 103 pm.

Note: Triplet carbene has lower energy then singlet carbene because in singlet carbene there is more inter-electronic repulsions as both the electrons exist in the same orbital, whereas in triplet carbene the two electrons exist in different orbitals, making it possess less energy.

What are Carbocations?

Carbocations have a sextet of electrons on the carbon-containing the positive charge and hence termed ‘cation’. It is sp2 hybridised and it has an empty p-orbital. The shape is planar. It is generally formed by heterolytic cleavage of a carbon-heteroatom bond.

What are Carbanions?

They are generated by heterolytically cleaving a group attached to carbon without removing the bonded electrons. This makes the carbon have a pair of electrons, thereby, imparting a negative charge on the carbon. CH3 is isoelectronic with NH3 and it is sp3 hybridised and the shape is pyramidal owing to the presence of a lone pair of electrons.

Free Radicals

They are formed by homolytic cleavage of carbon bond. The shape of the species formed is planar and the carbon is sp3 hybridised with an odd electron being placed in the p-orbital. If the free radical is relatively stable, then they may possess a planar structure.

Transition state of Organic Reactions

We saw the intermediates that could be formed in an organic reaction, now let us look into transition states and the difference between an intermediate and a transition state.

Intermediates are formed in a multi-step reaction, but some reactions can occur in a single step without having to form an intermediate. These reactions will occur by going through a transition state. This can be clear by looking at the energy profile diagram for a reaction, R→P

The transition state or TS, corresponds to the highest energy in the reaction, after which it can give either the product or in the case of a reversible reaction, the reactants.

Consider a reaction, A→D with the following steps,

A→B

B→C

C→D

The energy profile for this reaction is given below,

We can see that B and C are the products of a reaction and hence they are termed as intermediates. The highest energy of a particular reaction should be the transition state.

From this example, we can show that intermediates are isolable, that is, they can be isolated. On the other hand, TS are not isolable because we assume the reaction to take place via a TS but we cannot isolate it.

Types of Reagents

Reagents are the chemicals that we add to bring about a specific change to an organic molecule. A general organic reaction can be written as:

Substrate + Reagent → Product

Where substrate is the organic molecule to which we add the reagent. The Reagents are generally classified according to their ability to either donate or abstract electrons as

  • Electrophiles
  • Nucleophiles

Electrophiles

Electrophiles are those reagents that are deficient of electrons. It can be generalised that all positive charge containing species are electrophiles. Eg) H+,NO2+,CH3+,Cl+

Neutral molecules that are electron deficient can also act as electrophiles. Lewis acids like AlCl3 and BF3 are examples of neutral electrophiles.

Nucleophiles

Nucleophiles are those reagents that have the excess of electrons, or electron rich. They seek for bonding centers with other nuclei and hence the name, nucleophile. It can be generalized that negative charge containing species are nucleophiles. Eg) H,CH3 ,Cl

Neutral molecules with a lone-pair of electrons on the heteroatom can act as a nucleophile. Eg) H2O, NH3, CH3OH

Types of Reactions in Organic Chemistry

The reactions in Organic Chemistry are broadly classified into six categories.

Substitution Reactions

R-X + Y → R-Y +X

Where R-X is the substrate, Y is the reagent (which can be electrophilic or nucleophilic) and X is called the leaving group. The term substitution means one group is replacing the other group.

Substitution can either be one of these types

  1. Nucleophilic substitution ( SN1,SN2, SNi)
  2. Electrophilic substitution (SE)
  3. Nucleophilic aromatic substitution (SNAr)
  4. Addition reactions

Here a reagent is added to the substrate. Addition reactions can be further divided into,

  1. Electrophilic Addition
  2. Nucleophilic Addition
  3. Elimination reactions

Elimination reactions can be said to be the reverse of an addition reaction, wherein a simple molecule (HX, H2O) is removed from the substrate. That is, a molecule is said to be eliminated from the substrate.

They can be classified further into E1, E2, E1CB

  1. Oxidation and reduction reactions
  2. Pericyclic reactions
  3. Molecular rearrangements

Field Effects

Inductive Effect

It is an electron delocalisation effect via σ bonds that arises due to the difference in electronegativities.

For example, in a σ bonded compound like C-C-C-Cl, the carbon attached to the chlorine atom can be referred to as the α-carbon, and the one adjacent that carbon as the ß-carbon and so on.

Now since Cl is more electronegative than carbon, it withdraws the electrons that are present via the σ bond toward itself, thereby making Cα fractionally positive. Since it is devoid of electrons, Cα now being slightly electropositive than Cß, pulls the sigma bonded electrons of Cα-Cß bond toward itself, and in the process, making Cß slightly electropositive. The electron withdrawing effect of Cl atom is being transmitted through the carbon chain via the σ bonds. This transmission of charges decreases rapidly with the number of intervening σ bonds. We can practically ignore this effect beyond Cß.

The arrow is pointed toward the more electronegative atom if a group withdraws the electron from carbon, it makes carbon slightly electropositive. Such groups are called –I groups, and the effect as –I effect. For example, -Cl,-Br,-CN and –NO2 are –I groups.

Groups that release electrons towards carbon are termed as +I groups, and the effect is termed as +I effect. For example, alkyl groups like –CH3 are +I groups.

Inductive effect of other groups relative to hydrogen is given below,

Electromeric Effect

It is the temporary delocalisation of π-electrons in a compound containing multiple covalent bonds. It is important to note that it is only a temporary effect, that is, it occurs only when a reagent is added.

It can be classified into two types:

  • Positive Electromeric Effect
  • Negative Electromeric Effect

Positive Electromeric Effect

When the π-electrons are given to the attacking reagent.

For example, the reactions alkenes and alkynes mostly occur via +E, this reaction is also called electrophilic addition.

The proton adds at C-1 as the π-electrons were given to the attacking reagent (H+). This results in the formation of a carbocation.

Negative Electromeric Effect

When the π-electrons are shifted to a more electronegative atom (O, N, S) joined via multiple bonds.

For example, the reactions of aldehydes and ketones occur predominantly by –E effect. It is also called nucleophilic addition.

The CN- ion adds to the C atom of the carboxy group opposite to the movement of the π-electron cloud.

Mesomeric Effect

Molecules possessing σ-bonds and π-bonds alternatively exhibit the mesomeric effect. The effect is exhibited due to the permanent delocalisation of π-bonds. This increases the number of resonating structures which makes the molecule more stable. Such kind of a system, where there are alternative σ-and π-bonds are called conjugated.

Types of Mesomeric Effect

  • Positive mesomeric effect
  • Negative mesomeric effect:

Positive Mesomeric Effect

This is exhibited when the direction of the delocalization of electrons is away from the position where the group is attached. Normally groups having a lone pair of electrons attached to a conjugated system push electrons into the conjugated system, that is, away from them.

Groups showing a +M effect are –OH,-OR,-NH2,-SH, -X etc.

Negative Mesomeric Effect

This is exhibited when the direction of the delocalization of electrons is towards the position where the group is attached. These groups are generally electron withdrawing.

What is Resonance in Organic Chemistry?

For certain molecules like carbonate ion (CO32-), one single Lewis structure would not be enough to explain all of the properties. In that case, the molecule is said to have more than one structure.

Each of those structures can explain some of the properties but not all of the properties. The actual structure of the molecule is a hybrid of all the possible structures (canonical forms). This phenomenon is called resonance.

If resonance occurs, each bond would be both a single bond and a double bond at the same time, that is, the bond order would lie between one and two.

Resonating structures should fulfil the following criteria:

  1. All atoms should have the same positions in all the structures
  2. There should have the same number of paired and unpaired electrons
  3. The structures should have almost the same energies

Note: Canonical forms do not have any existence in reality.

Resonance Energy

The energy difference between the most stable canonical form and the resonance hybrid. More the resonance energy, more is the stability.

Rules for finding out the most stable canonical form:

  1. The canonical form with no charges is the most stable
  2. The canonical form with more number of covalent bonds is more stable
  3. The canonical form where unlike charges are in close proximity are more stable
  4. If there are to be charged, the negative charge should be on an electronegative atom. Then this canonical form is said to be more stable.

Steric Hindrance

The structure and reactivity of many compounds in organic chemistry is greatly dictated by the presence of bulky groups or constituents in the molecule. This is called steric hindrance. It arises because of inter-electronic repulsions due to spatial crowding amongst bulky groups.

Using steric factors, we can conclude that trans-2-butene is more stable than cis-2-butene.

Stability of Intermediates

Carbocations

The stability of carbocations can be explained by

1. Inductive Effect: Types of carbocations

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

Methyl Primary (1°) Secondary (2°) Tertiary(3°)

We know that alkyl groups are +I groups, that is, they release electrons through the sigma bonds.

Since the carbon is deficient of electrons, we can say that as the number of methyl group increases, the stability of the carbocation increases. Since the electropositive carbon is satiated by the electrons given by the methyl groups via +I effect.

Therefore, the stability order of carbocations is of the order 3°>2°>1°>Methyl carbocation

Hyperconjugation

When a C-H σ-bond is in conjugation with a carbocation, this effect is observed. A carbocation has a vacant p-orbital. The bonded σ-electron pair of the C-H bond is displaced toward the vacant p-atomic orbital. This increases the electron density in the empty p-AO.

It is, therefore, a resonance effect where a C-H bond breaks and the σ-electron pair is delocalized to the vacant p-AO of the carbonation. Since the bond between C and H is broken, it is also called as ‘no bond resonance’. It is also referred to as the Baker-Nathan effect.

More the number of α-hydrogens in a carbocation, that many hyperconjugated structures would be possible. More the number of structures more is the stability of the species.

Note: If there is a possibility of resonance, then that would make it more stable. This is because resonance affects the entire structure.

Carbanions

The stability of carbanions can be explained using the inductive effect

Types of carbanions-

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

Methyl Primary (1°) Secondary (2°) Tertiary(3°)

Since alkyl groups are electron releasing by nature through induction, we can say that more the number of methyl groups attached to the carbon having a negative charge, less would be its stability.

This is because, the carbon already has a negative charge, to which the methyl groups push electrons via induction. This results in inter-electronic repulsions and destabilizes the species.

Therefore the stability order for carbanions is as follows

3°<2°<1°<Methyl carbanion

If there is a possibility of resonance, then that would make it more stable. This is because resonance affects the entire structure.

Free Radicals

The stability of free radicals follows the same trend as that of carbocations.

CH3. .CH2(CH3).CH(CH3)2 .C(CH3)3

Methyl Primary (1°) Secondary (2°) Tertiary(3°)

Therefore, the stability order of free radicals is of the order

3°>2°>1°>Methyl carbocation

This can be explained with the help of Hyperconjugation that we saw, but here there would be an overlap between the σ-bond of C-H and the odd electron in the p-orbital of carbon.

Note: If there is a possibility of resonance, then that would make it more stable. This is because resonance affects the entire structure.


Practise This Question

Which one of the following is an inexhaustible natural resource?