Before getting started the study of chemical reactions of organic chemistry we need to have command over some basics of reaction mechanism. Here are some basic terms which will be helpful for building the concepts of organic chemistry.
(i) Intermolecular Forces and Physical Properties
Ionic Compounds: Inter ionic forces are strong electrostatic forces and hold each ion in position, giving the compound a rigid structure and high melting point. Ionic compounds are generally soluble in water due to polarity of water molecules. Ions get hydrated in water and causes the ions to separate and disperse in water.
Non-ionic (covalent) compounds have molecules as structural units which are held together through very weak forces. These forces are of three types:
(A) Hydrogen bonding: Hydrogen bond is formed between hydrogen atom linked to highly electronegative element X (F, O, N) and another highly electronegative element, Y (F, O, N)
(B) Dipole-dipole interaction: In polar molecules one end has δ+ and other δ- charge. The oppositely charged ends of molecules attract each other resulting in dipole-dipole interactions.
(C) London (van der Waal?s) Forces: These are very weak forces which arise due to electrostatic attractions between the nuclear of one molecule and electrons of the other in non-polar compounds. These forces depend on molecular mass and surface area of molecules.
Molecular mass ∝ van der Waal's forces
Surface area ∝ van der Waal's forces
The relative order of attractions of the three intermolecular forces is:
H-bond > dipole-dipole > van der Waal's forces
Melting point, boiling point and solubility largely depend on the nature of intermolecular forces and molecular mass.
(ii) Electronic displacements in organic molecules
(A) Inductive effect: It is a permanent effect which arises due to displacement of electrons in a polar covalent bond towards the more electronegative element or group.
Inductive effect is always transmitted along a carbon chain. The magnitude of charge on C-atoms decreases as the distance from the source (more electronegative atom) increases.
?I effect: Any atom or group which attracts electrons more strongly than hydrogen is said to have ?I (electron withdrawing) group. The relative strength of ?I effect of various functional groups are as follows:
N+(CH3)3 > NO2 > CN > F > Cl > Br > I > CF3 > OH > OCH3 > C6H5 > H
+I effect: Any atom or group which attracts electron less strongly than hydrogen is said to have +I effect.
(CH3)3C > (CH3)2CH > CH3CH2 > CH3 > H
? Importance of Inductive effect:
(a) Reactivity of alkyl halide > alkanes
Thus, due to ?I effect of Cl group of CH3Cl is more reactive than CH4 .
(CH3)2CH—Cl is more reactive than CH3Cl because in isopropyl chloride +I effect of two methyl group increases the ?I effect of Cl atom by repelling electrons towards the sec-cation. Thus, isopropyl chloride is more reactive the methyl chloride.
(b) Strength of carboxylic acids:
? ClCH2COOH is more acidic than CH3COOH
Due to the ?I effect of ?Cl group polarity of ?OH group in chloroacetic acid increases making the release of H+ ion easier. CH3COOH does not have ?I effect causing group, so it is less acidic.
? HCOOH is more acidic than ethanoic acid
(B) Hyperconjugation: An alkyl group with at least one hydrogen atom on the α-carbon atom attached to an unsaturated carbon atom, is able to release electrons by the similar mechanism to that of electrometric effect.
This type of electron release due to the presence of a conjugated system is known as hyperconjugation. More the number of H?C bonds attached to the unsaturated system more will be the probability of electron release by this mechanism.
Effects of hyperconjugation:
(a) Stability of alkenes: Propylene is more stable than ethylene because in propylene there are three H?C hyperconjugated bonds.
Similarly, 2-methylpropene is more stable than but-1-ene.
(b) Stability of alkyl free radical:
More the number of hyperconjugated structure more is the stability of free radical.
(C) Electrometric effect: It is a temporary phenomenon which takes place under the influence of attacking reagent. It involves complete transfer of electrons in some reactions involving molecules where atoms are joined with double or triple bonds.
This effect is denoted as E-effect.
+E effect: If the attacking reagent bonds to the atom to which electrons of the double bond have been transferred, then it is +E-effect.
?E effect: If the attacking reagent bonds to the atom other than to which electrons have been transferred.
(D) Mesomeric effect or Resonance effect: It is produced in a molecule having conjugated system (alternate σ and π bond) due to transfer of π electrons from a multiple bond to an atom or a single covalent bond or from lone pair of electrons of an atom to the adjacent single covalent bond. It is denoted as M-effect or resonance effect.
+M effect: If the molecule has conjugated system and conjugated C-chain is attached to a more electronegative atom, transfer of electrons takes place away from the more electronegative atom
?M effect: If the molecule has conjugated system and conjugated C-chain is bonded to a more electronegative atom through a multiple bond, transfer of electrons take place towards the more electronegative atom.
Some common atoms or groups which cause +M or ?M effect are as follows:
? Mesomeric effect is a permanent effect and always operates in a non-reacting molecule.
? It affects physical and chemical properties of a molecule.
Thus, if a molecule can be assigned two or more structures and none of which is capable of explaining all its properties, then the actual structure is intermediate or resonance hybrid of these structures. The energy of resonance hybrid is lower than that of any of the resonating structures. The difference in energy between the actual structure and the most stable of the resonating structures is called resonance energy.
Rules for writing resonance structures
The resonance structures should have
Relative contribution of resonance structures
The major resonance structure is one which has
(iii) Breaking of a covalent bond
Breaking of a covalent bond between two atoms takes place in two ways.
(A) Homolytic fission takes place when the two atoms are usually of similar electronegativity.
(B) Heterolytic fission takes place when the two atoms are of different electronegativity.
The two types of bond fission produces three types of reaction intermediate:
(a) Free radical: It is produced due to homolytic fission of a bond in which one electron of the bonding pair goes to each of the departing atom or group. Free radical is electrically neutral species having a single unpaired electron. Free radicals are very reactive and have a short life span.
The stability of free radical is explained by hyperconjugation. Following is the relative order of stabilities of common free radicals:
benzyl > tertiary alkyl > secondary alkyl > primary alkyl > methyl > vinyl
Extra stability of aromatic and alkyl free radicals is due to resonance.
(b) Carbonium ion or carbocation: It is an organic species in which a carbon atom has six electrons in its valence shall and has a positive charge. Carbonium ions are classified into 1?, 2? and 3? depending upon the nature of the
C-atom bearing the charge.
The C- of carbonium ion is sp2 hybridized. The stability of carbonium ion is influenced by resonance and inductive effect. Resonance explains the stability of alkyl and benzyl carbonium ions:
Stability of alkyl carbonium ions is explained on the basis of inductive effect. Due to +I effect the alkyl groups release electrons towards +vely charged carbon and reduce its +ve charge. In doing so alkyl group itself becomes partially positive. This dispersal of the charge stabilizes the carbonium ion.
Greater the +I effect more is the stability of carbonium ion:
Electron withdrawing groups (?I groups) —CN, —NO2, —Cl, etc., makes a carbonium ion less stable due to depositions of more +ve charge on carbocation.
(c) Carbanion: It is negatively charged organic species in which ?ve charge is on C-atom. The C-atom in carbanion has 4 paris of electrons in its valence shall.
The C-atom of carbanion is sp3 hybridised.
Stability of carbanions is explained on the basis of resonance and inductive effect. Resonance explains the stability of those carbanions in which ?ve charge is in conjugation with the double bond.
Inductive effect explains the stability of simple alkyl carbanions. +I groups destabilise the carbanion because they increase ?ve charge of negatively charged carbon.
Thus more the number of alkyl groups attached to a carbanion, less is its stability.