Reaction Mechanism : Structure and Reactivity

Introduction :

In every reaction there are some intermediates as below :

1. Carbocations

2. Carbanions

3. Free Radicals

4. Carbenes

5. Nitrenes

Recall that a cation is a positively charged ion and an anion is a negatively charged ion.

Carbocations and Carbon Radicals, which are sp2 hybridized - not because they form a π bond, but because they have an empty or half-filled p orbital.

1. Carbocations

A species containing a positively charged carbon atom is called a Carbocation.

Carbocations were formerly called Carbonium Ions.

Carbocation stability: 30 > 2 0 > 1 0

Example of Carbocation

2. Carbanion

A species containing a negatively charged carbon atom is called a Carbanion.

Alkyl substitution increases the stability of a carbocation and decreases the stability of a carbanion.

Carbanion stability: 30 < 2 0 < 1 0

A carbon bonded to a metal is nucleophilic because most metals are less electronegative than carbon.

An organometallic compound is a compound that contains a carbon-metal bond.

Organolithium compounds and organomagnesium compounds are two of the most common organometallic compounds.

- The electrostatic potential maps show that the carbon atom attached to the halogen in the alkyl halide is an electrophile (it is blue-green)

- Whereas the carbon atom attached to the metal ion in the organometallic compound is a nucleophile is the type of carbanion.

Carbanion should stable then it will form like Wittig Reaction.

Example of Carbanion :

3. Radicals

A species containing an atom with a single unpaired electron is called a radical (also called a Free Radical)

Two type of Radical Formation :

When a bond breaks such that both of its electrons stay with one of the atoms, the process is called Heterolytic Bond Cleavage or Heterolysis.

When a bond breaks such that each of the atoms retains one of the bonding electrons, the process is called Homolytic Bond Cleavage or Homolysis.


Relative Stability of radicals

Reactions of Radicals:

a) Initiation Steps :

- The first step is an initiation span step because it creates radicals.

- The second step is also an initiation step because it forms the chain-propagating radical

b) Propagation Steps :

A propagation step is a step that propagates the chain.

Steps 3 and 4 are propagation steps.

In step 3, the radical reacts to produce another radical.

In step 4, the radical produced in the first propagation step reacts to form the radical that was the reactant in the first propagation step.

The two propagation steps are repeated over and over. Hence, the reaction is called a radical chain reaction.

c) Termination Steps :

Steps 5, 6 and 7 are termination steps.

- In a Termination step , two radicals combine to produce a molecule in which all the electrons are paired, thus ending the role of those radicals in the radical chain reaction.

- Any two radicals present in the reaction mixture can combine in a termination step, so radical reactions produce a mixture of products.

Radical Cyclization Examples :

4. Carbenes:

A Carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons.

The two classes of Carbenes are : a) Singlet Carbenes b)Triplet Carbenes.

a) Singlet Carbenes :

Singlet carbenes are spin-paired.

In the language of valence bond theory, the molecule adopts an sp2 hybrid structure.

b)Triplet Carbenes :

Triplet carbenes have two unpaired electrons.

Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur atoms, and halides directly bonded to the divalent carbon.

Bond angle in Carbenes

Reactions of Carbenes

Acidity and Basicity

5. Nitrenes:

A nitrene (R - N:) is the nitrogen analogue of a carbene.

The nitrogen atom has only 5 valence electrons and is therefore considered anelectrophile. A nitrene is a reactive intermediate and is involved in many chemical reactions.

Nitrene C-H Insertion : A nitrene can easily insert into a carbon to hydrogen covalent bond yielding an amine or amide.

A singlet nitrene reacts with retention of configuration. In one study a nitrene, formed by oxidation of a carbamate with potassium persulfate gives an insertion reaction into the palladium to nitrogen bond of the reaction product of palladium(II) acetate with 2-phenylpyridine to methyl N-(2-pyridylphenyl)carbamate in a cascade reaction.

A nitrene intermediate is suspected in this C–H insertion involving an oxime, acetic anhydride leading to an isoindole.