Organic chemistry basic part 2

 Any factor that disperse the positive charge on the carbon stabilize the carbocation whereas any factor that intensify the positive charge on the carbon destabilize the carbocation.  Electron-donating groups attached to the carbocation decrease the positive charge on carbon thereby stabilizing them. Electron-withdrawing groups attached to carbocation intensify the positive charge on carbon thereby destabilizing them.

Tertiary > secondary > primary > methyl

Electrophilic substitution reactions

Alkyl groups show +I effect. They releases electrons in the direction of positive charge thereby dispersing the positive charge. Thus,the carbocation gets stabilized. Greater the number of alkyl groups attached to positive carbon, greater would be the charge dispersal and greater would be the stability of carbocation.

Free radical substitution reaction

Addition reaction

Carbocation is sp² hybridized. It has a vacant unhybridized p-orbital. σ –p hyperconjugation is possible. Greater the number of α-hydrogens attached to positive carbon, greater will be the number of hyperconjugative structures and greater would be stability. Tertiary has greater number of σ hydrogens, then secondary,primary. Methyl carbocation has no α-hydrogens. Thus stability of carbocations are in the order 3°>2°>1°

Allyl carbocation disperses positive charge by resonance and thereby stabilizes it.

nucleophilic addition reaction

 Benzyl cation disperses positive charge by resonance and thereby stabilizes it. Greater the number of phenyl groups attached to the positively charged carbon, greater will be the number of resonance structures,greater will the dispersal of positive charge and greater will be the stability.

electrophilic addition reaction

free radical addition reaction

Carbon bearing a negative charge is called carbanion.

 They have 8 valence electrons around carbon atom.  Electron rich species.  They are nuleophiles and Lewis bases.

Elimination reaction

Depending upon whether the two groups are removed from the same carbon or adjacent carbons,

 They have 8 valence electrons around carbon atom.  Electron rich species.  They are nuleophiles and Lewis bases.

explanation

α- elimination reaction

α- elimination reaction:

1. By heterolytic fission of a bond 2. By the decomposition of carboxylate ion . 3. By the decomposition of organometallic compounds

 Carbon atom in a carbanion is spᶟ hybridized.  Out of the four spᶟ hybrid orbitals, three of them form σ- bonds with three groups or atoms and remaining one contains unshared electron pair.  Carbanion has pyramidal shape with bond angle between 97° and 100°.

α- elimination reaction

β – elimination reaction

 Any factor that disperse the negative charge on the carbon stabilize the carbanion where as any factor that intensify the negative charge on the carbon destabilize the carbanion.  Electron-donating groups attached to the carbanion increases the negative charge on carbon thereby destabilizing them. Electron-withdrawing groups attached to carbanion disperse the negative charge on the carbon thereby stabilizing them. Examples:

Methyl > primary > secondary > tertiary

β – elimination reaction:

The order can be explained on the basis of Inductive effect. Alkyl groups show +I effect. They releases electron in the direction of negative charge thereby intensifying the negative charge on carbon. Greater the number of alkyl groups attached to the negative carbon, greater the intensification of negative charge on carbon and lower is its stability. CH₃⁻ > CH₃CH₂⁻ > (CH₃)₂CH⁻ > (CH₃)₃C⁻

β – elimination reaction

Allyl carbanion disperses negative charge by resonance and thereby stabilize them.

β – elimination reaction:

 The negative charge on benzyl carbanion can enter into conjugation with phenyl ring, causing delocalization of negative charge and greater stability.

rearrangement reaction

Greater the number of phenyl groups attatched to the negative carbon,greater will be the dispersal of negative charge and more stable would be the carbanion.

rearrangement reaction

explanation

rearrangement reaction

Redox reaction

The cyclopentadienyl anion is a six π- electron system and is aromatic. The six π- electrons are completely delocalized over the pentagonal ring bringing stability to the anion through resonance

explanation

redox reaction

redox reaction

As the s-character of carbon increases, the electrons are more attracted to the nucleus thereby lowering energy . the order of stability is in explanation

Free radicals are atoms or groups having an unpaired electron.

 They are neutral and highly reactive.  Short lived species  Paramagnetic due to the presence of unpaired electron

(i) Photochemical homolysis ( ii ) Thermal homolysis (iii) Gain of electron by a carbocation or loss of electron by a carbanion.

 Carbon atom is sp² hybridized in a free radical.  The three sp² hybrid orbitals form σ bonds with three atoms or groups.  Unpaired electron resides in an unhybridized p-orbital.  Carbon free radicals have trigonal planar shape with bond angle 120

Tertiary > secondary > primary >methyl Eg: (CH₃)₃C• > (CH₃)₂CH• > CH₃CH₂• > CH₃•

The order can be explained on the basis of hyperconjugation. Greater the number of α- hydrogens, greater would be the number of hyperconjugative structures and greater would be the stability.

If resonance stabilization of free radicals is possible, such free radicals are more stable. Resonance stabilization become possible when delocalization of unpaired electron is possible over aromatic system.

Benzyl free radical is resonance stabilized through delocalization of unpaired electron.

Larger the number of phenyl groups attached to the carbon having unpaired electron, greater will be the extend of delocalization, greater will be the number of resonance structures and more stable will be the free radical.

1. Substitution reactions 2. Addition reactions 3. Elimination reactions 4. Rearrangement reactions. 5. Redox reactions

An atom or a group of the substrate molecule is replaced by an atom or group from reage

a) Nucleophilic substitution reaction b)Electrophilic substitution reactions c)Free radical substitution reaction