General Organic Chemistry (GOC)- For NEB

General Organic Chemistry (GOC)- For NEB

General Organic Chemistry (GOC)- For NEB

Concept to understand reaction mechanism

  1. Electronic displacement in covalent bond
  2. Nature of reagent
  3. Bond clevage

1. Electronic displacement in Covalent bond

  • Inductive Effect
     

Inductive effect is permanent effect. It is the partial shifting or displacement of sigma electrons towards the more electronegative atom of sigma bond. For example, in the the compound C-X where X is more electronegative than C, so the electron is more attracted towards the atom X which results the latter attains partial negative charge while carbon atom wil attain the partial positive charge. The inductive effect is shown by arrow pointing towards the more electronegative elements or the direction of electron displacement. For example:-

https://khullanote.com/wp-content/uploads/2020/06/20200625_231700-1-1024x243.jpgFig:- Displacement of electron pair in chain

Inductive effect of C-H bond is considered as as zero. There are two types of inductive effects:-

    • Positive Inductive effect (+I-effect) Atoms or group of atoms which donate electrons towards the carbon chain atome are said to have +I-effect. It is also called as electro donating or electron realeasing groups. So, they are less electronegative groups. The order of +I-effect is:- CH_{2}^ {-}CH2− > NH^ {-}NHO^ {-}O > COO^ {-}COO >-(CH_{3})_{2}-C-CH_{3}−(CH3)2CCH3 >H-C-(CH_{3})_{2}HC−(CH3)2 >-CH_{2}-CH_{3}CH2CH3>HCH_{3}HCH3
    • Negative Inductive Effect(-I-effect) Atoms or group of atoms which withdraw electrons from carbon atom are said to have -I -effect. It is also known as electro withdrawing or electro attracting groups.Here, permanent displacement of sigma-electrons occurs away from the carbon chain. The order of -I -effect is:- -N^+ F_{3}N+F3 > -N^+ R_{3}N+R3>-NO_{2}NO2 >-CN > -CHO> -COR>-COOH > F> Cl > Br > I >-OR >-OH > -C(CH)(i.e -C triple bond CH) >NH_{2}NH2 ~ph(i.e. phenol)>CH=CH_{2}CH=CH2 >H (-I -effect=0)

      Applications of Inductive effect
    1. Carboncation & carbon free radical both are e^-e deficient (incomplete octet). So, both have similar tendency to gain e^- ses. +I -effect is directly proportional to stability and -I -effect is indirectly propertional to the stability.
    2. For carbanion, +I -effect is indirectly proportional to stability & -I -effect is directly proportional to stability.
    3. Reactivity of alkyl halide (R-X) :- Reactivity of Halides directly proportional stability of intermediate which is directly propoertional to +I -effect and inversely proportional to -I -effect.
    4. Reactivity order of alcohol:- (CH_{3})_{3} -C -OH(CH3)3COH > (CH_{3})_{2} -CH-OH(CH3)2CHOH > CH_{3}- CH_{2} -OHCH3CH2OH>CH_{3} -OHCH3OH
    5. Reactivity of carbonyl compounds:- Reactivity of carbonyl compound is directly proportional to -I -effect but inversely proportional to +I -effect and directly proportional to the positive charge on sp^{2}sp2-C
    6. Dipole moment:- Inductive effect is directly proportional to the dipole moment i.e. with increase in inductive effect the dipole moment increase.
    7. Acidic strength and basic strength:-
      • Acidic strength is directly proportional to K_{a}Ka value and stability of conjugate base and is inversely proportional to the pk_{a}pka . Acid strength is indirectly proportional to +I -effect.
      • Basic strength:- Basic strength is directly proportional to the +I -effect but inversely proportional to the -I -effect.
  • Electromeric Effect
     

Electromeric effect (E -effect) is a temporary effect. The displacement of shared pair of π-electrons in the presence of attacking reagent is called electromeric effect. It is dominant over inductive effect. It takes place in multiple bond system only. For e.g:- C=H, C=O etc. E-effect is classified into two:-

    • +E -effect:- The shared pair of electron moves towards the attacking reagent. Eg:- Electrophilic addition reaction of alkenes and alkynes.
    • -E -effect:- The shared pair of electrons move away from the attacking reagent. Eg:- Nucleophilic addition reaction in carbonyl compounds.
       
  • Hyperconjugation Effect
     

It is also known as Baker Nathan Effect/ Anchimeric effect/ No resonance/ sigma-π conjugation. The delocalization of sigma electron is hyperconjugation. ( Things to understand:- The displacement of sigma electrons is called inductive effect.). It is a permanent effect. It is stronger than inductive but weaker than resonance.

Application of Hyperconjugation

    • Stability of Alkene:- stability of alkene is directly proportional to the number of alpha hydrogen and reactivity but inversely proportional to the reactivity.
    1. Heat of Hydrogenation and heat of combustion:- Heat of combustion and heat of hydrogenation is inversely proportional to the Number of alpha-hydrogen.
    2. Stability of alkyl carbonation and alkyl free radical:- Stability of alkyl carbocation and stability of alkyl free radical is directly to the Number of alpha hydrogen.
  • Resonance Effect/ Mesomeric Effect:- Term was given by Ingold. It is displacement/shifting /movement of pie e^-e / lone pair within the compound. It is intramolecular phenomenon. Resonance is hypothetical phenomena. Actual structure of the compound is resonance hybrid which is formed by all the contributing structures. Conjugation is the most important part for resonance.

    Conjugation and conjugated compounds:- Given atom or group of atoms is said to be in conjugation with an unsaturated system if:-
  • It is directly linked to one of the atoms of the multiple bond through a single bond OR
  • It has pie bond, positive charge, negative charge, odd electron or lone pair of electron.
    Structure In Which Resonance is possible:-
  • When pie bond, sigma bond and +ve charge/ vacant p-orbital comes respectively:-
    • When pie bond, sigma bond and -ve charge comes respectively:- https://khullanote.com/wp-content/uploads/2020/07/IMG_20200712_110732-1024x196.jpg
    • When pie bond, sigma bond and free radical comes respectively:- https://khullanote.com/wp-content/uploads/2020/07/IMG_20200712_152020-1024x242.jpg
    • When pie bond, sigma bond and pie bond comes respectively:-https://khullanote.com/wp-content/uploads/2020/07/IMG_20200712_152646-1024x165.jpg
    • When lone pair ,sigma bond and ‘+’ charge/vacant orbital comes respectively:-https://khullanote.com/wp-content/uploads/2020/07/IMG_20200712_152840-1024x173.jpg
  • Types Of Resonance:- Group which is in conjugation with conjugated system is divided into 2 categories:- 1.+R/+M group:- It gives electron to the conjugation system. It increases the electron density on benzene ring. They are ring activators and orthopara directors. Example:- If lone pairs electrons /’-‘ve charge are directly attached to benzene.
    https://khullanote.com/wp-content/uploads/2020/07/IMG_20200712_155733-1-1024x385.jpg2.-R/-M group:- The group which draws electrons from the conjugated system is known as -R group. They decrease the electron density so are ring deactivators. They are metadirective. Example:- If the more electronegative atom than π bond is attached directly to the ring.
    https://khullanote.com/wp-content/uploads/2020/07/IMG_20200712_161906-1024x375.jpg
    Bond length (Shortening of C-C single bonds adjacent to multiple bonds):-Hyperconjugation, like conjugation and resonance, also affects bond length, eg:- 
    H_{3}C -CH3CC bond length is 1.46 A° in contrast to normal 1.54A° (in propane). The reason is the partial double bond character acquired makes it shorter.
    2. Nature Of Reagent:-Attacking reagent maybe divided into two main types:-
    • Electrophile or Electrophilic reagent:- They are electron deficient species. They tends to gain electrons, and are attracted by ‘-ve’ charge density. It can be classified into following types:- Type 1:- Positively charged with incomplete valence shell. Example:- R, NO_{2}NO2, NO,SO_{3}HSO3H etc.
      Type 2:- Neutral with incomplete Valence shell. Examples:- 
      R^{•}RR-O^{•}RO etc.
      Type 3:- With complete valence shell but expandable or vacant d-orbital species. Example:- 
      PCl_{3}PCl3SbCl_{5}SbCl5SnCl_{2}SnCl2ZnCl_{2}ZnCl2etc.
      Type 4:- Neutral, complete valence shell, non expandable; it breaks its existing bond and creates vacant orbital. Example:- 
      CO_{2}CO2SO_{2}SO2SO_{3}SO3etc.
      Strength of electrophile:- Type 1>Type 2> Type 3> Type 4
    • Necleophiles Or Nucleophilic Reagent [Nucelus Loving]:- They are electron donating species. They are electron rich i.e. they normally posses an unshared electron pair which they can donate. They attack regions of low electron density (positive centers) in the the substrate molecule. They are classified into 3 categories:-
      • Charged nucleophiles
      • Ambident nucleophiles
      • Neutral nucleophiles

C. Bond cleavage or fission or breaking of a covalent bond:- When the 2 atoms joined by a covalent bond are separated, the process is termed fission or cleavage of bond. Organic reaction is a process in which breaking and formation of covalent bond/s take place. There are 2 types of bond cleavage:-
1. Homolysis/symmetrical/ Non-polar clevage:- In this fission, the covalent bond is broken in such a way that each resulting species gets its own electron. Bond cleavage occurs between atoms having nearly equal electronegativity. As a result of homolytic fission, neutral species with unpaired electrons (odd electrons) are formed which are called free radicals. They show paramagnetic behaviour. Conditions for homolysis are:-
1). High temperature (≥500°)
2). Electricity
3). Light(sunlight)
4). Peroxide
5). Free Radical [@HELPR]

2Heterolysis /Unsymmetrical /Polar bond cleavage:- It is unsymmetrical bond fission. The covalent bond is broken in such a way that one species (i.e less electronegative) is deprived of its own electron, while the other species gains both the elctrons. It occurs between two atoms of having significant difference in electronegativities.
Conditions for Heterolysis:-
1). Significant Electronegative difference.
2). Polar nature of bond.
3). Polar nature of bond.
4). Polar solvent like 
H_{2}OH2O, HCOOJ of high dielectric constant.
5). Low temperature.

#General organic chemistry#Inductive effect