effect of nucleophile on substitution reaction|organic chemistry

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1. Maria Arshad BS Chemistry (2109-17) Presentation: Organic Chemistry (theory-1)

2. FACTORS AFFECTING SUBSTITUTIONS REACTIONS FACTORS: Nature of substrate structure Nature of the solvent Nature of the Nucleophile Nature of the leaving group

3. 3. Nature of the nucleophile In nucleophilic substitution, the atoms of the attacking nucleophile which gets bonded to the substrate is called attacking atom As the electrons of the attacking atom undergo coordination hence the most effective nucleophile would be that which can easily donate electrons for coordination

4. Difference between base and nucleophile BASE NUCLEOPHILE Basicity refers Nucleophilicity refers to coordination with to coordination with H+ Carbon.

5. BASE NUCLEOPHILE Basicity is a thermo- Nucleophilicity is a -dynamic property and kinetic property,. i.e it Depends on the Ka of is a measure of reaction The equilibrium. Rate Where B- base combine with proton of acid. All nucleophile are basic, though basicity and nucleophiicity are not always synonymous .

6. EFFECT ON SN1 REACTION

7. EFFECT ON SN2 REACTION SN2 reaction is nucleophile induced reaction substrate nucleophile product leaving group Reaction rate is directly proportional to nucleophile concentration which appears in the rate law.

8. FACTORS ON WHICH NUCLEOPHILICITY DEPENDS: following are the Factors on which nucleophilicity depends The electronegativity of the donor atom The size and polarizibility of the donor atom The density of the electrons on donor atom The nature of the solvent used The size of the nucleophile as a whole

9. Trend of nucleophilicity and basicity along the period Nucleophilicity order along a period runs parallel with basicity Explanation: Strong nucleophile which easily donate electrons so So as we go along the period from left to right Electronegativity increases nucleus strongly attract the e- so it will not be easy to donate electrons nucleophilicity Basicity : Strong base which easily donate electrons so along the period Electronagativity increases so difficult to donate electrons so basicity So the order will be same i.e

10. Another concept of basicity strong acid which easily donate H+ So along the period from left to right The electronegativity the nucleus strongly attracts the e- Even the atom of F will attract the e- of H-atom as well..so it iis easy to donate H+ so acidity increases along the period so, CH4<NH3<H2O<HF After removal of H+ conjugate bases will form as acid conjugate base Acid strong conjugate base so F- will be the weakest base So basicity decreases along the period

11. Trend of nucleophilicity and basicity along the group Order of nucleophilicity along the group is just opposite to the basicity Explanation: strong nucleophile which easily donate e- As we go down the group the size atom polarizibility hold of nucleus on e- so easily donate e- so nucleophilicity down the group so the order will be :F-< Cl -<Br-<I- BASICITY: Strong acid which easily donate H+ Down the group Size of atom increases so bond length increases between HI as compared to HF Bond between HI is weaker than HF so easily donate H+ So acidity HF< HCl < HBr <HI

12. After removal of H+ ion conjugate bases will be formed

13. Acidity order STRONG ACID HF< HCl < HBr <HI Basicity order weaker conjugate base F-> Cl ->Br->I- Nucleophilicity order: strong nucleophile F-< Cl -<Br-<I- Basicity down the group Nucleophilicity down the group hence proved ”the order of nucleophilicity is just opposite to the order of Basicity ”

14. AMBIENT NUCLEOPHILE: A Nucleophile which have more than one donor atom is called ambient nucleophile For example : CN-,SCN-,NO2 etc An ambient nucleophile is often characterized by the presence of soft donor atom (less electronegative and more polarizable). hard donor atoms ( more electronegative and less polarizable).

15. FOR-EXAMPLE In CN- C is soft nucleophile and N is hard In first example C of CH3-Br is soft electrophile so it combines with C (soft nucleophilic atom) of CN. Via SN2 In 2 nd example due to presence of Ag, Br leaves out rapidly and form AgBr .so in this case the C of CH3 become hard electrophilic site so it combines with N(hard nucleophilic atom ). Via SN1

16. With same donor atom: With same donor atom,nucleophilicity is proportional to the electron density on the donor atom. For example: CH3COO- is less basic and weaker nucleophile than CH3O-. Reason: Because of less charge density on oxygen atom due to resonance.

17. EXAMPLE PhO - is weaker than CH3O- though it is better nucleophile than CH3COO-. Reason: negative charge is resonance stabilized so electrons are not easily available

18. CHARGED NUCLEOPHILE: A Charged nucleophile is always stronger than the correspondiing neutral one Reason: because of the more concentrated electrons. For example : CH3O->CH3OH In CH3O- electrons are more concentrated so stronger nucleophile .

19. ADJACENT LONE PAIRS ON NUCLEOPHILE: Nucleophilicity is enhanced by the presence of unshared pair of electrons adjacent to the donor atom. Reason: The ground state of the nucleophile is destabilized by the repulsion of the adjacent electron pairs and makes it more reactive towards SN2 It is also called α -effect.

20. INFLUENCE OF SOLVENT ON NUCLEOPHILICITY: In polar aprotic solvent like DMF,DMSO The cationic part of the nucleophilic reagent is solvated strongly so the nucleophile is free and hence its nucleopohilicity is proportional to the concentration of negative charge on the donor atom. In case of polar aprotic solvent( hydrogen absent) nucleophilicity of F- is much greater than that of I- due to more conc. Electrons because of its small size. So the order of nucleophilicity in polar aprotic solvent is F- > Cl - >Br- >I-

21. IN POLAR PROTIC SOLVENT In polar protic solvent the order will be reverse Reason: In polar protic solvent H+ is present it will react with nucleophile as F- is more electronegative and smaller in size so H+ will strongly bind with F and form HF. While I- is larger in size so H+ will bind but bond will be weaker so it will be easily broken down. So electrons are available. So I- will be strong nucleophile . Order will be F- < Cl -<Br-< I-

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