Prashant jodhawat chemistry project.pptx

From Wikipedia, the free encyclopedia

Triethylboron is a trialkylborane.
A borane is a compound with the formula BRxHy although examples include multi-boron derivatives. A large family of boron hydride clusters is also known. In addition to some applications in organic chemistry, the boranes have attracted much attention as they exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes, are also a well developed class of compounds.[1]

History
The development of the chemistry of boranes led to innovations in synthetic methods as well as structure and bonding. First, new synthetic techniques were required to handle diborane and many of its derivatives, which are both pyrophoric and volatile. Alfred Stock invented the glass vacuum line for this purpose.[2] The structure of diborane was correctly predicted in 1943 many years after its discovery.[3] Interest in boranes increased during World War II due to the potential of uranium borohydride for enrichment of the uranium isotopes and as a source of hydrogen for inflating weather balloons. In the US, a team led by Schlesinger developed the basic chemistry of the anionic boron hydrides and the related aluminium hydrides. Schlesinger's work laid the foundation for a host of boron hydride reagents for organic synthesis, most of which were developed by his student Herbert C. Brown. Borane-based reagents are now widely used in organic synthesis. Brown was awarded the Nobel Prize in Chemistry in 1979 for this work.[4]

Synthesis
Main article: Organoboron chemistry
Most boranes are prepared directly or indirectly from diborane. Diborane reacts with alkenes to give alkylboranes, a process known as hydroboration:

B2H6 + 2 CH2=CHR → 2 BH2(CH2CH2R)
B2H6 + 4 CH2=CHR → 2 BH(CH2CH2R)2
B2H6 + 6 CH2=CHR → 2 B(CH2CH2R)2
Alkyl and aryl boranes can also be produced by alkylation of chloroboranes and boronic esters.

Classes of boranes
Some specialty boranes used in organic synthesis
Thexylborane
Thexylborane

9-Borabicyclo[3.3.1]nonane ("9-BBN")
9-Borabicyclo[3.3.1]nonane ("9-BBN")

Disiamylborane ("Sia2BH")
Disiamylborane ("Sia2BH")

Alpine borane
Alpine borane

Diisopinocampheylborane
Diisopinocampheylborane
Binary boron hydrides
The parent boranes are binary boron hydrides, starting with borane (BH3) and its dimer diborane (B2H6). Pyrolysis of these species leads to higher boranes, such as tetraborane and pentaborane. These two are early members of the boron hydride clusters.

Primary and secondary boranes
This family of boron hydrides includes mono- and dialkylboranes. The simplest members readily engage in redistribution reactions:

2 BH2(CH3) → BH(CH3)2 + 0.5 B2H6
With bulky substituents, primary and secondary boranes are more readily isolable and even useful. Examples include thexylborane and 9-BBN. Almost all primary and secondary boranes are dimeric with bridging hydrides.

Tertiary boranes
Most work focuses on trialkyl

1. ACID HYDROLYSIS OF OCTAHEDRAL COMPLEXES A SEMINAR PRESENTED BY PRASHANT JODHAWAT M.Sc. (CHEMISTRY) SEMESTER - I DEPARTMENT OF CHEMISTRY S.P.C. GOVERNMENT COLLEGE, AJMER 2023-2024

2. CONTENTS Introduction Acid Hydrolysis S N 1 Mechanism S N 2 Mechanism Simple acid hydrolysis reaction • Factors to decide the type of mechanism. References

3. Introduction :- The ligand substitution reaction in which a ligand present in a coordination sphere of a complex species is replaced by H 2 O molecule and OH - ion, is known as hydrolysis. It is of two types :- (a) Acid hydrolysis :- In which a ligand is replaced by H 2 O molecule and there is a formation of aqua complex. [Co(NH 3 ) 5 Cl] 2+ + H 2 O → [Co (NH 3 ) 5 (H 2 O)] 3+ + Cl - (b) Base hydrolysis :- In which a ligand is replaced by OH - ion and formation of hydroxo complexes takes place. [Co(NH 3 ) 5 Cl] 2+ + OH - → [Co (NH 3 ) 5 (OH)] 3+ + Cl -

4. Acid Hydrolysis :- The ligand substitution reaction occurring in neutral or acidic solution and in which a ligand present in the coordination sphere of a complex species is replaced by H 2 O molecule, is called acid hydrolysis. [Co(NH 3 ) 5 Cl] 2+ + H 2 O → [Co (NH 3 ) 5 (H 2 O)] 3+ + Cl - Aqua Complex Above reaction is an acid hydrolysis reaction. In which Cl - is the leaving ligand, H 2 O is the entering group, NH 3 is the inert ligand, which remains attached to the product.

5. S N 1 or Dissociative mechanism : - In this reaction one of the ligand dissociates from the reactant to form a reaction intermediate with lower coordination number and than this reacts with entering ligand and forms the product. Step I st :- [Co (NH 3 ) 5 Cl ] 2+ → [Co(NH 3 ) 5 ] 3+ ( C.No . = 6) intermediate ( C.No . = 5) Step IInd :- [Co(NH 3 ) 5 ] 3+ + H 2 O → [Co(NH 3 ) 5 (H 2 O)] 3 + ( C.No . = 5) ( C.No . = 6) Rate α [Co (NH 3 ) 5 Cl ]

6. Diagrammatic Representation

7. S N 2 or Associative mechanism :- In this, reaction intermediate is formed by including the incoming ligand in the coordination sphere and has higher coordination number than reactants or products and then leaving group leaves the complex and formation of products with lower coordination no. takes place. Step I st :- [Co (NH 3 ) 5 Cl ] 2+ + H 2 O → [Co(NH 3 ) 5 (H 2 O) Cl ] 2+ ( C.No . = 6) T.S. ( C.No . = 7) Step IInd :- [Co(NH 3 ) 5 (H 2 O) Cl ] 2+ → [Co(NH 3 ) 5 (H 2 O)] 3 + ( C.No . = 7) ( C.No . = 6) Rate α [Co (NH 3 ) 5 Cl ] [H 2 O]

8. Diagrammatic Representation The first step is slow since the incoming ligand H 2 O causes steric hinderance .

9. Simple acid hydrolysis reactions :- This type of reactions occurs in acidic conditions, but the rate of reaction does not depend on the concentration of acid. It is a first order reaction. (a) S N 1 Mechanism :- [Co (NH 3 ) 5 Cl ] 2+ → [Co(NH 3 ) 5 ] 3+ + Cl - [Co (NH 3 ) 5 ] 3+ + H 2 O → [Co (NH 3 ) 5 (H 2 O)] 3+ Rate α [Co (NH 3 ) 5 Cl ] (b) S N 2 Mechanism :- [Co (NH 3 ) 5 Cl ] 2+ + H 2 O → [Co(NH 3 ) 5 (H 2 O) Cl ] 2+ [Co(NH 3 ) 5 (H 2 O) Cl ] 2+ → [Co(NH 3 ) 5 (H 2 O)] 3 + Rate α [Co (NH 3 ) 5 Cl ] [H 2 O]

10. ● Factors to decide the type of Mechanism :- Charge on the complex :- During the hydrolysis of several octahedral complexes the rate of hydrolysis of a complex decreases with the increase in the charge on the complex.

11. Example :- The rate of hydrolysis of cis [Co (en) 2 Cl 2 ] + is some hundred times faster than that of cis [Co (en) 2 Cl (H 2 O)] 2+ cis [Co (en) 2 Cl 2 ] + + H 2 O → cis [Co (en) 2 Cl (H 2 O)] 2+ + Cl - (charge = +1) cis [Co (en) 2 Cl (H 2 O)] 2+ + H 2 O → cis [Co (en) 2 (H 2 O) 2 ] 3+ + Cl - (charge = +2) Explanation :- The above observation can be explained by S N 1 mechanism of hydrolysis reaction, since the increase in the positive charge on the complex makes the dissociation of the leaving group ( Cl - ) from the metal ion more difficult and hence the rate of hydrolysis becomes slow. But in S N 2 mechanism the rate remains unchanged.

12. (b) Basicity of leaving group L - :- If we study the rate of hydrolysis of complexes, [Co (NH 3 ) 5 L] 2+ which contain different L - ligands , we find that the rate of hydrolysis of these complexes decreases with the increase of the basicity of L- ligands . CF 3 COO - < CCl 3 COO - < CHCl 2 COO - < CH 2 ClCOO - < CH 3 CH 2 COO - Basicity of L - ligands increases → Rate of hydrolysis decreases → Explanation : Since the strength of Co 3+ - L - Bond is directly proportional to the basicity of L - ligand , with the increase in the basicity of L - ligands Co 3+ - L - bond becomes stronger and hence the rate of hydrolysis decreases.

13. (c) Inductive effect of the inert group :- It has been seen that the rate constants of acid hydrolysis reaction : [Co(en) 2 (A- Py ) Cl ] 2+ + H 2 O → [Co(en) 2 (A- Py )(H 2 O)] 3+ + Cl - Increase with the CH 3 substitution in pyridine. In this reaction A- Py stands for various derivatives of pyridine which are obtained by removing one of the H- atoms of pyridine by CH 3 group A- Py is an inert ligand , since it remains coordinated to the metal in the product, [Co (en) 2 (A- Py ) (H 2 O)] 3+ . The increase in rate constants is due to the inductive effect caused by the increasing CH 3 substitution which results in distorting electron density towards Co- atom and thus helps the dissociation of Cl - ion (Leaving group).

14. (d) Steric effects :- In the complexes of trans [Co (AA) 2 Cl 2 ) + type, if the bidentate ligand AA is NH 2 – CH 2 – CH 2 -NH 2 , NH 2 -CH 2 -CH 2 -CH 2 -NH 2 , NH 2 -CH 2 -CH (CH 3 ) NH 2 then the bulk of the ligand , (AA) increase, i.e., the ligand (AA) becomes more bulky. Due to the increase in the bulk of the ligand the sterric overcrowding of the ligand round the central metal ion (CO 3+ ) also increases. Due to the increase in the overcrowding of the ligand round the central metal ion (CO 3+ ) another ligand cannot be taken up by the complex, i.e. there is no possibility of S N 2 mechanism. The removal of a ligand will reduce the overcrowding of the ligand round the central metal ion. This gives the evidence of S N 1 mechanism.

15. References :- Selected Topics in Inorganic chemistry by : Dr. Wahid U. Malik Dr. G.D. Tulsi Dr. R.D. Madan Advanced Inorganic chemistry by : S.K. Agarwal Keemti Lal Special Thanks to : Mritunjay Sahu (Senior)

16. THANK YOU FOR BEING A WONDERFUL AUDIENCE