Organometallic Compounds

Question 1

What is LiAlH4 reagent?

Answer 1

→Lithium aluminium hydride also known as LAH, is a reducing agent that is commonly used in modern organic synthesis.
→It is a nucleophilic reducing agent that is best suited for reducing multiple polar bonds such as C=O.
→The LiAlH4 reagent is capable of converting aldehydes to primary alcohols, ketones to secondary alcohols, amides and nitriles to amines, epoxides to alcohols and lactones to diols.
→LiAlH4 cannot reduce isolated non-polar multiple bonds such as C=C. The double or triple bonds in conjugation with the multiple polar bonds, on the other hand can be reduced.

Question 2

Give the preparation LiAlH4.

Answer 2

Preparation:
The reaction of lithium hydride and aluminium chloride produces lithium aluminium hydride.
4LiH +AlCl3→LiAlH4 + 3LiCl
In addition to this method, the industrial synthesis requires the initial preparation of sodium aluminium hydride under high pressure and temperature.
Na + Al +2H2→NaAlH4
Following that, LiAlH4 is synthesized via salt metathesis reaction as follows:
NaAlH4 +LiCl →LiAlH4 + NaCl
It proceeds in a high yield . Filtration is used to remove LiCl from an ethereal solution of LAH.

Question 3

Give the properties of LiAlH4.

Answer 3

Physical properties:
1. It is a strong reducing agent.
2. Molar mass of LiAlH4 is 37.97 g/mol.
3.Density of LiAlH4 is 917kg/me
4.Melting Point is 150°C
5.Boiling point is184°C
Chemical properties:
1. Although LiAlH4 is a white solid, commercial samples are usually grey due to impurities.
2. LiAlH4 reduces the nitriles to primary amines.
Eg: Acetonitrile reduces to ethyl amine in the presence of LiAlH4.
H3C-C triple bond N→{(1)2 eq. LiAlH4 (2) H30+} H3C-CH2NH2
3. LiAlH4 is used to convert haloalkanes and haloarenes to their corresponding hydrocarbons.
4. It has high solubility in diethyl ether.
5.However, due to the presence of catalytic impurities, it may spontaneously decompose in it.
6. Despite it’s low solubility,THF is the preferred solvent for LAH
7.Usually the reactions are carried out with an excess of LAH. To remove mixture if any, a small amount of the reagent is added to the solvent.

Question 4

What is grignard reagent?

Answer 4

Grignard reagents are among the most versatile organometallic reagents . In 1990 the French Chemist Francois Auguste Victor Grignard discovered that organohalides react with magnesium in ether solvents to form organomagnesium compounds. The organomagnesium halides are known as Grignard reagents.
Alkyl halides reacts with magnesium metal in diethyl ether (ethoxy ethane)(C2H5)2O to form compounds that contain a metal carbon bond. Methyl bromide, for example, forms methylmagnesium bromide.
CH3CH2Br +Mg→CH3CH2MgBr
(Ethoxy ethane)
Grignard reagents are our first source of carbanions Lewis structure of the CH3- ion suggests that carbanions can be Lewis base or electron pair donors.
H
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[ H - C: ]
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H

Question 5

How is Grignard reagent prepared?

Answer 5

The grignard reagents are prepared by the action of activated magnesium on organic halides in suitable solvents like diethyl ether Et2O, tetrahydrofruon,THF in anhydrous conditions. The halides can be Cl, Br, I.
R-X +Mg→R-Mg-X
(Anhydrous Et2O or THF)

Question 6

State the applications of Grignard reagent.

Answer 6

1. Production of organometallic compounds:
   Like ( organotin compounds,organosilicon compounds,organoboron compounds, ) as well as primary materials and intermediates of pharmaceuticals and agrochemicals.
   Organotin compounds: These can be produced by reacting grignard reagents with tin tetrachloride.