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Online Study MaterialCourse: B.Sc. (Hons./Subs.) Part-1

Subject: Organic Chemistry

Paper-II

Unit 5 (Group B) - Organometallic compounds

Topic- Grignard reagent and its application in synthesis

Developed by

Dr. Abhay Nanda Srivastava

Assistant Professor

Department of Chemistry

Nitishwar Mahvidyalaya, Muzaffarpur

(A constituent unit of B.R.A. Bihar University, Muzaffarpur.)

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Organometallic Compounds

The compound in which metal atom is directly bonded to a carbon atom (i.e.

metal carbon bond) is called as organometallic compound. For example;

Organomagnesium halides (R-Mg-X), Tetraethyl lead [Pb(C2H5)4], Diethyl

zinc [Zn(C2H5)2], Triethyl aluminum [Al(Et3)], Ferrocene [(C5H5)2Fe], etc.

Victor Grignard was awarded with Nobel Prize in 1912 for the

development of Organomagnesium halides (R-Mg-X) which is named as

Gringard reagent in his honor.

Classification of organometallic compounds : Organometallics have been

classified as :

(1) Ionic organometallic compounds: In these compounds carbon atom of the

organic moiety (group) is attached to highly electropositive metal ions such as

Na, K, etc. For examples K+(C5H5)-, Na+(C6H5)-, Na+(C6H5)3C- etc.

(2) σ–bonded (Covalent) organometallic compounds: Compounds such

as RMgX, R2Zn, R4Pb, R3Al, R4Sn etc, contains M-C σ–bond and are called σ-

bonded organometallic compound.

Pb Si

i) Structure of Tetraethyl lead ii) Structure of Tetraethyl silane

(3) π-bonded organometallic compounds: The transition metals binds to

unsaturated hydrocarbons and their derivatives using their d-orbitals. Here metal

atom is bonded to ligands in such a way that donations of electrons and back

acceptance by the ligand is feassible. These are called π–orbitals of the ligand.

These are called π–complexes. For example: (i) π-cyclopentadienyl iron

complex: Ferrocene; [Fe(η5-C5H5)2], Bis(cyclopentadienyl) iron (II). It is

a π bonded sand-witch compound. The number of carbon atoms bonded to the

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metal ion is indicated by superscript on eta (ηx) i.e. η5 in this complex. Its

structure is.

(ii) Dibenzene chromium (π–complex): It is also a bonded sand-witch

compound. Its formula is [Cr(η6-C6H6)2] and structure is.

(iii) Alkene complex (π–complex): Zeise’s salt KPtCl3(η2-C2H4); Potassium

trichloroethylene platinate (IV). It is a π bonded complex. η2 indicates that two

carbons of ethylene are bonded to metals.

Fe Cr

i) Structure of Ferrocene ii) Structure of Dibenzene chromium

(3) Complexes containing both σ– and π–bonding characteristics: Metal

carbonyls, compounds formed between metal and carbon monoxide belong to

this class. Metal carbonyls have been included in organometallics. These are of

two types:

(i) Mononuclear carbonyls: Contain one metallic atom per molecule. e.g

Ni(CO)4.Fe(CO)5, Cr(CO)6

(ii) Poly-nuclear carbonyls: Contain two or more metallic atoms per molecule.

e.g., Mn2(CO)10, Fe(CO)9, Fe(CO)12

Synthesis of organometallic compounds: Organometallic compounds can be

synthesized by following methods:

(1.) Reductive substitution of organic halides with metals: Organometalic

compounds are often prepared by direct action of metals on organic halides in

ether solvent.

+ Mg(C2H5)2O

R X Mg XR (Alkyl magnesium halide)

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+ 2 M(C2H5)2O

R X Mg XR + M-X [M = Li, Na, K; X = Cl]

+ 2 Li(C2H5)2O

CH3I CH3Li + Li-I

+ 2 Na (C2H5)2OC2H5I C2H5Na + NaI

n-C2H9 Br(C2H5)2O

+ 2 Li n-C4H9 Li + Li-Br(n-butyl lethium)

+ 2 Zn2 C2H5Br (C2H5)2O(C2H5)2Zn + ZnBr2(Diethyl zinc)

+ Sn2 CH3Cl (CH3)2SnCl2Under pressure

625 KOver Cu

+ Ge2 C2H5Cl (C2H5)2GeCl2

The metal is sometime more reactive when present in an alloy or amalgam. For

example:

+ Pb / Na Alloy4 C2H5I (C2H5)4Pb + 4 NaI

4 C2H5I + Na / Hg amalgam (C2H5)2Hg + 2 NaI

(2.) By The use of alkylatimg agents: When reagents like Grignard reagents

and alkyl lithium reagents are made to react with halides of metals or non-

metals in presence of ether as solvent, organometallic compounds are formed.

For example:

SnCl4 + 4 n-C4H9 Li + 4 LiCl(n-C4H9)4Li(C2H5)2O

PbCl4 + C2H5MgBr (C2H5)4Pb + 4 MgBrCl(C2H5)2O

CdCl2 + 2 C2H5MgCl (C2H5)2Cd + 2 MgCl2(C2H5)2O

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Applications of organometallic compounds

(1) Grignard reagent (RMgX) has been extensively used for synthesis of

various organic compounds: The chemical behavior of R-Mg-X resembles that

of carbanions, species that contain a negatively charged carbon. Grignard

reagents are strong bases and strong nucleophiles. Except for hydrocarbons,

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ethers, and tertiary amines, almost all organic compounds react with Grignard

reagents. Many of these reactions are used for synthetic purposes, notably those

with carbonyl compounds (e.g., aldehydes, ketones, esters, and acyl chlorides),

with epoxides, and with halogen compounds of certain metals (e.g., zinc,

cadmium, lead, mercury) to form the alkyl derivatives of those metals.

(2) Wilkinson's hydrogenation process: Wilkinson's catalyst [(PH3P)3RhCl]

i.e. tris (triphenylphosphine) chlororhodium (I) is used as a homogeneous

catalyst for the hydrogenation of alkenes.

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Mechanism:

The last step regenerates the catalyst, which can then cause hydrogenation of

another molecule of alkene.

(3) Formation of carbon-Carbon bond: rhodium complexes have also been

used to synthesize compounds in which the formation of C-C bond is required.

An example is synthesis that follows:

The first step, a ligand exchange. occurs by a combination of ligand association

– dissociation steps and incorporates the methyl group into the coordination

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sphere of rhodium. The next step, an oxidative addition, incorporates the phenyl

group into the rhodium coordination sphere. Then, in the last step, a reductive

elimination joins the methyl group and the benzene ring to form toluene.

(4) As catalyst in polymerization reaction: Zeigler Natta catalyst (composed

of a transition metal salt, generally TiCl4 and trialkyl aluminium) are used as

heterogeneous catalysts in the polymerisation of alkenes.

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