Online Study Material · Synthesis of organometallic compounds: Organometallic compounds can be...
Transcript of Online Study Material · Synthesis of organometallic compounds: Organometallic compounds can be...
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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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