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Class notes Chemistry Notes - Titration, Spectroscopy (AC102)
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Complete notes on organometallic compounds
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144 ENGINEERING CHEMISTRY
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Activation energy
bl
with kinetic energy (E)
with a catalyst
Number of molecules
y.
Activation energy
without a catalyst
it
rs
ve
Kinetic energy (E)
ni
Fig. 6.5
Give experimental evidence that different catalyst has no influence on the
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yield of a reaction.
Ans. It has been observed that at the same temperature the yield of SO3 is the same,
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whether the catalyst used be Pt, Fe2O3 or V2O5 .
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Q. 6. Give important uses of negative catalysts of technical importance.
Ans. An important type of negative catalyst is termed anti-oxidants. Aromatic amines,
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thiourea, etc., are used as antioxidants for prolonging the life of rubber. Widest technical use
of negative catalyst is TEL as antiknock compound in motor fuel. n-propyl gallate, lentoxy
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anisole are added to retard the rancidity of oil.
Q. 7. What is autocatalysis?
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Ans. When one of the products of a reaction itself acts as a catalyst for that reaction the
phenomenon is called autocatalysis.
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Q. 8. Draw a graph for an autocatalytic reaction.
a
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Completion of reaction
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Rate of reaction
/k
Sigmoid curve
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tp
Time
ht
Fig. 6.6
Ans. In autocatalysis reaction as the catalytic product is gradually formed, the rate of
reaction increases. The rate is maximum when the reaction is complete (Fig. 6.6).
Q. 9. Why is the hydrolysis of ethyl acetate autocatalytic?
Ans. As traces of moisture initiate the reaction, CH3COOH is formed acts as catalyst.
CH3COOC2H5 + H2O → CH 3COOH + C2H5OH
catalyst
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CATALYST 145
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Q. 10. What are homogeneous and heterogeneous catalysts? Give examples.
Ans. If the catalyst is in the same phase as the reactants, it is a homogeneous catalyst.
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If the catalyst is in a different phase, it is a heterogeneous catalyst.
Example: (i) Homogeneous catalyst = Hydrolysis of ester, both acid and base catalysed.
y.
(ii) Heterogeneous catalyst (contact catalyst) = In Habers process for manufacture of
ammonia (Fe + Al2O3 + K2O).
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Q. 11. What are enzymes?
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Ans. Enzymes are proteins in nature and are the catalysts for biochemical reactions.
Q. 12. Do you know any use of enzymes in detergent?
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Ans. Some washing powders contain proteasesenzymes that remove blood stains on
dirty clothes.
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Q. 13. Mention the main characteristics of an enzyme.
Ans. Enzymes are highly specific. Each enzyme catalyses a particular reaction. Every
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enzyme has an active site, that is, just right shape and size for the substrate molecules. Each
enzyme works best at a particular temperature and pH.
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CATALYTIC APPLICATIONS OF ORGANOMETALLIC COMPLEXES
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Effective Atomic Number (E.A.N.)
To explain the formation of a complex of a metal with a ligand, it was suggested by
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Sidgwick that metal ions will tend to accept the electron pairs from donors, i.e., ligand until
they have obtained a sufficient number of electrons, so that a metal ion in the resulting complex
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has an effective atomic number of the nearest inert gas. This rule will help to understand the
formation of a complex which in turn will lead to get a better explanation of complex compounds
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acting as catalysts in various reactions.
So, the effective atomic number (E.A.N.) of a metal is obtained by deducting the number
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of electrons lost in the metal ion formation, then adding the number of electrons gained by co-
ordination i.e., two electrons from each ligand having one co-ordination centre. The rule can be
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exemplified by the following table:
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E.A.N. Determination
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Atomic no. Co-ordination Electrons Electrons
Metal ion of metal number lost in ion added by E.A.N.
/k
formation co-ordination
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Fe2+ 26 6 2 12 36 (Kr)
Co3+ 27 6 3 12 36 (Kr)
tp
Cu+ 29 4 1 8 36 (Kr)
ht
Ni2+ 28 4 2 8 34 (Se)
The 18-electron Rule
This is another way of expressing noble gas electron number rule i.e., effective atomic
number rule regarding complex formation. The statement of 18-electron rule is that the valence
shell of metal atom will attain 18 electrons by saturation of (n 1) d, ns and np orbitals of
metals by the ligands. Thus, the saturation will occur as follows:
10 (for (n 1) d subshell) + 2 (for ns subshell) + 6 (for np subshell) = 18 electrons.
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146 ENGINEERING CHEMISTRY
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To illustrate, let us cite the following examples:
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Compounds Central metal Electronic confi- No. of electrons Total no. of
atom with oxida- guration of the donated by electrons in
y.
tion number central metal of ligands (n 1) d, ns
(n1) d subshell and np levels
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(i) Ni (CO)4 Ni(0) 3d10 2 × 4 (four CO 10 + 8 = 18
groups) = 8 electron
rs
valence shell
configuration
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(ii) Fe(CO)5 Fe(0) 3d8 2 × 5 (five CO 8 + 10 = 18
groups) = 10 electron
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valence shell
configuration
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The attainment of 18 electrons in the valence shell of the metal atom or satisfying 18-
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electron rule is one of the useful criteria of elucidating the structure of metal carbonyls.
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Structure and bonding in organometallic complexes, the 16-and 18-electrons rule:
The role of metals in catalytic cycles during some chemical reactions:
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• Hydrogenation using (Ph3P)3RhCl
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The hydrogenation of unsaturated organic compounds i.e., mostly alkenes is an important
industrial reaction. Almost all large scale hydrogenation processes are carried out using Raney
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Ni as catalyst, which acts heterogeneously. Homogeneous systems are used for pharmaceutical
industries. Organometallic chemists made the process very popular. The hydrogenations of
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alkenes and alkynes have been studied extensively. (Ph3P)3 RhCl, which is generally known as
Willkinsons catalyst, acts as a homogeneous catalyst for the hydrogenation of alkenes and
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alkynes. The catalyst is not able to reduce other organic functional groups, though it is a very
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reactive compound. It is dissociated (only 5%) into 14 electron species in pure solvents, thus:
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(Ph3P)3 RhCl (Ph3P)2RhCl + PPh3
The species (Ph3P)2 RhCl is of low coordinating power.
/k
The action of the Willkinsons catalyst is represented by the following cycle. In the cycle
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there are four co-ordination compounds (II V). The 14-electron species (II) is formed by
dissociation of phosphene ligand from (I). The structure (II) is seen to possess a vacant
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coordination site shown by a square. This is because stable Rh(I) complexes are generally four
coordinated (16-electron species).
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The 14-electron species (II) which takes two hydrogen atoms to give (III) by oxidative
addition (III) is still unsaturated and has a vacant site, so it can readily accept π electrons from
the alkene to give (IV). The stage IV is very very significant in the overall hydrogenation
process as in this stage, the substrates alkene, hydrogen are now bonded to the same metal
atom. Next step is the rearrangement of the coordination site to give V. (V) undergoes reductive
elimination to release the alkane and regenerate the 14-electron species.
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CATALYST 147
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L3RhCl
(I)
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L+ –L H2
L L
y.
addition of H2
Rh
it
Cl
C—C 14e
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(II)
H H
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H
L H
ni
Rh
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Cl L
16e
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H
L C—CH (III)
Rh
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Cl L H C—C
L H
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16e
(V) Rh
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Cl L
C C PH3P = L
ch
18e
(IV) Vacant site =
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Fig. 6.7 Catalytic applications of organometallics.
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Highlights:
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• Oxidative addition refers to the increase in coordination site of the central metal.
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• Reductive elimination refers to the decrease in coordination site of the central metal.
/k
• Hydroformylation
The reaction of an alkene with synthesis gas i.e., mixture of carbon monoxide and
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hydrogen (CO + H2) to produce an aldehyde, is known as hydroformylation. The reaction,
which takes place in presence of a catalyst is as follows:
tp
Catalyst
ht
R.CH=CH2 + CO + H2 → RCH2CH2CHO + R CH CH 3
|
CHO
Several transition metal compounds are used as catalyst in hydroformylation. But among
them, catalysis by cobalt or rhodium compounds is of interest. Industrially cobalt carbonyl is
used as catalyst at 130° 170°C and under 200 300 atm pressure. As this needs high pressure
reactor the initial installation cost of the plant is high for an industrial process.
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