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BONDING AND ELECTRON DISTRIBUTION

PRESENTED BY:

ROSHNI ANN BABY

M-PHARM PART 1

PHARM.CHEMISTRY

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CONTENTS

Atomic structure and atom modelsQuantum mechanicsChemical BondLocalized chemical bondingHybridizationDelocalized chemical bondingAromaticity

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ATOM• Atoms are made up of 3 types of

particles electrons , protons  and neutrons . These particles have different properties. Electrons are tiny, very light particles that have a negative electrical charge (-).  

• Protons are much larger and heavier than electrons and have the opposite charge, protons have a positive charge.(+)

• Neutrons are large and heavy like protons, however neutrons have no electrical charge.

• Each atom is made up of a combination of these particles.

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• Protons and neutrons form the nucleus and they are surrounded by the electron

• *atomic number is the number of protons*atomic mass is the number of protons and neutronsEg. Helium It has 2 protons and 2 neutrons so its atomic number is 2 and its atomic mass is 4

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Particle Charge Mass (g) Mass (amu)Proton +1 1.6727 x 10-24 g 1.007316

Neutron 0 1.6750 x 10-24 g 1.008701Electron -1 9.110 x 10-28 g 0.000549

Protons and neutrons have almost the same mass, while the electron is approximately 2000 times lighter.Protons and electrons carry charges of equal magnitude, but opposite charge. Neutrons carry no charge (they are neutral).Atoms in their natural state have no charge, that is they are neutral. Therefore, in a neutral atom the number of protons and electrons are the same. If this condition is violated the atom has a net charge and is called an ion. Two atoms with the same number of protons, but different numbers of neutrons are called isotopes.

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• Atoms have sizes on the order of 1-5 Ao and masses on the order of 1-300 amu.

• If an atom were the size of Ohio stadium, the nucleus would only be the size of a small marble. However, the mass of that marble would be ~ 115 million tons.

• The negatively charged electron is attracted to the positively charged nucleus by a Coulombic attraction.

• The protons and neutrons are held together in the nucleus by the strong nuclear force.

• Chemical reactivity of an atom is dependent upon the number of electrons and protons, and independent of the number of neutrons.

• The mass and radioactive properties of an atom are dependent upon the number of protons and neutrons in the nucleus.

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Atomic Mass Unit• The unified atomic mass unit (symbol: u)

or dalton (symbol: Da) is the standard unit that is used for indicating mass on an atomic or molecular scale (atomic mass).

• One unified atomic mass unit is approximately the mass of a nucleon and is equivalent to 1 g/mol.

• It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660538921(73)×10−27 kg.

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Different Atom models• A schematic presentation of the plum pudding model of the

atom; in Thomson's mathematical model the "corpuscles" (or modern electrons) were arranged non-randomly, in rotating rings

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• Ernest Rutherford, bombarded a sheet of gold foil with alpha rays—by then known to be positively charged helium atoms—and discovered that a small percentage of these particles were deflected through much larger angles than was predicted using Thomson's proposal. Rutherford interpreted the gold foil experiment as suggesting that the positive charge of a heavy gold atom and most of its mass was concentrated in a nucleus at the center of the atom—the Rutherford model

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• Ernest Rutherford, bombarded a sheet of gold foil with alpha rays—by then known to be positively charged helium atoms—and discovered that a small percentage of these particles were deflected through much larger angles than was predicted using Thomson's proposal. Rutherford interpreted the gold foil experiment as suggesting that the positive charge of a heavy gold atom and most of its mass was concentrated in a nucleus at the center of the atom—the Rutherford model

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• In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with attraction provided by electrostatic forces rather than gravity.

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Concept of Orbit & Orbital• An orbit is a circular path followed by an electron around the

nucleus.• An atomic orbital is a mathematical function that describes the

wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus. The term may also refer to the physical region or space where the electron can be calculated to be present.

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• Each orbital in an atom is characterized by a unique set of values of the three quantum numbers n, ℓ, and m, which correspond to the electron's energy, angular momentum, and an angular momentum vector component, respectively. Any orbital can be occupied by a maximum of two electrons, each with its own spin quantum number. The simple names s orbital, p orbital, d orbital and f orbital refer to orbitals with angular momentum quantum number ℓ = 0, 1, 2 and 3 respectively. These names, together with the value of n, are used to describe the electron configurations.

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Quantum numbers

Name Symbol

Orbital meaning Range of values Value examples

principal quantum numbern shell 1 ≤ n n = 1, 2, 3, …

azimuthal quantum number (angular momentum)

ℓsubshell (s orbital is listed as 0, p orbital as 1 etc.)

0 ≤ ℓ ≤ n − 1for n = 3:ℓ = 0, 1, 2 (s, p, d)

magnetic quantum number, (projection ofangular momentum)

mℓenergy shift (orientation of the subshell's shape)

−ℓ ≤ mℓ ≤ ℓfor ℓ = 2:mℓ = −2, −1, 0, 1, 2

spin projection quantum numbermsspin of the electron (−½ = "spin down", ½ = "spin up")

−s ≤ ms ≤ sfor an electron s = ½,so ms = −½, ½

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Dual nature of matter & Planck’s constant• Wave–particle duality is a theory that proposes that all matter

exhibits the properties of not only particles, which have mass, but also waves, which transfer energy. A central concept of quantum mechanics, this duality addresses the inability of classical concepts like "particle" and "wave" to fully describe the behavior of quantum-scale objects.

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• The Planck constant (denoted h, also called Planck's constant) is a physical constant that is the quantum of action in quantum mechanics. The Planck constant was first described as the proportionality constant between the energy (E) of a charged atomic oscillator in the wall of a black body, and the frequency (ν) of its associated electromagnetic wave. This relation between the energy and frequency is called the Planck relation:

E = hv h=planck’s constant

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Hund's rule of maximum multiplicity

• The three rules are:• For a given electron configuration, the term with maximum

multiplicity has the lowest energy. The multiplicity is equal to , where is the total spin angular momentum for all electrons.

• For a given multiplicity, the term with the largest value of the total orbital angular momentum quantum number has the lowest energy.

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• For a given term, in an atom with outermost subshell half-filled or less, the level with the lowest value of the total angular momentum quantum number (for the operator ) lies lowest in energy. If the outermost shell is more than half-filled, the level with the highest value of is lowest in energy.

• In short, electron pairing in s,p,d,f orbital cannot take place until each orbital with same subshell fill one electron each.

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Pauli’s exclusion principle• The Pauli exclusion principle is the quantum

mechanical principle that no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is anti-symmetric with respect to exchange of the particles.

• For example, in an isolated atom no two electrons can have the same four quantum numbers; if n, ℓ, and mℓ are the same, ms must be different such that the electrons have opposite spins, and so on.

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Aufbau Principle• Aufbau principle is used to determine the electron

configuration of an atom,molecule or ion. The principle postulates a hypothetical process in which an atom is "built up" by progressively adding electrons. As they are added, they assume their most stable conditions (electron orbitals) with respect to the nucleus and those electrons already there.

• According to the principle, electrons fill orbitals starting at the lowest available (possible) energy levels before filling higher levels (e.g. 1s before 2s).

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• The number of electrons that can occupy each orbital is limited by the Pauli exclusion principle.

• If multiple orbitals of the same energy are available, Hund's rule states that unoccupied orbitals will be filled before occupied orbitals are reused (by electrons having different spins).

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Introduction to bonding

CHEMICAL BONDForce that holds atoms together.•Compound are formed from chemically bound atoms or ions.•Bonding only involves the valence electrons•Greatest stability is reached when outer shell is filled.•Ionic and covalent bonds- tendency of atoms to attain stable atomic configuration

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IONIC BONDS

Transfer of electrons from one atom to another. Force holding cations and anions together

A• • B A+ B-••Ionicbond

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Formation of Ionic Bonds

Eg : Formation of sodium chloride, calcium bromide

NaCl

Na Cl•••

•••••+ Na1+ + Cl

••

••••••

1-

2s22p63s1 3s23p5 2s22p6 3s23p6

8 v.e.

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COVALENT BONDS

Two atoms share one pair of electrons

A • • B A •• BElectronsshared

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Examples….

Formation of H2O

H • H • O•• •••• O•

• ••••

H •H•

O••••

H

H

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Quantum mechanics

• Erwin Schrodinger• Motion of an electron in terms of its energy• Wave equation-electrons shows properties not only

of particles but also of waves • Series of wave functions corresponding to different

energy level of electrons• The differential equation - Schrodinger equation

and its solution - wave function, Ψ.• Most suitable to understand atomic and molecular

structure

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Localized Chemical Bonding

Electrons shared by two and only two nuclei

Covalent Bonding Schrodinger equation - Equation which serves

mathematical model for electrons

∂2Ψ/ ∂x2 + ∂2Ψ/∂y2 + ∂2Ψ/∂z2 + 8π2m/h2 (E – V)Ψ = 0

m-mass of electron, h-plancks constant, E - total energy V- potential energy of electron Ψ – wave function(expresses the probability of finding the electron)

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Molecular orbital method

• Bonding – overlap of atomic orbitals• Atomic orbitals combine to form molecular orbitals• Molecular orbitals clouds that surround the nuclei of

two or more atoms• In localized bonding-two orbitals are present• One bonding orbital (lower energy)and the other

antibonding (higher energy)

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• Orbitals of lower energy fill first• Antibonding orbital remain empty in ground state• Greater overlap stronger bond• Sigma orbitals- molecular orbitals formed by overlap

of two atomic orbitals when centers of electron density are on the axis common to two nuclei

• Bonds sigma (σ) bonds anti bonding (σ ̽ )

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VALENCE BOND METHOD

• Chemical bond - overlap of atomic orbitals. • In hydrogen molecule, the 1s orbital of one

hydrogen atom overlaps with the 1s orbital of the second hydrogen atom to form a molecular orbital called a sigma bond.

• Wave equation written for each possible electronic structures

• Total ψ obtained by summation of as many of these Ψ = C 1 Ψ 1 + C 2 Ψ 2 +……

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Multiple valence

• An atom with valence of 2 or more-forms bonds by using atleast two orbitals

• Examples: Oxygen• Two half filled orbitals – valence of 2• They overlap with orbitals of two other atoms and

forms an angle of 90 • Two available orbitals are p-orbitals which are

perpendicular

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HYBRIDIZATION

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals

Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules.

It is an integral part of valence bond theory.

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“sp” hybrid orbitals

• BeF2 : Be has no unpaired electrons. But has a valence of 2 forms 2 covalent bonds.Valence bond theory predicts that each bond is an overlap of one Be 2s e- and one 2p e- of F. However, Be’s 2s e-

are already paired. So…

To form 2 equal bonds with 2 F atoms:

1. In Be, one 2s e- is promoted to an empty 2p orbital.

2. The occupied s and p orbitals are hybridized (“mixed”), producing two equivalent “sp” orbitals.

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3. As the two “sp” hybrid orbitals of Be overlap with two p orbitals of F, stronger bonds result than would be expected from a normal Be s and F p overlap and is observed as a linear molecule with 2 equal-length Be-F bonds

4. These orbitals point in exactly opposite direction.

5. The angle between the BeF2 bonds must be 180 °

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“sp2” hybrid orbitals

• Eg: Boron trifluoride• Boron –only one unpaired electron, occupies 2p orbital• Need 3 unpaired electron• Promote one of the 2s electron to a 2p orbital• Three equivalent hybrid orbital's

BF3 observed as trigonal planar molecule.

Bond angle observed is 120º.

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sp3 hybrid orbitals• Ground state and excited state electronic configuration of C

• ↑_ ↑_ ↑_ ↑_

• ↑↓ ↑_ ↑_ __

• The hybridization of a s and three p orbitals led to 4 sp3 hybrid orbitals for bonding.

• Compounds involving sp3 hybrid orbitals: CF4, CH4, : NH3, H2O::

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“sp3d2” hybrid orbitals (or d2sp3)

• SF6 : observed as octahedral; forms 6 equal-length bonds

One s + three p + two d → Six sp3d2 orbital

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Shapes of orbitals

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Multiple BondsMultiple Bonds Sigma bondSigma bond (s) → A bond where the line of electron density A bond where the line of electron density is concentrated symmetrically along the line connecting the is concentrated symmetrically along the line connecting the two atoms.two atoms.

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Pi bondPi bond (p) → A bond where A bond where the overlapping regions exist the overlapping regions exist above and below the above and below the internuclear axis (with a nodal internuclear axis (with a nodal plane along the internuclear plane along the internuclear axis).axis).

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Example: HExample: H22C=CHC=CH

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Multiple Bonds

In triple bonds, as in acetylene, two sp orbitals form a σ bond between the carbons, and two pairs of p orbitals overlap in π fashion to form the two π bonds.

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Electro negativity• Measure of power to attract electrons sharing in

covalent bonds• Greatest for atoms in upper right corner of

periodic table• Lower for atoms in the lower left corner• Pauling scale for electronegativity based on bond

energy• Electronegativity –obtained from nmr spectra• Greater electronegativity lower the electron

density around proton

Pauling scale

• The Pauling scale is the most commonly used.

• Fluorine (the most electronegative element) is given a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7.

Dipole moment

• Results from charge seperation• Polar-centre of negative charge doesnot

coincide with centre of positive charge• Dipole-two equal and opposite charges

separated in space• Dipole moment-magnitude of charge ,e

multiplied by the distance ,d between the centers of charge

µ = e x d

Inductive and field effect

• The polarization of a σ bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is called inductive effect.

• The effect that operates directly through space or solvent molecule is called field effect

• An atom like fluorine which can pull the bonding pair away from the atom it is attached to is said to have a negative inductive effect.

• It arises due to electro negativity difference between two atoms forming a sigma bond.

• It is transmitted through the sigma bonds. • The magnitude of inductive effect decreases while

moving away from the groups causing it. • It is a permanent effect. • It influences the chemical and physical properties of

compounds.• Inductive effects are sometimes given symbols: -I (a

negative inductive effect) and +I (a positive inductive effect).

Types of inductive effect

1) Negative inductive effect (-I): The electron withdrawing nature of groups or atoms is called as negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect:

NH3+ > NO2 > CN > SO3H > CHO > CO > COOH >

COCl > CONH2 > F > Cl > Br > I > OH > OR > NH2 > C6H5 > H 2) Positive inductive effect (+I): It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect. C(CH3)3 > CH(CH3)2 > CH2CH3 > CH3 > H

Bond length

• Distance between the center of nuclei of the two bonded atoms

• Expressed in angstrom unit• Ionic compounds-radii of the two concerned ions• Double and triple bond radii are 13 % and 22 % less

than corresponding single bond radius• Carbon bonds shortened by increasing s character.

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Factors affecting bond length

• Electronegativity- as electronegativity increases bond length decreases

• Delocalization• Hybridization - bond length decreases as the s

character increases

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Bond angle

Angle between the directions of two covalent bonds Depends on nature of bond present S orbital spherically symmetrical-overlap another orbital

equally in all directions P orbitals mutually perpendicular-expected bond angle 90

º For water bond angle expected 90 º , measured bond

angle 104º 31’(VSEPR Theory) Divergence due to two factorsRepulsion between atoms or groups attached to the

central atomHybridization of bonding orbitals (s character increases

bond angle increases)

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Bond energies

• Amount of energy associated with each bond as it exist in molecule is called bond energy

• Dissociation energy-energy to cleave a bond• Summation of bond energy gives heat of formation of

molecule from its atoms• In diatomic molecules bond energy is determined by

measuring the heat of formation of the molecule heat of dissociation of the molecule

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Characteristics of covalent bond energy

• In case bonded atom has lone pair of electrons bond between such atoms is weaker due to the electrostatic repulsion between them

• Since 2s orbitals are closer than 2p electrons to the nucleus they are more tightly held (increase in s character, bond energy increases)

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Delocalized bonding

One or more bonding orbitals that spread over three or more such bonding

In valence bond method several possible Lewis structures (canonical forms) are drawn

Ψ = C 1 Ψ 1 + C 2 Ψ 2 + ……

Representation of real structure as two or more canonical forms is called Resonance

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Delocalized π bonds in C6H6

C-C p-bonds result from overlap of one non-hybridized p-orbitals from each C

Delocalization of e- in p-bonds results in a “double-donut” shaped e-

cloud above and below the molecular carbon plane.

RESONANCE THEORY

• Resonance theory states that if more than one resonance form can be drawn for a molecule, then the actual structure is somewhere in between them.

• Furthermore, the actual energy of the molecule is lower than might be expected for any of the contributing structures.

• If a molecule has equivalent resonance structures it is much more stable than either canonical would be – hence the extra stability of benzene (called resonance energy).

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Resonance in benzene

Each resonance form contribute 39% and 7.3 % respectively to the actual molecule of benzeneEach C-C bond is not half way between a single and a double bond but less.Energy of actual molecule lessDifference in energy between actual molecule and the Lewis structure of lowest energy is called resonance energy

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Rules of resonance All canonical forms must be bonafide Lewis

structures Positions of the nuclei must be same in all the

structures All atoms taking part in the resonance must lie in a

plane All canonical forms must have same number of

unpaired electrons The energy of actual molecule lower than any other

form All canonical forms donot contribute equally to true

molecule.each form contributes in proportion to its stability

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Resonance effect

Mesomeric effect•Permanent effect in which the π electrons are transferred from a multiple bond to a single covalent bond•Decrease in electron density•+M and – M effect•+M when transference of electron pair is away from the atom•– M when towards the atom

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– M effect

Eg: -NO 2 -CHO,-SO3H etc

–R or –M effect

C C CH O — C — C CH — O–

(–R effect of –CHO group)

+

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+M effect

Egs: halogens,-OH, -NH 2, -NR 2 etc

C C OH — C — C–

O H

(+M or +R effect of –OH group)

+

+R or +M effect

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Hyper conjugation

(a) Involves and bond orbitalsσ π

(b) σ- π conjugation(c) More the number of hyper conjugative structures, more will be the stability of ion or molecule

H – C – C

H H

H H

+H – C = C – H

H

H H

+

H C = C

H H

H H

+H H

H H+

H – C = C

Structure of ethyl carbonium ion

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(d) The number of hyper conjugative structures in an alkene is obtained by the number of C — H bonds attached to the carbon bonded directly to the double bonded carbon atoms.

CH CH

H

H

CH2CH CH

H

H

CH2

–

+CH CH

H

H

CH2

–+

CH CH

H

H

CH2

–

+

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Cross conjugation

• Three groups present, two of which are not conjugated with each other although each is conjugated with the third

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aromaticity

• Benzene and other organic compounds which resembles benzene in certain characteristic properties are called aromatic compounds.

• These characteristic properties constitute what is commonly known as aromatic character or aromaticity

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Structure of benzene

• Molecular formula:C 6H 6

• Unsaturated nature: since saturated compounds having 6 carbon atoms,benzene is expected to be an unsaturated hydrocarbon which is indicated by

• 1.it adds 6 chlorine atoms in the presence of sunlight

• 2.it forms a triozonide,C 6 H 6(O3 )3

• 3.It may be catalytically hydrogenated to cyclohexane by taking 3 molecules of hydrogen

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• Saturated behavior:benzene does not give the following characteristic reactions of the unsaturated compounds

1.does not decolorise potassium permanganate solution

2.does not decolorise bromine water in the dark

3.does not add halogen acids

•Special status to benzene

Benzene is an unsaturated compound containing 3 double bonds but yet behaves like a saturated compound

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• Open chain structure discarded :benzene forms only monosubstituted derivative ,i.e all the 6 hydrogen atom of benzene are equivalent.

• Cyclic structure to benzene : benzene with hydrogen under pressure in presence of raney nickel at 2000 gives cyclohexane(cyclic compound)

• kekule’s structure: Kekulé (1865) conceived a cyclic structure , but this would imply

alternating single and double bonds

(C-C = 1.47Å, C=C = 1.34Å).

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MO THEORY

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• Each MO can accommodate 2 electrons, so for benzene we see all electrons are paired and occupy low energy MO’s (bonding MO’s). All bonding MO’s are filled. Benzene is therefore said to have a closed bonding shell of delocalised p electrons and this accounts in part for the stability of benzene

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Hückel’s Rule77

Annulenes(cyclopolyenes).

• Monocyclic compounds with alternating single and double bonds are termed Annulenes. :

• benzene is [6] annulene • COT is [8] annulene

• Remember Hückel’s rule predicts that annulenes will be aromatic if i) they have (4n + 2) p electronsii) they have a planar C skeleton

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• [14] annulene [16] annulene

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The definitions:-If, on ring closure, the p electron energy of an open

chain polyene (alternating single and double bonds) decreases the molecule is classified as aromatic.

If, on ring closure, the p electron energy increases, the molecule is classified as antiaromatic.

If, on ring closure, the p electron energy remains the same the molecule is classified as non-aromatic e.g. COT (just a polyene).

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antiaromaticity

• Planar cyclic conjugated species less stable than corresponding acyclic unsaturated species are called antiaromatic.

• Cyclic compounds which have 4n π electrons are called antiaromatic compounds.

• This characteristic is known as anti aromaticity.

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Aromaticity and nuclear magnetic resonanceIn addition to high degree of stability and their

tendency to participate in substitution rather than addition reactions, aromatic compounds have unique NMR spectra.

NMR has been applied successfully for determining whether a compound has closed ring of electrons or not.

A compound having closed loop of electrons can sustain an induced ring current and hence it will be aromatic in nature

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• When an external magnetic field is imposed upon an aromatic ring ,the closed loop of π electrons begins to circulate in a plane at right angles to the direction of the applied field.

• This electron circulation generates an induced magnetic field tries to ‘oppose’ (‘neutralise’) applied filed B0. But (since magnetic lines of force are continuous) at the position of the protons of benzene the applied field is reinforced by the field produced by the circulation of p electrons.

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• Thus the proton lying in the former region are shielded while those lying in the latter region are deshielded.

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Aromatic Ions• Cyclopentadiene is unusually acidic (pKa 16)• has a sextet of 6π – electrons ,meets Huckels rule• Has high resonance energy• Confirmed by the isolation and thus stability of its

salts

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cycloheptatriene• pKa is 36

• Loss of HYDRIDE is unusually easy, however, because it leads to an aromatic cation – tropylium ion.

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• The tropylium cation is planar and the C – C distance is 1.40 A

CH+

CH+

CH+

CH+

CH+

CH+

CH+

+

canonical structures of tropylium cation

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Tropylium cationHas 6 π electrons – show aromaticity on the basis

of Huckel’s ruleThe positive charge is uniformily distributed among

the 7 carbon atom of the ionPropertiesAre high melting solids Can readily reducedVery easily alkylated at room temperatureRearranged to benzaldehyde by oxidizing agents

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Benzenoid Aromatic Compounds• benzene exhibits unusual stability compared to

“cyclohexatriene” structure

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• Also

Difference (357 – 207 = 150 kJ/mol) is called the “Resonance Energy” of benzene

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• Benzenoid Compounds (fused benzene rings) have similar “aromatic” properties to benzenee.g.

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non-benzenoid aromatic compound

• An interesting non-benzenoid aromatic compound is Azulene, which has large resonance energy and a large dipole moment.

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Heterocyclic Aromatic Compounds

• many compounds we find in nature are cyclic compounds with an element other than carbon in the ring. These are called Heterocyclic compounds. Further, some are aromatic compounds - can be termed heteroaromatic.

• The degree of aromaticity (extra stability) may vary as the heteroatom changes.

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• In electronic terms pyridine is related to benzene

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• Pyrrole has electrons arranged differently – related to the cyclopentadienyl anion.

• Similar electronic configurations for furan and thiophene

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Theoretical criteria for aromaticity

• 1.It must have cyclic clouds of delocalised π –electrones above and below the plane of the molecule.

• 2. The π –clouds must contain a total of (4n+2) π – electrones,where n is an integer i.e its value may be 0,1,2,3……..This rule is known as Huckles rule .

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Aromatic characters

• Thermal stability• Electrophilic substitution rather than addition

reaction• Cyclic flat molecules• Resistance to oxidation• Unique nuclear resonance spectra

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3 membered carbocyclic compounds

• By Huckel’s rule,the simplest aromatic system (n=0)shoud contain only 2 π electrons – cyclopropenyl cation

• Cyclopropenyl cation may represented as a resonance hybrid of the following 3 structures

C+

H

HH

C+

H

HH

C+

H

H H

canonical structures of cyclopropenyl cation

+

resonance hybrid

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5 membered carbocyclic compounds

Cyclopentadiene is a typical diene and lacks aromatic charecterestics by its small resonance energy(3 kcal/mole) .

But ,if one of the atoms constituting the cyclopentadiene has an unshared pair of electrones,the system can also have aromatic sextet- thus shows aromaticity.

It is evidenced from the following pointshas a sextet of 6π – electrons ,meets Huckels ruleHas high resonance energyConfirmed by the isolation and thus stability of its salts

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Ferrocene(dicyclopentadienyl iron)

Is a metallocene,formed between cyclopentadienyl anion and transition metals like cobalt ,nickel,ruthenium,osmium,titaniumand vanadium.

PropertiesOrganic solid,m p.172Cyclopentadienyl anion are equidistant

from ferrous anion i.e 3.4 A0 .• zero dipole moment• C-H stretching band at 2075cm

-

-

..

.

.

.

.

..

.

...

Fe++

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• C-C bond length 1.41 A• No restricted rotation• There are 12 π electrons ,iron is in zero valence state• It is a stable compound ,when heated to 470 degree –

remaines unchanged • Resist the attack of acids and bases• Resistant to hydrogenation and refusal to add maleic

anhydride ,undergo electrophilic substitution reaction .halgenation and nitration are not possible owing to the ease of oxidation

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References

• Advanced organic chemistry-Jerry March• Morrison and Boyd Organic Chemistry• Organic chemistry - Ingold• Organic chemistry- O.P Agrawal• Stereo chemistry and the chemistry of natural products

–I . L. Finar

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