Geometry is tetrahedron.

All the valencies of Carbon are satisfied by -bonds Saturated Carbon.

Rotation about C C is free Conformational Isomers.

Saw-Horse projections

Eclipsed

Staggered

Newman Projections

3 kcal (torsional energy)

Potential Energy Changes during rotation

Pote

ntial Energ

y

Rotation

3 kcal

Me Me

H

H H

H

Me Me

H

H H

H

Eclipsed

Me

Me

H

H

H

H

Gauche

Me

Me H

H H

H

Partially Eclipsed

H

Me

H

H

Me

H

Staggered

Potential Energy Changes during rotation Pote

ntial Energ

y

Rotation

H

Me

H

H

Me

H

3.4 kcal

4.4-6.1 kcal

Different arrangements of atoms that can be converted into one another by rotation

about single bonds are called conformations.

Conformers cannot be isolated

Always inter-converted. Positions not frozen.

sp3 carbon is least electronegative of all other hybrid carbon atoms.

Geometry is Triangular Planar.

All the valencies of Carbon are not satisfied by -bonds Unsaturated Carbon.

Rotation about C=C or C-C (in a ring) is restricted geometrical isomers are possible.

CC

Me

H

Me

H

CC

H

Me

Me

H

H

Me

H

Me

H

Me

Me

H

Different arrangements of atoms in space arising out of hindered rotation about C-C

or C=C are called geometrical isomers.

sp2 carbon has higher electronegativity than sp3 carbon.

Geometry is Linear.

All the valencies of Carbon are not satisfied by -bonds Unsaturated Carbon.

Has highest electronegativity among carbon atoms of various hybridizations.

Polarization of - bonded electrons towards the atom of higher electronegativity.

The standard of reference is H atom.

Pushing electrons away is +I effect while pulling electrons towards is –I effect.

Lets check the inductive effect of CH3

H X + -

C X H

H

H

-

-

CH3 has +I effect

Lets replace one H of CH3 with another CH3 and then check.

C X H

H

H

-

-

CH2 CH3 X

-

Increasing C chain

increases +I effect

but the rate of

increase decreases.

Lets replace one H of CH3 with another CH3 and then one H of CH2 with a CH3.

CH2 CH2 X

- CH3

CH X -

CH3

CH3

I effect decreases

with distance.

Lets replace the H with a sp2 carbon.

H X + -

H2CCH

sp2 carbon has –I effect. So does sp carbon.

Inductive effect is a particle effect. This means it is the manifestation of the particle nature

of electrons.

Inductive effect dissipates with distance.

Inductive effect is a permanent effect.

Inductive effect is a weak effect.

Order of decreasing inductive effects:

Inductive effect is an ‘always’ effect.

+I groups: O- > CO-2 > CR3 > CHR2 > CH2R > D

-I groups: NR3+ > SR2

+ > NH3+ > NO2 > SO2R > CN > CO2H > F > Cl > Br > I >

OAr > COOR > OR > COR > SH > OH > C CR > Ar > CH=CR2

Delocalization of electrons.

It arises out of the wave nature of electrons.

Since wave nature cannot be drawn, we represent canonical forms which exhibit particle

nature and whose hybrid the actual structure can be assumed to be.

The actual structure of the molecule/ion is the weighted mean of the canonical or parent

structures. But the energy of the molecule is lower than all the canonical forms.

While drawing the structures of canonical forms positions of atoms are to remain fixed

while that of electrons changes.

Molecules showing resonance must be conjugated and the orbitals appropriately oriented.

Resonance effect is also called Mesomeric Effect.

Resonance effect is a permanent effect.

Resonance effect is a strong effect, generally stronger than Inductive Effect.

Resonance is a relative effect. It is a need based effect.

Basic movement of electrons:

ABC +AB-C

ABC +ABC-

A : A+ :

A : +A

A -A

A -A

AB : +AB-

1. Double bond conjugated with +ve charge:

+CCC

+CH2CHCHCHCH2 CH2CHCHCHCH2 +

CH2CHCHCHCH2 +

I II III

Stability order of resonating structures having +1 charge:

1. Of all such resonating structures the one in which octet of all atoms is complete is most stable and

therefore most contributing.

2. Next is that resonating structure in which +ve charge is on the most electropositive element. And then in

that order.

3. Resonating structures which are similar have equal stability.

4. Among resonating structures in which charge is on similar atoms but the structures are different, the

stability is decided by inductive effects.

I = III > II

2. Double bond conjugated with double bond:

CCCC

CH2CHCHCHCHCH2 CH2CHCHCHCH+CH2- CH2CHCH+CHCHCH2

-

+CH2CHCHCHCHCH2-

II III

IV

I

Stability order of resonating structures having net charge zero:

1. Of all such resonating structures the one in which no atom has any charge is most stable.

2. Next is that resonating structure in which charge separation is the least and then in that order.

I > II > III > IV

Evidences of resonance:

1. All bonds are between a single and a double bond.

2. On treating the molecule with Br2 in CCl4 we get 4 products, 1,2; 3,4; 1,4 and 1,6.

CH2CHCHCHCHCH2

Br Br

CH2CHCHCHCHCH2

Br Br 1,2 product 1,4 product

CH2CHCHCHCHCH2

Br Br

3,4 product

CH2CHCHCHCHCH2

Br Br 1,6 product

1,6 product > 1,2 product > 1,4 product > 3,4 product

1,6 product is most stable because the double bonds are conjugated and non-terminal. 1,2

is next since double bonds are conjugated but one of them is terminal. 1,4 is next as one of

the double bonds is non-terminal, while 3,4 is the least as both the bonds are terminal.

3. Double bond conjugated with –ve charge:

-CCC

-OCHCHCHCH2 I

OCHCHCHCH2 -

II

OCHCHCHCH2 -

III

Stability order of resonating structures having -1 charge:

1. Of all such resonating structures the one in which the –ve charge is on the most electronegative atom is

most stable and then in that order.

2. Resonating structures which are similar have equal stability.

3. Among resonating structures in which charge is on similar atoms but the structures are different, the

stability is decided by inductive effects.

I > II > III

4. Double bond conjugated with lone pair:

:CCC

H2NCHCHCHCH2

I

H2NCHCHCHCH2

-

II

+

H2NCHCHCHCH2

+ -

III

Stability order of resonating structures having net charge zero:

1. Of all such resonating structures the one in which no atom has any charge is most stable.

2. Next is that resonating structure in which charge separation is the least and then in that order.

I > II > III

5. Double bond conjugated with unpaired electron:

CCC

CH2CHCHCHO

I CH2CHCHCHO

II CH2CHCHCHO

III

Stability order of resonating structures having no charge and having 1 unpaired electron:

1. Of all such resonating structures the one in which the unpaired electron is on the least electronegative

atom is most stable and the in that order.

2. Resonating structures which are similar have equal stability.

3. Among resonating structures in which the unpaired electron is on similar atoms but the structures are

different, the stability is decided by inductive effects.

I > II > III

6. Lone pair conjugated with +ve charge:

:NC+

H2NCHCHCH2

+

I

H2NCHCHCH2

+

II

H2NCHCHCH2

+

III

Stability order of resonating structures having +1 charge:

1. Of all such resonating structures the one in which octet of all atoms is complete is most stable and

therefore most contributing.

2. Next is that resonating structure in which +ve charge is on the most electropositive element. And then in

that order.

3. Resonating structures which are similar have equal stability.

4. Among resonating structures in which charge is on similar atoms but the structures are different, the

stability is decided by inductive effects.

III > I > II

7. Lone pair conjugated with unpaired electron:

:NC :NH2CH2 NH2CH2

+ -

I II

Stability order of resonating structures having no charge and having 1 unpaired electron:

1. Of all such resonating structures the one in which the unpaired electron is on the least electronegative

atom is most stable and the in that order.

2. Resonating structures which are similar have equal stability.

3. Among resonating structures in which the unpaired electron is on similar atoms but the structures are

different, the stability is decided by inductive effects.

I > II

The conjugation need not be necessarily via a single bond. It can be via a double bond as

well. CH2=C=C=CH2

Wherever we have studied that a double bond is conjugated with something, the double

bond could also be a triple bond.

Molecules that satisfy the following conditions are said to be aromatic. Aromatic molecules

have extra stability compared to non-aromatic or anti-aromatic molecules.

1. The molecule should be cyclic.

2. It must have (4n+2) electrons in the periphery of the molecule.

3. The molecule must show cyclic delocalization.

4. All the atoms must be sp2 hybridized in at least one resonating structure.

In order to check whether a molecule is aromatic or not, we would first learn how to make a

molecule aromatic.

To do this we would first make an empty ring structure aromatic.

Lets start with the smallest ring structure, a 3 member ring.

We’ll first add one electron to each of the peripheral atoms.

We would add / remove electrons if required in order to make the

electrons equal to (4n+2). In the present case the closest value of

(4n+2) to 3 is 2. So we remove one electron from any of the atom.

If we remove then we put a +ve charge, if we add then we

put a –ve charge.

+

Now we join the other electrons by bonds.

The structure made this way will be aromatic. The first 2 conditions are anyway met. The

last condition is also met. Lets check the 3rd condition by drawing all the resonating

structures of the compound.

To draw the resonating structures in a cyclic molecule, we first choose a direction of the

arrow (clockwise or anticlockwise) and maintain this direction for moving electrons in all

resonating structures. Next we choose whether we put the arrow inside or outside the ring. +

+ +

This ion is called Troponium ion.

We can see that all bonds have become double once, each atom has had the positive

charge once. Therefore the molecule shows cyclic delocalization.

The closest value of (4n+2) to 4 is either 2 or 6. Since we have already

seen the removal of electrons, lets add electrons this time.

We add or remove from adjacent atoms because we need to join the

leftover electrons.

- -

Lets draw all the resonating structures and then check.

- -

-

- -

-

-

-

-

-

-

-

-

Benzene Naphthalene Anthracene

Phenanthrene

-

-

+ +

+

Pyrene

Now let us find out how to assess whether a given molecule or ion is aromatic or not.

Count the no. of atoms that make up the ring and draw another ring of the same no.

using only carbon atoms.

Now make this ring aromatic as done before.

If all that is present in your structure is present in the given structure then it is aromatic

otherwise not. Remember a negative charge is equivalent to a lone pair.

N

H

Pyrrole

N

N

H Imidazole

N

Pyridine

N

N

N

N

Pyrimidine

N N

Pyridazine

+ -

Pyrazine

+

CCHCH

H

H

H

H

CCHCH

CH3

CH3

CH3

H

<

Resonance involving single bonds.

Different as compared to resonance as it involves -bonds (generally CH bond).

Also known as no bond resonance.

Hyperconjugation is a permanent effect.

Weaker than Resonance but stronger than Inductive Effect.

1. Single bond conjugated with double bond:

CCHCH

H

H

H

H

CCHCH

H

H

H

H

-

+

CCHCH

H

H

H

H

- + CCHCH

H

H

H

H

-

+

C CH H

H +

+

+

H

CH2 -

2. Single bond conjugated with +ve charge:

CCH2

H

H

H +

CCH2

H

H

H

+

CCH2

H

H

H +

CCH2

H

H

H

+

3. Single bond conjugated with unpaired electron:

CCH2

H

H

H

CCH2

H

H

H

CCH2

H

H

H

CCH2

H

H

H

4. Single bond conjugated with -ve charge

CCH2

Cl

Cl

Cl

CCH2

Cl

Cl

Cl

-

CCH2

Cl

Cl

Cl -

CCH2

Cl

Cl

Cl -

A sudden effect (temporary) due to the presence of other molecules or groups in the same

molecule.

Causes permanent change in the nature of the molecule.

Responsible for reactions between a neutral molecule and charged ion or between two

neutral molecules.

The movement of electrons due to the presence of another molecule or ion leading to the

reaction is electromeric effect.

CH2 CH2

E

B

-

+

+