1. Resonance occurs when a molecule can be represented by two or more structures that differ only in electron arrangements, not atom positions. 2. The actual structure of molecules like benzene and acetate ion are resonance hybrids that are intermediate between hypothetical resonance structures. 3. Resonance hybrids are more stable than any single contributing resonance structure. The extra stability is called the resonance energy.

1. RESONANCE

3. Displacement, shifting, movement of π electrons with in a compound.
• A number of organic compounds cannot be accurately represented by one
structure.
• For example

4. • This structure has three carbon-carbon single bonds and three carbon-
carbon double bonds. However, it has been determined experimentally that
all carbon-carbon bonds in benzene are identical and have the same bond
length(1.39 Å). furthermore, the carbon-carbon bond length of (1.39 Å) is
intermediate between the normal carbon-carbon double bond length(1.34 Å)
and the normal carbon-carbon single-bond length (1.54 Å). Actually two
alternative structures can be written.

5. These two structures differ only in the position of electrons. Neither 1 nor 2 is a
correct representation of benzene. The actual structure of benzene lies somewhere
between these two structures this phenomena in which two or more structures can be
written for a compound which involve identical positions of atoms is called resonance.

7. • The actual structure of the molecule Is said to be a resonance hybrid of
various possible alternative structures. The alternative structures are referred
to as the resonance structures or canonical forms.
• A double headed arrow (↔) between the resonance structures is used to
represent the resonance hybrid. thus in case of benzene (1) and (2) represent
the resonance structures. Actual structure of the molecule may be
represented as hybrid of these two resonance structures or by the single
structural formula.

8. • The resonance hybrid is more stable than any one of the various resonance
structures. The difference in energy between the hybrid and the most stable
resonance structure is known as the resonance energy. Resonance energy can be
determined by the difference between the calculated and experimental heats of
combustion(energy given off as heat when one mole of compound is burned) of
the compound.
• For example, it has been calculated that hypothetical structure1 and 2 would have a
heat of combustion of 797kcal/mol. The measured value for the heat of
combustion of benzene is 759 kcal/mol. Therefore,the resonance energy of
benzene is (797-759)kcal/mol or 38 kcal/mol. The benzene is said to be “stabilized”
by resonance energy of 38 kcal/mol.

9. • Another specie that is not correctly represented by single structure is acetate
ion. As in the case of benzene, acetate ion is a hybrid of two resonance
structures. Both carbon-oxygen bonds in the acetate ion are identical and
have the same bond length (1.26Å). The carbon oxygen bond length of
1.26Å is intermediate between the normal carbon-oxygen double-bond
length 1.26Å and the normal carbon-oxygen single bond length 1.43Å.

10. ≡

11. Rules governing resonance
• Resonance occurs whenever a molecule a molecule can be represented by two or
more structures differing only in the arrangements of electrons without shifting any
atoms. Resonance only involves the delocalization of electrons.
• Resonance structures are not actual structures for the molecule. They are
nonexistent and hypothetical.
• Resonance structures are interconvertible by one or a series of short electron-shifts.
• Resonance hybrid represents the actual structure of the molecule. The structure of
the resonance hybrid is intermediate between the various resonance structures and
is not a mixture of them.

12. • Resonance hybrid is represented by a double headed arrow (↔).this should
not be confused with two arrows (⇄) used to denote equilibrium between
two different compounds.
• Resonance hybrid is more stable than any of its contributing forms
(resonance structures).
• Resonance always increases the stability of a molecule and lessens its
reactivity.

13. Mesomeric Effect

14. Definition
• The mesomeric effect refers to the polarity produced in a molecule as a
result of interaction between two π bonds or a π bond and lone pair of
electrons. The effect is transmitted along a chain in a similar way are
inductive effects.
• It involves π electrons of double and triple bonds.

15. • The mesomeric effect is of great importance in conjugated compounds.in
such systems the π electrons get delocalized as a consequence of mesomeric
effect giving a number of resonance structures of molecule.
• Consider a carbonyl group. The oxygen atom is more electronegative than
the carbon atom.as a result the π electrons of the carbon oxygen double
bond get displaced towards the oxygen atom. This gives the following
resonance structures.

16. • The mesomeric effect is represented by a curved arrow. The head of the
arrow indicates the movement of a pair of π electrons. If the carbonyl group
is conjugated with a carbon-carbon double bond, the above polarization will
be transmitted further via the π electrons.

17. • The mesomeric effect like the inductive effect may be positive or negative.
Atoms which lose electrons towards a carbon atom are said to have a +M
Effect. Those atoms or groups which draw electrons away from a carbon
atom are said to have a –M Effect.

18. • Some common atoms or groups which cause +M or –M effects are listed
below.
• +M Effect groups
• Cl, Br, I, NH2, NR2, OH, OCH3
• -M Effect Groups
• NO2, CN, >C=O

19. • The +M effect of NH2

20. • The –M effect of the nitro group

21. • The mesomeric effect like the inductive effect results in a permanent state of
the molecule. It does not depend upon the presence of reagent.
• The inductive and mesomeric effects indicate the charge distribution in a
molecule. Thus they provide an effective way of determining the point of
attack of electrophiles and nucleophiles on the molecule.