If electron-withdrawing groups (generally with multiple bonds) are attached to the conjugate system, `pi`-electron displacement takes place towards such groups (but away from conjugate system). This is said to have `-M` effect. See fig.1.

`*` Benzyl charbonium ion is resonance stabilised. See fig.2.

`*` Greater the number of `C-H` bonds at `alpha`-carbon to the unsaturated system, greater wiII be the electron-release and thus greater the hyperconjugation effect. See fig.3.

`*` Any primary radical (like the radial) has hyperconjugative interaction, which is not possible in the simple methyl radical. Thus ethyl radical is more stable than methyl radical. On the same line `2^(o)` alkyl radical is more stable than `1^(o)` alkyl radical, and `3^(o)` alkyl radial is more stable than `2^(o)` alkyl radical.

`3^(o) > 2^(o) > 1^(o) > CH_3`

Based on the various effects we have studied, everything being equal `HX` is a stronger acid than `HY` if

(i) `X` is more electronegative atom than `Y(HF > H_2O > NH_3 > CH_4)`

(ii) The `H-X` bond is weaker than the `H- Y` bond `(HI > HBr > HCl > HF)`

(iii) `X` is a group bearing more electronegative atoms closer to the site of negative charge in the conjugate base `X^(-)` than in `Y^(-)` `(Cl_3C CO_2H > Cl_2CHCO_2H > ClCH_2CO_2H > CH_3CO_2H)`

(iv) `X^(-)` is less sterically blocked from solvation than `Y^(-)` `(MeOH > EtOH > i-PrOH > t-BuOH)`

(v) `X^(-)` has greater fractional `s-`character than `Y^(-) (RCequivCH > R_2C = RH > R_3C-CR_2H)`

(vi) The negative charge in `X^(-)` can be delocalised over a large number of atoms than `Y^(-)` (`CH_2=CH- CH=CH-CH_3 > CH_2 = CH-CH_3 > CH_3CH_2CH_3`) or `CH_3COCH_2COCH_3 > CH_3COCH_3 > CH_3CH_2CH_3)`

(vii) The negative charge in `X^(-)` is more stabilised by aromaticity than in `Y^-` (cyclopentandiene > `1,3-`pentadiene > cycloheptatriene)

(viii) The negative charge in `X^(-)` can be delocalised on to a more-electronegative atom than in `Y^(-) (CH_3COCH_3 > CH_3CH=CHCH_3 > CH_3CH_2CH_3` or `CH_3CHO > CH_3CO_ 2CH_3 > CH_3CON` `(CH_3)_2)`.

A group of atom is said to have `+M` effect when the direction of electron-displacement is away from it. Such groups have lone pair of electrons, and release the pair conjugation with an attached unsaturated (conjugated) system. Examples executing `+M` effect are : See fig.1.

`=>` This effect extends the degree of delocalisation and imparts stability to the molecule.

`=>` The `+M` effect of halogen atom in vinyl halide and aryl halide explains their low reactivity. See fig.2.

`=>` Lone pair on `N` in aniline gets delocalised and thus basic nature of aniline is less than `NH_3`. See fig.3.

`=>` Phenoxide ion gets stabilized by resonance. See fig.4.

a) No real existence : Resonance structures exist only on paper. Resonance structures are useful because they allow us to describe molecules, radicals and ions for which a single Lewis structure is inadequate. We write two or more lewis structures, calling them resonance structures of resonance contributors. We connect these structures by double headed arrows ( `lleftrightarrow`) and we say that the real molecule, radical, or ion is a hybrid of all of them.

b) `text(In writing resonance structures we are only allowed to move electrons)` :

Position of nuclei of the atoms must remain same in all resonance structures e.g. See fig.1.

c) All of the structures must be proper lewis structures. See fig.2.

d) Charge separation should be low since to separate charge energy is required, therefore structure in which opposite charges are separated have greater energy and hence less stable. See fig.3.

e) All the atoms taking part in the resonance i.e., covered by delocalized electrons, must lie in a plane or nearby so, the reason for planarity is maximum overlap of the `p`-orbitals.

f) All resonating structure must have the same number of unpaired electrons.

The difference in energy between the hybrid and the most stable cannonical structure is called as Resonance energy. See fig.

-It is delocalisation of sigma electron.

-Also known as sigma-pi-conjugation or no bond resonance.

`text(Occurrence)` :

Alkene, alkynes

Free radicals (saturated type) carbonium ions (saturated type)

`text(Condition :)`

Presence of a `-H` with respect to double bond, triple bond carbon containing positive charge (in carbonium ion) or unpaired electron (in free radicals)

`text(Example :)` See fig.1.

`text(Note :)` Number of hyperconjugative structures = number of `alpha`-Hydrogen. Hence, in above examples structures i, ii, iii, iv are hyperconjugate structures (H-structures).

Hyperconjugation is a permanent effect.

`text(Effects of hyperconjugation :)`

i) `text(Bond Length :)`

Like resonance, hyperconjugation also affects bond lengths because during the process the single bond in compound acquires some double bond character and vice-versa. Eg. `C-C` bond length in propene is `1.488 A^o` as compared to `1.334 A^o` in ethylene. See fig.2.

ii) `text(Dipole moment :)`

Since hyperconjugation causes these development of charges, it also affects the dipole moment of the molecule..

iii) `text(Stability of carbonium Ions :)`

The order of stability of carbonium ions is as follows :

Tertiary > Secondary > Primary.

Above order of stability can be explained by hyperconjugation. In general the number of hydrogen atoms attached to a carbon atoms, the more hyperconjugtaion forms can be written and thus greater will be the stability of carbonium ions. See fig.3.

iv) `text(Stability of Free radicals :)`

Stability of Free radicals can also be explained as that of carbonium ion.

`(CH_2)overset(.)(C) > (CH_2)_2overset(.)CH > CH_2overset(.)CH_2 > overset(.)CH_2`

v) `text(Orientation influence of methyl group :)`

The o, p-directing influence of the methyl group in methyl benzenes is attributed partly to inductive and party of hyperconjugation effect. See fig.4.

The role of hyperconjugation in o,p-directing influence of methyl group is evidenced by the part that nitration of p-iso propyl toluene and p-tert-butyl toluene form the product in which `-NO_2` group is introduced in the ortho position with respect to methyl group and not to isopropyl or t-butyl group although the latter groups are more electron donating than Methyl groups. See fig.5.

i.e., The substitution takes place contrary to inductive effect. Actually this constitutes an example where hyperconjugation overpowers inductive effect.