Question

Phynyl group is not an electron donating group but C6H5CH=CH2 reacts very fast with HCL.

Answer 1

.This is explained according to following effects (opposing each other) based on the molecule these groups are attached with (e.g: CH3Cl, CH3-CH=CH2 etc).

Inductive Effect:

It is a sort of permanent effect operating in polar covalent bonds. The induction of a permanent dipole in a covalent bond between two unlike atoms of different electronegativies is called the inductive effect.

Explanation: When a carbon is bonded to an atom of different electronegativity compare to carbon then charges (fractional/partial) will be induced on both.

For your case, lets say there is a bond formed between carbon and halogen. As halogen is more electronegative, so Carbon will get a partial positve charge and halogen, a partial negative charge.

On the other hand we also know the bond formed between two neutral carbon atoms has no dipole moment and the electrons are symmetrically present between the two nuclei because both atoms have the same tendency to attract the shared pair of electrons. In other words they both are neutral.

But what happens when one of them gets a charge more than the other? Suddenly the electronic cloud will get disturbed and the bond will tend to move towards stabilisation. This stabilisation is brought about by creating a dipole moment between the two atoms. So the electron pair will move towards the atom which is electron deficient (α-carbon in this case) to stabilise it, thus leaving the other atom (β-carbon in this case) with a partial positive charge. This partial positive charge is stabilised by the other atom (γ-carbon, if present) attached to the new electron deficient atom (β-carbon). Thus in this way the displacement of electrons (due to positive inductive effect here) shifts from one atom to the other in chain.

(Example in the pic)

Note: The inductive effect of an atom or a group of atoms diminishes rapidly with distance. In fact, the inductive effect is almost negligible beyond two carbon atoms from the active atom or group.

So according to Inductive effect, the order of electron donation (positive I-effect) for given groups should be:

methyl < ethyl < phenyl

Hyperconjugation:

When the alkyl group is attached to an unsaturated system such as -CH=CH2 group, the order of the inductive effect simply gets reversed. This anamoly is explained on the basis of “hyperconjugation”.

It may be regarded as a sigma-pi orbital overlap, analogous to the pi-pi orbital overlap.

Explanation: In hyperconjugation, the sigma C-H bond on the α-carbon is delocalized with the empty p-orbital of a C=C or a carbocation. The resulting H+ does not change its position.

[Remember! This is a hypothetical form which contributes to the actual structure of the molecule. The only effect this delocalization has is that in the resonance hybrid (actual molecule) the C-H bond under consideration will be weaker than a normal C-H bond].

Now as ethyl group (-CH2-CH3) has only two hydrogen atoms attached to the α-carbon atom directly linked to the double bonded carbon atom and methyl (-CH3) has three, so the hyperconjugation in ethyl will be lesser than methyl resulting in more electron donating power of methyl than ethyl.