Which halogen best delocalises electron density to benzene ring ?

 The smaller the size of halogen atom, smaller will be C-X bond. Also shorter the C- X bond, more effective wiII be overlap of p-orbitals, and more extensive will be delocalisallon. Hence fluorine effectively delocalised it's lone pairs electron with benzene ring due to smallest and most electronegativity.

Therefore order of best delocalisation of electron density to benzene ring should be.

                                  “   F > Cl > Br > I “

+M effect of fluorine is dominated over its -I effect while -I effect of other halogens is dominated over its + M effect.

More related Questions:

Penta-1,3-diene is more stable than penta-1,4 -diene. Why?

Phenoxide ion is more stable than an alkaoxide. why?

Why does aqueous sodium bicarbonate solution dissolve carboxylic acids but not phenol though they are also acidic ?

Why enol form of Beta Dicarbonyl compound more stable than those of monocarbonyl compounds?

Why enol form of phenol is more stable than its keto isomers?

Reactivity order of Pyrrole, Furan and Thiophene towards Electrophilic substitution :

Phenyl group is known to extract negative inductive effect, but each phenyl ring in biphenyl is more reactive than benzene towards Electrophilic substation. Why?

Chloroform is more acidic than fluoroform why?

Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ? 

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?

Why is the Tropylium carbocation less stable than the tricyclopropylmethyl carbocation?

Arrange the following groups in order of their increasing tendency for nucleophilic addition reactions -COO-, -COOH, -COOCH3, -COCH3, -CHO, -COCI, -CONH2. Justify your answer.

Oxymercuration demercuration hydration procedure is superior to acid- catalysed hydration of most alkenes. Why ?

Out of Acet.yl chloride and acetic anhydride, acetic anhydride is preferred for acylation reactions. Explain why?

Only aldehydes, methyl ketones and cyclic ketones react wilh sodium bisulphite.explain why?

Carbonyl compounds do not undergo nucleophillc substitution reactions.why?