Question

Reactivity order of halides for dehydrohalogenation is:

Answer 1

Hint : We know that dehydrogenation or dehydration describes the removal of hydrogen atoms whereas; dehalogenation describes the removal of halogens. Therefore, as the name suggests, dehydrohalogenation suggests the removal of hydrogen and a halogen group.

Complete Step By Step Answer:
Let us describe the dehydrohalogenation in detail; Dehydrohalogenation reactionIt is in general, the synthesis reaction of alkene (or alkyne) from alkyl halides; by the elimination of HX (H represents hydrogen and X represents halogen) from adjacent atoms. The alkyl halides or haloalkanes are usually the bromo- or iodo- or less commonly chloro- substituted alkanes. It is normally carried out in methanol with sodium methoxide, in ethanol with sodium ethoxide or in tert-butyl alcohol with potassium tertbutoxide; along with the presence of heat.
We know that alkyl halides are the chain of hydrocarbons in which hydrogen are replaced by halogens. The reactivity of the alkyl halide is decided by the ease with which the halide leaves the substrate. The electron pair from the nucleophile (nuc) attacks the substrate(R-LG) forming a new bond, while the leaving group (LG) departs with an electron pair.
Reactivity order of halides for dehydrohalogenation is $\,R-I>R-Br>R-Cl>R-F$. Alkyl iodides are most reactive and alkyl fluorides are least reactive. Iodide ion is a good leaving group, as the negative charge is on a larger atom and is dispersed. Also, $C-I~$ bond is weaker than $C-F~$ bond.

Note :
Remember that this reaction would be more precise if we call it a beta-dehydrohalogenation reaction as the hydrogen is being removed from the beta carbon i.e. the adjacent carbon to the carbon, halogen is attached. Also, if more than one alkene is produced during the reaction then, more substituted alkene is the major product of the reaction.