Which alkyl halide is least reactive with magnesium metal? A. CH_3 CH_2 CH_2 Br
B. CH_3 CH_2 CH_2 F
C. CH_3 CH_2 CH_2 I
D. CH_3 CH_2 CH_2 CI

B. CH_3 CH_2 CH_2 F

Explanation:

The alkyl group contains all univalent group derived from alkanes by the loss of a hydrogen atom. When an alkyl group combines or is bonded with an halogen  family (such as Flourine, Chlorine , Bromine , Iodine) , we have an alkyl halide such as the options given in the question.

Alkyl  halide bond with an sp³ Carbon. Depending on where the halide is bonded to , we can have the primary halide, secondary halide and the tertiary halide.

When alkyl halide react with magnesium metal, we have a reaction that is known as Grignard Reaction. The order of reactivity of alkyl halide with grignard reagents is :

F < Cl < Br < I .

This is because of the nature of the electronegativity and the bond strength.

As we move than the group from the top to the bottom, the electronegativity decreases and the bond strength increase, so as the size. Thus, this is exactly what the reactivity of alkyl halide with Grignard reaction is established for.

Thus since the fluorine have the highest electronegativity and the smallest bond size in the halogen family , then it will be the least reactive alkyl halide in reacting with magnesium metal.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.