What occurs during the backside attack of a nucleophile on a haloalkane?

During the backside attack of a nucleophile on a haloalkane, a crucial mechanism occurs that involves the nucleophile approaching the carbon atom from the opposite side of the leaving halogen atom. This attack leads to the formation of a transition state where the carbon-halogen bond is partially broken while a new bond between the nucleophile and the carbon is partially formed.

This scenario aligns with option B, which accurately describes that the nucleophile begins to form a covalent bond with the carbon atom while the carbon-halogen bond is still in the process of breaking. This specific mechanism is characteristic of the S_N2 reaction (substitution nucleophilic bimolecular), where the concerted nature of the reaction requires that both bonds (to the halogen and the nucleophile) exist in a transitional state simultaneously.

Other choices do not reflect the precise nature of the S_N2 mechanism. For instance, a statement about the bond being completely broken before a new bond forms would not capture the synchronous nature of bond formation and breaking necessary in this reaction type. Similarly, the notion that the nucleophile could be repelled would conflict with the intrinsic nature of nucleophilic attacks. Considering these details, option B provides the most accurate representation of the backside attack phenomenon in halo