Why Hydride Shift Did Not Occur in This Substitution Reaction Explained

Why Didn’t a Hydride Shift Occur in This Substitution Reaction?

A hydride shift did not occur in this substitution reaction because the mechanism proceeds via SN2, which is a concerted process that does not involve carbocation intermediates or rearrangements. Hydride shifts require a carbocation intermediate, which only forms in SN1 or E1 reactions. The presence of a strong nucleophile and a primary substrate ensures that the reaction follows the SN2 pathway instead, precluding hydride shifts.

SN2 Mechanism and Carbocation Absence

The key aspect is that SN2 reactions occur in a single concerted step. This means the nucleophile attacks the electrophilic carbon at the same time the leaving group departs. Because the pathway does not involve a carbocation intermediate at any stage, hydride shifts cannot take place. These shifts require the formation of a carbocation, which rearranges to form a more stable carbocation by moving a hydride ion. Since no carbocation forms, no rearrangement is possible.

Role of Strong Nucleophile and Primary Substrate

• The nucleophile ethoxide ion (EtO−) is strong.
• The substrate is a primary alkyl halide, which is less prone to carbocation formation.
• Together, these factors favor SN2 over SN1 since primary carbocations are highly unstable.
• The strong nucleophile reacts rapidly, leaving no time for carbocation formation.

Hydride Shifts Are Exclusive to Carbocation Intermediates

Hydride shifts occur only when a carbocation intermediate forms and can rearrange to a more stable carbocation.

In SN1 and E1 mechanisms, the carbocation may undergo such rearrangements to minimize energy. Since the current reaction does not form a carbocation, no hydride shift is needed or possible.

Concerted Reaction Precludes Rearrangements

Since SN2 occurs through a backside attack displacing the leaving group in one step, the nucleophile directly substitutes the leaving group without intermediate species.

This concerted mechanism excludes rearrangements such as hydride shifts.

Additional Considerations

• The substrate’s primary nature and strong nucleophile dominate the mechanism choice.
• While branching or steric effects can influence the reaction path, in this case, the SN2 pathway and absence of carbocation prevent hydride shifts.

Key Takeaways

• Hydride shifts require carbocation intermediates, which form only in SN1/E1 mechanisms.
• SN2 reactions proceed concertedly, forming no carbocation, so no hydride shifts occur.
• Strong nucleophiles and primary substrates favor SN2 and prevent carbocation formation.
• No intermediate rearrangement occurs; substitution happens directly at the electrophilic center.