Question

A hydride shift to a positively charged carbon is just another example of a Lewis base-Lewis acid reaction. Although it looks very different from the more obvious electrophile-nucleophile reactions we have studied, it really is quite similar. In these rearrangements, there is a very strong and strategically located electrophile and a weak, but nonetheless reactive enough nucleophile. Identify the nucleophile and electrophile in this kind of reaction.

Short answer

The nucleophile is the hydride ion (H-), and the electrophile is the positively charged carbon.

Step-by-step solution

- Understanding the Reaction

In a hydride shift, a hydrogen atom (with its pair of electrons, i.e., a hydride ion) moves from one carbon to another adjacent carbon, which is usually positively charged.

- Identify the Electrophile

The electrophile is the species that is deficient in electrons and seeks to obtain them. In this case, the positively charged carbon is the electrophile since it seeks electrons to fulfill its octet.

- Identify the Nucleophile

The nucleophile is the species that has a pair of electrons that it can donate. Here, the hydride ion (H-) acts as the nucleophile since it donates a pair of electrons to the positively charged carbon.

- Relating to Lewis Acid-Base Theory

According to Lewis theory, a Lewis acid accepts an electron pair whereas a Lewis base donates an electron pair. In this case, the positively charged carbon is a Lewis acid (electron pair acceptor) and the hydride ion is a Lewis base (electron pair donor).

Key concepts

Lewis acid-base reaction

A Lewis acid-base reaction is a fundamental concept in organic chemistry where electron pairs are transferred.
The Lewis acid is an electron pair acceptor, and the Lewis base is an electron pair donor.
In the context of the hydride shift mechanism, the positively charged carbon is the Lewis acid.
It accepts an electron pair. The hydride ion, which carries a pair of electrons, is the Lewis base.
This process helps stabilize the molecule by fulfilling the octet rule for the involved atoms.
Remembering these roles will ease understanding various organic chemistry reaction mechanisms that rely on similar principles.

Nucleophile

A nucleophile is a species that donates an electron pair to an electrophile.
It is usually rich in electrons and can be negatively charged or neutral with a lone pair of electrons.
In the hydride shift, the hydride ion (H-) acts as the nucleophile.
The hydride ion moves to the neighboring positively charged carbon to donate its electron pair and stabilize the molecule.
Common characteristics of nucleophiles include:

• Having a negative charge or lone pair
• Being rich in electrons
• Capable of forming a bond by donating a pair of electrons

Understanding nucleophiles' behavior is key to predicting and explaining many organic and inorganic reactions.

Electrophile

An electrophile is a species that seeks to accept an electron pair.
It is typically electron-deficient and may carry a positive charge or have a partial positive charge.
In the hydride shift mechanism, the electrophile is the positively charged carbon.
This carbon is in need of electrons to stabilize its electron-deficient state.
Characteristics of electrophiles include:

• Being electron-deficient
• Possessing a positive charge or partial positive charge
• Having an affinity for electrons

Recognizing electrophiles helps in understanding how and why certain reactions occur as they do.

Organic chemistry reaction mechanisms

Organic chemistry reaction mechanisms describe the step-by-step process through which reactants transform into products.
Understanding these mechanisms allows us to predict the outcome of reactions and design new ones.
For a hydride shift, knowing the roles of nucleophiles and electrophiles is crucial.
Key elements in organic reaction mechanisms include:

• Identification of reactants and products
• Step-by-step transformation, detailing electron movement
• Intermediate states and transition states in the reaction pathway
• Stability of intermediates and products

Mastering reaction mechanisms provides a deeper insight into organic chemistry, making complex reactions more manageable and predictable.