Basics in Organic Reactions Mechanisms
In organic chemistry, the mechanism of an organic reaction is defined as a complete step-by-step explanation of how a reaction of organic compounds happens. A completely detailed mechanism would relate the first structure of the reactants with the last structure of the products and would represent changes in structure and energy all through the reaction step.
Heterolytic Bond Breaking
Heterolytic bond breaking is also known as heterolysis or heterolytic fission or ionic fission. It is defined as breaking of a covalent bond between two different atoms in which one atom gains both of the shared pair of electrons. The atom that gains both electrons is more electronegative than the other atom in covalent bond. The energy needed for heterolytic fission is called as heterolytic bond dissociation energy.
Polar Aprotic Solvent
Solvents that are chemically polar in nature and are not capable of hydrogen bonding (implying that a hydrogen atom directly linked with an electronegative atom is not found) are referred to as polar aprotic solvents. Some commonly used polar aprotic solvents are acetone, DMF, acetonitrile, DMSO, etc.
Oxygen Nucleophiles
Oxygen being an electron rich species with a lone pair electron, can act as a good nucleophile. Typically, oxygen nucleophiles can be found in these compounds- water, hydroxides and alcohols.
Carbon Nucleophiles
We are aware that carbon belongs to group IV and hence does not possess any lone pair of electrons. Implying that neutral carbon is not a nucleophile then how is carbon going to be nucleophilic? The answer to this is that when a carbon atom is attached to a metal (can be seen in the case of organometallic compounds), the metal atom develops a partial positive charge and carbon develops a partial negative charge, hence making carbon nucleophilic.
![### Grignard Reaction: First Step Identification
**Question:**
Which of the following schemes is the first step in the following Grignard reaction?
**Schemes:**
1. **Scheme I:**
- The scheme depicts a Grignard reagent (BrMg) approaching the carbonyl group of acetone. The reaction arrow indicates the movement of electrons from the carbon atom of the Grignard reagent (bearing a partial negative charge) towards the carbon atom of the carbonyl group, suggesting the formation of a new carbon-carbon bond.
2. **Scheme II:**
- In this scheme, the carbonyl oxygen's lone pair electrons are suggested to attack the magnesium of the Grignard reagent (BrMg). The electrons from the carbonyl carbon bond shift, suggesting a different mechanism compared to Scheme I.
3. **Scheme III:**
- This scheme illustrates the lone pair of electrons on the oxygen of the carbonyl group forming a bond with the magnesium of the Grignard reagent (BrMg). The arrows and notations suggest a nucleophilic attack from the opposite direction to Scheme II.
4. **Scheme IV:**
- Similar to Scheme II, this scheme indicates the lone pair of electrons on the carbonyl oxygen attacking the magnesium atom from BrMg. However, the positioning and resultant movement of electrons are slightly different.
**Explanation of Graphs/Diagrams:**
- Each scheme represents potential mechanisms for the initial step in the Grignard reaction involving a carbonyl compound (acetone) and a Grignard reagent (BrMg).
- The arrows within each diagram demonstrate the flow of electron pairs and indicate which atoms are involved in bond formation or breaking.
- The dotted areas represent electron lone pairs on oxygen atoms which are capable of initiating nucleophilic or electrophilic attacks.
**Objective:**
Understand the nature and direction of electron flow in Grignard reagent's interaction with a carbonyl compound to predict the correct initial mechanistic step of the reaction.
By analyzing the schemes, learners might understand the correct mechanistic step by focusing on the nucleophilicity of the carbonyl oxygen and the electrophilicity of the Grignard reagent.](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fe4d0ca9f-229e-40d6-ac53-f10954775736%2Fe9016ca7-b63b-4bcc-9ef6-bd2c4bae1e6f%2F9mjmdsm_processed.jpeg&w=3840&q=75)
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