Reactive Intermediates
In chemistry, reactive intermediates are termed as short-lived, highly reactive atoms with high energy. They rapidly transform into stable particles during a chemical reaction. In specific cases, by means of matrix isolation and at low-temperature reactive intermediates can be isolated.
Hydride Shift
A hydride shift is a rearrangement of a hydrogen atom in a carbocation that occurs to make the molecule more stable. In organic chemistry, rearrangement of the carbocation is very easily seen. This rearrangement can be because of the movement of a carbocation to attain stability in the compound. Such structural reorganization movement is called a shift within molecules. After the shifting of carbocation over the different carbon then they form structural isomers of the previous existing molecule.
Vinylic Carbocation
A carbocation where the positive charge is on the alkene carbon is known as the vinyl carbocation or vinyl cation. The empirical formula for vinyl cation is C2H3+. In the vinyl carbocation, the positive charge is on the carbon atom with the double bond therefore it is sp hybridized. It is known to be a part of various reactions, for example, electrophilic addition of alkynes and solvolysis as well. It plays the role of a reactive intermediate in these reactions.
Cycloheptatrienyl Cation
It is an aromatic carbocation having a general formula, [C7 H7]+. It is also known as the aromatic tropylium ion. Its name is derived from the molecule tropine, which is a seven membered carbon atom ring. Cycloheptatriene or tropylidene was first synthesized from tropine.
Stability of Vinyl Carbocation
Carbocations are positively charged carbon atoms. It is also known as a carbonium ion.
![### Chemical Reaction Mechanisms: Drawing Major Products
Below are five distinct chemical reactions where you need to draw the major product of each reaction, based on the given starting materials and conditions. Here's a breakdown of each reaction:
#### Reaction a.
**Starting Material:**
- A terminal alkyne (triple bond between two carbon atoms in the middle)
**Reagents:**
- HCl
**Explanation:**
- When a terminal alkyne reacts with HCl, the hydrogen (H) will add to the carbon with more hydrogen atoms, and the chlorine (Cl) will add to the carbon with fewer hydrogen atoms. This is an anti-Markovnikov addition.
---
#### Reaction b.
**Starting Material:**
- A substituted alkyne (triple bond in the middle of a carbon chain with branches on the ends)
**Reagents:**
- H2
- Lindlar's catalyst
**Explanation:**
- This reaction involves hydrogenation. Using Lindlar's catalyst in the presence of H2 converts an alkyne to a cis-alkene.
---
#### Reaction c.
**Starting Material:**
- A terminal alkyne (triple bond at the end of a carbon chain)
**Reagents:**
- H2O, H2SO4 (sulfuric acid), and HgSO4 (mercuric sulfate)
**Explanation:**
- In the presence of these reagents, the terminal alkyne undergoes oxymercuration to form a ketone via Markovnikov addition.
---
#### Reaction d.
**Starting Material:**
- An internal alkyne (triple bond between two carbon atoms in the middle of a carbon chain)
**Reagents:**
- Na (sodium)
- NH3 (ammonia)
- -78°C (low temperature)
**Explanation:**
- This method is called the Birch reduction, which converts the alkyne to a trans-alkene.
---
#### Reaction e.
**Starting Material:**
- A terminal alkyne (triple bond at the end of a carbon chain with a branch)
**Reagents:**
- H2
- Pd/C (palladium on carbon catalyst)
**Explanation:**
- The presence of a palladium catalyst with hydrogen gas results in full hydrogenation, converting the alkyne to an alkane.
---
Through this exercise, students can understand the different outcomes based on reagent used with alkynes, revealing](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fd859d598-50bb-4ccc-a26f-3606b98e6f85%2Fd96cdd43-8c9c-47bd-8d86-093ccdf1cba2%2Fr01fxv_processed.jpeg&w=3840&q=75)
![### Organic Chemistry Reactions
**Example Reactions and Mechanisms**
**f. Hydroboration-Oxidation of Alkyne**
- **Starting Material**: Terminal alkyne
- **Reagents**:
1. \( \text{BH}_3 - \text{THF} \)
2. \( \text{NaOH}, \text{H}_2\text{O}_2, \text{H}_2\text{O} \)
- **Explanation**: This reaction involves the hydroboration-oxidation of an alkyne, where the alkyne reacts with borane (BH₃) in tetrahydrofuran (THF) followed by oxidation with hydrogen peroxide (H₂O₂) in a basic medium (NaOH).
**g. Addition of HBr to an Alkyne**
- **Starting Material**: Substituted alkyne
- **Reagent**: \( \text{HBr} \)
- **Explanation**: This reaction describes the addition of hydrobromic acid (HBr) to an alkyne. The alkyne undergoes electrophilic addition with HBr, which generally results in the formation of a bromoalkene.
**h. Partial Hydrogenation of Alkyne**
- **Starting Material**: Substituted alkyne
- **Reagents**: \( \text{H}_2 \) in the presence of Lindlar's catalyst
- **Explanation**: The alkyne undergoes partial hydrogenation with molecular hydrogen (H₂) using Lindlar's catalyst. This selectively reduces the alkyne to a cis-alkene without further reduction to an alkane.
**i. Reduction of Alkyne to Trans-Alkene**
- **Starting Material**: Substituted alkyne
- **Reagents**: \( \text{Na}, \text{NH}_3 \) at -78°C
- **Explanation**: In this reaction, the alkyne is reduced to a trans-alkene by dissolving metallic sodium (Na) in liquid ammonia (NH₃) under low-temperature conditions (-78°C).
**j. Hydrohalogenation of Cycloalkyne**
- **Starting Material**: Cyclohexyl alkyne
- **Reagent**: Excess \( \text{HCl} \)
- **Explanation](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fd859d598-50bb-4ccc-a26f-3606b98e6f85%2Fd96cdd43-8c9c-47bd-8d86-093ccdf1cba2%2Fwy6lwp8_processed.jpeg&w=3840&q=75)
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