Problem 6 - Basic Principles of Organic Chemistry

Question

a. Would you expect a carbon-nitrogen triple bond to be hydrogenated more, or less, easily than a carbon-carbon triple bond? b. Why is it difficult to hydrogenate a tetrasubstituted alkene such as 2,3 -dimethyl-2-butene?

Step-by-Step Solution

Answer

a. A carbon-nitrogen triple bond is hydrogenated less easily than a carbon-carbon triple bond. b. Tetrasubstituted alkenes are difficult to hydrogenate due to steric hindrance.

Step 1: Understanding Triple Bonds

Recognize that the hydrogenation of triple bonds reduces them to double bonds or single bonds. Carbon-nitrogen triple bonds are called nitriles, while carbon-carbon triple bonds are found in alkynes. The bond dissociation energy for nitriles is higher than that for carbon-carbon triple bonds.

Step 2: Analyzing Reactivity

Triple bonds with higher bond dissociation energies tend to be less reactive to hydrogenation. Since the C-N triple bond in nitriles is stronger than the C-C triple bond in alkynes, it is less easily hydrogenated.

Step 3: Consideration of Steric Hindrance in Alkenes

Tetrasubstituted alkenes, like 2,3-dimethyl-2-butene, have substantial steric hindrance due to substituents around the double bond, making it difficult for hydrogen to approach and react during hydrogenation.

Key Concepts

Carbon-Nitrogen Triple Bonds

Carbon-nitrogen triple bonds, also known as nitriles, are a fascinating aspect of organic chemistry due to their unique properties and structure. A triple bond between a carbon and a nitrogen atom is shorter and stronger than most other types of bonds because it includes one sigma and two pi bonds.
Nitriles play a crucial role in many chemical reactions, and their high bond dissociation energy makes them less reactive in hydrogenation compared to carbon-carbon triple bonds in alkynes.
This high bond strength means that more energy is required to break the bond, making hydrogenation less favorable. However, once successfully hydrogenated, nitriles are reduced to amines, which are useful in various chemical syntheses.
Pay special attention to nitriles in your reactions because their strength affects how easily reactions can proceed.

Alkynes

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond, making them a significant group in organic chemistry.
The carbon-carbon triple bond consists of one sigma bond and two pi bonds, which gives alkynes their linear geometry.

• Alkynes have higher bond dissociation energy than single and double bonds but less than nitriles.
• This lower energy requirement compared to nitriles makes alkynes more reactive in hydrogenation reactions.

When hydrogenated, alkynes can be reduced to alkenes and further to alkanes.

• The initial addition of hydrogen goes through the syn-addition mechanism, meaning hydrogen atoms add on the same side.

Understanding the structure and reactivity of alkynes is crucial for predicting the outcomes of hydrogenation processes.

Steric Hindrance

Steric hindrance is a significant factor in determining the reactivity of molecules during chemical reactions, particularly hydrogenation.
It refers to the repulsion between atoms or groups of atoms in a molecule that crowds a reactive site, making it difficult for reactants to approach and react efficiently.

• Tetrasubstituted alkenes like 2,3-dimethyl-2-butene are classic examples where steric hindrance plays a major role.
• The bulky groups around the double bond impede the access of hydrogen molecules and catalysts, inhibiting hydrogenation.

To overcome steric hindrance, reactions may require additional energy or specialized catalysts. By understanding steric hindrance, chemists can design better reaction pathways and potentially resolve issues related to reaction efficiency.

Bond Dissociation Energy

Bond dissociation energy (BDE) is a critical concept in understanding chemical reactions, especially in hydrogenation.
It is defined as the energy required to break a specific bond in a molecule in its gaseous state into separate atoms. The higher the BDE, the stronger the bond and the less reactive it generally is in chemical reactions.

• For triple bonds, BDE varies depending on the atoms involved, such as C-N in nitriles versus C-C in alkynes.
• Nitriles have higher BDE due to the stronger interatomic forces involved, hence they are less prone to hydrogenation.

Bond dissociation energy directly affects the process of adding hydrogen to a system. Recognizing BDE helps in estimating the energy needed for breaking and forming bonds during reactions, allowing for more precise control over chemical transformations.