Question

Elimination of HBr from 2-bromonorbornane gives only 2-norbornene and no 1-norbornene. How do you account for the regioselectivity of this dehydrohalogenation? In answering this question, you will find it helpful to look at molecular models of both 1-norbornene and 2-norbornene and to analyze the strain in each.

Short answer

Question: Explain the regioselectivity observed during the elimination of HBr from 2-bromonorbornane, resulting in only 2-norbornene and not 1-norbornene. Answer: The regioselectivity observed in the elimination of HBr from 2-bromonorbornane is due to the differences in strain present in the transition states leading to the formation of 2-norbornene and 1-norbornene. The transition state leading to 2-norbornene has less strain due to the bromine atom being farther away from the newly forming double bond, while the transition state leading to 1-norbornene has more strain due to the bromine atom being closer to the forming double bond. The higher strain makes the pathway leading to 1-norbornene less favorable, resulting in only 2-norbornene being formed during the reaction.

Step-by-step solution

Identify the starting material and products

The elimination reaction involves the conversion of 2-bromonorbornane to 2-norbornene and 1-norbornene. However, the reaction only produces 2-norbornene. So, first, we need to draw the molecular structures of these three compounds. 2-Bromonorbornane: ``` Br | /--C--\ / \ C C \ / \--C--/ | H ``` 2-Norbornene: ``` H | /--C===\ / \ C C \ / \--C--/ | H ``` 1-Norbornene: ``` H | /--C--\==C / \ C C \ / \--C--/ ```

Consider the transition states

During the elimination of HBr from 2-bromonorbornane, a transition state is formed in which the carbon-halogen bond and carbon-hydrogen bond are partially broken, leading to the formation of a double bond in the product. Consider the possible transition states for both 1-norbornene and 2-norbornene formations. Transition state for 2-norbornene: ``` Br H ||-- |\ /C --\ / C / \C / C C / \ / H \C --/ \-- H ``` Transition state for 1-norbornene: ``` H | /--C==C / \ C C \ / \--C--Br ```

Analyze the strain in the transition states

In the transition state leading to 2-norbornene, the bromine is further away from the newly forming double bond, reducing potential steric strain. On the other hand, in the transition state leading to 1-norbornene, the bromine atom is closer to the newly forming double bond, causing steric hindrance and greater strain. This higher strain in the transition state leading to 1-norbornene makes this pathway less favorable than the pathway leading to 2-norbornene.

Discuss regioselectivity

As a consequence of the differences in strain of the transition states, the dehydrohalogenation of 2-bromonorbornane happens regioselectively to form only 2-norbornene. The pathway leading to 2-norbornene is more favorable due to the decreased strain, while the high strain in the transition state leading to 1-norbornene makes this pathway less favorable. This regioselectivity results in only 2-norbornene being formed in the elimination of HBr from 2-bromonorbornane.

Key concepts

Elimination Reaction

An elimination reaction is a chemical reaction where two atoms or groups are removed from a molecule, resulting in the formation of a double or triple bond. This type of reaction is commonly used to generate unsaturated compounds, such as alkenes and alkynes.
For example, during the elimination of hydrogen bromide (HBr) from 2-bromonorbornane, a proton (H) and a halogen (Br) are removed from the molecule. This results in the formation of a double bond, converting the substrate into 2-norbornene.
An elimination reaction can proceed via different mechanisms, predominantly E1 or E2 pathways:

• E1 Mechanism: This is a two-step process involving the formation of a carbocation intermediate. The rate of reaction depends only on the concentration of the substrate.
• E2 Mechanism: This is a one-step concerted process where the elimination occurs as a single reaction step. Both the substrate and the base concentrations affect the rate of reaction.

The regioselectivity observed in the elimination of HBr from 2-bromonorbornane, resulting only in 2-norbornene, is a manifestation of these mechanistic pathways and the associated transition states.

Transition State

The concept of a transition state is pivotal in understanding chemical reactions, including elimination reactions. The transition state is a high-energy, unstable arrangement of atoms that occurs during the transformation of reactants into products.
In chemical reactions, there are energy barriers that must be overcome to convert reactants into products. The transition state represents the highest point on the energy landscape connecting the reactants and products.

• During the conversion of 2-bromonorbornane to 2-norbornene, a transition state is involved where the bonds to hydrogen and bromine are partially broken, and the new carbon-carbon double bond begins to form.
• The energy and structure of the transition state can determine which reaction pathway is preferred, often influencing regioselectivity.

Transition states are not typically observable or isolatable, but they are crucial for understanding why certain products form preferentially over others. For example, less steric hindrance in the transition state for 2-norbornene formation reduces energy, making this pathway more favorable.

Steric Strain

Steric strain refers to the repulsion that occurs when atoms are brought too close to each other, which can increase the energy of a molecule. This concept is especially important in understanding why certain chemical reactions occur preferentially in a particular manner.
In the studied elimination reaction, the formation of 2-norbornene compared to 1-norbornene is favored due to differences in steric strain during the transition states.

• For the transition state leading to 2-norbornene, the bromine and hydrogen atoms are more favorably positioned, resulting in less steric strain. This arrangement helps to stabilize the transition state, making this pathway more energetically favorable.
• In contrast, the transition state leading to 1-norbornene experiences more steric hindrance due to the proximity of bulky groups, which raises the energy of this state. Consequently, this pathway is less favorable.

Understanding steric strain helps to rationalize the regioselectivity observed in chemical reactions. By analyzing steric interactions, chemists can predict the outcome of reactions, such as in the elimination of HBr from 2-bromonorbornane, where only 2-norbornene is produced.