Question

2-methyl but-2-ene is less reactive than but-2-ene. This is due to (a) Resonance (b) Hyperconjugation (c) Electromeric effect (d) Inductive effect

Short answer

(b) Hyperconjugation

Step-by-step solution

Understanding the Reactivity

To understand why 2-methyl but-2-ene is less reactive than but-2-ene, we need to consider the substitution on the double-bonded carbon atoms. 2-methyl but-2-ene has a methyl group attached, whereas but-2-ene does not.

Considering Substituent Effects

Substituents attached to the double-bonded carbons can affect the electron density and stability of the alkene. In alkenes, hyperconjugation and inductive effects are common factors that influence reactivity.

Evaluating Hyperconjugation

Hyperconjugation occurs when electrons in \(\sigma\) bonds (usually C-H or C-C) interact with an adjacent empty or partially filled p-orbital or \(\pi\) orbital. The presence of an additional alkyl group in 2-methyl but-2-ene increases hyperconjugation, stabilizing the alkene and decreasing its reactivity.

Examining Other Options

Resonance and the electromeric effect are not applicable here as we are dealing with alkenes whose reactivity is not influenced by these effects in this context. The inductive effect, while present, is weaker than hyperconjugation when it comes to stabilizing alkenes.

Conclusion on Reactivity

The decreased reactivity is primarily due to hyperconjugation, where the additional alkyl group in 2-methyl but-2-ene provides more electron release through overlap, stabilizing the double bond and reducing reactivity.

Key concepts

Alkene Reactivity

In order to understand alkene reactivity, it is important to consider the structure of an alkene. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond gives alkenes unique reactivity properties. The double bond is highly reactive because it is a region of high electron density, making it an attractive site for electrophiles. However, not all alkenes react with the same speed or preference for reactions.
Alkene reactivity is influenced by the substituents attached to the carbon atoms involved in the double bond. The presence of additional substituents can either stabilize or destabilize the double bond, affecting reactivity. Generally, alkenes with fewer substituents are more reactive, and those with more substituents are less reactive due to factors we will explore such as hyperconjugation and substitution effects.

• Simple alkenes with minimal substitution like ethylene are more reactive.
• Complex alkenes with more substituents show reduced reactivity.

Hyperconjugation

Hyperconjugation is a key concept in understanding why some alkenes are less reactive. It involves the interaction between a filled c bond (such as C-H or C-C) and an adjacent c or a bond. This overlap allows for the delocalization of electrons, providing extra stability to the molecule.
In 2-methyl but-2-ene, the additional methyl group provides more possibilities for hyperconjugation. Each C-H bond adjacent to the double bond can participate, resulting in a more stable electron cloud around the double bond.
This stability from hyperconjugation reduces the attractiveness of the double bond to reactive species, meaning:

• The alkene is less likely to participate in reactions, thus reducing reactivity.
• Hyperconjugation can also reduce the energy of the molecule, aiding stability further.

Hyperconjugation is an important stabilizing factor that contrasts with initial expectations about reactivity.

Substitution Effects

The substitution effects on alkenes play a crucial role in their reactivity profiles. A substitution effect refers to how substituents (or groups attached to the main carbon chain) influence the chemical behavior of the molecule. These effects can manifest through electronic and steric interactions.
For alkenes, the type and number of substituents can substantially affect reactivity. In the case of 2-methyl but-2-ene, the methyl substituent increases stability. While it adds bulk to the structure, leading to steric hindrance, its main effect is electronic.

• Electronics: Substituents exert effects through inducing or donating electron density via hyperconjugation or inductive contributions.
• Sterics: Bulkier groups around the double bond can inhibit approach of reactants.

Though the inductive effect also plays a role, hyperconjugation is the dominant factor in stabilizing alkenes when additional alkyl groups are present. Understanding substitution effects allows chemists to predict the reactivity and select the optimal conditions for reactions involving alkenes.