Question

Write structures of all the alkenes which on hydrogenation give 2-methylbutane.

Short answer

The alkenes are 2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene.

Step-by-step solution

Understand the Target Alkane

The target alkane given in the problem is 2-methylbutane. It is important to know that alkenes upon hydrogenation in the presence of a catalyst produce alkanes. The bromine atom adsorbs onto the surface, attracting and slightly polarizing the Br-Br sigma bond, lowering the bonding molecular orbital. As a result, the molecule is activated, increasing the electrophilic strength of the Br-Br bond. 2-methylbutane has the structure: CH₃-CH(CH₃)-CH₂-CH₃.

Dehydrogenate to Determine Possible Alkenes

Hydrogenation of an alkene produces an alkane by adding hydrogen atoms across the carbon-carbon double bond. Therefore, to identify the alkenes that can be hydrogenated to 2-methylbutane, remove one hydrogen from one of the adjacent carbon atoms in the target alkane to create a double bond between them. Possible structures: 1. Remove H from C-1 and C-2 or insert double bond between: - CH₂=C(CH₃)-CH₂-CH₃ (2-methyl-1-butene) 2. Remove H from C-2 and C-3: - CH₃-CH=C(CH₃)-CH₃ (2-methyl-2-butene)

Consider Isomeric Alkenes

Consider other possible isomers that could form alkenes from 2-methylbutane upon removal of different hydrogen atoms: 1. Removing H from C-3 and C-4 creates: - CH₃-CH(CH₃)-CH=CH₂ (3-methyl-1-butene) 2. Explore double bonds in different locations to ensure all isomers are accounted for.

List All Possible Alkene Structures

Based on dehydrogenation, the possible alkenes which on hydrogenation can give 2-methylbutane are: 1. 2-methyl-1-butene: CH₂=C(CH₃)-CH₂-CH₃ 2. 2-methyl-2-butene: CH₃-CH=C(CH₃)-CH₃ 3. 3-methyl-1-butene: CH₃-CH(CH₃)-CH=CH₂. These structures cover all possible isomers that could become 2-methylbutane through hydrogenation.

Key concepts

Alkene Isomers

Alkene isomers are compounds that share the same molecular formula but differ in the arrangement of atoms around the carbon-carbon double bond. When studying alkenes, an understanding of alkene isomerism is crucial, especially for predicting the possible outcomes of reactions such as hydrogenation.

For instance, in the case of alkenes that hydrogenate to produce 2-methylbutane, it's important to recognize the different ways of forming double bonds in the carbon chain. By moving the position of this double bond, we create different isomers:

• 2-methyl-1-butene: The double bond is between the first and second carbon.
• 2-methyl-2-butene: The double bond is between the second and third carbon.
• 3-methyl-1-butene: The double bond is between the third and fourth carbon.

These isomers have unique structures and, therefore, distinct chemical properties, even though they all produce the same alkane upon hydrogenation. Identifying the different isomers ensures a complete understanding of all possible alkenes that can become 2-methylbutane through this reaction pathway.

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H₂) across the carbon-carbon double bond in an alkene, converting it into an alkane. This is an important reaction in organic chemistry, as it saturates the previously unsaturated molecule.

In this process:

• The double bond in the alkene is broken.
• Hydrogen atoms are added to the carbon atoms involved in the double bond.
• This leads to the conversion of the alkene to an alkane.

A catalyst, often a metal such as palladium, platinum, or nickel, is typically used to accelerate the reaction. Using 2-methylbutane as an example, hydrogenation is used to determine from which alkenes it can be formed by reversing the process (dehydrogenation). Each isomer of an alkene capable of being hydrogenated to 2-methylbutane demonstrates the versatility and predictability of this reaction.

Carbon-Carbon Double Bonds

Understanding carbon-carbon double bonds is central to grasping the chemistry of alkenes. These double bonds occur when two carbon atoms share two pairs of electrons, making the bond more rigid and less rotatable compared to single bonds.

The presence of a double bond influences:

• The geometry of the molecule, leading to possible "cis" and "trans" configurations.
• The reactivity of the molecule, as double bonds are reactive sites.
• The physical properties of the substance, like its boiling and melting points.

In 2-methylbutane, the exploration of alkenes involves understanding which carbon atoms can be part of a double bond. By evaluating the structure of 2-methylbutane and examining where double bonds might form, it becomes possible to consider all isomers that can lead back to the original alkane via hydrogenation. Recognizing carbon-carbon double bonds advances comprehension of reaction mechanisms and isomer formation in organic chemistry.