Question

Write structural formulas for two straight-chain alkenes with the formula \(\mathrm{C}_{5} \mathrm{H}_{10}\). Are these the only two structures that meet these specifications?

Short answer

There are only two straight-chain alkenes: 1-pentene and 2-pentene.

Step-by-step solution

Understanding Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The general formula for alkenes is \\[\mathrm{C}_n\mathrm{H}_{2n},\] indicating that the number of hydrogen atoms is double the number of carbon atoms, confirming the given formula \(\mathrm{C}_{5}\mathrm{H}_{10}\) belongs to this family.

Drawing the First Alkene

Start with a five-carbon straight chain and place a double bond at the beginning. The structure is as follows:\[\begin{align*}\mathrm{CH}_2 = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3\end{align*}\]This is called 1-pentene.

Drawing the Second Alkene

Move the double bond to the second position in the chain. The structural formula becomes:\[\begin{align*}\mathrm{CH}_3 - \mathrm{CH} = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_3\end{align*}\]This structure is known as 2-pentene.

Confirming All Structures

In the structural formula \(\mathrm{C}_5\mathrm{H}_{10}\), each carbon-carbon double bond is in the first or second position of a five-carbon straight chain. Other placements would create the same molecules as previously drawn due to symmetry or lead to isomers that are not straight-chain.

Checking For Additional Structures

Since any double bond placement not only changes the position number in the name and not the actual molecule, 1-pentene and 2-pentene are the only two distinct straight-chain alkenes possible with formula \(\mathrm{C}_5\mathrm{H}_{10}\).

Key concepts

Structural Formula

A structural formula is a representation showing the arrangement of atoms in a molecule. For alkenes, it highlights not only the types of atoms but also how they connect through bonds, especially the important carbon-carbon double bond. This is crucial in understanding how the molecules are structured and how they might react chemically.

For example, in a molecule like 1-pentene, the structural formula would be displayed as follows:

• \( \mathrm{CH}_2 = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3 \)

This illustrates each carbon (C) and hydrogen (H) atom and shows the double bond between the first two carbon atoms.
Structural formulas are essential in chemistry because they give a clear picture of a molecule's geometry, which can help predict physical and chemical properties.

Being able to draw and understand structural formulas is a key skill in chemistry, especially when dealing with different isomers of a molecule which might have the same chemical formulas but different structural arrangements.

Carbon-Carbon Double Bond

The carbon-carbon double bond is a defining feature of alkenes. Unlike single bonds, where electrons are shared between two atoms, a double bond involves four electrons shared between the carbon atoms.
This unique bonding gives alkenes different chemical properties compared to alkanes, which only have single bonds.

The presence of the double bond makes alkenes generally more reactive. They can undergo various chemical reactions, like addition reactions, much more readily than alkanes.
In the structural formulas for 1-pentene and 2-pentene, the double bonds are shown clearly:

• 1-pentene: \( \mathrm{CH}_2 = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3 \)
• 2-pentene: \( \mathrm{CH}_3 - \mathrm{CH} = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_3 \)

The position of the double bond can significantly affect a molecule's properties, giving rise to different isomers that do not have the same geometric arrangement.

This change in arrangement often affects how a molecule reacts or how its physical properties manifest, making the study of carbon-carbon double bonds crucial to organic chemistry testing.

Straight-Chain Alkenes

Straight-chain alkenes are alkenes where all the carbon atoms are connected in a linear sequence without branches. This structure contrasts with branched alkenes, which have alkyl groups branching off the main chain.

In the exercise, both 1-pentene and 2-pentene represent straight-chain alkenes. This lack of branching often results in higher boiling points because the linear shape allows for better packing, leading to stronger intermolecular forces between molecules.

• 1-pentene: The double bond occurs at the first carbon. Its formula is \( \mathrm{CH}_2 = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3 \).
• 2-pentene: The double bond is found between the second and third carbon atoms. Represented as \( \mathrm{CH}_3 - \mathrm{CH} = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_3 \).

Studying straight-chain alkenes helps us understand fundamental concepts in organic chemistry, such as isomerism, where different arrangements of atoms can lead to different molecules, despite having the same chemical formula.
This understanding is pivotal, especially when predicting the behavior of alkenes under different physical and chemical conditions.