Question

Draw structures and name the four alkenes that have the formula \(\mathrm{C}_{4} \mathrm{H}_{8}\)

Short answer

1-butene, cis-2-butene, trans-2-butene, and 2-methylpropene.

Step-by-step solution

Introduction to Isomers

Alkenes are hydrocarbons with at least one carbon-carbon double bond. Isomers are compounds with the same formula but different structures. Here, we need to find different structural isomers of an alkene with the formula \(\mathrm{C}_4 \mathrm{H}_8\).

Identify the Carbon Skeletons

Start with identifying possible carbon skeletons. For \(\mathrm{C}_4 \mathrm{H}_8\), the possible main carbon chain lengths are four. The skeletal structures can be: a straight chain (but-ene) and a branched chain (like 2-methylpropene).

Draw and Name the Straight Chain Isomers

1. Draw a straight chain of four carbon atoms. The first isomer would be a double bond at the end: \(\mathrm{CH}_2=\mathrm{CHCH}_2\mathrm{CH}_3\), named 1-butene.2. Move the double bond one position: \(\mathrm{CH}_3\mathrm{CH}=\mathrm{CHCH}_3\), named 2-butene.

Consider Carbon Atom Arrangement Variations

Explore branching options to form another structural isomer:1. Branch by creating a chain of three carbon atoms plus a methyl group (2-methylpropene). The structure is \(\mathrm{CH}_2=C(\mathrm{CH}_3)\mathrm{CH}_3\).

Identify Possible Stereoisomers

Given \(\mathrm{C}_4 \mathrm{H}_8\), there is an opportunity for stereoisomers in \(\mathrm{CH}_3\mathrm{CH}=\mathrm{CHCH}_3\) (2-butene). The groups can be arranged across the double bond as either 'cis' (same side) or 'trans' (opposite sides). Hence, 2-butene has 'cis-2-butene' and 'trans-2-butene.'

Conclusion: List of Isomers

The four alkenes are: 1-butene, cis-2-butene, trans-2-butene, and 2-methylpropene. Each has a unique structure or stereochemistry while having the formula \(\mathrm{C}_4 \mathrm{H}_8\).

Key concepts

Structural Isomers

In the world of organic chemistry, isomers are like siblings in a family—they share a common formula but have different structures. **Structural isomers** specifically refer to compounds that have the same molecular formula but differ in how their atoms are connected, showing different structures. These differences can include variations in the placement of the carbon backbone or the functional groups attached.

For alkenes with the formula \(\mathrm{C}_4 \mathrm{H}_8\), structural isomers are formed by arranging the four carbon atoms in different ways. This includes creating a straight chain, such as in *1-butene*, or a branched chain, like in *2-methylpropene*. The main focus is how the carbon atoms are linked together to form the backbone of the molecule.

You can think of structural isomers as different blueprints for building a small house, each using an identical set of building blocks. It's fascinating to see how altering the arrangement of these blocks gives rise to compounds with distinct properties, emphasizing the importance of form and connectivity in organic chemistry.

Stereoisomers

While structural isomers deal with different connectivity, **stereoisomers** introduce us to the 3D world of molecular geometry. Stereoisomers have the same sequence of bonded atoms, but they differ in the spatial arrangement of these atoms. In alkenes, this is most often seen in the orientation of the groups around a double bond.

For *2-butene*, there are two main types of stereoisomers: *cis-2-butene* and *trans-2-butene*. **Cis-** means that the substituent groups (in this case, the methyl groups, \(\mathrm{CH}_3\)) are on the same side of the double bond. **Trans-**, on the other hand, indicates that the groups are on opposite sides.

This seemingly simple change in geometry can lead to differences in the boiling points, melting points, and even reactivity of compounds. It's akin to the difference between your right hand and left hand—they mirror each other, but they are not identical.

Carbon Skeletons

The term **carbon skeleton** refers to the basic chain or framework of carbon atoms in a molecule. This framework serves as the fundamental structure around which the rest of the molecular geometry is built.

In the context of the alkene formula \(\mathrm{C}_4 \mathrm{H}_8\), we can explore various carbon skeletons, like single straight chains or branched configurations. A *straight chain* carbon skeleton simply links the carbon atoms in a single, unbranched line, forming compounds like *1-butene*.

Branched chains arise when a carbon "branch," such as a methyl group, diverts from the main path. This results in structures like *2-methylpropene*. The carbon skeleton significantly impacts the chemical properties, because it dictates the distribution of the rest of the molecule's atoms.

Double Bond Position

In alkenes, a defining feature is the presence of a carbon-carbon **double bond**. The position of this double bond within the carbon skeleton profoundly affects the isomer's chemical nature.

For molecules like \(\mathrm{C}_4 \mathrm{H}_8\), placing the double bond at different positions generates distinct isomers. Moving the double bond within a straight chain creates different alkenes such as *1-butene* and *2-butene*, showcasing how even a small shift in the bond position changes the entire structure.

This adjustment in the double bond position also influences molecular properties such as stability and reactivity. It’s a bit like moving a door on a wall—it changes how you interact with the room even though the room itself hasn’t changed. This is crucial for understanding how molecules behave and interact in various chemical reactions.