Question

Draw the condensed structural formula for each of the following alkenes: (a) 2-pentene (b) 3-heptene (c) 1-propene (d) 4 -nonene

Short answer

2-pentene: \( CH_3-CH=CH-CH_2-CH_3 \); 3-heptene: \( CH_3-CH_2-CH=CH-CH_2-CH_2-CH_3 \); 1-propene: \( CH_2=CH-CH_3 \); 4-nonene: \( CH_3-CH_2-CH_2-CH=CH-CH_2-CH_2-CH_2-CH_3 \).

Step-by-step solution

Understand Alkene Structure

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The general formula for alkenes is C_nH_{2n}, where \( n \) is the number of carbon atoms.

2-Pentene Structure

2-Pentene is an alkene with 5 carbons where the double bond is between the 2nd and 3rd carbon. The condense structural formula is: \( CH_3-CH=CH-CH_2-CH_3 \).

3-Heptene Structure

3-Heptene is an alkene with 7 carbons where the double bond is between the 3rd and 4th carbon. The condensed structural formula is: \( CH_3-CH_2-CH=CH-CH_2-CH_2-CH_3 \).

1-Propene Structure

1-Propene is an alkene with 3 carbons where the double bond is between the 1st and 2nd carbon. The condensed structural formula is: \( CH_2=CH-CH_3 \).

4-Nonene Structure

4-Nonene is an alkene with 9 carbons where the double bond is between the 4th and 5th carbon. The condensed structural formula is: \( CH_3-CH_2-CH_2-CH=CH-CH_2-CH_2-CH_2-CH_3 \).

Key concepts

Condensed Structural Formula

A condensed structural formula is a more compact version of the structural formula of a molecule. Instead of showing a detailed structure with bonds represented by lines, a condensed formula compresses this information on one line. This format makes it easier to read and write, especially for complex molecules.
In a condensed structural formula for an alkene, you'll see each carbon atom listed, connected by dashes that represent bonds, with attached hydrogen atoms either written immediately after them or implied if not explicitly shown. For example, in 2-pentene, represented as \( CH_3-CH=CH-CH_2-CH_3 \), you can see that a double bond is present between the second and third carbon atoms. This style is favored in organic chemistry for its simplicity in displaying the essential structure of the molecule.
However, understanding the arrangement is crucial, as the placement of double bonds and substituents can significantly influence a molecule's properties and reactivity.

Carbon-Carbon Double Bond

The hallmark of alkenes is the presence of a carbon-carbon double bond, designated as \( \text{C}=\text{C} \). This double bond consists of one sigma (σ) bond and one pi (π) bond, the latter occurring due to the overlap of p orbitals. This double bond is not just a simple link; it also imparts rigidity and influences the chemical behavior of the alkene.

• Alkenes are named such that the lowest numbered carbon in the double bond determines the number. In 3-heptene, the number '3' indicates the double bond starts at the third carbon.
• The presence of the double bond allows for geometric (cis-trans) isomerism, giving rise to different spatial arrangements of the atoms attached to the bonded carbons.

Understanding the position and nature of this bond is essential as it affects the molecule's stability, reactivity, and the type of reactions it can undergo, such as addition reactions, which are characteristically favored by the electron-rich double bond.

Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of hydrogen and carbon atoms. They form the foundational elements of organic chemistry, and are broadly divided into several classes based on the types of bonds between carbon atoms. Alkenes fall under unsaturated hydrocarbons due to their ability to form additional bonds through their carbon-carbon double bonds.
Hydrocarbons are classified due to their structure and types:

• Saturated hydrocarbons, like alkanes, feature only single bonds.
• Alkenes, the focus of this exercise, have at least one double bond and follow the general formula \( C_nH_{2n} \).
• Alkynes contain at least one triple bond.

The specific nature of alkenes being unsaturated gives them unique properties, such as the ability to (easily) add atoms to themselves via reactions, which makes them very important in industrial and biological processes.