Question

Draw the condensed structural formula for each of the following compounds: (a) 3-ethyl-2-heptene (b) 4,4 -dimethyl-2-pentyne

Short answer

3-ethyl-2-heptene: CH3-CH=C(CH2CH3)-CH2-CH2-CH3; 4,4-dimethyl-2-pentyne: CH3-C≡C-CH(CH3)2.

Step-by-step solution

Understand the Nomenclature for 3-ethyl-2-heptene

The name 3-ethyl-2-heptene implies that the parent chain is heptene, which consists of seven carbon atoms with a double bond starting at carbon 2. Additionally, an ethyl group is attached to the third carbon.

Draw the Structural Backbone for 3-ethyl-2-heptene

Firstly, draw the heptene chain with a double bond between carbons 2 and 3: CH3-CH=CH-CH2-CH2-CH2-CH3. Then, place the ethyl group on the third carbon: CH3-CH=C(CH2CH3)-CH2-CH2-CH3.

Condense the Formula for 3-ethyl-2-heptene

The condensed structural formula includes both the main chain and any branches or substituent groups. Represent it as: CH3-CH=C(CH2CH3)-CH2-CH2-CH3, indicating the ethyl group (CH2CH3) on the third carbon.

Understand the Nomenclature for 4,4-dimethyl-2-pentyne

The name 4,4-dimethyl-2-pentyne suggests the parent chain is pentyne, a five-carbon chain with a triple bond starting at carbon 2. There are two methyl groups attached to the fourth carbon.

Draw the Structural Backbone for 4,4-dimethyl-2-pentyne

Start with the pentyne chain with a triple bond between carbon atoms 2 and 3: CH3-C≡C-CH2-CH2. Add two methyl groups to the fourth carbon: CH3-C≡C-CH(CH3)2.

Condense the Formula for 4,4-dimethyl-2-pentyne

The condensed structural formula shows the parent chain and attached substituents. Condense it to: CH3-C≡C-CH(CH3)2 where the two methyl groups are represented by (CH3)2 on the fourth carbon.

Key concepts

Condensed Structural Formula

In organic chemistry, a condensed structural formula is a compact way of representing molecules. It shows each carbon atom and the hydrogen atoms attached to it in a sequential manner. Unlike a full structural formula, it doesn't display the individual bonds, making it less cluttered.

This method of representation provides essential information in a streamlined format, helping chemists quickly grasp the molecular structure. For instance, in 3-ethyl-2-heptene, the condensed formula looks like this: \( \text{CH}_3-\text{CH}=\text{C(}\text{CH}_2\text{CH}_3\text{)-CH}_2\text{-CH}_2\text{-CH}_3 \). Here, every part of the molecule is expressed succinctly, with the ethyl group shown as \( \text{(CH}_2\text{CH}_3\text{)} \) off the third carbon.

Condensed structural formulas are beneficial because:

• They reduce visual complexity while retaining structural information.
• They are useful for quick reference and understanding.
• They are easier to write and reproduce, saving time in chemical communication.

Alkenes and Alkynes

Alkenes and alkynes are types of hydrocarbons, which are compounds made entirely of hydrogen and carbon atoms. The terms describe the presence of carbon-carbon multiple bonds, either double (alkenes) or triple (alkynes).

**Alkenes** have at least one double bond between carbon atoms. This bond introduces rigidity and a geometric configuration to the molecule. For example, in 3-ethyl-2-heptene, there's a double bond starting at the second carbon, altering the shape and properties compared to a single bond.

**Alkynes** contain one or more triple bonds. These bonds are even stronger than double bonds, affecting the chemical reactivity of the compounds. In 4,4-dimethyl-2-pentyne, there is a carbon-carbon triple bond starting at the second carbon, giving the compound distinct physical and chemical characteristics.

The presence of double or triple bonds influences:

• The melting and boiling points of the compounds.
• The stereochemistry and possible isomers.
• The types of reactions they can undergo, such as addition reactions.

Carbon Chains and Substituents

Carbon chains form the skeletal backbone of organic compounds. They vary in length and branching, giving rise to a vast number of possible molecules and diverse properties. Understanding the primary carbon chain and its substituents is key in organic nomenclature.

A *straight chain* consists of continuous carbon atoms, with no branching. For example, heptene and pentyne in the provided compounds are primary carbon chains to which substituents are attached.

*Substituents* are groups of atoms attached to this main chain, altering the compound's properties. These can include alkyl groups like methyl or ethyl. In 3-ethyl-2-heptene, an ethyl group is attached to the third carbon, while 4,4-dimethyl-2-pentyne has two methyl groups on the fourth carbon.

The function of substituents and carbon chains includes:

• Defining the molecular structure and complexity.
• Influencing physical properties such as solubility and melting point.
• Providing specific spots for chemical reactions to occur, making some sites more reactive than others.