Question

Write structural formulas for the following alkanes. (a) 3-ethylpentane (b) 2,2-dimethylbutane (c) 2-methyl-3-ethylheptane (d) 2,3-dimethylpentane

Short answer

Short Answer Question: Using the IUPAC nomenclature, draw the structural formula for the following alkane: 2,3-dimethylpentane. Short Answer: The structural formula for 2,3-dimethylpentane is as follows: CH3-CH(CH3)-CH(CH3)-CH2-CH3.

Step-by-step solution

(a) 3-ethylpentane Structure

To draw the structural formula for 3-ethylpentane, follow these steps: 1. Draw a straight chain of 5 carbon atoms (pentane) in a horizontal line. 2. Add hydrogens to each carbon atom so that each has a total of four bonds. 3. On the third carbon atom (counting from left or right), replace one hydrogen atom with an ethyl group (-CH2-CH3). The resulting structural formula for 3-ethylpentane is: CH3-CH2-CH(CH2-CH3)-CH2-CH3

(b) 2,2-dimethylbutane Structure

To draw the structural formula for 2,2-dimethylbutane, follow these steps: 1. Draw a straight chain of 4 carbon atoms (butane) in a horizontal line. 2. Add hydrogens to each carbon atom so that each has a total of four bonds. 3. On the second carbon atom (counting from left or right), replace two hydrogen atoms with two methyl groups (-CH3). The resulting structural formula for 2,2-dimethylbutane is: CH3-CH(CH3)2-CH2-CH3

(c) 2-methyl-3-ethylheptane Structure

To draw the structural formula for 2-methyl-3-ethylheptane, follow these steps: 1. Draw a straight chain of 7 carbon atoms (heptane) in a horizontal line. 2. Add hydrogens to each carbon atom so that each has a total of four bonds. 3. On the second carbon atom (counting from left or right), replace one hydrogen atom with a methyl group (-CH3). 4. On the third carbon atom (counting from left or right), replace one hydrogen atom with an ethyl group (-CH2-CH3). The resulting structural formula for 2-methyl-3-ethylheptane is: CH3-CH(CH3)-CH(CH2-CH3)-CH2-CH2-CH2-CH3

(d) 2,3-dimethylpentane Structure

To draw the structural formula for 2,3-dimethylpentane, follow these steps: 1. Draw a straight chain of 5 carbon atoms (pentane) in a horizontal line. 2. Add hydrogens to each carbon atom so that each has a total of four bonds. 3. On the second carbon atom (counting from left or right), replace one hydrogen atom with a methyl group (-CH3). 4. On the third carbon atom (counting from left or right), replace one hydrogen atom with a methyl group (-CH3). The resulting structural formula for 2,3-dimethylpentane is: CH3-CH(CH3)-CH(CH3)-CH2-CH3

Key concepts

Structural Formulas

In organic chemistry, a structural formula represents the arrangement of atoms within a molecule. It provides more detail than a simple molecular formula, showing how the atoms are connected. For alkanes, which are saturated hydrocarbons, the structural formula reveals all carbon (C) and hydrogen (H) bonds, making it evident how each carbon is connected and how many hydrogen atoms are bonded to it. Alkanes follow the general formula of \( C_nH_{2n+2} \).

To create a structural formula, start with the carbon backbone. For example, in 3-ethylpentane, draw five carbon atoms in a straight chain (since pentane refers to five carbons). Then, add hydrogen atoms so each carbon achieves four bonds. This fulfills the alkane's saturated nature.

The beauty of structural formulas lies in their specificity. They allow chemists to visualize the exact nature of the molecule—where branches occur, and which carbons host additional groups. This is crucial for understanding chemical properties and reactivity.

Substituted Hydrocarbons

Substituted hydrocarbons are alkanes that have one or more hydrogen atoms replaced by other atoms or groups of atoms, called substituents. These compounds are key in organic chemistry due to their varied properties and reactivities.

For example, the substituents in the given exercises include the ethyl (\(-CH_2-CH_3\)) and methyl (\(-CH_3\)) groups. In the molecule 3-ethylpentane, an ethyl group substitutes a hydrogen atom on the third carbon of the pentane chain. Similarly, in 2,2-dimethylbutane, two methyl groups substitute hydrogens on the second carbon of the butane chain.

This substitution causes a significant change in the molecule's characteristics, such as its boiling point, melting point, and solubility. Understanding these changes is vital for predicting how substituted hydrocarbons will behave in different conditions, making them a topic of great interest in chemical synthesis and industrial applications.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with carbon-containing compounds. It's central to chemical sciences because carbon can form stable bonds with many elements, allowing for a diversity of compounds that are the basis of life and materials we use daily.

Alkanes are a starting point in organic chemistry, as they are simple hydrocarbons with single bonds that establish basic concepts like hybridization and molecular geometry. Substituted alkanes, like those explored here, illustrate how adding different groups alters molecular behavior.

Organic chemistry principles, such as the concept of functional groups and the idea of isomerism (different compounds with the same molecular formula), are fundamental to creating and analyzing new compounds, whether for pharmaceuticals, plastics, or fuels. By mastering the structures of simple alkanes and their derivatives, students lay groundwork for more complex molecular design and synthesis.

Nomenclature

Nomenclature in organic chemistry is the systematic method for naming organic compounds. It ensures that each compound's name provides insight into its structure.

For alkanes, names are derived from the number of carbon atoms in the longest continuous chain. Substituents are mentioned with their position (the smallest number possible along the chain from either end) and type (like methyl or ethyl).

Let's consider 2-methyl-3-ethylheptane:

• "Heptane" indicates a seven-carbon chain.
• "2-methyl" means a methyl group on the second carbon.
• "3-ethyl" means an ethyl group on the third carbon.

This systematic approach is essential for clarity in communication among chemists worldwide. By learning these naming rules, students can derive a significant amount of information about the compound simply from its name, which is invaluable for chemical identification and synthesis.