Question

Draw the structural formula for each of the following straight-chain alkanes. a. pentane b. ethane c. undecane (eleven carbon atoms) d. methane

Short answer

The structural formulas for the given straight-chain alkanes are: a. Pentane: CH3-CH2-CH2-CH2-CH3 b. Ethane: CH3-CH3 c. Undecane: CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3 d. Methane: CH4

Step-by-step solution

Identifying Alkanes Formula

Alkanes are hydrocarbons with only single bonds between the carbon atoms and the general formula \(C_nH_{2n+2}\). Straight-chain alkanes are alkanes in which all the carbon atoms are connected in a linear arrangement, with no branching.

a. Drawing the Structural Formula for Pentane

Pentane has 5 carbon atoms (n=5) due to "pent" meaning 5. Using the general formula for alkanes, we can calculate the number of hydrogen atoms: \(H_{2n+2} = H_{2(5)+2} = H_{12}\). Hence, the molecular formula for pentane is C5H12. The structural formula is: CH3-CH2-CH2-CH2-CH3

b. Drawing the Structural Formula for Ethane

Ethane has 2 carbon atoms (n=2). The number of hydrogen atoms can be calculated using the general formula: \(H_{2n+2} = H_{2(2)+2} = H_{6}\). The molecular formula for ethane is C2H6. The structural formula is: CH3-CH3

c. Drawing the Structural Formula for Undecane

Undecane, as "undec" means eleven, has 11 carbon atoms (n=11). Using the general formula for alkanes, we can again calculate the number of hydrogen atoms: \(H_{2n+2} = H_{2(11)+2} = H_{24}\). The molecular formula for undecane is C11H24. The structural formula is: CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3

d. Drawing the Structural Formula for Methane

Methane, having only 1 carbon atom (n=1), is the simplest alkane. So, calculating the number of hydrogen atoms from its general formula: \(H_{2n+2} = H_{2(1)+2} = H_{4}\). The molecular formula for methane is CH4. The structural formula is: CH4

Key concepts

Understanding Alkanes

Alkanes are a class of hydrocarbons that are recognized by their simplified structure and chemical behavior. They are saturated hydrocarbons, meaning each carbon atom is bonded to the maximum possible number of hydrogen atoms. This characteristic gives alkanes a relatively simple composition and a consistent pattern in their formulas. Each carbon atom in an alkane forms four single covalent bonds, which can be with either another carbon or a hydrogen atom.

What truly sets alkanes apart is their lack of more complex functional groups, making their reactivity quite low. This simplicity translates into an easily recognizable molecular formula pattern, making alkanes a perfect starting point for anyone diving into organic chemistry. Alkanes follow a general formula of \(C_nH_{2n+2}\), where \(n\) is the number of carbon atoms.

Straight-chain alkanes are where all carbon atoms are linked in a single linear sequence without any branching. Pentane, as given, is an example where the structural formula showcases the carbon atoms connected one after the other, following a straight chain pattern.

Decoding the Molecular Formula

The molecular formula of a compound reveals key details about its composition. For alkanes, the molecular formula, like \(C_5H_{12}\) for pentane, provides a concise representation of its constituent elements and their proportions. This formula arises from the alkane's general formula \(C_nH_{2n+2}\). It's a critical feature as it reflects both the number of carbon atoms and hydrogen atoms present.

To decode this formula:

• The subscript of carbon (\(C\)) denotes the number of carbon atoms in the molecule.
• The subscript of hydrogen (\(H\)) is derived from the \(H_{2n+2}\), resulting from twice the carbon atoms plus two.

This formula not only indicates the quantity but also helps depict the bonding structure. It acts as a foundation for drawing structural diagrams of alkanes, breaking down a complex 3D molecular structure into understandable symbolic representations.

Characteristics of Hydrocarbons

Hydrocarbons are organic compounds composed solely of hydrogen and carbon atoms. They are the simplest organic compounds, yet they form the backbone of chemistry. Alkanes fall under the category of saturated hydrocarbons due to their single-bond nature. This saturation impacts their physical properties such as melting and boiling points, which increase with longer carbon chains due to intermolecular forces like van der Waals forces.

Hydrocarbons like alkanes are abundant in nature, forming the primary components of fossil fuels. Their straightforward structure, seen in examples such as ethane or methane, helps in easily identifying their type based on combustion properties and energy content. This simplicity doesn't limit their utility; rather, it exemplifies their adaptability and significance as a basic building block in synthetic organic chemistry.

The Role of Single Bonds

Single bonds in alkanes are one of the most fundamental types of chemical bonds. Each carbon atom in an alkane connects to other atoms through four single bonds, maintaining stability and forming a tetrahedral shape around the carbon atoms.

These bonds occur due to the sharing of one pair of electrons between two atoms, which provides stability and shapes the simpleness of alkane molecules. For example, the single covalent bonds in ethane (CH3-CH3) allow for free rotation of the atoms around them, giving alkanes flexibility and contributing to their low reactivity.

Single bonds ensure that alkanes remain saturated, without room for addition reactions under normal conditions. This quality is essential for predicting reactions and behaviors in various chemical contexts, thus serving as a fundamental concept in understanding the chemistry of alkanes and related hydrocarbons.