Question

Write a structural formula and a condensed structural formula for each alkane. (a) methane (b) pentane (c) butane (d) propane

Short answer

The condensed structural formulas are: (a) CH4, (b) C5H12, (c) C4H10, (d) C3H8. The structural formulas are the corresponding chain structures with single bonds between carbons and the appropriate number of hydrogens to satisfy each carbon's four bonds.

Step-by-step solution

Understanding Alkanes

Alkanes are hydrocarbons with the general chemical formula CnH2n+2, where n is the number of carbon atoms. They consist only of single bonds between the carbon atoms and are saturated compounds.

Drawing Structural Formulas for Methane

Methane is the simplest alkane with only one carbon atom. The structural formula is a single carbon with four hydrogen atoms bonded to it, forming a tetrahedral shape.

Writing Condensed Structural Formula for Methane

Since methane has only one carbon atom and four hydrogen atoms, its condensed structural formula is simply CH4.

Drawing Structural Formulas for Pentane

Pentane has five carbon atoms. Draw a chain of five carbon atoms with each carbon atom (except the ends) bonded to two hydrogen atoms, and the end carbon atoms bonded to three hydrogen atoms each.

Writing Condensed Structural Formula for Pentane

The condensed structural formula for pentane is C5H12. This represents a chain of five carbon atoms each bonded to the appropriate number of hydrogen atoms to satisfy the alkane formula CnH2n+2.

Drawing Structural Formulas for Butane

Butane contains four carbon atoms. Draw a chain of four carbon atoms with each carbon atom (except the ends) bonded to two hydrogen atoms, and the end carbon atoms bonded to three hydrogen atoms each.

Writing Condensed Structural Formula for Butane

The condensed structural formula for butane is C4H10, corresponding to four carbon atoms and the hydrogen atoms arranged to fulfill the general alkane formula.

Drawing Structural Formulas for Propane

Propane consists of three carbon atoms. Draw a chain of three carbon atoms with the middle carbon atom bonded to two hydrogen atoms, and the end carbon atoms each bonded to three hydrogen atoms.

Writing Condensed Structural Formula for Propane

Propane's condensed structural formula is C3H8. This represents three carbon atoms with hydrogen atoms completing the alkane formula.

Key concepts

Alkane Chemical Formula

Alkanes are fundamental organic compounds that make up a significant portion of many substances around us, including fuels like propane and butane. As beginners in organic chemistry, understanding alkanes starts with their chemical formula. The simplest way to define an alkane is through its general formula, which is (CnH2n+2). In this, 'n' represents the total number of carbon (C) atoms present; hydrogen (H) atoms are always double the carbon count, plus two more. For example, with methane (CH4), the smallest alkane, 'n' is 1, which leads to 2*(1)+2 = 4 hydrogen atoms. Remembering this formula allows students to easily deduce the composition of any alkane.

It’s crucial to acknowledge that the alkane formula signifies only single carbon-carbon bonds and no double or triple bonds – a characteristic which classifies them as saturated hydrocarbons. These single bonds contribute to the relatively low reactivity of alkanes compared to other hydrocarbon families.

Drawing Structural Formulas

Visualizing molecules plays a crucial role in chemistry, and drawing structural formulas is a skill that provides insight into the molecular architecture of compounds. When it comes to alkanes, structural formulas represent the bonding arrangement between atoms. For instance, drawing methane involves sketching a carbon atom with four hydrogen atoms connected, resembling a tetrahedron. This three-dimensional shape is a direct consequence of the four single covalent bonds spreading out to minimize repulsion - a concept known as VSEPR (Valence Shell Electron Pair Repulsion) theory.

To draw structures of more complex alkanes, one must simply connect carbon atoms in a chain, ensuring that each carbon is bonded to the correct number of hydrogens. A trick that often helps is to start with the carbon backbone and then fill in the hydrogen atoms, making sure that each carbon makes four bonds in total. It’s a handy exercise for developing a deeper understanding of molecular structures and organic chemistry as a whole.

Condensed Structural Formulas

Condensed structural formulas provide a bridge between the fully detailed structural formulas and the chemical formula. They are especially useful when dealing with complex molecules where drawing every single bond can become cumbersome. Instead of showing each bond explicitly, a condensed formula lumps together atoms in a way that still makes it clear how they are connected. For example, the condensed formula for pentane, C5H12, tells you there are five carbons connected in a straight chain, and enough hydrogen atoms to satisfy all the carbon atoms' valence requirements.

It’s crucial for students to acknowledge that while condensed formulas are streamlined, they still must represent the alkane accurately. Each carbon atom must have four bonds, and single lines (or dashes) usually represent the bonds in structural formulas, but are omitted in the condensed version for brevity and simplicity. This form of notation is beneficial for quickly conveying structural information without too much complexity.

Hydrocarbons

Hydrocarbons are compounds composed solely of hydrogen and carbon atoms. They serve as the backbone for organic chemistry and are classified based on their bonding structure into alkanes, alkenes, and alkynes, to name a few. Alkanes belong to the simplest class of hydrocarbons with only single bonds, denoted as saturated hydrocarbons. This saturation refers to the maximum number of hydrogen atoms that can bond with the carbon atoms.

Understanding hydrocarbons is paramount as they are not only used in fuel but also as the starting material for the synthesis of plastics, pharmaceuticals, and other essential chemicals. The diversity of hydrocarbons is immense because the carbon atoms can link in various forms - as straight chains, branches, or rings. Learning about hydrocarbons provides a solid foundation upon which many other topics in organic chemistry are built.

Organic Chemistry

Organic chemistry is the branch of chemistry that studies the structure, properties, composition, reactions, and synthesis of carbon-containing compounds. Carbon's ability to form stable bonds with many elements, primarily hydrogen, oxygen, nitrogen, and other carbon atoms, leads to an incredible variety of molecules. Organic molecules range from simple substances like methane to complex molecules like DNA.

Mastering organic chemistry involves not only memorizing formulas and structures but also understanding how these molecules behave and interact. The concepts of alkanes and their structural formulas discussed earlier are foundational lessons in organic chemistry, which will then lead to exploring more complex topics such as functional groups, isomerism, and reaction mechanisms. With its applications extending to medicine, engineering, and biochemistry, organic chemistry is a gateway to understanding how microscopic molecular changes can lead to macroscopic consequences in the material world.