In this explainer, we will learn how to write and interpret the names and formulas of alkanes and describe trends in physical properties.
Alkanes are saturated organic compounds that contain covalently bonded carbon and hydrogen atoms. Alkanes are some of the least reactive organic compounds and they experience some of the weakest intermolecular forces.
Alkanes are saturated hydrocarbons that contain covalently bonded carbon and hydrogen atoms.
Each carbon atom in an alkane is covalently bonded to four other atoms. This can be seen in the following figure. Some carbon atoms are bonded to two other carbon atoms and two other hydrogen atoms. Other carbon atoms are bonded to just one carbon atom and three hydrogen atoms. All of the carbon and hydrogen atoms are, however, always linked with single covalent bonds. The following image shows three representative alkane molecules. The top line presents the molecules using a ball-and-stick model, and the second and third lines present the molecules using molecular formula and displayed formula representation methods.
Covalent bonds are made up of negatively charged electrons, and this explains why there is always electrostatic repulsion between all of the four covalent bonds around any one central carbon atom. The covalent bonds are pushed away from each other and end up taking on a shape that minimizes the strength of the repulsive electrostatic forces. The four covalently bonded atoms end up being spread out like the corners of a tetrahedron, and there is an angle of between each one of the four covalent bonds.
Methane () is the simplest alkane. It contains just one carbon atom that is covalently bonded to four hydrogen atoms. Ethane () is a slightly more structurally complex molecule, and it contains two carbon atoms and six hydrogen atoms. Methane and ethane are the first two members of the straight-chain alkane homologous series and propane () is the third member.
Example 1: Identifying the Name of the Simplest Alkane
What is the name of the alkane with the chemical formula ?
Alkane molecules are relatively simple organic compounds that contain covalently bonded carbon and hydrogen atoms. Some of the alkane molecules are very long and complex and others are very simple and short. Methane () is the simplest alkane, and it contains just one carbon atom and four covalently bonded hydrogen atoms. We can use this explanation to determine that option A must be the correct answer for this question.
The straight-chain alkane homologous series has the general formula, where is the number of carbon atoms and is the number of hydrogen atoms. The formula can be used to determine the molecular formula for any short straight-chain alkane such as ethane or any long straight-chain alkane such as decane. The formula can also be used to determine how many hydrogen atoms there are in an alkane that has some known number of carbon atoms. For example, we can determine that propane has eight hydrogen atoms because the “prop-” term suggests that propane has three carbon atoms and when .
Example 2: Determining the Molecular Formula of an Unknown Alkane Compound
What is the molecular formula of an alkane with 7 carbon atoms?
The straight-chain alkane must be heptane because it has seven carbon atoms. We can determine the number of hydrogen atoms in heptane with the general formula for the straight-chain alkane homologous series. The straight-chain alkane homologous series is described by the general formula , where is the number of carbon atoms and is the number of hydrogen atoms. We can determine that heptane has sixteen hydrogen atoms because we know that and . The calculations suggest that heptane has the formula . This line of reasoning can be used to determine that option C must be the correct answer for this question.
The general formula can only be used for straight-chain alkanes such as methane and ethane. This formula only works when there are groups at the endpoints of an alkane molecule, and this is not true for the series of cyclic alkanes. Cyclic alkanes usually only contain groups, and this is why we use the formula to determine the molecular formula of cyclic alkanes such as cyclopentane and cyclohexane.
Example 3: Identifying the General Formula for the Cyclic Alkane Homologous Series
What is the general formula for the group of compounds to which the following displayed formula is related?
General formulas are simple mathematical formulas that describe the relative abundance of atoms in separate classes or groups of molecules. The figure depicts a cycloalkane molecule, and cycloalkane molecules generally have two hydrogen atoms for every carbon atom. The correctly predicts that there will be twice as many hydrogen atoms as carbon atoms in a cycloalkane molecule. We can use these statements to determine that option D has to be the correct answer for this question.
The composition of alkanes can become quite complex, and the IUPAC has developed a naming system that helps organic chemists identify and classify different types of alkane molecules. The IUPAC base (root term) is determined from the number of carbon atoms in the longest continuous chain of carbon atoms and the IUPAC prefix is determined from the number and type of any side chains. The suffix will always be the “-ane” term because the IUPAC system uses this suffix to indicate that a molecule is a saturated hydrocarbon.
|Number of Carbon Atoms||Base (root)||Suffix||Parent Alkane Name||Parent Alkane Formula||Alkyl Group Name||Alkyl Group Formula|
We will use the IUPAC naming system to classify a single organic compound. The displayed formula for the unknown compound is shown below.
The longest carbon chain contains seven carbon atoms. This suggests that the base or root is the “hept-” term. There are no side chains. This suggests that we should leave the prefix blank. The molecule is a saturated hydrocarbon. This implies that the suffix is “-ane.” We can put these terms together to determine that this molecule should be called heptane.
It is much more challenging to determine the name for a highly branched hydrocarbon, but we can take a look at just one example so you can better understand the IUPAC naming system. The following figure shows the displayed formula of the next unknown hydrocarbon. The unknown hydrocarbon has the structural formula .
The longest carbon chain contains four carbon atoms, and this implies that the base or root is the “but-” term. There are two methyl group side chains, so the prefix must include the “dimethyl-” term. The methyl groups must be located at the second carbon atom if we are counting carbon atoms from left to right and at the third carbon atom if we are counting carbon atoms from right to left. We will count from left to right because this gives us smaller numbers and we always want the smallest set of numbers possible. If we follow this logic to its endpoint, we should be able to see that this molecule is called 2,2-dimethylbutane.
There are an almost unlimited number of different alkanes, but it is interesting to note that there are general trends between the size of an alkane and its physical properties. It has been found that longer alkanes generally have higher boiling and melting points. This trend can be understood if we consider the different chain–chain dispersion interaction forces between short-chain alkanes and longer-chain alkanes.
The dispersion forces depend on both the size and the shape of the interacting alkane molecules. Dispersion forces are stronger when there is more surface area for contact between neighboring molecules. There is generally more surface area for contact between adjacent molecules that are longer and have more carbon atoms, but this simple relationship can be complicated by alkane chain branching. There tends to be more distance between highly branched alkanes because these molecules cannot effectively stack on top of each other.
Example 4: Understanding the Relationship between Alkane Chain Length and Alkane Melting Point
Which of the following alkanes has the highest melting point?
Alkane melting point values are inextricably linked with alkane molecule chain lengths. Longer straight-chain alkanes have higher melting points than shorter straight-chain alkanes. We can determine that hexane has six carbon atoms and that it is the longest molecule from this list of comparable straight-chain alkane molecules. Hexane has the highest melting point because it is the longest of the listed straight-chain alkane molecules. We can use this line of reasoning to determine that option E is the correct answer for this question.
Definition: London Dispersion Force
The London dispersion force is a temporary attractive interaction that exists between any two adjacent molecules that are capable of forming dipoles.
There is a similar relationship between the viscosity and density of a straight-chain alkane and its length. Longer straight-chain alkanes have higher viscosity and density values and shorter straight-chain alkanes have lower viscosity and density values.
|Straight-Chain Alkane||Chemical Formula||Density (g/mL) at|
The length of an alkane similarly determines its flammability value. Flammability is defined as the ease with which a combustible substance can be ignited. Longer alkane molecules tend to have lower flammability values and shorter alkane molecules tend to have higher flammability values.
The volatility of a straight-chain alkane can also be determined from its length. Longer straight-chain alkanes have more surface area for contact and they experience stronger intermolecular interactions. The strong dispersion forces make alkanes less liable to spontaneously dissociate from a liquid.
Example 5: Understanding the Relationship between Chain Length and Viscosity
Which of the following alkanes has the greatest viscosity?
Alkane viscosity values are inextricably linked with alkane molecule chain lengths. Longer straight-chain alkanes have higher viscosity values than comparable short-chain alkanes. Octane is the longest molecule from this list, and we can therefore surmise that it is the most viscous hydrocarbon. We can use this line of reasoning to determine that option A is the correct answer for this question.
Hydrocarbons are almost always obtained from oil deposits that are discovered underground.
Chemists extract crude oil from large underground reservoirs and then they use fractionating columns to separate the crude oil mixture. The process not only enables chemists to isolate hydrocarbons from other organic compounds, but also enables chemists to separate one type of alkane from another.
The alkanes are then used as a source of energy for planes, trucks, cars, and ships. The energy is obtained by heating the alkanes with a flame and allowing them to react with a plentiful supply of oxygen. This chemical reaction is called complete combustion, and it is always exothermic. The following algebraic formula describes the process of complete combustion for different types of alkanes:
It is important to appreciate that alkanes can only completely combust when they have enough oxygen gas to react with. Alkanes will only be able to incompletely combust if they cannot interact with enough oxygen molecules. Complete and incomplete combustion can be easily distinguished from each other because incomplete combustion produces carbon monoxide and complete combustion does not.
Definition: Complete Combustion
The complete combustion of an alkane is an exothermic reaction that produces carbon dioxide and water molecules.
- Alkanes are saturated hydrocarbons that contain covalently bonded carbon and hydrogen atoms.
- Each carbon atom in any alkane will form four covalent bonds.
- IUPAC nomenclature specifies how chemists should identify and classify different alkanes.
- Alkane chain–chain dispersion interaction strength varies with chain length.
- Longer alkane chains generally have higher phase transition temperatures, viscosity values, and densities.
- Longer alkane chains are generally less volatile.