Alkane Nomenclature and Properties

Organic compounds can be classified into different groups based on their functional groups, properties, and structures. One of the most important groups of organic compounds is alkanes and cycloalkanes. Alkanes are hydrocarbons that contain only single covalent bonds between carbon atoms and are also known as saturated hydrocarbons. Cycloalkanes are cyclic hydrocarbons where each carbon atom is bonded to two other carbons and two hydrogens. In this lesson, we will discuss the nomenclature and properties of alkanes and cycloalkanes.

Nomenclature of Alkanes

The nomenclature of alkanes follows a systematic naming system that is based on the number of carbon atoms in the molecule. The prefix "meth-" is used for one carbon, "eth-" for two carbons, "prop-" for three carbons, "but-" for four carbons, and so on. The suffix "-ane" is added at the end to indicate that the compound is an alkane. For example, a hydrocarbon with five carbon atoms would be named pentane, and a hydrocarbon with ten carbon atoms would be named decane.

In addition to the main chain, alkanes can have different substituents or branches attached to the main chain. The substituents are named using prefixes such as "methyl-", "ethyl-", "propyl-", "butyl-", etc. The location of the substituent on the main chain is indicated by a number. The number is chosen so that the substituent has the lowest possible number on the main chain. If there are multiple substituents, they are listed in alphabetical order. 2-methylpentane is shown as an example below:

Properties of Alkanes

Alkanes are non-polar molecules due to the absence of any polar functional groups. This leads to low solubility in polar solvents like water and high solubility in non-polar solvents like hydrocarbons. The boiling point of alkanes increases with an increase in the number of carbon atoms because the surface area of the molecule increases, which leads to stronger intermolecular forces. Alkanes are generally unreactive due to the stability of the C-C single bond.

Nomenclature of Cycloalkanes

The nomenclature of cycloalkanes is similar to that of alkanes. The prefix "cyclo-" is added to the name of the alkane with the same number of carbons in the ring. For example, a cyclic hydrocarbon with six carbon atoms would be named cyclohexane. Cycloalkanes can also have substituents attached to the ring. The location of the substituent on the ring is indicated by a number, and the number is chosen so that the substituent has the lowest possible number. If there are multiple substituents, they are listed in alphabetical order.

Properties of Cycloalkanes

Cycloalkanes have properties similar to alkanes. They are non-polar and have low solubility in polar solvents. The boiling point of cycloalkanes also increases with an increase in the number of carbon atoms. However, cycloalkanes have different conformations due to the ring structure, which can affect their physical properties. Cycloalkanes with fewer than seven carbons have ring strain that can cause them to be highly reactive and undergo ring-opening reactions. Cycloalkanes with seven or more carbons are more stable and have similar reactivity to alkanes.

Summary

Alkanes and cycloalkanes are important groups of organic compounds. Alkanes are hydrocarbons that contain only single covalent bonds between carbon atoms, while cycloalkanes are cyclic hydrocarbons. The nomenclature of alkanes and cycloalkanes follows a systematic naming system based on the number of carbon atoms and any substituents attached to the main chain or ring. Alkanes and cyclocalkanes are relatively stable, limiting the number of reactions they participate in.

Test Your Knowledge:

What is the IUPAC name of the following alkane:

Which of the following statements about cycloalkanes is true?

a) Cycloalkanes have higher boiling points than alkanes of the same number of carbons.

b) Cycloalkanes with fewer than seven carbons are more stable than those with seven or more carbons.

c) Cycloalkanes are more reactive than alkanes due to their double bonds.

d) Cycloalkanes can undergo ring-opening reactions due to the ring strain caused by their bond angles.