11 3.1 Nomenclature and Physical Properties of Alkanes and Cycloalkanes
Chapter 3.1 Learning Objectives
- Be able to apply the IUPAC rules for naming alkanes and cycloalkanes.
- Given the IUPAC name, be able to draw alkanes and cycloalkanes.
- Be able to relate the IMF with the physical properties of alkanes.
3.1.1 Introduction
Hydrocarbons are made up of carbon and hydrogen atoms. Saturated hydrocarbons contain only C-C and C-H single bonds. Unsaturated hydrocarbons contain multiple bonds (such as double and triple bonds) between carbon atoms. Alkanes are saturated hydrocarbons. All C atoms in an alkane are surrounded by four groups, making them sp3 hybridized and tetrahedral, and all bond angles are 109.5°. General formula of alkanes is CnH2n+2.
3.1.2 Nomenclature of Alkanes
In this chapter, we will learn about nomenclature, structure and reactivity of alkanes and cycloalkanes. In the early 19th century, organic compounds’ names were selected by their discoverer. These are referred to as common names and many of them such as morphine are still in use today. As the library of organic compounds grew, it became essential for a systematic method of naming compounds to be established. In 1892, a group of chemists, now referred to as the International Union of Pure and Applied Chemistry (IUPAC) met in Switzerland and developed a system of organic nomenclature called the Geneva rules, now referred to as IUPAC nomenclature. Both IUPAC/systematic and common names will be discussed. The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity concerning which chemical compound the name refers to: each chemical name should refer to a single substance.
Additional Resources
- This video is an introduction to IUPAC nomenclature.
- Watch this video on the basics of organic nomenclature.
- A step by step instructional guide on naming alkanes can be found here.
- How to name straight chain alkanes?
- How to name branched chain alkanes?
Exercise: Name the given alkanes using IUPAC rules.
Exercise: Draw the structure of the alkane for each given name.
Check out this resource for naming cycloalkanes.
3.1.3 Physical Properties of Alkanes
Alkanes show regular increases in both boiling point and melting point as molecular weight increases an effect due to the presence of weak dispersion forces between molecules. Only when sufficient energy is applied to overcome these forces does the solid melt or liquid boil. As you might expect, dispersion forces increase as molecular size increases, accounting for the higher melting and boiling points of larger alkanes.
Another effect seen in alkanes is that increased branching lowers an alkane’s boiling point. Thus, pentane has no branches and boils at 36.1 °C, isopentane (2-methylbutane) has one branch and boils at 27.85 °C, and neopentane (2,2-dimethylpropane) has two branches and boils at 9.5 °C. Similarly, octane boils at 125.7 °C, whereas isooctane (2,2,4-trimethylpentane) boils at 99.3 °C. Branched-chain alkanes are lower-boiling because they are more nearly spherical than straight-chain alkanes, have smaller surface areas, and consequently have smaller dispersion forces.
A plot of melting and boiling points versus number of carbon atoms for the C1–C14 straight-chain alkanes. There is a regular increase with molecular size.
Exercise: Name the cycloalkanes using IUPAC rules.
Exercise: Draw cycloalkanes for each given name.
