Difference Between Alkanes and Alkenes

Main Difference – Alkanes vs Alkenes

Hydrocarbons are compounds composed of C and H atoms. These hydrocarbon molecules have different combinations of C and H atoms and have different geometries according to the spatial arrangement of the molecules. Since thousands of hydrocarbons have been discovered so far, there are different classifications for these compounds. Alkanes and alkenes are such two categories. Both these types of hydrocarbons are composed of branched, unbranched and cyclic hydrocarbons. The main difference between alkanes and alkenes is that alkanes are saturated hydrocarbons whereas alkenes are unsaturated hydrocarbons.

Key Areas Covered

1. What are Alkanes
      – Definition, Nomenclature, Properties, and Reactions
2. What are Alkenes
      – Definition, Nomenclature, Properties, and Reactions
3. What are the Similarities Between Alkanes and Alkenes
      – Outline of Common Features
4. What is the Difference Between Alkanes and Alkenes
      – Comparison of Key Differences

Key Terms: Aliphatic, Alkanes, Alkenes, Branched Alkanes, Crude Oil, Cyclic Alkanes, Hydrocarbons, Linear Alkanes , Natural Gas, Saturated Hydrocarbons, Unsaturated Hydrocarbons

What are Alkanes

Alkanes are saturated hydrocarbons having the chemical formula C_nH_2n+2 (where n is a whole number). Alkanes are called hydrocarbons because they are composed of C and H atoms. All these atoms are linked to each other via single covalent bonds. Since there are no double or triple bonds, alkanes are called saturated hydrocarbons.

Alkanes are a broad group of organic molecules. They are named according to the number of carbon atoms and the side groups that they are comprised of. The smallest alkane is methane. In methane, a central carbon atom is bonded to 4 hydrogen atoms. The IUPAC nomenclature of alkanes is based on Greek prefixes. Some examples are given in the below table.

According to the structure of alkanes, there are three major types of alkanes. They are;

• Linear alkanes
• Branched alkanes
• Cyclic alkanes

Linear alkanes are straight chain alkanes having no side groups attached to the alkane molecule. Branched alkanes are molecules having alkyl groups as side groups, but they are saturated molecules. Alkyl groups are alkanes lacking one hydrogen atom, thus having a vacant point to get attached to a carbon atom. Cyclic alkanes are ring structures.

Figure 1: Different Forms of Alkanes

The main sources of alkanes are crude oil and natural gases. These fuels are composed mainly of alkane molecules. Therefore we can obtain alkanes through petroleum refining. A process called fractional distillation separates these alkanes.

All alkanes are colorless and odorless. The melting points and boiling points increase with the increase of the number of carbon atoms. Under standard temperature and pressure conditions, some alkanes are liquids, and some are gaseous compounds. This difference is due to their different boiling points. Alkanes show isomerism. An alkane molecule may have structural isomerism or stereoisomerism according to its structure and spatial arrangement of the molecule.

One of the major reactions that alkanes undergo is the combustion reaction. The combustion of alkanes includes the reaction between alkane and molecular oxygen. This reaction produces carbon dioxide, water, and heat. Moreover, alkanes can undergo halogenation reactions. There, molecules like Cl₂ react with alkanes when sufficient energy is provided. This reaction produces alkyl halides.

What are Alkenes

Alkenes are unsaturated hydrocarbons having the chemical formula C_nH_2n. These are called hydrocarbons because alkenes are only composed of C and H atoms. They are called unsaturated because they are composed of at least one C=C double bond. Therefore, there cannot be any alkene having only one carbon atom. The simplest alkene is C₂H₄. Small alkenes are gases at room temperature. Other alkenes are liquids. But alkenes having a very large number of carbon atoms are in a waxy solid state.

Figure 2: General Structure of Alkenes

In the above image, R1, R2, R3, and R4 are alkyl groups that can be either identical or different from each other.

An alkene may have one or more double bonds. Most of the times, these double bonds are arranged in an alternative manner. Alkenes are colorless and nonpolar. Alkenes are considered as reactive molecules due to the presence of a double bond. This double bond is composed of a sigma bond and a pi bond. This pi bond can be broken down; this is called the opening up of the double bond. Then electrophiles can be attached to these carbon atoms that was in a double bond. These are called addition reactions of alkenes.

In addition, polymerization is a major reaction that alkenes are able to undergo. The opening up of the double bond allows alkenes to undergo polymerization. Here, one alkene molecule is called a monomer. This leads to the production of very important polymers.

Alkenes show geometrical isomerism. This is also called cis-trans isomerism. When naming an alkene, one should also consider the cis-trans geometry of these molecules.

Similarities Between Alkanes and Alkenes

• Alkanes and alkenes are hydrocarbons.
• Small alkane molecules and small alkene molecules are gases at room temperature.
• The atoms in alkanes and alkenes are bonded to each other through covalent bonds.
• Both are nonpolar molecules.
• Therefore both are insoluble in water and other polar solvents.
• Both alkanes and alkenes can be burnt to form heat along with carbon dioxide and water vapor.

Difference Between Alkanes and Alkenes

Definition

Alkanes: Alkanes are saturated hydrocarbons having the chemical formula C_nH_2n+2.

Alkenes: Alkenes are unsaturated hydrocarbons having the chemical formula C_nH_2n.

Chemical Bonding

Alkanes: Alkanes are composed of only single bonds.

Alkenes: Alkenes are composed of double bonds as well as single bonds.

Chemical Formula

Alkanes:  The chemical formula for alkanes is C_nH_2n+2.

Alkenes: The chemical formula for alkenes is C_nH_2n.

Polymerization

Alkanes: Alkanes cannot undergo polymerization.

Alkenes: Alkenes can undergo polymerization.

Pi Bonds

Alkanes: Alkanes have no pi bonds.

Alkenes: Alkenes are composed of pi bonds.

Conclusion

Alkanes and alkenes are important hydrocarbons. Although there are similarities between alkanes and alkenes such as nonpolar behavior and insolubility in water, they have many distinct features. The main difference between alkanes and alkenes is that alkanes are saturated hydrocarbons whereas alkenes are unsaturated hydrocarbons.

References:

1. “Alkanes: Definition, Properties, Formula & Examples.” Study.com, Available here. Accessed 21 Aug. 2017.
2. Libretexts. “Reactivity of Alkenes.” Chemistry LibreTexts, Libretexts, 28 Feb. 2017, Available here. Accessed 21 Aug. 2017.

Image Courtesy:

1. “Saturated C4 hydrocarbons ball-and-stick” By Fvasconcellos – Own work (Public Domain) via Commons Wikimedia

2. “Alkene General Formula V.1” By Jü – Own work (CC0) via Commons Wikimedia