Physical Properties

Organic - Alkanes

Alkanes are the simplest of the hydrocarbons. Hydrocarbons are molecules that consist of carbon and hydrogen only. Alkanes are made up of exclusively carbon-hydrogen bonds, and single carbon-carbon bonds.1 Another word for alkane is saturated hydrocarbon, because there is a hydrogen in every available location on the molecule.2 Examples of alkanes are methane, ethane, propane, butane, etc.

Isomerism

Alkanes are not just single carbon linked chains. If there are branches of carbons going off of the carbon chain, as long as the molecule still is single carbon-carbon bonds, and carbon-hydrogen bonds only, it is still an alkane. For example, butane and 2-methyl-propane both have molecular formulas of C4H10, but they are structural isomers.3 A structural isomer is a molecule with the same molecular formula, but arranged in a different way.

Physical properties

Structure

Alkanes have the structure of CnH2n+2. All the carbons are sp3 hybridized, having 4 single bonds to either carbon or hydrogen. Using standard conditions (25oC, 1atm), alkanes up to 4 carbons (i.e. butane) are gases, C5H12 to C17H36 are liquids, and higher than C18H38 are solids. 

Boiling point

The boiling points of alkanes directly correspond to the size of the molecule. Alkanes with higher molecular weight will have higher boiling points. When comparing straight carbon chain alkanes with their structural isomers, the straight chain will have a higher boiling point.4 This is because of Van der Waals forces.1 Van der Walls forces can be described as small intermolecular attractions because of temporary dipole moments in the molecule. So, the larger molecules have more temporary dipoles, and therefore are more difficult to separate into the gas phase (boil). As a general trend for linear alkanes, the boiling point of an alkane increases 20-30oC for every carbon added. 5

Melting point

The trends for boiling points in alkanes are very similar for melting points as well; the larger the molecule, the higher the melting point. However, odd-numbered alkanes (odd number of carbons) have lower melting points than even-numbered alkanes. This is because the even-numbered alkanes pack closer together more easily, making it more difficult to break down the solid material.6

Solubility

Alkanes are almost completely insoluble in water.4 They are, however, soluble in organic solvents. Liquid alkanes tend to be good solvents themselves for other organic molecules.1

History

Explore the discoverer's biography, including general facts about his life and anecdotes regarding how he made this particular discovery. Also see other significant scientific discoveries built largely on this concept and other real-world applications in history that may not still be relevant.

Use/Application through History

Before the invention of electricity, alkanes were used as fuel for light.5 Another early use that is still being used today (and adapted to the use of electricity) is paraffin wax. Parrafin wax is a mixture of alkanes that are solid at room temperature (20-40 carbons), that liquefies at around 37oC.6 It can be used as wax, but is more useful today as an electrical insulator. Additionally, in the food industry it is used to lubricate baking tins, or coat fruit to make them appear shiny.7

Alkanes can be found in the universe too! For example, Jupiter, Saturn, Uranic and Neptune all have alkanes in their atmospheres. Titan, a satellite of Saturn, also rains liquid methane, and there is a methane-spewing volcano. Meteorites also have track amounts of alkanes in them. Methane gas is found in the Earth’s atmosphere as well. Organisms like Archaea, that live in the gut of cows, are the primary producer of methane in our planet’s atmosphere.8

 

Concept Definition

Study the primary definition of this concept, broken into general, basic, and advanced English definitions. Also see the mathematical definition and any requisite background information, such as conditions or previous definitions.

General Science

Molecules that are made up carbons and hydrogens with only single bonds.

Basic

All carbons are sp3

Mathematical Definition

CnH2n+2

Background Information

The alkanes were originally called paraffins, which came from the Latin phrase parum affinis, which means little affinity. This phrase was used due to the low reactivity of these hydrocarbons. When the alkanes do react the reactions are usually quite vigorous.

 

Vocabulary

Learn important vocabulary for this concept, including words that might appear in assessments (tests, quizzes, homework, etc.) that indicate the use of this concept.

 Important Vocabulary

Term Context
Alkanes Molecules that consist of hydrogens and carbon boned singularly together. 
Hydrocarbons Molecules made up of carbon and hydrogen only.
Parrafin wax Mixture of molecules that are solid at room temperature (20-40 carbons) and liquifies at around 37 degrees Celsius. It is used in electrical conductors, and in the food industry as a lubricator and for food appearance.
Saturated hydrocarbons A molecule made up of carbon-carbon chains, where a hydrogen is in every available location; another name for alkane. 
Structural Isomers Molecule with the same molecular formula, but arranged differently in space. 
Van der Waals forces Small intermolecular forces of attraction that are due to small dipole moments in molecules.

 

Videos

Browse relevant videos from the Journal of Chemical Education's (JCE) Chemistry Comes Alive! library and other video sources.

Physical Properties

Alkanes/Alkenes/Alkynes


Duration: 0 s s; Size: 164 KB

Comments (0)