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Straight-Chain Alkanes

Physical properties, structure of linear, saturated hydrocarbons

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Straight-Chain Alkanes

Minibus prototype that runs on propane

Credit: Courtesy of Yoichi Robert Okamoto, Environmental Protection Agency
Source: http://commons.wikimedia.org/wiki/File:PROTOTYPE_OF_A_MINI-BUS_WHICH_WILL_HAVE_A_MAXIMUM_SPEED_OF_SEVEN_MPH_AND_WILL_RUN_ON_PROPANE_GAS-LATER_TO_BE..._-_NARA_-_549652.tif
License: CC BY-NC 3.0

How can we rethink transportation?

As our country looks at the prospect of oil shortages in the future, we are searching for alternative transportation fuel sources. One very viable possibility is propane gas. Power and acceleration for propane-powered vehicles are comparable to gasoline-powered vehicles and fuel efficiency is greater. Propane has a higher octane rating than regular gasoline, leading to much longer engine life. When properly structured, propane engines can produce lower amounts of air pollution. We are seeing a growing use of propane in buses, trucks, and police cars. Pictured above is a prototype of a minibus that will run on propane fuel. Maybe your next car will burn propane.

Straight-Chain Alkanes

Hydrocarbons

A hydrocarbon is an organic compound that is made up of only carbon and hydrogen. A hydrocarbon is the simplest kind of organic molecule and is the basis for all other more complex organic compounds. Hydrocarbons can be divided into two broad categories. Aliphatic hydrocarbons are hydrocarbons that do not contain the benzene group or a benzene ring. Aromatic hydrocarbons contain one or more benzene rings. In this concept, we will discuss the aliphatic hydrocarbons.

Alkanes

An alkane is a hydrocarbon in which there are only single covalent bonds. The simplest alkane is methane, with the molecular formula CH4. The carbon is the central atom and makes four single covalent bonds to hydrogen atoms.

Various representations of the structure of methane

Credit: (A) CK-12 Foundation - Steven Lai; (B) and (C) Ben Mills (Wikimedia: Benjah-bmm27)
Source: B) http://commons.wikimedia.org/wiki/File:Methane-CRC-MW-3D-balls.png; (C) http://commons.wikimedia.org/wiki/File:Methane-CRC-MW-3D-vdW.png
License: CC BY-NC 3.0

Methane is the simplest hydrocarbon and is shown with a structural formula, a ball-and-stick model, and a space-filling model.[Figure2]

The next simplest alkane is called ethane (C2H6) and consists of two carbon atoms with a single covalent bond between them. Each carbon is then able to bond to three hydrogen atoms. The alkane series progresses from there, increasing the length of the carbon chain by one carbon at a time. Structural formulas for ethane, propane (C3H8), and butane (C4H10) are shown below.

Nomenclature of three straight-chain alkanes

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

These alkanes are called straight-chain alkanes because the carbon atoms are connected in one continuous chain with no branches. Naming and writing structural and molecular formulas for the straight-chain alkanes is straightforward. The name of each alkane consists of a prefix that specifies the number of carbon atoms and the ending –ane. The molecular formula follows the pattern of CnH2n+2 where n is the number of carbons in the chain. Table below lists the first ten members of the alkane series.

First Ten Members of the Alkane Series

Name

Molecular Formula

Condensed Structural Formula

Boiling Point (°C)

Methane

CH4

CH4

-161.0

Ethane

C2H6

CH3CH3

-88.5

Propane

C3H8

CH3CH2CH3

-42.0

Butane

C4H10

CH3CH2CH2CH3

0.5

Pentane

C5H12

CH3CH2CH2CH2CH3

36.0

Hexane

C6H14

CH3CH2CH2CH2CH2CH3

68.7

Heptane

C7H16

CH3CH2CH2CH2CH2CH2CH3

98.5

Octane

C8H18

CH3CH2CH2CH2CH2CH2CH2CH3

125.6

Nonane

C9H20

CH3CH2CH2CH2CH2CH2CH2CH2CH3

150.7

Decane

C10H22

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

174.1

Note that the table shows a variation of a structural formula called a condensed structural formula. In this formula, the covalent bonds are understood to exist between each carbon and the hydrogens associated with it, as well as between carbon atoms. This table also shows that the boiling points of the alkanes steadily increase as the length of the carbon chain increases. This is due to an increase in the strength of the intermolecular attractive forces and is a general feature of other organic molecules as well.

 

 

Review

  1. What is a hydrocarbon?
  2. What is an alkane?
  3. Name the alkane that has five carbons in its chain.

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Image Attributions

  1. [1]^ Credit: Courtesy of Yoichi Robert Okamoto, Environmental Protection Agency; Source: http://commons.wikimedia.org/wiki/File:PROTOTYPE_OF_A_MINI-BUS_WHICH_WILL_HAVE_A_MAXIMUM_SPEED_OF_SEVEN_MPH_AND_WILL_RUN_ON_PROPANE_GAS-LATER_TO_BE..._-_NARA_-_549652.tif; License: CC BY-NC 3.0
  2. [2]^ Credit: (A) CK-12 Foundation - Steven Lai; (B) and (C) Ben Mills (Wikimedia: Benjah-bmm27); Source: B) http://commons.wikimedia.org/wiki/File:Methane-CRC-MW-3D-balls.png; (C) http://commons.wikimedia.org/wiki/File:Methane-CRC-MW-3D-vdW.png; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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