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Saturated Hydrocarbons

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Saturated Hydrocarbons

You can tell that this sponge is saturated with water because when it is squeezed, the water pours out. When something is saturated with water, it already holds as much water as possible. Compounds called hydrocarbons, which contain only carbon and hydrogen, can also be saturated, but not with water.

What Are Saturated Hydrocarbons?

Saturated hydrocarbons are hydrocarbons that contain only single bonds between carbon atoms. They are the simplest class of hydrocarbons. They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogen. You can see an example of a saturated hydrocarbon in the Figure below . In this compound, named ethane, each carbon atom is bonded to three hydrogen atoms. In the structural formula, each dash (-) represents a single covalent bond, in which two atoms share one pair of valence electrons.

Q: What is the chemical formula for ethane?

A: The chemical formula is C 2 H 6 .

What’s in a Name?

Saturated hydrocarbons are given the general name of alkanes . The name of specific alkanes always ends in – ane . The first part of the name indicates how many carbon atoms each molecule of the alkane has. The smallest alkane is methane. It has just one carbon atom. The next largest is ethane with two carbon atoms. The chemical formulas and properties of methane, ethane, and other small alkanes are listed in the Table below . The boiling and melting points of alkanes are determined mainly by the number of carbon atoms they have. Alkanes with more carbon atoms generally boil and melt at higher temperatures.

Alkane Chemical Formula Boiling Point(°C) Melting Point(°C) State (at 20 °C)
Methane CH 4 -162 -183 gas
Ethane C 2 H 6 -89 -172 gas
Propane C 3 H 8 -42 -188 gas
Butane C 4 H 10 0 -138 gas
Pentane C 5 H 12 36 -130 liquid
Hexane C 6 H 14 69 -95 liquid
Heptane C 7 H 16 98 -91 liquid
Octane C 8 H 18 126 -57 liquid

Q: The Table above shows only alkanes that have relatively few carbon atoms. Some alkanes have many more carbon atoms. What properties might larger alkanes have?

A: Alkanes with more carbon atoms have higher boiling and melting points, so some of them are solids at room temperature.

Shapes of Alkanes

Structural formulas are often used to represent hydrocarbon compounds because the molecules can have different shapes and a structural formula shows how the atoms are arranged. Hydrocarbons may form straight chains, branched chains, or rings (See Figure below ). You can read about each type below.

A) In a straight-chain molecule, all the carbon atoms are lined up in a row like cars of a train. The carbon atoms form the “backbone” of the molecule. B) In a branched-chain molecule, at least one of the carbon atoms branches off from the backbone. C) In a cyclic molecule, the chain of carbon atoms is joined at the two ends to form a ring. Each ring usually contains just five or six carbon atoms, but rings can join together to form larger molecules. A cyclic molecule generally has higher boiling and melting points than straight-chain and branched-chain molecules.

Summary

  • Saturated hydrocarbons are hydrocarbons that contain only single bonds between carbon atoms. As a result, each carbon atom is bonded to as many hydrogen atoms as possible.
  • Saturated hydrocarbons are given the general name of alkanes. The name of specific alkanes always ends in –ane and has a prefix to indicate how many carbon atoms it has.
  • Hydrocarbons may form straight chains, branched chains, or rings. Ring-shaped hydrocarbons are called cyclic hydrocarbons.

Vocabulary

  • alkane : Saturated hydrocarbon that contains only single bonds between carbon atoms.
  • saturated hydrocarbon : Hydrocarbon that contains only single bonds between carbon atoms.

Practice

Read the short article on the following Web page, and then answer the questions below. http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/rocks/fuelsrev1.shtml

  1. What is the general chemical formula for alkanes?
  2. How many hydrogen atoms are there in the alkane that has five carbon atoms? Draw the structural formula for this alkane.
  3. If an alkane has eight carbon atoms, what state is it likely to be in at room temperature? What is its chemical formula?

Review

  1. What is a saturated hydrocarbon?
  2. How are saturated hydrocarbon compounds named?
  3. What are the name and chemical formula of the saturated hydrocarbon that has the following structural formula seen in the Figure below ?

  4. Compare and contrast straight-chain, branched-chain, and cyclic alkanes.

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