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Methane, the simplest hydrocarbon, is composed of four hydrogen atoms surrounding a central carbon. The bond between the four hydrogen atoms and the central carbon spaced as far apart as possible. This results in a tetrahedral shape with hydrogen atoms projecting upward and off to three sides around the central carbon. Ethane is composed of two carbons connected by a single bond. Each carbon also has three hydrogen atoms connected to it. The hydrogens are spaced as far apart from each other and from the other carbon so again the shape is tetrahedral. Ethene, like ethane, is composed of two carbon atoms, but in this case the carbons are connected by a double bond. Each carbon also has two hydrogen atoms connected to it, for a total of three bonds. The three bonds are spaced as far apart as possible around carbon, which means they are all on the same plane and pointing off in three directions. As a result, the molecule is planar, or flat.
When carbon forms single bonds with other atoms, the shape is tetrahedral. When two carbon atoms form a double bond, the shape is planar, or flat. Single bonds, like those found in ethane, are able to rotate. Double bonds, like those found in ethene cannot rotate, so the atoms on either side are locked in place.

Hydrocarbon rings

So far, the hydrocarbons we have discussed have been aliphatic hydrocarbons , which consist of linear chains of carbon atoms. Another type of hydrocarbon, aromatic hydrocarbons , consists of closed rings of carbon atoms. Ring structures are found in hydrocarbons, sometimes with the presence of double bonds, which can be seen by comparing the structure of cyclohexane to benzene in [link] . Examples of biological molecules that incorporate the benzene ring include some amino acids and cholesterol and its derivatives, including the hormones estrogen and testosterone. The benzene ring is also found in the herbicide 2,4-D. Benzene is a natural component of crude oil and has been classified as a carcinogen. Some hydrocarbons have both aliphatic and aromatic portions; beta-carotene is an example of such a hydrocarbon.

Four molecular structures are shown. Cyclopentane is a ring consisting of five carbons, each with two hydrogens attached. Cyclohexane is a ring of six carbons, each with two hydrogens attached. Benzene is a six-carbon ring with alternating double bonds. Each carbon has one hydrogen attached. Pyridine is the same as benzene, but a nitrogen is substituted for one of the carbons. No hydrogens are attached to the nitrogen.
Carbon can form five-and six membered rings. Single or double bonds may connect the carbons in the ring, and nitrogen may be substituted for carbon.

Isomers

The three-dimensional placement of atoms and chemical bonds within organic molecules is central to understanding their chemistry. Molecules that share the same chemical formula but differ in the placement (structure) of their atoms and/or chemical bonds are known as isomers    . Structural isomers (like butane and isobutene shown in [link] a ) differ in the placement of their covalent bonds: both molecules have four carbons and ten hydrogens (C 4 H 10 ), but the different arrangement of the atoms within the molecules leads to differences in their chemical properties. For example, due to their different chemical properties, butane is suited for use as a fuel for cigarette lighters and torches, whereas isobutene is suited for use as a refrigerant and a propellant in spray cans.

Geometric isomers , on the other hand, have similar placements of their covalent bonds but differ in how these bonds are made to the surrounding atoms, especially in carbon-to-carbon double bonds. In the simple molecule butene (C 4 H 8 ), the two methyl groups (CH 3 ) can be on either side of the double covalent bond central to the molecule, as illustrated in [link] b . When the carbons are bound on the same side of the double bond, this is the cis configuration; if they are on opposite sides of the double bond, it is a trans configuration. In the trans configuration, the carbons form a more or less linear structure, whereas the carbons in the cis configuration make a bend (change in direction) of the carbon backbone.

Art connection

Part A shows butane and isobutene are structural isomers. Both molecules have four carbons and ten hydrogens, but in butane the carbons form a single chain, while in isobutene the carbons form a branched chain. Part B shows cis-2 butene and trans-2 butene each consist of a four-carbon chain. The two central carbons are connected by a double bond resulting in a planar, or flat shape. In the cis isomer, both terminal CH3 groups are on the same side of the plane, and two hydrogen atoms are on the opposite side. Imagine a person with arms stretched out and upwards and legs spread apart, with a glove on the left hand and a sock on the left foot: this represents a cis configuration. In cis-butene the terminal CH3 groups are on opposite sides of the plane. Now, imagine a person with outstretched arms and legs, but this time with a glove on the left hand and a sock on the right foot: this is what a trans configuration looks like. Part C shows two enantiomers, each with different arrangement of hydrogen, bromine, chlorine and fluorine around a central carbon. The molecules are mirror images of one another.
Molecules that have the same number and type of atoms arranged differently are called isomers. (a) Structural isomers have a different covalent arrangement of atoms. (b) Geometric isomers have a different arrangement of atoms around a double bond. (c) Enantiomers are mirror images of each other.

Which of the following statements is false?

  1. Molecules with the formulas CH 3 CH 2 COOH and C 3 H 6 O 2 could be structural isomers.
  2. Molecules must have a double bond to be cis - trans isomers.
  3. To be enantiomers, a molecule must have at least three different atoms or groups connected to a central carbon.
  4. To be enantiomers, a molecule must have at least four different atoms or groups connected to a central carbon.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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