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Chapter 22: Problem 33

There is only one compound that is named 1,2 -dichloroethane, but there are two distinct compounds that can be named 1,2-dichloroethene. Why?

Short Answer

Expert verified
The difference between 1,2-dichloroethane and 1,2-dichloroethene lies in the bonding and geometry of the molecules. 1,2-dichloroethane has single bonds, allowing free rotation and no geometric isomers. In contrast, 1,2-dichloroethene has a double bond between the carbon atoms, restricting free rotation and leading to two distinct geometric isomers: the cis-isomer (Z)-1,2-dichloroethene and the trans-isomer (E)-1,2-dichloroethene. This results in there being only one compound named 1,2-dichloroethane, while two distinct compounds can be named 1,2-dichloroethene.

Step by step solution

01

Recall the molecular structures of ethane and ethene

Ethane (C2H6) is an alkane, which means it has single bond between the two carbon atoms and has 6 saturated hydrogen atoms. The basic structural formula of ethane is C-C. Ethene (C2H4) is an alkene, which means it has a double bond between the two carbon atoms and has 4 saturated hydrogen atoms. The basic structural formula of ethene is C=C.
02

Understand the difference in bonding between ethane and ethene

In ethane, each carbon atom forms three single bonds to either H atoms or the other carbon atom. Therefore, each carbon in ethane is said to be sp^3 hybridized, forming a tetrahedral geometry around each carbon atom. In ethene, each carbon atom forms 2 single bonds to H atoms, and a double bond to the other carbon atom. This means each carbon in ethene is sp^2 hybridized, forming a planar trigonal geometry around each carbon atom.
03

Identify the molecular structure of 1,2-dichloroethane

In 1,2-dichloroethane, two chlorine atoms replace two of the hydrogen atoms on the adjacent carbon atoms in ethane. The resulting structural formula is: Cl-CH_2-CH_2-Cl. There are no geometric isomers for this molecule because the carbon-chlorine and carbon-carbon bonds are single bonds, and free rotation can occur around these bonds.
04

Identify the molecular structures of 1,2-dichloroethene

In 1,2-dichloroethene, two chlorine atoms replace two hydrogen atoms on adjacent carbon atoms in ethene. This results in two possible structures, with the following structural formulas: 1. Cl-CH=CH-Cl (the chlorine atoms are on the same side) - called the cis-isomer or (Z)-1,2-dichloroethene 2. Cl-CH=CH-Cl (the chlorine atoms are on opposite sides) - called the trans-isomer or (E)-1,2-dichloroethene The reason there are two distinct compounds is due to the presence of a double bond between the carbon atoms, which restricts free rotation and results in different geometries for the molecule. These two compounds are called geometric isomers and will exhibit different physical and chemical properties.
05

Conclude the explanation

There is only one compound named 1,2-dichloroethane because it is derived from ethane, which has single bonds, allowing free rotation and no distinct geometric isomers. There are two compounds that can be named 1,2-dichloroethene because they are derived from ethene, which has a double bond between the carbon atoms, restricting free rotation and resulting in two distinct geometric isomers (cis and trans).

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