Chemistry_Grade_10-12 (1).pdf

94 representing organic compounds there are a number

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9.4 Representing organic compounds There are a number of ways to represent organic compounds. It is useful to know all of these so that you can recognise a molecule however it is shown. There are three main ways of representing a compound. We will use the example of a molecule called 2-methylpropane to help explain the difference between each. 9.4.1 Molecular formula The molecular formula of a compound shows how many atoms of each type are in a molecule. The number of each atom is written as a subscript after the atomic symbol. The molecular formula of 2-methylpropane is: 152
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12 9.4 C 4 H 10 9.4.2 Structural formula The structural formula of an organic compound shows every bond between every atom in the molecule. Each bond is represented by a line. The structural formula of 2-methylpropane is shown in figure 9.3. C C C H H H H H H C H H H H Figure 9.3: The structural formula of 2-methylpropane 9.4.3 Condensed structural formula When a compound is represented using its condensed structural formula, each carbon atom and the hydrogen atoms that are bonded directly to it are listed as a molecular formula, followed by a similar molecular formula for the neighbouring carbon atom. Branched groups are shown in brackets after the carbon atom to which they are bonded. The condensed structural formula below shows that in 2-methylpropane, there is a branched chain attached to the second carbon atom of the main chain. You can check this by looking at the structural formula in figure ?? . CH 3 CH(CH 3 )CH 3 Exercise: Representing organic compounds 1. For each of the following organic compounds, give the condensed structural formula and the molecular formula . (a) H C H H C H C H C H H H 153
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9.5 CHAPTER 9. ORGANIC MOLECULES - GRADE 12 (b) C H H C H C H C H H H C H H H 2. For each of the following, give the structural formula and the molecular formula . (a) CH 3 CH 2 CH 3 (b) CH 3 CH 2 CH(CH 3 )CH 3 (c) C 2 H 6 3. Give two possible structural formulae for the compound with a molecular for- mula of C 4 H 10 . 9.5 Isomerism in organic compounds It is possible for two organic compounds to have the same molecular formula but a different structural formula . Look for example at the two organic compounds that are shown in figure 9.4. C C C C H H H H H H H H H H C C C H H H H H H H C H H H Figure 9.4: Isomers of a 4-carbon organic compound If you were to count the number of carbon and hydrogen atoms in each compound, you would find that they are the same. They both have the same molecular formula (C 4 H 10 ), but their structure is different and so are their properties. Such compounds are called isomers . Definition: Isomer In chemistry, isomers are molecules with the same molecular formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently.
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