Organic chemistry - Formulae, functional groups and terminology (3)
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1.
Consider the following homologous series: Alkanes (methane, ethane, propane, butane). Complete the table below by filling in the missing information.
| Name of Series: | Alkanes |
| General Formula: | CnH2n+2 |
| Functional Group: | Single C-C single bond (alkane) |
| Example Compounds: | Methane, Ethane, Propane, Butane |
The table above completes the information about the alkane homologous series. It demonstrates the key characteristics of a homologous series: a consistent general formula, a specific functional group, and a series of related compounds.
2.
Draw the structural formulas of all the structural isomers of the molecular formula C4H8. Label each isomer clearly. Explain briefly how the number of structural isomers relates to the number of possible arrangements of the atoms.
The structural isomers of C4H8 are:
- CH2=CH-CH2-CH3 (1-butene)
- CH3-CH=CH-CH2-CH3 (2-butene)
- CH3-C(CH3)=CH-CH3 (2-methyl-2-butene)
- CH3-CH=C(CH3)-CH3 (2-methyl-1-butene)
Explanation of the relationship between the number of isomers and arrangements:
The number of structural isomers can be determined by considering the possible arrangements of the atoms in the molecule. For C4H8, we have four carbon atoms and eight hydrogen atoms. The number of possible arrangements is affected by the presence of a double bond. The double bond restricts the number of possible arrangements compared to a straight chain. The formula for calculating the number of structural isomers is: 2n-1, where 'n' is the number of carbon atoms. In this case, n=4, so the formula gives 24-1 = 23 = 8. However, this formula doesn't account for the different types of double bonds (position of the double bond). Therefore, a systematic approach is needed to list all possible isomers, as shown above. The double bond introduces branching and different connectivity patterns, leading to multiple distinct structural isomers.
| Isomer Name | Structural Formula |
| 1-Butene | CH2=CH-CH2-CH3 |
| 2-Butene | CH3-CH=CH-CH3 |
| 2-Methyl-1-butene | CH3-CH=C(CH3)-CH3 |
| 2-Methyl-2-butene | CH3-C(CH3)=CH-CH3 |
3.
The following statements are about homologous series. State whether each statement is True or False. Explain your answers briefly.
- All members of a homologous series have the same functional group.
- The first four members of a homologous series differ by 12 units of mass.
- Compounds in a homologous series have identical physical properties.
- The general formula of a homologous series is always the same for all members.
- True. This is the defining characteristic of a homologous series.
- True. A homologous series differs by a constant mass unit, which is typically a hydrogen atom (12 amu).
- False. While they share similarities, physical properties like boiling point and melting point increase as the size of the molecule increases.
- True. The general formula indicates the number of carbon atoms and the type of functional group present in all members of the series.