Melting & Boiling points of organic molecules Organic Compounds

What type of isomerism is shown in the following pairs of molecules? Starter What type of isomerism is shown in the following pairs of molecules? CH3CH2OCH3 and CH3CH2CH2OH CH3CH2CH2OH and CH3CH(OH)CH3 CH3CH2CH2CH2CH3 and CH3CH(CH3)CH2CH3

Melting And Boiling Crude oil contains hundreds of different alkanes which can be separated by fractional distillation This separation is possible because the boiling points of the alkanes are different, increasing as their chain length increases. Alkane Methane Ethane Propane Butane Pentane Hexane Heptane Boiling point (0C) -164 -89 -42 -1 36 69 98

Bonds in hydrocarbons The boiling point increases because the intermolecular forces between the molecules, called Van Der Waals forces become stronger. These forces hold the molecules together but, once broken, the molecules move apart and become a gas. The greater the intermolecular forces between the molecules the greater the boiling point.

Bonds in hydrocarbons What is the difference between an intermolecular and intramolecular force? Draw two molecules of water and mark on the diagram an intermolecular force and an intramolecular force.

Effect of chain length on boiling point Intermolecular forces act between molecules in close surface contact. The Van der Waals forces between the molecules will be greater and so more energy is required to overcome those forces. Increasing chain length (greater surface area) The longer the chain length higher the boiling point

Effect of branching on boiling point Isomers of alkanes have the same molecular mass. Branching decreases the surface area meaning weaker Van Der Waals forces and a lower boiling point. Alkane Pentane 2-methylbutane 2,2-dimethylpropane Boiling point (0C) 36 28 9 Increasing branching (lower surface area) The greater the branching in an isomer the lower the boiling point

Knowledge check Kerosene is obtained from crude oil. Name the process used to obtain kerosene from crude oil and explain how the process works (2 marks) Explain why the straight chain isomer of C10H22 has a higher boiling point than any of its branched-chain structural isomers. (2 marks) Explain why the boiling points increase down the alkane homologous series. (2 marks) Describe how you could lower the boiling point of pentane without changing it’s molecular formula (2 marks)

End of topic test Next lesson we will have our end of topic test. You MUST get a good grade in this test or you will be resitting it. Best revision tools: Create a set of revision cards for topic ‘2.4 Organic Compounds’ using your notes and textbook. Go back over worksheets and try the questions again Try past paper questions