Organic Chemistry Structures
What do I need to know? • Translate between molecular, structural and ball and stick representations of simple organic molecules • Describe how the functional group affects the property of an organic compound and understand that alkanes are unreactive towards aqueous reagents because C—C and C—H bonds are unreactive; • Write balanced chemical reactions including for burning hydrocarbons including state symbols
Representations of organic molecules • There are a number of different ways to represent organic molecules. • Ball and stick – this is just like molymods
Representations of organic molecules • Structural formula – this is where we show the covalent bonds between atoms as a line • Semi-structural (molecular) – this is where we write out the formula but do not include bonds; these are implied egCH3CH2OH
Molecular formula– this simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way they are joined together. • EgC2H6O • This is the least helpful type of formula as it could be one of two (or more) different chemicals
Rules of organic molecules Generally speaking Carbon must make four bonds Nitrogen must make three bonds Oxygen must make two bonds Hydrogen must make one bond A double bond counts as two bonds eg C=C or C=O. A triple bond counts as three bonds.
AfL - Quiz • Draw the structural formula for butanol • Write the molecular formula for butanol • Draw the structural formula for hexane • Write the molecular formula for hexane • Write the molecular formula for an alkane with 25 carbon atoms. • How many bonds does oxygen make in methanol? 7. Give an example of a use for ethanol 8. Give an example of a use for methanol
Butanol C4H10O Hexane C6H14 C25H52 2 Fuel/feedstock for synthesis/solvent/used in perfume Solvent, antifreeze, feedstock for adhesives and plastics
Understanding reactivity • Alkanes are unreactive towards aqueous reagents because C-C and C-H bonds are unreactive. • What about organic molecules that have different bonds? • We call families of different types of bonded atoms FUNCTIONAL GROUPS • An example is the –OH group or alcohol group.
Alkanes and combustion • Because of the hydrocarbon chain alkanes burn readily releasing large amounts of energy. • Alkanes are therefore used as fuels. • When they burn completely they make carbon dioxide and water. eg octane (found in petrol) C8H18 +12 ½ O2 8CO2 + 9H2O
Balanced chemical equations Write the balanced chemical equation for burning ethanol in air as a fuel and burning pentane as a fuel (include state symbols).
Answers Ethanol 2C2H5OH(l) + 6O2(g) 4CO2(g) + 6H2O(l) Pentane C5H12(l)+ 8O2(g) 5CO2(g) + 6H2O (l)
Alcohols and the Manufacture of Ethanol C7.1 and C7.5
What do I need to know? 1. The characteristic properties of alcohols are due to the presence of an –OH functional group 2. Know a range of methods for synthesising ethanol and limitations of fermentation reactions 3. Be able to explain why bioethanol is important for sustainability
Functional groups - reminder • Look back at your table of functional groups. • Write a short paragraph to explain why different organic chemicals have different properties in terms of functional groups. • Use examples such as “carboxylic acids are acidic because they have a –COOH group”.
Can you recognise the functional group? • Circle which of these are alcohols?
Answer • Alcohols have an –OH group
Properties and uses of alcohols Properties: • volatile liquid (evaporates quickly at room temperature – more than water) • colourless • burns readily in air because of the hydrocarbon chain • good solvent
Uses of ethanol and methanol Ethanol: biofuels, solvents, feedstock for synthesis Methanol: cleaner, feedstock for synthesis Feedstock is the name we give to an “ingredient” on a chemical plant
Reactions of different functional groups • This is illustrated very well by comparing the reaction of sodium with ethanol, hexane and water. • You have seen this reaction. Fill in the following table and compare with the mark scheme:
How do we make ethanol? • Fermentation is a key process for obtaining ethanol. It is relatively cheap and requires wheat or beet sugar. • The process involves the anaerobic respiration of yeast at temperatures between 20 and 40°C and at pH 7.
Conditions for fermentation Why is temperature important? • Outside an optimum temperature the yeast does not work (high temperatures kill the yeast). Why do you think pH is important? • Outside an optimum pH the yeast does not work (extremes of pH kill the yeast). Why do you think it is important to shut out oxygen? • To make ethanol the yeast must respire anaerobically (without oxygen). What effect will increasing ethanol concentration have on the yeast? • Eventually the ethanol concentration will be too high for the fermentation to continue. This means only a dilute solution can be made.
How do we obtain a concentrated solution? • Ethanol has a different boiling point to water. We can therefore separate water and ethanol using distillation.
Working out masses • We can use the useful relationship • Where Mr is the molecular mass • egMr of ethane C2H6 is (2 X 12) + (6 x 1) = 30