Topic 6 - ORGANICS I
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Features of a homologous series
• Same general formula • Show a trend in physical properties • Have similar chemical properties
What type of bonds are all single bonds?
Sigma bonds
What are the properties of sigma bonds?
• End on overlap • Large overlap • Directly between nuclei • Strong attraction
What are the properties of pi bonds?
• Side on overlap • Smaller overlap • Not directly between nuclei • Significantly weaker
What type of bonds are double bonds?
A sigma bond surrounded by 2 pi bonds
What are structural isomers?
Molecules with: - Same molecular formula - Different structural formula
What are geometric isomers?
• Same molecular formula • Different structural formula • Restricted rotation about a C=C double bond • Different groups on the C atoms
What is a hydrocarbon?
Molecules made of hydrogen and carbon atoms only
Definition of an alkane
Hydrocarbons with the general formula CnH2n+2
What is the trend of boiling points as alkane size increases?
Bp/mp increases
Why does boiling point increase as alkane size increases?
• London forces increase • More electrons per molecule • More energy required to overcome IMFs
What state is C4H10?
Gas
What state is C5H12?
Liquid
What is miscibility?
The ability of two solutions to combine and form a homogeneous solution
Are alkanes miscible in H2O?
No - CnH2n+2 and H2O cannot form hydrogen bonds
Are alkanes miscible in cyclohexane?
Yes - Both CnH2n+2 and C6H12 only form London forces
Can alkanes bond with other reactants?
Not likely - Alkanes have sigma bonds which are very strong - Hence low reactivity
What type of reactions can alkanes undertake?
• Combustion reaction • Substitution with halogens
Describe the process of obtaining fuel from perch
• Dead matter from the seabed is compressed and heated, producing hydrocarbons • Drilling removes the crude oil from the sea • Fractional distillation separates the crude oil into fractions • Cracking converts long hydrocarbon chains to shorter, more useful chains • Reformation converts these chains to shorter, thick branched isomers
What are the main fractions produced in fractional distillation? Lightest->heaviest
• Refinery gases • Gasoline • Kerosene • Diesel • Fuel oil • Bitumen
What are the conditions necessary for cracking?
~ 650oC - Catalysts such as aluminium oxide/silica dioxide
Why is reformation useful?
• Isomers are short, thick and branched Isomers pack less closely together, therefore: - Weaker London forces - More efficient combustion
What are the products of complete alkane combustion?
• Highest oxides • CO2 • Exothermic, releases energy Only when there is sufficient O2
What are the products of incomplete alkane combustion?
• Lower oxides • CO2, CO and C When there is insufficient O2