ORGANIC CHEMISTRY: ALCOHOLS
All alcohols contain the hydroxyl functional group, -O-H, and many
belong to an homologous series which has the general formula CnH2n+1OH;
this first Table shows some data for five straight-chain homologues.
 Systematic name 
Condensed
 formula
Boiling pt.
   / °C
 Density
 / g cm-³
 Heat of Combustion
   (DH) / kJ mol-¹ 
 Methanol
 CH3OH
    65
  0.791
       -715
 Ethanol
 C2H5OH
    78
  0.798
      -1370
 Propan-1-ol
 C3H7OH
    97
  0.804
      -2010
 Butan-1-ol
 C4H9OH
   118
  0.810
      -2650
 Pentan-1-ol
 C5H11OH
   138
  0.815
      -3320

1.  Use the data in this Table to state, as precisely as possible, the 
pattern between molecular formula and the:
Boiling point  As the molecular formula increases by CH2, the boiling
point increases by about 20°C.
Heat of combustion  As the molecular formula increases by CH2, the heat
of combustion increases by about 650 kJ mol-¹.
                                                                    [3]
2.  Extrapolate these data to predict the boiling point and the heat of
combustion for hexan-1-ol.  ca. 158°C ; ca. -3970 kJ mol-¹
                                                                    [2]
3.  Why are propan-1-ol and propan-2-ol considered to be isomeric?
They have the same molecular formula but different structural formulae.
                                                                    [1]
4.  The symbol equations for the combustion of methanol and propan-1-ol
are as follows:
       2CH3OH(g) + 3O2(g) —————————————————————® 2CO2(g) + 4H2O(g)
      2C3H7OH(g) + 9O2(g) —————————————————————® 6CO2(g) + 8H2O(g)
(a) Construct the symbol equation for the combustion of pentan-1-ol.

    2C5H11OH(g) + 15O2(g) —————————————————————® 10CO2(g) + 12H2O(g)                                                                   [2]
(b) How many moles of air are required for the (complete) combustion of
1.0 mole of pentan-1-ol?  As air contains about 20% oxygen, the number
of moles of air required = 7.5 ÷ 0.2 = 37.5 (... ca. 0.9 dm³ of air)
                                                                    [2]
(c) Suggest two products, other than carbon dioxide and water, that
might be formed when pentan-1-ol is burnt in a limited supply of air.
Carbon monoxide and Carbon  (... soot)
                                                                    [2]
(d) Calculate the energy released from the combustion of one molecule 
of pentan-1-ol (1 mole = 6.022 × 10²³ particles).
Energy released = (3320 × 10°) ÷ (6.022 × 10²³) = 551 × 10-²° kJ
                                                                    [2]
5.  Apart from their use as fuels and as synthetic intermediates, 
alcohols are used extensively as non-aqueous solvents; in particular, 
and in contrast to water, they dissolve a wide range of covalently
bonded solutes. Although ethanol is drunk socially in beverages (e.g., 
beers and spirits), most alcohols are extremely toxic. Name two human 
organs which are damaged by solvent abuse.  Brain / Liver / Kidney
                                                                    [2]

6.  In industry, large quantities of alkenes are obtained via the
catalytic cracking of petroleum oil fractions. However, the most
convenient method of preparing a small quantity of any alkene is by
catalytic dehydration of the corresponding alcohol; e.g., propene can
be prepared from propan-1-ol using the apparatus shown in the diagram.
(a) Alcohols can be dehydrated by heating with concentrated sulfuric 
acid. Suggest and explain one reason why this dehydrating agent is less
commonly used.  As a safety precaution, because concentrated sulfuric
acid is very corrosive.
                                                                    [2]
(b) Explain the purpose of the Bunsen valve.  As a safety precaution, 
by preventing cold water 'sucking back' into the hot test-tube.
                                                                    [2]
(c) Suggest what would be observed if, in the above experiment, the 
water was replaced with alkaline potassium manganate(VII).  Little or
no gas would be collected until the purple solution had decolourized 
(and a brown precipitate had formed).
                                                                    [2]
(d) The equation for this dehydration reaction can be written as:
Complete this second Table using these bond energies (in kJ mol-¹):
C-C (346); C-O (360); C-H (413); O-H (463); and C=C (610).
  Bonds broken  
 Energy absorbed 
   / kJ mol-¹
  Bonds formed  
 Energy released 
   / kJ mol-¹
     7 C-H 
      2891
     6 C-H 
      2478
     2 C-C 
       692
     1 C-C 
       346
     1 C-O
       360
     1 C=C
       610
     1 O-H 
       463
     2 O-H 
       926
           Total =      4406
           Total =      4360
                                                                    [4]
Calculate the heat energy change (DH) for the above reaction.
Energy absorbed - energy released = 4406 - 4360 = DH +46 kJ mol-¹
                                                                    [2]
(e) State and explain, using Le Chatelier's Principle, what would be 
the effect of high pressure on this dehydration reaction.  The yield of 
propene would be lower, because the position of equilibrium would move 
to oppose the increase in pressure.
                                                                    [2]

Dr. R. Peters Next Contents' List & Teacher's Notes