Reactions of organic compounds
QUICK POINTS
The general formula of the alkanes is Cn H2n+2, while that of the alkenes is Cn H2n.
Alkanes are saturated compounds (contain single bonds) and so react by substitution. Alkenes are unsaturated (possessing a double bond) and so undergo addition reactions.
Alkenes can be differentiated from alkanes, as they decolourise bromine and acidified potassium permanganate solution. This is also used as a positive test for the C=C double bond.
Alkanes and alkenes burn in air to produce carbon dioxide and water. If oxygen is limited, carbon monoxide and carbon can also be formed. Alkenes will produce a more sooty flame due to a higher proportion of carbon than alkanes.
Alkanes undergo substitution reactions. This occurs when atoms in the molecule are replaced by other atoms.
Alkenes undergo addition reactions. This occurs when atoms or radicles are added to the molecule.
QUESTION 1
But-1-ene can be obtained from alkanes such as nonane, C9H20, by cracking.
(a)What is cracking?
(b)Write the equation for the cracking of nonane to give but-1-ene. Note the reaction conditions.
(c) How can but-1-ene be converted to butan-1-ol? Name the reagent used.
(d) Write an equation to show the combustion of butan-1-ol.
(e) How could butan-1-ol be converted to the ester butyl propanoate?
ANSWERS
(a) Cracking is the process of breaking up long-chain alkanes into shorter chains.
(b) C9H20 (l) ––– C5H12 (l) + C4H8 (g) Conditions: High temperatures/pressures. Old Harbour High’s Schools’ Challenge Quiz team (from left): Captain Dujain Jordan, Maurice Miller, Chevaughn Gordon and Stallone Francis. They went up against Magotty High School in TVJ’s Schools’ Challenge Quiz 2019 Competition and won the match.
(c) But-1-ene can be converted to butan-1-ol by reacting with steam in the presence of phosphoric acid (catalyst).
(d) C4H9OH (l) + 6O2 (g) ––– 4CO2 (g) + 5H2O (l). (e) Butan-1-ol can be converted to butyl propanoate by reacting with propanoic acid in the presence of dilute sulphuric acid as a catalyst.
QUESTION 2
Esters are made by reacting an alcohol with a carboxylic acid.
(a) Which acid and alcohol react together to form the following ester?
(b) What type of reaction is this an example of?
ANSWERS
(a) The ester is formed from CH3OH (methanol) and CH3CH2COOH (propanoic acid).
(b) This is an example of a condensation reaction
QUESTION 3
The structures of four hydrocarbons, A, B, C, and D, are shown below.
(a) Which two of these structures have the same relative molecular mass?
(b) Which two of these structures will decolourise aqueous bromine?
(c) Write the equation for the reactions in (b) and name the products formed.
ANSWERS
(a) Hydrocarbons B and D have the same molecular formula C4H8, thus they will have the same relative molecular mass.
(b) Alkenes decolourise aqueous bromine. Hydrocarbons B and D are alkenes, hence will decolourise bromine.
(c) C4H8 (g) + Br2 (aq) ––– C4H8Br2 (l) dibromobutane That is all for this week.