(a) The letters R,S,T represent an alkene, an alkene and a terminal alkyne respectively. Which of R, S and T typically undergo(es) the following reactions? ...
Question 1 Report
(a) The letters R,S,T represent an alkene, an alkene and a terminal alkyne respectively. Which of R, S and T typically undergo(es) the following reactions?
(i) Addition reaction;
(ii) Combustion;
(iii) Substitution reaction.
(b)(i) Name the process by which an alkanol can be converted to an alkene.
(ii) Write the name and structural formula of the third member cf the alkanol series.
(iii) State what would be obtained if a primary alkanol reacted with excess acidified KMnO\(_4\) solution.
(c)(i) Give one chemical test for alkanoic acids.
(ii) Write an equation to show how methanoic acid reacts with ethanol in the presence of mineral acids.
(iii) What is the role of mineral acid in the reaction in (c) ii?
(d) In an experiment, cassava was pressure-cooked to release starch granules, followed by treatment with malt for about 2 hours at 55°C. Yeast was then added and the mixture was left for 2 days at 27°C. An organic product J and a gas H were obtained.
(i) Identify H
(ii) State the class of carbohydrates to which starch belongs and explain what happened to the starch during treatment with malt.
(iii) Draw a labelled diagram of a suitable set-up for obtaining a sample of J from the reaction mixture.
Answer Details
(a) There is an inconsistency in the question wording: it says that R and S are both alkenes, but the reaction pattern expected requires R to be an alkane, S an alkene, and T a terminal alkyne.
- Addition reaction: S and T. Alkenes and alkynes are unsaturated, so substances can add across their carbon–carbon multiple bonds.
- Combustion: R, S and T. All are hydrocarbons and burn in oxygen to form carbon(IV) oxide and water.
- Substitution reaction: R and T. Alkanes undergo substitution reactions, while the hydrogen atom at the end of a terminal alkyne can be replaced.
(b)(i) The process is dehydration, which is the removal of water from an alkanol to form an alkene.
(b)(ii) The third member of the alkanol homologous series is propanol. One acceptable structural formula is propan-1-ol:
\[ \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \]
Propan-2-ol, \(\text{CH}_3\text{CHOHCH}_3\), is also an acceptable isomer of propanol.
(b)(iii) A primary alkanol is oxidised by excess acidified potassium manganate(VII) solution to the corresponding alkanoic acid.
(c)(i) Add aqueous sodium hydrogencarbonate or sodium carbonate to the acid. An alkanoic acid gives effervescence because carbon(IV) oxide gas is produced. The gas turns limewater milky.
(c)(ii) Methanoic acid reacts with ethanol to form the ester ethyl methanoate:
\[ \text{HCOOH}+\text{CH}_3\text{CH}_2\text{OH}\xrightarrow{\text{mineral acid}}\text{HCOOCH}_2\text{CH}_3+\text{H}_2\text{O} \]
(c)(iii) The mineral acid acts as a catalyst: it increases the rate of esterification without being used up.
(d)(i) Gas H is carbon(IV) oxide, \(\text{CO}_2\).
(d)(ii) Starch is a polysaccharide (a complex carbohydrate). Malt contains the enzyme diastase, also called amylase. At \(55^\circ\text{C}\), this enzyme hydrolyses starch into the simpler sugar maltose:
\[ (\text{C}_6\text{H}_{10}\text{O}_5)_n+n\text{H}_2\text{O}\xrightarrow{\text{diastase}}n\text{C}_{12}\text{H}_{22}\text{O}_{11} \]
The yeast then ferments the sugar to produce ethanol, J, and carbon(IV) oxide.
(d)(iii) Product J is ethanol. It can be obtained from the fermented mixture by simple distillation.
For this separation, ethanol is collected as the distillate because it vaporises and condenses before most of the water and non-volatile material in the fermented mixture.
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