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CHEMISTRY, M7 2018 HSC 10 MC

Which row of the table correctly matches the reaction type with the reactant(s), catalyst and product(s)?
 

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`D`

Show Worked Solution
  • The dehydration of an alkanol \(\ce{(C2H5OH)}\) uses a concentrated acid catalyst to produce an alkene \(\ce{(C2H4)}\) and water.

`=>D`

CHEMISTRY, M7 2019 HSC 34

The following reaction scheme can be used to synthesise ethyl ethanoate.
 


 

Outline the reagents and conditions required for each step and how the product of each step could be identified.   (7 marks) 

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Step 1:

  • To synthesise chloroethane (A) into ethanol (B), \(\ce{NaOH}\) is added and heated. \(\ce{KMnO4 / H+}\) is then added and heated.
  • The mixture is then treated with concentrated sulfuric acid and refluxed.
  • Ethanol (B) can be identified using infrared spectroscopy by looking for a broad absorption between 3230 cm ¯1 and 3550 cm ¯1, which indicates the presence of an \(\ce{O-H}\) bond. This absorption would not be present in chloroethane (A).
  • Alternative ways to identify ethanol include: mass spectrum analysis (single ion peak at m/z = 46), reactivity tests, and \( \ce{^1H NMR}\) spectrum analysis (3 signals vs 2 for chloroethane). 

Step 2:

  • Ethanol (B) can be converted into ethanoic acid (C) by combining it with a strong oxidant like sodium carbonate, which produces carbon dioxide bubbles, confirming the presence of a carboxylic acid.
  • Ethanol will not react as above and the compounds can be distinguished.
  • Alternative ways to identify ethanoic acid include: IR or \( \ce{^13C NMR}\) spectrum analysis, litmus indicators, mass spectrum analysis (ion peak at m/z = 60 vs m/z = 46) 

Step 3

  • Ethyl ethanoate (D) can be synthesised by heating a mixture of ethanol, ethanoic acid and concentrated sulfuric acid under reflux.
  • A \( \ce{^1H NMR}\) spectrum can be used to identify ethyl ethanoate as it will have 3 signals versus ethanol and ethanoic acid that will only have 2 each.
  • Alternative ways to identify ethyl ethanoate include: a distinct smell, no \(\ce{O-H}\) peaks in the IR spectrum or mass spectrum analysis (ion peak at m/z = 102).
Show Worked Solution

Step 1:

  • To synthesise chloroethane (A) into ethanol (B), \(\ce{NaOH}\) is added and heated. \(\ce{KMnO4 / H+}\) is then added and heated.
  • The mixture is then treated with concentrated sulfuric acid and refluxed.
  • Ethanol (B) can be identified using infrared spectroscopy by looking for a broad absorption between 3230 cm ¯1 and 3550 cm ¯1, which indicates the presence of an \(\ce{O-H}\) bond. This absorption would not be present in chloroethane (A).
  • Alternative ways to identify ethanol include: mass spectrum analysis (single ion peak at m/z = 46), reactivity tests, and \( \ce{^1H NMR}\) spectrum analysis (3 signals vs 2 for chloroethane). 

Step 2:

  • Ethanol (B) can be converted into ethanoic acid (C) by combining it with a strong oxidant like sodium carbonate, which produces carbon dioxide bubbles, confirming the presence of a carboxylic acid.
  • Ethanol will not react as above and the compounds can be distinguished.
  • Alternative ways to identify ethanoic acid include: IR or \( \ce{^13C NMR}\) spectrum analysis, litmus indicators, mass spectrum analysis (ion peak at m/z = 60 vs m/z = 46) 

Step 3

  • Ethyl ethanoate (D) can be synthesised by heating a mixture of ethanol, ethanoic acid and concentrated sulfuric acid under reflux.
  • A \( \ce{^1H NMR}\) spectrum can be used to identify ethyl ethanoate as it will have 3 signals versus ethanol and ethanoic acid that will only have 2 each.
  • Alternative ways to identify ethyl ethanoate include: a distinct smell, no \(\ce{O-H}\) peaks in the IR spectrum or mass spectrum analysis (ion peak at m/z = 102).

♦♦ Mean mark 38%.

CHEMISTRY, M8 2022 HSC 33

Analyse how a student could design a chemical synthesis process to be undertaken in the school laboratory. In your response, use a specific process relating to the synthesis of an organic compound, including a chemical equation, and refer to:

  • selection of reagent(s)
  • reaction conditions
  • any potential hazards and any safety precautions to minimise the risk
  • yield and purity of the product(s).   (8 marks)
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Selecting reagents

  • The student could synthesise ethyl ethanoate through esterification between acetic acid and ethanol.
  • Both readily available in the school laboratory and are relatively safe.

Acetic acid + ethanol  ⇌  Ethyl ethanoate + water

  • Concentrated sulfuric acid should be used as the acid catalyst as it is a strong acid that is also readily available. 

Reaction conditions

  • Increasing the temperature of the system increases the reaction rate because it increases the average kinetic energy of the reactant molecules, and thus increases the likelihood of successful collisions, producing more product.
  • Additionally, the addition of reactants would increase the likelihood of successful collisions, thus increasing the reaction rate.
  • The reaction should also be undertaken under reflux allowing vaporised molecules to condense and return back to the reaction vessel, increasing the amount of reactants, and thus increasing the rate of reaction. 
  • Concentrated sulfuric acid should also be utilised as a catalyst in order to speed up the reaction and lower the activation energy. 

Potential hazards and safety precautions

  • The acetic acid and sulfuric acid used is corrosive and may cause skin and eye burns, therefore, appropriate lab coat and safety glasses should be utilised.
  • The organic reactants are highly flammable and may cause fires. The reaction mixture should be heated on a hot plate or heating mantle instead of a bunsen burner.
  • Refluxing may cause pressure build-up, therefore, ensure the reflux condenser is open.
  • Superheating and bumping may occur in apparatus. Boiling chips should be utilised to provide nucleation sites allowing liquids to boil smoothly. 

Yield and purity

  • Concentrated sulfuric acid, used as a catalyst, also acts as a dehydrating agent that removes water from the system and improves yield.
  • When the reaction reaches equilibrium, the ester can be separated from the mixture by adding excess sodium carbonate solution in order to neutralise the acid.
  • Transfer to a separation funnel to separate the organic layer (containing the ester) from the aqueous layer.
  • Then use fractional distillation to separate the ester from the organic layer.
Show Worked Solution

Selecting reagents

  • The student could synthesise ethyl ethanoate through esterification between acetic acid and ethanol.
  • Both readily available in the school laboratory and are relatively safe.

Acetic acid + ethanol  ⇌  Ethyl ethanoate + water

  • Concentrated sulfuric acid should be used as the acid catalyst as it is a strong acid that is also readily available. 

Reaction conditions

  • Increasing the temperature of the system increases the reaction rate because it increases the average kinetic energy of the reactant molecules, and thus increases the likelihood of successful collisions, producing more product.
  • Additionally, the addition of reactants would increase the likelihood of successful collisions, thus increasing the reaction rate.
  • The reaction should also be undertaken under reflux allowing vaporised molecules to condense and return back to the reaction vessel, increasing the amount of reactants, and thus increasing the rate of reaction. 
  • Concentrated sulfuric acid should also be utilised as a catalyst in order to speed up the reaction and lower the activation energy. 

Potential hazards and safety precautions

  • The acetic acid and sulfuric acid used is corrosive and may cause skin and eye burns, therefore, appropriate lab coat and safety glasses should be utilised.
  • The organic reactants are highly flammable and may cause fires. The reaction mixture should be heated on a hot plate or heating mantle instead of a bunsen burner.
  • Refluxing may cause pressure build-up, therefore, ensure the reflux condenser is open.
  • Superheating and bumping may occur in apparatus. Boiling chips should be utilised to provide nucleation sites allowing liquids to boil smoothly. 

Yield and purity

  • Concentrated sulfuric acid, used as a catalyst, also acts as a dehydrating agent that removes water from the system and improves yield.
  • When the reaction reaches equilibrium, the ester can be separated from the mixture by adding excess sodium carbonate solution in order to neutralise the acid.
  • Transfer to a separation funnel to separate the organic layer (containing the ester) from the aqueous layer.
  • Then use fractional distillation to separate the ester from the organic layer.

♦♦ Mean mark 52%.

CHEMISTRY, M7 2022 HSC 11 MC

Cyclohexanol is an alcohol and undergoes a dehydration reaction with sulfuric acid as shown.
 

What is the major organic product of this reaction?
 

 

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`B`

Show Worked Solution
  • When alcohols are dehydrated using concentrated \(\ce{H2SO4}\), an \(\ce{OH}\) group and a \(\ce{H}\) atom from the adjacent carbon is eliminated to form an alkene.

`=> B`

CHEMISTRY, M7 2021 HSC 26

A sequence of chemical reactions, starting with 2-methylprop-1-ene, is shown in the flow chart.

  1. Complete the flow chart by drawing structural formulae for compounds `text{A}`, `text{B}`, `text{C}`, and `text{D}`.   (4 marks)
     
     

     
  2. Reflux is used in the synthesis of methyl 2-methylpropanoate.
  3. Provide TWO reasons for using this technique.   (2 marks)

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a.   Compound A:

Compound B:

Compound C:

Compound D:


 

b.  Reasons for reflux technique:

  • Reflux heats the reaction mixture which increases the average kinetic energy, and thus increases the reaction rate.
  • Heating causes the volatile substances to form vapour molecules. Refluxing uses a condenser to cool the vapour molecules into liquids, and thus retains the substances.
Show Worked Solution

a.   Compound A:

Compound B:

Compound C:

Compound D:


 

b.  Reasons for reflux technique:

  • Reflux heats the reaction mixture which increases the average kinetic energy, and thus increases the reaction rate.
  • Heating causes the volatile substances to form vapour molecules. Refluxing uses a condenser to cool the vapour molecules into liquids, and thus retains the substances.

♦ Mean mark (b) 46%.

CHEMISTRY, M7 2021 HSC 13 MC

A chemist synthesises a substance using the following pathway.

\[\ce{ X ->[{hydration}] {Y} ->[{oxidation}] Z}\]

What are compounds `text{X, Y, Z}` ?

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`C`

Show Worked Solution

By elimination:

  • Hydration reaction is an addition reaction that can only occur on alkenes, thus  `X` = prop-1-ene  (eliminate A and B)
  • `Y` = propan-2-ol
  • The oxidation of secondary alcohol creates a ketone, thus `Z` = propanone

`=> C`

CHEMISTRY, M7 2021 HSC 7 MC

Methanol undergoes a substitution reaction using hydrogen bromide.

Compared to methanol, the product of this reaction has a

  1. lower boiling point.
  2. lower molecular mass.
  3. greater solubility in water.
  4. different molecular geometry at the carbon atom.
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`A`

Show Worked Solution
  • The product of the substitution reaction between methanol and hydrogen bromide is bromomethane.
  • Methanol contains an OH functional group and thus can form strong hydrogen bonds.
  • Bromomethane can only form dipole-dipole forces which are weaker than hydrogen bonds. As a result, bromomethane requires less energy to break these intermolecular forces, resulting in a lower boiling point than methanol.

`=> A`