55 Nucleophilic substitution reaction mechanism Post 16 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

In this activity, students use mechanism cards to represent the stages in a nucleophilic substitution reaction of an halogenoalkane. The cards act as a prop to foster thinking and discussion about an organic reaction mechanism.

Learning objectives

Students will:

  • understand the mechanism of nucleophilic substitution reactions.

Sequence of activities

Organise students to work in pairs. Describe the procedure for the introductory practical activity. Supervise the students as they carry it out.

In a plenary, ask for:

  • explanations for the formation of a precipitate
  • the role of the ethanol
  • the role of the water.

Share the learning objective with students.

Give support while students:
  • work individually
  • write a description of the mechanism by which bromoethane reacts with hydroxide ions to form ethanol.
Ask students to work in pairs. Give a set of Mechanism cards to each pair.

Circulate, support with prompts, and confirm correct mechanisms while they use the cards to represent the nucleophilic substitution mechanism for bromoethane and hydroxide ions.

Circulate and support while the students:
  • work individually
  • modify and/or add to their original mechanism description in light of their work with the cards.
Give each student an Additional reaction card. Circulate and support with prompts while students:
  • work in pairs
  • take turns in explaining to their partner the mechanism for their additional reaction, using a diagram of the reaction mechanism to help them.
Ask students to:
  • identify, below their reaction mechanism diagram, the ideas they found the most difficult
  • describe what they need to focus on to further develop their understanding about nucleophilic substitution reactions
  • hand in the sheet.
Give written feedback on the individuals’ response and reflections that is encouraging and supportive.

Assessment for learning commentary

Questions to examine the students’ observation and grasp of the topic give the teacher an indication of the level of understanding. They give the students a starting point for the activity and help to clarify the learning objective.

By using cards to explore reaction mechanisms, the students confront their own level of knowledge which they further assess by working with a partner.

These assessments are formalised at the end of the session, allowing the teacher to guide the student to additional support.

Resources

Props

Download Word Download PDF Additional reaction card for each student
Download Word Download PDF Set of Mechanism cards for each pair of students

Equipment

For each pair of students

  • 250 cm3 beaker
  • Two test-tubes
  • Bunsen burner, tripod and gauze (or supply of hot water)
  • Ethanol
  • Bromobutane
  • 0.01 mol dm-3 Silver nitrate solution.

Procedure for introductory activity

  • Heat some water in a beaker to about 50 °C and extinguish the flame.
  • Place two test-tubes in the beaker.
  • In one test-tube place 1 cm depth of ethanol and then add two drops of bromobutane.
  • In the other test-tube place 1 cm depth of silver nitrate solution.
  • After about five minutes add the silver nitrate solution to the other test-tube.
  • Observe what happens.