BEFORE INTERVENTION

PROBLEM

Reflection on previous teaching and learning session

From the experience, students always struggle to write the mechanism of free radical substitution reaction of alkanes correctly. Though they were being taught to write the mechanism in lecture and tutorial classes repeatedly, many still could not write the mechanism when questions were being given to them. Examples of in-class activity conducted in class to enhance students’ understanding in free radical substitution mechanism of alkanes include PSPM standard question (Figure 1) and fill-in-the-blank type of question (Figure 2).

Figure 1: In-class activity using PSPM standard question

Figure 2: Fill-in-the-blank type of question as one of the in-class activities for learning free radical substitution mechanism of alkanes.

From the in-class activity, the weak students were identified. A pre-test and questionnaire were then conducted to get important insight into students’ weaknesses in writing free radical substitution mechanism of alkanes.

From the pre-test, students’ weaknesses are being analysed. Students’ weakness includes inability to write the name of steps involves in the free radical substitution reaction of alkanes: Initiation, propagation and termination (Figure 3 and 4). 

Figure 3: The student could not list out correctly the steps involved the free radical substitution mechanism of alkanes. 

Figure 4: Another student who could not list out the steps in free radical substitution of alkanes.

For initiation step, students' problem include could not place the single electron at correct site on the intermediates formed (Figure 5), fail to draw the arrow correctly to show the bond breaking (Figure 6) and unable to draw correct reactant to be involved in initiation step (Figure 7). 

Figure 5: The student did not place the single electron correctly in the intermediates formed in initiation step. 

Figure 6: The student did not draw the arrow correctly as the arrows were started at the ends of the bond and full-headed arrows were used for homolytic cleavage.

Figure 7: The student did not know which reactant was involved in the initiation step. 

For propagation step, most students could give correct reactant and intermediate involve in first step of propagation. However, many could not remember the reactant and intermediate involve in second step of propagation (Figure 8). Some were also confused with what reactant and intermediate that are involved in second step of propagation (Figure 9).

 

Figure 8: The student could not remember the reactant and intermediate involved in second step of propagation, could not draw the arrow to show simultaneous bond breaking and bond forming in propagation step and could not draw the product and intermediate formed in each step of propagation. 

Figure 9: Another student who could not remember the reactant and intermediate involved in second step of propagation, could not draw the arrow to show simultaneous bond breaking and bond forming in propagation step and could not draw the product and intermediate formed in each step of propagation.

For termination step, many made mistakes on writing correct intermediates involved in termination step and drawing arrows to show bond forming, even though they could still deduce the products formed in termination step correctly (Figure 10 and 11). 

Figure 10: The student could only write one mechanistic step in termination correctly. The arrow and the intermediate for another mechanistic step are wrongly drawn. This probably implies that the student learnt termination step through rote learning without understanding the bond forming incident which takes place in termination and the proper way of drawing the arrows to show bond forming. 

Figure 11: The arrow and the intermediate for mechanistic steps in termination are wrongly drawn. However, this student could draw the products formed for each mechanistic step in termination correctly.

Issues of concern

The issue of students struggle to write the free radical substitution of alkanes mechanism correctly is one of the main concerns as it implies that 

1. students are not mastering the basic skill of organic chemistry by using arrows to show bond breaking or bond forming or simultaneous bond breaking and bond forming.
2. they might struggle to write mechanism of other classes of compounds in next few chapters.
3. lecturers might need to change in their approach of teaching this mechanism to allow students to have different perception on the mechanism, particularly on the movement of electrons which describes the bond breaking or bond forming or both taking place at each step of reaction mechanism.  

Hence, B-B B-F Model and guided worksheets were prepared to expose students with the free radical substitution mechanism differently from conventional method to help students to see clearly where bond forming or bond breaking or simultaneous bond breaking and bond forming was/were taking place and allow them writing the products formed correctly for each mechanistic step of free radical substitution reaction of alkanes.