Chemical and physical properties of pheromones

Suggest some chemical and physical properties of a pheromone described and answer some problems related to it.

Introduction

Teachers who have not used the problems before should read the section Using the problems before starting.

Prior knowledge

Knowledge that ethers are chemically unreactive, concept of bond length and familiarity with Avogadro’s number. A detailed knowledge is unnecessary as students are encouraged to consult textbooks and data books during the exercise.

Resources

Scientific calculators, data books and textbooks should be available for reference.

Possible solutions

Question (ii) can be answered precisely; all the others require – to a greater or lesser extent – judgement and/or reasonable ‘guesstimates’.

(i) Properties of saturated hydrocarbons; if students have met ethers they may over-estimate the flammability of the pheromone1 by comparing it with the volatile ethoxyethane. They are not expected to know about the epoxide ring, but in the trials some groups did expect that the ring would be unstable.2 The physical properties will be similar to those of a C19 alkane, one of the bigger molecules in diesel, so it probably would (just) be detectable by smell if left open in a small room.

The following calculations offer an excellent opportunity for discussing significant figures.

(ii) Formula C₁₉H₃₈O; molecular mass = 282

1 mole or 6.02 x 10²³ molecules has a mass of 282 g

                                                                             6.02 x 10²³ x 10^–13

the number of molecules in 10^–13 g = ———————— = 2 x 10⁸

                                                                                          282

(iii) Distance between carbons (C–C single bond length) = 0.154 nm

(The actual distance will be 80 % of this because sin 54^o = 0.8; in terms of order of magnitude this can be ignored.)

The approximate length of the molecule is therefore 17 x 0.15 nm = 2.5 nm (2.5 x 10^–9 m).

Thus the molecules in 10-13 g of pheromone placed end to end will stretch approximately 2 x 10⁸ x 2.5 x 10^–9 m or about 0.5 m.

(iv) The data given in the passage are inadequate for such a calculation, but it is sometimes necessary to make decisions based on inadequate information. Students are asked about the maximum volume where there is a possibility of the pheromone being sensed by the moth. If they are stuck, it helps to ask ‘What information do you need to know in order to do the calculation?’, and then tell them to make guesses at these figures. The students’ assumptions could well be quite different from those made below, but could be equally valid.

Some possible assumptions are that:

• the male gypsy moth can detect a single molecule of pheromone;
• the molecule has to be present in the approximate volume of a gypsy moth; and
• the volume of a male gypsy moth is about 0.5 cm3 or 0.5 x 10-3 dm3.

On this basis, 1 molecule per 0.5 x 10^–3 dm³ is required.

There are 2 x 10⁸ molecules, so the volume is 0.5 x 10^–3 x 2 x 10⁸ = 105 dm³ or 100 m³, ie a large room 10 m x 4 m x 2.5 m high.

This will be a maximum figure: the real figure is likely to be substantially smaller.

Suggested approach

During trialling the following instructions were given to students and proved to be extremely effective:

1. Discuss each of the problems in turn. In each case decide whether a precise answer is appropriate; where it isn’t, use your common sense to make sensible estimates. You can divide the work amongst yourselves but keep each other informed of progress. Discussion can play a vital part in working out solutions to problems like this where divergent thinking is needed. Several minds working together on a problem can stimulate ideas that one on its own could not manage. About 10 minutes should be spent on the initial discussion, with further discussion as required.
2. Describe briefly what you did. Include how and why you decided upon the estimates.
3. Working as a group, prepare a short (ca 5-minute maximum) presentation to give to the rest of the class. If possible all group members should take part: any method of presentation (such as a blackboard, overhead projector, etc) can be used.

Outline the problem, describe what you did and explain the approximations and ‘guesstimates’ you made. After the presentation, be prepared to accept and answer questions and to discuss what you did with the rest of the class.