Are Red Table Grapes Sold in Stores Always Red?

Structure. Structure. Structure. As we all know as chemistry educators, structure is the 'go to' concept when trying to figure out why molecules do what molecules do. In this blog I describe a short GChem 1 classroom activity (length = 15-20 minutes, depending) where students eventually draw a 2-D rendition of the 3-D structure of a plant growth-promoter based on the correct interpretation of information related to the bond types in the molecule of interest. My teaching emphasizes the applications of chemistry, especially as related to food production and agricultural practices, and this activity focuses on the structure of a molecule that promotes the change in color of red table grapes on the vine from green to red, a chemical process in viticulture called veraison (veer-ray-zohn).

The activity is prefaced with a short lecture or 'story' to set the stage and perhaps enhance interest in the topic or promote the applications of chemistry to reality. Post-lecture students then engage in proposing a structure of the molecule of interest. The activity concludes with students comparing their proposed structure to the structures proposed by other students. As is common in my teaching style, I typically have students compare their answer(s)- in this instance, a proposed structure of a molecule- to other students as I believe as an educator that students need to defend their conclusions, or perhaps grow comfortable with the fact and reality that they indeed may be wrong and someone else right. Upon comparison one of four possibilities then is possible.

1. Student A's structure is correct and student B's structure is incorrect.

2. Student A's structure is incorrect and student B's correct.

3. Neither students' structure is correct. Or...

4. Both students have the correct structure and the structure is supported by the bond type information.

In its entirety (lecture 'story' + students figuring out structure of molecule + comparing structures) the activity takes no more than 20 minutes to complete, depending of course on the number of student questions asked during the lecture 'story' component (described below) and also time taken to compare their proposed structure to another student(s). First, I'll give insight into the lecture material that starts the activity. Then I'll describe the bond type information and expected results.

This activity starts out with the following question,

I then talk a bit about grape growing regions around the world and ask the following question below.

As much as possible I incorporate into my teaching 'big picture' science concepts. In the question above if students do not know already, then I want them to know that the climatic seasons in the southern hemisphere are opposite those in the northern hemisphere at any given time during the 365.25 day/year earth-orbiting-the-sun calendar. I have Goggle Earth ready to go to point out, if a hint is needed, the location of Argentina. Most students answer the question above correctly.

The lecture 'story' leading up to the activity continues. Supported by a series of slides, I continue with the 'story' as I shift into discussion about how grape ripening and color change require not only heat units (warmth) but also a trigger mechanism initiated by nighttime cool temperatures. With average nighttime low temperatures creeping ever higher over the last two decades and cold (not cool) nights becoming less frequent, it is becoming more problematic for growers of red table grapes to get the necessary distinct color change (veraison) in the requisite time period to begin harvesting and then ship their product off to market. The longer a grower allows a red table grape variety to stay on the vine waiting to change color naturally (and with less predictability) with the day-night temperature fluctuations, the more likely hungry and opportunistic insects will move in to munch on the grapes (meaning the application of more topical pesticides), a destructive soil fungus or leaf powdery mildew may invade (meaning the application of more topical or systemic fungicides), birds swoop in and feast on grapes still on the vine, a wind storm severely damages the crop, or something else beyond the control of the grower causes irreparable damage to a grape grower's crop. 

So what does a red table grape grower do? Accelerate the color change process. How? Using a variety of plant growth regulators singly or in combination. I then show/discuss the images below.

Images: additional visual cues, the author

One plant growth regulator used- singly or in combination with another growth regulator such as abscisic acid- is ethephon. With the lecture 'story' complete and context provided, now it is time for students to apply their knowledge of bonding to draw a 2-D structure of ethephon. Below is the information given to students regarding ethephon.

Note that I do not provide the name of the plant growth regulator ethephon. Why? Because a student can just look it up on wikipedia (and no doubt some would). And also note that the table above is incomplete. Why give students everything? Obviously before drawing a 2-D structure supported by the bond type information, students first need to complete the table and then interpret and apply the information correctly to draw the structure of ethephon.

At this point in the semester my GChem 1 students have used their organic model kits to build a number of molecules comprised of CHNOX (= halogen) atoms in varying combinations- saturated/unsaturated, cyclic/linear/branched, and so on and have drawn in 2-D their fair share of molecules from the 3-D models built. So going into this activity my students know carbon forms four bonds (single, double, or triple), hydrogen one, oxygen two (single or double), nitrogen three (single, double, or triple), and any halogen one. But they do not know about phosphorus. But via plausible deduction from the information above students can glean that phosphorus can form five bonds. Some students figure this out; others do not. But that is why I have students compare their drawn structure to other students' structures. Some students end up teaching others. (Or what else happens is that without me knowing some just Goggle how many bonds phosphorus can form. Fine go ahead. I am not in the business of policing their chemistry education.)

After comparing their drawn structures students find that two constitutional isomers of ethephon exist. Some will have drawn the 2-chloro isomer (top structure, image below) and others will have drawn the 1-chloro isomer. Orange = P, Red = O; colored stars on carbons are for visual reference.   

Both structures above are in agreement with the bond type information provided so which isomer is the actual plant growth regulator used? From this point you basically have two instructional choices. The easy choice- you can just tell them that the 2-chloro isomer (top) is the plant growth regulator used. The not as easy choice- you can do a JiTT (Just-in-Time-Teaching) activity involving stereochemistry and asymmetric carbons. The question following a brief JiTT about molecular stereochemistry could be, Q: The isomer that is the plant growth regulator does not rotate plane-polarized light (PPL). Based on this fact and the two structures shown, which isomer is the plant growth regulator?

Some stereochemistry as a background. The 2-chloro isomer (top) is achiral (= does not have an asymmetric carbon and as a molecule does not rotate PPL) whereas the 1-chloro isomer (bottom) is chiral* on account of the asymmetric carbon (asymmetric = four different atoms or group of atoms bonded to a saturated, sp3 carbon). Being chiral the 1-chloro isomer rotates PPL so the two constitutional isomers can be differentiated** based on their rotation or non-rotation of PPL.

What about the use of differences in common physical properties to figure out which isomer is the one used? I tried to find some physical properties data like density and/or melting/boiling temperatures for both but was unsuccessful, though admittedly I did not expend too much effort trying to find the information. Though I suppose I could just make up some stuff like, VP (vapor pressure): top isomer > bottom isomer. So then one could ask, Q: The isomer used as the plant growth regulator has the lower boiling temperature of the two shown. Which isomer then- top or bottom- is the structure of the plant growth regulator used to accelerate the change in color of table grapes? 

Hope you enjoyed this brief post and perhaps can use this molecular structure activity in your classroom in some way. Thanks for reading....

*: limiting chirality to carbon atoms even though other atoms exhibit chirality

**: as long as the 1-chloro isomer is not racemic (= equal concentrations of the two (chiral) enantiomers)