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Reactions Involving Alkanes, Alkenes & Arenes

posted Dec 15, 2016, 7:55 AM by Grace Ong   [ updated Dec 17, 2016, 1:20 AM ]

The following post was first posted on Blogger on Sunday, 24 March 2013.



The following slides describe the main reactions involving alkanes, alkenes and arenes. The following mechanisms are included:
  • Free-radical substitution mechanism of alkanes
  • Electrophilic addition of alkenes
  • Electrophilic (aromatic) substitution of arenes

(To view an expandable version, click here to open slides on SlideShare.)

WeWW! Regarding that green tea demonstration you saw during your China trip...

posted Dec 5, 2016, 9:38 PM by Grace Ong   [ updated Dec 6, 2016, 1:32 AM ]

The following post was first posted on Blogger on Friday, 15 March 2013.



Pardon my tardiness! I've been feeling a little overwhelmed with work and health issues lately, so the following post has been sitting in my drafts folder since Lunar New Year!

The post may seem a little stale now... but the video posted must seem familiar to those who have been to a green tea plantation in Hangzhou.

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I've been drowning myself with plenty of green tea lately to counter all that feasting over the Lunar New Year period.

green tea leaves

I bought these Longjing (龙井) green tea leaves from Meijiawu Tea Village (杭州梅家坞) in Hangzhou, China, last June. As with any guided tour to China, a visit to the famous tea plantations comes with a mandatory sales pitch on their tea products. For those of you who have been herded to such a sales talk while in Hangzhou, you will find the following demo very familiar:

(Watch this video on YouTube.)

Okay, granted that the guy in this video is not the pretty Chinese lady performing the demo at the tea plantation, but the procedure is the same:
  • Water is added to some rice grains.
  • An iodine tincture is added to the rice-water mixture and a blue-black colour results.
  • A green tea infusion is added to the blue-black water and stirred.
  • Voila! The blue-black colour of iodine is decolourised by the green tea infusion!
  • Conclusion: green tea is a powerful antioxidant!
After this 'amazing' demo, everyone present gets suckered into buying packets of Longjing tea leaves or bottles of green tea extract. I, too, found myself most happily and willingly suckered into parting with my money for 2 kg of Longjing tea leaves. And no, it's not because of the demo. Really. In vivo studies such as this have shown that tea catechins are indeed effective antioxidants. The many health benefits aside, I absolutely love the delicate fragrance of Longjing tea. (Of course, the very convincing sales people helped too!)

So, does the above experiment actually demonstrate the antioxidant properties of green tea? Any tea which has undergone minimal oxidation, e.g. white and green tea, contains plenty of polyphenolic compounds called catechins:

examples of green tea catechins

Phenolic groups are highly activating. When aqueous iodine is added to a phenol, electrophilic aromatic substitution of benzene hydrogens by iodine occurs readily:

electrophilic substitution of phenol by iodine

Note: (i) Only the para mono-substituted product is shown here.
(ii) It is also possible for iodine to substitute at the ortho-positions.

From a chemical perspective, I think what happens in the demo is as follows:
  • A starch solution is prepared by mixing rice grains with water.
  • Upon adding an iodine tincture, which contains iodine and potassium or sodium iodide dissolved in ethanol, a blue-black complex of starch and iodine results. I believe this step was added for its 'dramatic' effect, as the blue-black colour is so deep that its intensity is not reduced even with dilution. A reader more correctly pointed out that water, instead of green tea, was added to the starch-iodine solution as a control to show that the blue-black colour could not be removed by dilution.
  • Upon adding the green tea, the blue-black colour disappears, as the polyphenols present in green tea readily undergo electrophilic aromatic substitution reactions with iodine.
While a redox reaction has indeed occurred: the phenols have been oxidised at the benzene carbons where iodine has substituted the hydrogens, while iodine has been reduced to iodide, the actual antioxidant mechanism is much, much more complicated than this. In fact, research such as this has shown that the mechanism involves free radicals. Simply put, free radicals in our bodies cause cell and even DNA damage. Antioxidants such as catechins help reduce the damage by 'mopping up' these free radicals by forming less reactive radicals or non free-radical molecules.

Whether or not the above demo actually shows the antioxidant prowess of green tea, it was fun to watch. More importantly, to the sales staff, when the blackish colour of iodine disappears magically, it sends out a powerful and convincing sales message to the layman: Green tea cleans up your system efficiently and effectively, so for your health's sake, (buy and) drink plenty of it!

And that was how everyone at the demo ended up buying packets of tea leaves and bottles of green tea extract - items they probably would not consume unless they were already habitual chinese tea drinkers!

P.S. Leo Kwan ranked tea leaves in order of their catechin content on his website called Tea Guardian. I don't know if he had based his list on empirical evidence, but he does seem to know a lot about tea.

The Trials and Tribulations of a Chemistry Teacher I

posted Nov 16, 2016, 10:40 PM by Grace Ong   [ updated Nov 16, 2016, 10:41 PM ]

The following post was first posted on Blogger on Thursday, 28 March 2013.



Sigh.

silly student answer

In case you are wondering, compound F is

structure of benzyl chloride
benzyl chloride

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