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The Stereoisomers of Organic Molecules with More Than One Chiral Carbon Atom

posted Nov 16, 2016, 6:12 AM by Grace Ong   [ updated Nov 16, 2016, 6:50 AM ]

The following post was first posted on Blogger on Wednesday, 1 January 2014.

Happy New Year, everyone!

This is a little embarrassing... I'm starting 2014 with a blog post which I had wanted to publish some time back, but promptly forgotten as I got too busy at school.

I came across the following question (and its proposed answers) a few months ago.

structure elucidation with incorrectly drawn enantiomers

This is a relatively straightforward question, but the solution isn’t quite as straightforward, particularly in relation to the three-dimensional structures of compounds B and C

Both (geometric) isomers of A, as shown in the given answers above, undergo mild oxidation with cold, dilute potassium manganate(VII) solution to produce butane-2,3-diol:

structural formula of butane-2,3-diol

This diol has two chiral carbon atoms, and a plane of symmetry between them.

A consequence of this structure is that while it is possible for butane-2,3-diol to have a maximum of 22 = 4 stereoisomers, in reality, it only has three – a pair of optically active enantiomers and an optically inactive meso compound.

Since the question states that the products of this oxidation, B and C, rotate light in opposite directions, they must be the two enantiomers of butane-2,3-diol. At this point, this conclusion is congruent to the answers proposed.

What I do not quite agree with, however, is the simplistic three-dimensional representation of B and C in the given answers; the teacher who drew these structures had used the ‘standard’ convention for drawing enantiomers given under Section 10.1 of the H2 Chemistry Syllabus 9647, using only one of the asterisked carbons as a chiral centre reference point. While this works fine for molecules with only one chiral carbon, this is not so for molecules with more than one chiral carbon, such as the butane-2,3-diol answer required by the question above.

The following diagrams illustrate what I mean:

meso compound of butane-2,3-diol

enantiomers of butane-2,3-diol

And M is a diastereoisomer of B and C.

As you can see, the overall chirality of butane-2,3-diol is dependent on the spatial orientation of the H atoms, −OH and −CH3 groups around both chiral carbon atoms. In other words, the only way to tell if the molecule has a plane of symmetry (and therefore achiral) is to draw wedge-and-dash bonds around not one, but both chiral carbon atoms.

Addendum (02 January 2014):

The reaction between compound A and cold, alkaline potassium manganate(VII) is an example of a syn stereoselective hydroxylation of an alkene to form a diol.

Syn hydroxylation of cis-but-2-ene gives meso butane-2,3-diol (M),

syn hydroxylation of cis-but-2-ene

while that of trans-but-2-ene results in racemic butane-2,3-diol (B and C).

syn hydroxylation of trans-but-2-ene

Thus, if the reaction starts off with a mixture of both cis- and trans- isomers of A, the diol formed will be a racemic mixture of both enantiomers, i.e. B and C, as well as the meso compound, M.

The Boring Magnesium Oxide & Water Reaction

posted Nov 1, 2016, 1:27 AM by Grace Ong   [ updated Nov 1, 2016, 1:27 AM ]

The following post was first posted on Blogger on Sunday, 26 January 2014.

I finally have a post on an inorganic chemistry topic!

My school year began with Group 2 elements, a topic which my students were asked to complete on their own during the year-end holidays. During a tutorial discussion, I asked my students what they would observe when water is added to solid magnesium oxide. As expected, given their lack of practical chemistry experience, a good number of them told me that they would observe either all or some of the solid dissolving to give a clear, alkaline solution.

They only got the alkaline solution part right though. I must point out that when one adds water to magnesium oxide, one does not see an observable change. There is a reaction – no doubt about it, but it cannot be simply ‘observed’; you only know that a reaction has occurred if you test the pH of the mixture with litmus or pH paper.

Before water is added ...
solid MgO in a test tube
a white solid of magnesium oxide, MgO

After water is added ...
suspension forms when water is added to solid MgO

magnesium is insoluble in water a suspension of white solid (of magnesium hydroxide) is observed,
i.e. white solid remains insoluble.

Solid magnesium oxide reacts with water to form solid magnesium hydroxide, Mg(OH)2.
MgO(s) + H2O(l) → Mg(OH)2(s)

On standing ...
litmus test on MgO suspension

MgO reacts with H2O to form Mg(OH)2 the white solid (of magnesium hydroxide) settles to the bottom of the test-tube.
Supernatant liquid above solid is tested alkaline, as red litmus paper turns blue.
(Notice that there is no discernible change in quantity of white solid after the reaction.)

Magnesium hydroxide is only sparingly soluble in water. Thus the amount of solid does not decrease significantly for a change to be observed.
Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH(aq)
The solution above the white solid is tested alkaline due to the presence of (a very low concentration of) OH ions.

Interestingly, there is a big jump in ‘observed reactivity’ with water from magnesium oxide to calcium oxide: there is no observable change when magnesium oxide reacts with water, but calcium oxide reacts so vigorously with water that the amount of heat evolved is enough to bring the mixture to a boil, as the home video below demonstrates!

A note of caution!
If you are planning to try this experiment at home using a glass container, make sure it is made of borosilicate glass (e.g. Pyrex®;) rather than silica-glass. The latter is soluble in hot, concentrated alkali solution, or in molten alkali!

The Wothers Guide to the Periodic Table - The Alkali Metals

posted Nov 1, 2016, 12:10 AM by Grace Ong   [ updated Dec 17, 2016, 1:16 AM ]

The following post was first posted on Blogger on Thursday, 30 January 2014.

This is a highly entertaining public lecture on Group 1 elements which I would like share with my readers. A good introduction to the chemistry of alkali metals! Lots of 'visual effects' too!

The lecturer is Dr Peter Wothers, who is a Teaching Fellow in the Department of Chemistry, University of Cambridge and a Fellow and Director of Studies in Chemistry at St Catharine’s College, Cambridge. He has a whole collection of just-as-entertaining open lectures. Enjoy!

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