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Articles Posted in October 2013 (previously hosted on Blogger)


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WeWW! Chemistry Haiku

posted Nov 16, 2016, 6:49 AM by Grace Ong   [ updated Nov 16, 2016, 6:50 AM ]

The following post was first posted on Blogger on Friday, 4 October 2013.


Haiku (俳句) is an ancient form of Japanese poetry, in three lines of five, seven and five on (音) or sounds. When writing a haiku in English, the on may be interpreted as syllables.

A haiku is traditionally used to convey an experience of nature. The subject of modern haiku however covers a wider range of topics, from love to even humour.

The following pieces are my first attempt at writing haiku, written in between consultation sessions with my students, on the eve of their H2 Chemistry paper. The subject of these haiku, as you might have guessed, is Chemistry, or the studying of Chemistry.

Boron and barium
Oh what lovely green observed
In O-two they burn		 Dipolar ions
Can't get along with water
So in cliques they form


Oh carbanion!
Thru you an F abandons
An aryl fluoride		 Rote is learning not!
For it makes your senses dull
Dimwit you become

They are not very good, I know. sad face icon

For more (and better, funnier) Chemistry haiku, read the works of Dr Steve Hardinger's students. (Dr Steve Hardinger is a UCLA University Distinguished Senior Lecturer in Organic Chemistry.)

Drawing Condensed Structural Formulae

posted Oct 25, 2016, 2:07 AM by Grace Ong   [ updated Oct 25, 2016, 2:17 AM ]

The following post was first posted on Blogger on Wednesday, 30 October 2013.


A student called me on the day he received his H2 Chemistry promotional examination paper, sounding a little indignant that he was marked wrong for writing the following structural formula:

HOH3C

Poor fellow, I had to break it to him gently that I would have marked him wrong too.

While it was obvious that he had meant CH3OH, no teacher, however lenient, would have accepted his (incorrect) answer.

Despite repeated reminders, I still find students reversing every single atom in a condensed structural formula of a group. This is particularly prevalent in the drawing of
  • geometrical isomers

    E.g. 1
    incorrectly drawn structure - inverted carbon chains in geometric isomers

  • optical isomers

    E.g. 2
    incorrectly drawn structure - optical isomers with inverted carbon chains

  • groups on the left-hand side of a benzene ring or a ring structure

    E.g. 3

    incorrectly drawn structures of carbon chains left of benzene rings and cycloalkanes
With heteroatoms, i.e. atoms other than carbon and hydrogen, such reversal is fine. For example, it is acceptable to write HO−R, HO2C−R (or HOCO−R), H2N−R and O2N−R.

representing heteroatoms in structural formulae

Such reversal however is not acceptable in the representation of carbon chains. The correct way to represent an ethyl group is CH3CH2−R, and not H3CH2C−R. While Cambridge examiners sometimes give the benefit of the doubt in the latter case (but not always!), they still prefer to see the correct representation of carbon chains, i.e. atoms, or groups of atoms (enclosed by parentheses), which are joined to carbon atoms in the carbon chain should be written after the carbon atom to which they are connected.

Let's explore a few examples to illustrate what is meant by the 'correct representation' of carbon chains.

Case 1: Drawing of geometric isomers

The correct representation of geometric isomers in E.g. 1 above is as follows:

correctly represented carbon chains in geometric isomers

Note that the H atoms are written after the carbon atom to which they are connected.

Some teachers, I know, insist that correct atoms be shown connected to each carbon in the C=C double bond, so alternatively, geometric isomers may also be represented as

an alternative correctly represented carbon chain in geometric isomers

Again, the H atoms are written after the carbon atom to which they are connected.

Case 2: Drawing of optical isomers of compounds with one chiral carbon

Like all stereoisomers, optical isomers have the same molecular and structural formulae. They show the same atom connectivity and differ only in the three-dimensional orientation of their atoms in space. Their differences, therefore, cannot be effectively represented by two-dimensional structures.

Section 10.1 of the H2 Chemistry Syllabus 9647 clearly indicates that when drawing a pair of optical isomers, candidates should use three-dimensional structures according to the convention used in the example below:

representation of optical isomers - Syllabus 9647
(Source: Section 10.1, 2014 H2 Chemistry Syllabus 9647)

Following this convention, the correct representation of optical isomers in E.g. 2 above should be:

correct representation of carbon chains in optical isomers

Alternatively, to show that correct atoms are connected to the chiral carbon, the following representation may be used:

alternative representation of carbon chains in optical isomers

The following shows the optical isomers of a cyclic compound whose chiral carbon is part of the ring structure:

representing ring structures in optical isomers

Case 3: Drawing groups on the left-hand side of a benzene ring or a ring structure

The correct representation of the two compounds given in E.g. 3 above is as follows:

correct representation of carbon chains to the left of benzene rings

If the correct atoms are to be shown connected to the benzene ring or ring structure, the following representation may be used:

alternative representation of carbon chains to the left of benzene rings

Note:
  1. The condensed structural formula has its limitations as it is not effective in illustrating complex molecules. I therefore strongly encourage students to learn how to draw skeletal formula well.
  2. If a question calls for a structural formula (as opposed to a displayed or full structural formula), students are free to use displayed (most time-consuming), condensed or skeletal (least time-consuming) formula to represent their compound. A hybrid of these formulae is also acceptable.

    Skeletal formulae of compounds in used in E.g. 1 and 3 above

    examples of skeletal formulae

    Example of a partial skeletal, partial condensed structural formula
    (Source: Section 10.1, 2014 H2 Chemistry Syllabus 9647)

    example of partial skeletal formula - Syllabus 9647

  3. When writing structural formulae (including displayed formulae), thebenzeneconvention for representing the aromatic ring is preferred by Cambridge examiners.
  4. Unlike the old Syllabus 9258, which carries the statement, 'The symbol cyclohexane for cyclohexane is acceptable' under Section 10.1, the recent Syllabus 9647 no longer includes this statement. This suggests that Cambridge examiners now require students to show all bonds when drawing cyclic compounds.

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