Calculating UV λmax of molecules using the Woodward-Fieser Rules

Dear First Year students,

This is a short tutorial on calculating UV absorption maximum (λmax) using the Woodward-Fieser rules. I hope this would help to clear any doubts you may have, so that you will understand the topic better and do well during the coming exam.

Let's look at the questions in the previous quiz.. They are good examples for us to understand how to arrive at our λmax.

The first question:

Not so difficult,isn't it. The second question:

To sum it up, follow these general steps:

1.     First, determine the parent or base structure be it a diene, α,β -unsaturated ketone, or an aromatic ring. This would tell you which rule to use

2.     Look for any double bonds extending this diene or α,β -unsaturated ketone.

3.     Treating this conjugated system as a whole, go along the chain looking for alkyls or auxochromes directly attached to it, anything far from the chain not counted.

4.     Lastly, look for other elements such as exocyclic double bonds and homodiene element, if there are rings involved.

I hope that this tutorial will help you. Please also study the tutorials for other examples. 

I wish you all the best for exams and happy studying! 

Best wishes, Dr Lee

A tutorial on IR Spectroscopy

Dear students,

This is a tutorial on reading IR spectra to make your preparation for the exam complete. I take the opportunity to discuss the IR question from the previous quiz.

Here is the spectrum given in the quiz:

You were asked to choose between two compounds, C and D as to which the spectrum belongs to. C is an aldehyde (1-pentanal) and D is a carboxylic acid (pentanoic acid).

Let’s look closely at the spectrum. The most obvious peak is the intense and sharp carbonyl (C=O) stretch at 1727 cm^-1 signalling the presence of a carbonyl which both compounds have in common, both are carbonyl compounds. Next, look closer at a moderately intense peak at 2719 cm^-1. There are actually two peaks at ~ 2700 and ~2800 cm^-1, one of the peaks being half hidden behind the bigger saturated C-H stretch bands. This two peaks show the presence of C-H stretch of an aldehyde. So, this is clear indication that this spectrum belongs to compound C.

Some of you may have been misled by a rather medium size peak at around 3500 cm^-1. This is NOT an O-H stretch. It is probably an overtone of our intense carbonyl band at  ~1700 cm^-1. If you remember the IR spectrum of myristic acid from our Nugmeg myristicin practical, you will see that the O-H stretch band of myristic acid is distinctively different: it is broader, smooth, and spreads over a larger region (overlapping the saturated C-H bands). Have a look at it again.

An actual IR spectrum of compound D, pentanoic acid (it’s also called valeric acid) is shown below:

Hope this little discussion clears the haze regarding IR spectrum.

I include here a video on reading IR spectra (a substitute tutorial teacher ) that I find useful for your preparation. Pleasant viewing and good luck for the exams.

Source: Youtube channel Sarutahiko1 (www.youtube.com/watch?v=XIWc9eT476c)

Welcome to Good Molecules!

Dear visitors,

Welcome to Good Molecules! 

This is a place where you can find everything related to Pharmaceutical Chemistry, Medicinal Chemistry, Organic Chemistry and Pharmacognosy, useful and interesting to all Pharmacy students.

More postings will be available soon. There will be topics of interest, side-stories, fun and entertaining stuff coming soon.

Please check for postings of materials related to the current topics.

Best wishes
Dr Lee