As a K-12 teacher the project gets the students to understand the preparation of iodoform along with the mechanism.
Preparation of iodoform
Preparation of iodoform or the iodoform test is notorius for its poor yield and can be a frustrating experience. We took it upon ourselves to identify the conditions to maximize the yield of iodoform.
What is iodoform? Iodoform is Tri-iodomethane (CHI3), a pale yellow colored precipitate that has antiseptic properties. The formation of Iodoform is characterised by its pale yellow color and its characteristic odor.
What is the iodoform test?
Presence of the methyl carbonyl group (CH3CO-) in aldehydes and ketones or the CH3-CH-(OH) group in alcohols is detected with the iodoform test. Ethanal is the only aldehyde that will give the iodoform test.
CH3COR +NaOI-->R COONa+ CHI3
To optimize the conditions for the preparation of iodoform.
The preparation of iodoform may sound to be a mundane project. However, our investigation was different. In the K-12 lab, students often test for organic functional groups. One of the tests they perform is the iodoform test for methyl ketones, ethanal(acetaldehyde) and ethanol. However, the prescribed test rarely gives them a thick pale yellow precipitate. What is observed is a faint white milky solution or a miniscular yield of fine yellow matter at the bottom of the test-tube.
In our lab as part of the K-12 project we decided to streamline the reaction conditions to come up with a recipe that would give us a bountiful precipitate.
We decided to study both the conditions as well as the functional group that would give us a large and noticeable amount of iodoform.
Our variables -
i. the functional group of the substrate
We tested three substrates - an aldehyde, a ketone and an alcohol viz. ethanal, acetone or propanone and ethyl alcohol were used in the study.
ii. the method used
In our first method we use sodium carbonate and potassium iodide (KI) for the preparation. In our second method, we use NaOH along with potassium iodide while in the third method iodine crystals along with NaOH were used to prepare the iodoform product.
iii. the addition of sodium hypochlorite –
we first compared the precipitates obtained before and after addition of the sodium hypochlorite as addition of the bleach (NaOCl) was to be our winning find.
Since time was a constraint, our experimental set up was designed to help us determine in one single run if the three organic compounds would yield us a sufficient amount of iodoform.
We had a set of 9 small labelled 100 mL beakers set up for our experiment.
In the first set of the three 100 mL beakers, we added 5 mL of each of the organic functional groups. Each of these was subjected to subsequent addition of sodium carbonate and KI. The beakers were incubated in a hot water bath at 70oC for approximately 10-15 minutes.
To our second set we added solid KI and NaOH solution to form iodoform to which each of the three organic compounds were added.
To the third set of beakers containing the three organic compounds, we added iodine crystals and NaOH solution to form iodoform.
With the three methods described earlier we observed some amount of iodoform however the yield was very low .
Then, to each of these beakers we added some sodium hypochlorite solution as an additional experimental step.
There was a noticeable and instant gratification in the form of a thick pale yellow precipitate. There was dramatic improvement in the yield upon the addition of sodium hypochlorite regardless of the procedure used for the preparation.
In our comparative study between the three methods, we observed that for Ethanol as well as acetaldehyde, the yield was maximum with Sodium Carbonate and powdered Iodine.
For Acetone, the yield was maximum with Iodine crystals and Sodium hydroxide.
On addition of bleach, the yield increases substantially in all the three methods. The addition of bleach, increases the yield as bleach acts as an oxidising agent. The Cl- displaces I - and makes I2 more readily available for the reaction which increases the yield
This experimental work was carried out by Saumya Joshi and Shreya Shridhar students of Grade 12 CBSE at R.N. Podar Sr. Secondary School, Khar under the guidance of Ms Anjali B. Gharpure