Iodoform (CHI3) is the iodine analogue of chloroform. It is a pale yellow crystalline solid (m.p. 119°C), having a characteristic odour. It is used as a mild antiseptic and disinfectant. It is also used in the preparation of many medicinal ointments used as pain-relievers.
lodoform can be prepared by treating any organic compound containing CH3CH(OH) - group (e.g., ethanol, 2-propanol, 2-butanol) or CH3CO– group (e.g., propanone, 2-butanone) with iodine in presence of sodium hydroxide. In the laboratory, it is usually prepared from either ethanol or propanone. The chemical reactions involved are:
(a) With ethanol
2NaOH + I2 → NaOI + NaI + H2O
CH3CH2OH + NaOI → CH3CHO + NaI + H2O
CH3CHO + 3NaOI → I3CCHO + 3NaOH
I3CCHO + NaOH → CHI3 + HCOONa
(b) With Propane
CH3COCH + 3NaOI → CH3COCI3 + 3NaOH
CH3COCI3 + NaOH → CHI3 + CH3COONa
Preparation of iodoform
(i) Dissolve 5 g of iodine in 5 ml propane or ethanol in a 100 ml conical flask or round bottomed (R.B.) flask.
(ii) Add 5% NaOH solution in small portions while continuously shaking the flask. Cool the flask from time to time under tap water. The addition of NaOH solution is further continued till the brown colour of iodine just disappears.
(iii) Allow the flask to stand for 5-10 minutes.
(iv) Decant off the clear supernatant liquid.
(v) Filter the iodoform, wash with little cold water and then dry on a filter paper.
Crystallization of iodoform
(i) Place the crude iodoform in a 100 ml conical flask.
(ii) Add small amount of rectified spirit and heat it on a water bath.
(iii) Add more rectified spirit slowly till the iodoform dissolves.
(iv) Filter the solution quickly through a fluted filter paper into a beaker.
(v) Cool the solution in ice. The iodoform will crystallize rapidly.
(vi) Filter the crystals on a Buchner funnel, dry the crystals between the ibids of the filter paper.