Electrophilic+Aromatic+Iodination+of+Vanillin

=Introduction=

The purpose of this lab is to iodinate the aromatic compound vanillin, through an electrophilic aromatic substitution. In electrophilic aromatic substitution reactions, a hydrogen atom attached to an aromatic system is replaced by an electrophile. The electrophilic aromatic substitution of vanillin occurs as a two-step addition-elimination mechanism, beginning as an addition reaction and finishing as an elimination reaction. A carbocation intermediate is formed when the pi bond electrons in the aromatic ring are attacked by an electrophilic reagent. A leaving group, hydrogen, is then eliminated by a nucliophile, forming the substituted product, 5-iodovanillin.



Iodination reactions typically involve the use of a mixture of iodine and a powerful oxidizing agent. In this lab, a combination sodium iodide, and household bleach (NaOCl) will be used as the oxidizing agents, in aqueous ethanol. Bleach is used break the bond between two iodine atoms resulting in an iodide radical that will start the reaction. Once the reaction is complete, the solution will be acidified to promote the precipitation of the 5-iodovanillin. However, the color removing effects of the bleach used to start the reaction, renders the universal pH indicators useless Therefore, sodium thiosulfate will be added to the solution before this step in order to neutralize the bleach. This is considered a green experiment because the reaction is efficient and selective, and offers more environmentally benign reaction conditions through the use of a less hazardous oxidizing agent. The reagents and solvents used in this lab minimize the formation of by products and eliminate wasted starting material and solvent.

SUPER nice!! Great intro.

=Procedure=

The procedure for the experiment can be found on pages 186-187 in Doxsee and Hutchison's Green Organic Chemistry.

Relevant Compounds

 * Name:** 2-propanol (NIST)
 * Cas #:** 67-63-0
 * Molecular Formula:** C 3 H 8 O


 * Name:** 5-iodovanillin (ChemBook)
 * Cas #:** 5438-36-8
 * Molecular Formula:** C 8 H 7 IO 3


 * Name:** Ethanol (NIST)
 * Cas #:** 64-17-5
 * Molecular Formula:** C 2 H 6 O


 * Name:** Hydrochloric acid (NIST)
 * Cas #:** 7647-01-0
 * Molecular Formula:** HCl


 * Name:** Sodium Hypochlorite (ChemBook)
 * Cas #:** 7681-52-9
 * Molecular Formula:** NaOCl


 * Name:** Sodium Iodide (NIST)
 * Cas #:** 7681-82-5
 * Molecular Formula:** NaI


 * Name:** Sodium Thiosulfate (NIST)
 * Cas #:** 7772-98-7
 * Molecular Formula:** Na 2 O 3 S 2


 * Name:** Vanillin (NIST)
 * Cas #:** 121-33-5
 * Molecular Formula:** C 8 H 8 O 3

=Data=


 * Reaction**

1.003g of vanillin was dissolved with 20mL of ethanol in a 100mL round bottom flask. Once dissolved, 1.178g of sodium iodide was added to the solution and cooled to 0⁰ C via an ice bath. The resulting solution was an opaque milky white/yellow color. 11mL of aqueous sodium hypochlorite solution was added drop-wise via a separatory funnel over 10 minutes while being stirred. The calculation for drop rate can be found in the analysis section. The solution changed in color throughout the course of the reaction from its initial opaque white/yellow color to a more translucent red/brown. The resulting mixture was then allowed to warm to room temperature while continuing to be stirred.


 * Workup and Isolation**

10 mL of sodium thiosulfate solution was added to the mixture. This was followed by the addition of 4 mL of hydrochloric acid. The resulting change in pH was monitored via pH paper until the mixture had a pH of 1-2. At this point, a pale precipitate began to form in the mixture. With instructor assistance, ethanol was removed with a rotary evaporator. The resulting mixture looked similar to what went into the rotary evaporator, however, it was much more concentrated. The flask was cooled in an ice bath for an additional ten minutes and the resulting precipitate was collected via vacuum filtration. The weight of the recovered crude product was 1.351g. Were the instructions to "acidify" the contents of the flask? Lowering the pH to the level you did was probably unnecessary....though at this point you sure had a good yield. I would interpret "acidify" as asking the pH to get below 7, no need to go further.

1.351g of crude product was placed into a 100 mL erlenmeyer flask. Over heat, warm 2-propanol was added to the flask in order to dissolve the crude product. In total, 65 mL of 2-propanol was added. At this point, not all of the crude product had been dissolved. However, the next step in the procedure was carried out in fear of having too much solution for the precipitate to form in. 15ml of hot water was added was added to the solution. Again, the desired cloudiness had not been achieved, but the next step was carried out for the same reasons discussed before. 8 more mL of 2-propanol was added and the solution was placed into an ice bath to cool. After 15 minutes, it became clear that no precipitate was forming. An attempt was made to collect any precipitate that was formed via vacuum filtration. A small sample was recovered. Sad!

However, its weight could not be determined to the .000g. There was enough recovered product to fill approximately 1 / 16 of inch in a capillary tube. The melting point was observed to be 156.4⁰C-170.0⁰C.

=Analysis=


 * Reaction**

Drop rate was calculated via the following conversion:




 * Workup and Isolation**


 * % Recovery Crude Product:**




 * % Recovery Finished Product:**

I appreciate how you have laid out these equations. Thank you.
 * Product Purity:**



=Conclusion=

The purpose of this lab was to iodinate vanillin through electrophilic aromatic substitution. This is an important reaction because it gives organic chemists a place in aromatic molecules that more desirable substituents can be added by future reactions. The final product produced from this reaction was 5-iodovanillin. The percent yield for the crude product was 73.71%. Due to limitations during the lab, there was not enough final product collected to obtain a weight; therefore, the percent yield for the final product was 0%.

The literature melting point of the final product, 5-iodovanillin, is 183°C - 185°C. The melting point range obtained for the final product in this lab was 156°C - 170.0°C. The lower than literature melting point could be due to some impurities in the final product. It was also difficult to observe the melting point with such a small sample.

A limitation that occurred during this lab was the final product would not precipitate out of the solution. The final sample was so small, a mass could not be obtained to the .000g. This error could be due to the amount of 2-propanol added to the crude product during the work-up and isolation steps. Only enough 2-propanol to dissolve a majority of the crude product should have been used. Instead, too much 2-propanol was added in an effort to get all of the crude material to dissolve. This error is attributed to the experimenters lack of experience when working with recrystallization on the macro scale. I think the experimenters also have a legitimate concern regarding the way the procedure is written! I'll have to try this out myself, again, one of these days. I think the potential to change those instructions exists, and that they could be made clearer as a result.

Resources
//Chemical book//. (2008). Retrieved from http://www.chemicalbook.com/

Doxsee, K. M., & Hutchison, J. E. (2004). //Green organic chemistry: Strategies, tools, and laboratory experiments//. (pp. 186-187). Belmont: Brooks/Cole.

P.J. Linstrom and W.G. Mallard, Eds., NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg MD, 20899, [|http://webbook.nist.gov], (retrieved February 15, 2012).