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The Hive => Stimulants => Topic started by: ADDkid on June 25, 2004, 04:11:00 PM

Title: Hofmann rearrangement?
Post by: ADDkid on June 25, 2004, 04:11:00 PM
I know this is a stupid question, however I tried the search engine.
If a strong sodium hydroxide solution is added to sodium bromide would that form the hypobromite that is needed for the Hofmann reaction.  I was unsure if the molar ratios of the Na ions needed to be equal?  Does the bromine Lewis acid need to be present.  And if so would HBr be a better choice when it is neutralized with Hydrogen peroxide, and then slowly added Sodium hydroxide to the solution?
Title: hypobromite
Post by: WizardX on June 25, 2004, 06:47:00 PM

If a strong sodium hydroxide solution is added to sodium bromide would that form the hypobromite that is needed for the Hofmann reaction.



NO!

http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0008 (http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0008)



2 NaOH + Br2 ==> NaOBr + NaBr

A solution of 42 g. (1.05 moles) of sodium hydroxide in 360 ml. of water is placed in a 1-l. three-necked, round-bottomed flask fitted with a mechanical stirrer and a thermometer and is cooled to ?5° in an ice-salt bath. The stirrer is started, and 43 g. (0.263 mole) of bromine is added from a separatory funnel at such a rate that the temperature remains below 0° (addition time about 5 minutes). The ice-cold solution is diluted with 240 ml. of dioxane (Note 1) that has previously been cooled to 13–14° (Note 2). This solution is kept at 0° until required.


2. The temperature of the hypobromite solution is kept below 10° to avoid the formation of sodium bromate.

Title: Wizard you should have read the whole post, I...
Post by: ADDkid on June 25, 2004, 08:26:00 PM
Wizard you should have read the whole post, I mention that adding hydrogen peroxide to HBr until neutral, and then adding the NaOH, I don't believe that the soultion needs to be concentrated so the water that is formed from adding the peroxide should not matter.
Title: Temperature
Post by: WizardX on June 26, 2004, 05:47:00 PM
ADDkid: NaOBr is temperature sensitive. Lets take a closer look at the procedure marked in red.

2 NaOH + Br2 ==> NaOBr + NaBr

A solution of 42 g. (1.05 moles) of sodium hydroxide in 360 ml. of water is placed in a 1-l. three-necked, round-bottomed flask fitted with a mechanical stirrer and a thermometer and is cooled to -5° in an ice-salt bath. The stirrer is started, and 43 g. (0.263 mole) of bromine is added from a separatory funnel at such a rate that the temperature remains below 0° (addition time about 5 minutes). The ice-cold solution is diluted with 240 ml. of dioxane (Note 1) that has previously been cooled to 13–14° (Note 2). This solution is kept at 0° until required.


2. The temperature of the hypobromite solution is kept below 10° to avoid the formation of sodium bromate.

I mention that adding hydrogen peroxide to HBr until neutral, and then adding the NaOH



If you intend to make the Br first (hydrogen peroxide to HBr until neutral) and add the NaOH, YOU MUST CONTROL THE TEMPERATURE DURING THE REACTION.