[Wad224]

Hi,
I am trying to isolate benzoic acid from a mixture of benzoic acid, methoxybenze, and trimethylpentane. I'm thinking of just adding hot water to the mixture and have the benzoic acid dissolve in water and then crystallize it by cooling the solution... would that work? I want to do an extraction so that none of the benzoic acid is left in the methoybenzene / trimethylpentane mixture.



thank you so much for your help.

[sjb]

It may work, but I'm not sure of the solubility of anisole in water, hot or otherwise.

It's true that water is a preferred recrystallisation solvent for benzoic acid, but this may depend on the other impurities. Is there any other property of the benzoic acid you can use to effect a useful extraction?

[Wad224]

Thanks for your reply. After thinking about it, I think it would be best to extract the benzoic acid through chemically active extraction. (With 10% Na2CO3, a weak base.) will I be able to do recrystalization of Benzoic acid with Na2CO3 after the extraction?

[Ch3mTutor]

Sample answer:

Dissolve the entire mixture in dichloromethane, and extract out benzoic acid as its sodium salt using 0.1-0.5 M NaOH(aq). Before recrystallizing, it is necessary to regenerate the free acid by acidification with HCl(aq) (done so with an external ice-bath to dissipate any heat released). The free acid may be extracted with fresh ethyl acetate, diethyl ether or dichloromethane. Removal of solvent in vacuo affords reasonably pure benzoic acid. Recrystallize from hot deionized water if further purification is desired.

[sunhee]

Hello.

  I am doing this lab what it is named as "Multi-step purification" tomorrow, and I am having trouble to understand the concept based questions in the below. If anybody know, please help me!!

 1. Why does Benzoic acid become soluble in water when it reacts with Na2CO3? (hint: weak acid and weak base approach)

 2. After the benzoic aicd product from the reaction, HCl is added to the solution. Explain why the product of the reaction will precipitate from the solution.

Thank you for your time!!

[ARGOS++]

Dear sunhee;

Please re-read the answer given from Ch3mTutor above and think about what you know about salt (Where are salt soluble?)!

Maybe a reaction equation can be of additional help.

Good Luck!
                    ARGOS⁺⁺

[sunhee]

Thank you for the reply.

 My trouble is that, I am not understanding what you are trying to tell me now. Not only English is my second language but also, I am lacking of solid chemistry study background. I feel embarrased to tell you that I don't know how to do this.  All I am thinking is when Na2CO3 disscoiates into Na+ and CO32- in the water, then, it will be something like, NaOH, HCO3-. there, I see the salt of NaOH.  Then, what should I think next?


Dear sunhee;

Please re-read the answer given from Ch3mTutor above and think about what you know about salt (Where are salt soluble?)!

Maybe a reaction equation can be of additional help.

Good Luck!
                    ARGOS⁺⁺

[ARGOS++]

Dear sunhee;

Not too bad!,  -  what you know about NaHCO₃.

What you also should know about salt is, that at least all Sodium salts are quite good water-soluble.
The second you should also know is that the stronger acid displaces the weaker in its salt.

Now as you know that Benzoic acid ( C₆H₅COOH )  isn’t well water-soluble, but is a stronger acid than H₂CO₃, are you able to write the equation for the extraction?:

C₆H₅COOH   +         .      ----->     .

Good Luck!
                    ARGOS⁺⁺

[sunhee]

Mr. Argos,

  Is the reaction would be something like ;

 C6H5COOH + NaHCO3 -> C6H5COONa + H2O + CO2

this? am I missing water in the reactant side? 
   
  I am sorry for keep asking you.  Thank you for your time.

Dear sunhee;

Not too bad!,  -  what you know about NaHCO₃.

What you also should know about salt is, that at least all Sodium salts are quite good water-soluble.
The second you should also know is that the stronger acid displaces the weaker in its salt.

Now as you know that Benzoic acid ( C₆H₅COOH )  isn’t well water-soluble, but is a stronger acid than H₂CO₃, are you able to write the equation for the extraction?:

C₆H₅COOH   +         .      ----->     .

Good Luck!
                    ARGOS⁺⁺

[ARGOS++]

Dear sunhee;

Bravo!

If you write it this way:
C₆H₅COOH   +   Na⁺  +  HCO₃^-      ----->     C₆H₅COO^-  +  Na⁺   +  H₂CO₃
        then you see better what’s going on, and why the Benzoic acid is now water-soluble.

And you are right, that H₂CO₃ will decompose into water and CO₂ gas!

Good Luck!
                    ARGOS⁺⁺