[friedpork]

I have some homework questions after synthesizing isoamyl acetate.

1)The first question is assuming that the reaction had gone to completion after the 20 minutes of refluxing, what compound(s) besides the desired product, isoamyl acetate, would be in the reaction flask?

I assume that the only think left in the reaction flask is water? In order to balance the equation water is needed.

2)The second question is using balanced chemical equations wherever possible and incorporating structural formulae, how you removed each of these compound(s) from the reaction mixture during the isolation/purification of the product, isoamyl acetate.

I believe that the isolation/purification reaction portion of hte product is during the distillation. I thought the way water is removed from the reaction mixture is because water and isoamyl acetate have different boiling points. The boiling point of the mixture increases as the vapour is removed from the reaction mixture. So I don't entirely understand how to incorporate balanced equations or structural formulae for this question.

3) The third question is a stoichiometry question. It asks if the reaction between acetic acid and isoamyl alcohol could only obtain a maximum yield of 67% of the desired ester, if you needed to produce 45 grams of isoamyl acetate, what are the minimum weights of both the acetic acid and isoamyl alcohol that you would need for this experiment? What would be the minimum volumes of both acetic acid and isoamyl alchol that you would need to measure out for this experiment?

I'll show what I have gotten...

45 grams x 0.67 = 80 grams
80 grams x mol / 130.18 grams (mw of isoamyl acetate) = 0.61 mols
0.61 mols x 88.15 grams/ mol (mw of Isoamyl alcohol) = 54 grams isoamyl acetate
54 grams x ml / 0.813 g (desnity of isoamyl acetate) = 66 mL isoamyl acetate

0.61 mols x 60.16 grams/mol (mw of acetic acid) = 37 grams acetic acid
37 grams * ml/ 1.049 g (density of acetic acid) = 35 mL

I know there is a lot of information in this post. I don't expect all of it to be answered at once. If I could get some help for at least the first 2 questions that would be greatly appreciated.

[discodermolide]

I think you should check your calculation(s) in part 3. Hint: How does 45 x 0.67 = 80?

[friedpork]

45 * 0.67 = 30.15
45 +30.15 = 80.15 This is what I originally meant
I did this because your maximum yield is only 67% so your theoretical yield needs to 80 grams in order to get 45 grams of isoamyl acetate

[Dan]

45 * 0.67 = 30.15
45 +30.15 = 80.15 This is what I originally meant
I did this because your maximum yield is only 67% so your theoretical yield needs to 80 grams in order to get 45 grams of isoamyl acetate

No. You can easily see that something is wrong by considering that 67% of 80 g is 53.6 g (not 45 g as you require). What was your logic in this calculation?

If the yield is 67%, this means that:

Actual yield = 0.67(Theoretical yield)

Rearrange the equation to find the theoretical yield - the amount you have to theoretically aim for to obtain an actual yield of 45 g.

[friedpork]

45/0.67 = 67.16 grams. Is that better?

[Dan]

Much better.

You will have to post your experimental procedure for help with the first two.

[friedpork]

For question 3, is the rest of it correct? It's just the initial finding of the yield was wrong and the other steps were correct?

My procedure for this experiment included

1)setting up 15ml isoamyl alcohol and 20ml glacial acetic acid into reflux in a 100 ml RB flask
2)After 20 minutes of boiling, I would remove the heat source (thermowell), wait for it to cool and then pour the organic layer into a separatory funnel and wash it with 2x25 ml water, 2x15 ml 5% sodium bicarbonate and 1x15 ml saturated NaCl
3) Then i would pour the ester into a erlenmeyer flask and dry it with magnesium sulfate for 10 minutes, filter off the dryign agent and then set the ester for distillation
4) I collected the distillate between 130 and 140 degrees.

Pretty much my procedure in a nut shell

[Dan]

For question 3, is the rest of it correct? It's just the initial finding of the yield was wrong and the other steps were correct?

No, because your numbers in the following calculations are based on an incorrect calculation of 80 g.

1)setting up 15ml isoamyl alcohol and 20ml glacial acetic acid into reflux in a 100 ml RB flask

For Q1: What is the limiting reagent (a balanced chemical equation will help you here)? The reagent in excess will be left over after the reaction as well as the water produced

2)After 20 minutes of boiling, I would remove the heat source (thermowell), wait for it to cool and then pour the organic layer into a separatory funnel and wash it with 2x25 ml water, 2x15 ml 5% sodium bicarbonate and 1x15 ml saturated NaCl
3) Then i would pour the ester into a erlenmeyer flask and dry it with magnesium sulfate for 10 minutes, filter off the dryign agent and then set the ester for distillation

These are purification steps - what is happening? From your revised answer to Q1, how/why are the impurities (almost completely) removed in this process?

[friedpork]

The limiting reagent is the isoamyl alcohol. So therefore the acetic acid is in excess. The water wash is to help remove some of the water from the product. The sodium bicarbonate wash is to neutralize the acid?? I think...

The drying agent is used to remove the rest of the water in the reaction mixture.

[discodermolide]

How can a water wash help remove water?
What else can the water wash remove? What else is water soluble here?

[friedpork]

ahh.. I guess the acetic acid is soluble in water so the water wash is used to help remove some of the acetic acid

[Dan]

Close, what is the purpose of the bicarbonate?

[friedpork]

As far as I understand, the purpose of the bicarbonate is to neutralize the acetic acid, is it not?

[Dan]

Yes. You have said that the acetic acid is washed out with water - but there is no acetic acid left because it was neutralised with bicarbonate:

http://en.wikipedia.org/wiki/Acid-base_extraction

[friedpork]

I'm not entirely sure if I understand the purpose of the water then... I do the water wash first before I use the bicarbonate. So are you saying that the water wash does not wash out the acetic acid?