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# HELP .. Very confused with atom economy ? watch

1. 2-bromo-2-methylpropane,C4H9Br can be used to produce methylpropene,C4H8. 2-bromo-methylpropane reacts with sodium ethoxide, NaOC2H5 to produce ethanol,C2H5OH and sodium bromide , NaBr along with methylpropene. using the relative formula masses of the reactants and products,
work out the atom economy for this reaction.

What I did was i worked out the rfm of the desired product which was methylpropene,C4H8 = 56
Then I wrote down the equation

So C4H9Br + NaOC2H5 --> C2H5OH + NaBr + C4H8

Then I added all the rfm's of all the products (C2H5OH + NaBr + C4H8)
So ((12x2) + 5 +16+1) + (23+79.9) + ((12x4)+8) = 204.9

So
atom economy = molecular mass of desired product / sum of molecular masses of all products x100

(56/204.9)x 100 = 27.3304.. So 27.3% 3s.f

Unfortunately my answer is wrong and the right answer is 41% ??

Why is that?
2. (Original post by FluffyCherry)
2-bromo-2-methylpropane,C4H9Br can be used to produce methylpropene,C4H8. 2-bromo-methylpropane reacts with sodium ethoxide, NaOC2H5 to produce ethanol,C2H5OH and sodium bromide , NaBr along with methylpropene. using the relative formula masses of the reactants and products,
work out the atom economy for this reaction.

What I did was i worked out the rfm of the desired product which was methylpropene,C4H8 = 56
Then I wrote down the equation b

So C4H9Br + NaOC2H5 --> C2H5OH + NaBr + C4H8

Then I added all the rfm's of all the products (C2H5OH + NaBr + C4H8)
So ((12x2) + 5 +16+1) + (23+79.9) + ((12x4)+8) = 204.9

So
atom economy = molecular mass of desired product / sum of molecular masses of all products x100

(56/204.9)x 100 = 27.3304.. So 27.3% 3s.f

Unfortunately my answer is wrong and the right answer is 41% ??

Why is that?
I do hate this atom economy stuff. There seems to be no consistency to how things are calculated.

To get 41% the maths is

Mr[C4H8]/Mr[C4H9Br]
3. (Original post by TeachChemistry)
I do hate this atom economy stuff. There seems to be no consistency to how things are calculated.

To get 41% the maths is

Mr[C4H8]/Mr[C4H9Br]
This is really confusing, how am I supposed to know whether to use the formula of atom economy or do what you just did?

Besides why would they include the reaction of 2-bromo-methylpropane with sodium ethoxide ???
4. (Original post by FluffyCherry)
This is really confusing, how am I supposed to know whether to use the formula of atom economy or do what you just did?

Besides why would they include the reaction of 2-bromo-methylpropane with sodium ethoxide ???
I agree with you. I'd have worked out the atom economy in the exact same way that you did.
5. True back of an envelope mechanism.

6. (Original post by TeachChemistry)
I agree with you. I'd have worked out the atom economy in the exact same way that you did.
(Original post by TeachChemistry)
True back of an envelope mechanism.

(Original post by TeachChemistry)
I do hate this atom economy stuff. There seems to be no consistency to how things are calculated.

To get 41% the maths is

Mr[C4H8]/Mr[C4H9Br]
I see, thanks anyway
7. Where is the Q from? Who says that the answer is 41%?

Whoever they are, they are wrong, it is 27.3%, even Wikipedia agrees!
8. (Original post by Pigster)
Where is the Q from? Who says that the answer is 41%?

Whoever they are, they are wrong, it is 27.3%, even Wikipedia agrees!
The question is from the Edexcel AS chemistry book, and I've got the answers to the book with me. You see I think you're right cos I've noticed some mistakes in the answers to student book itself in other questions.
9. How very dare you : ) Of course I'm right.

(also we are right, it is a mistake)

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