[Mikasa Ackerman]

Hey everyone,
I was looking through a few questions over a nitration of bromobenzene reaction and had a few questions.
First:
Melting points of pure 2- and 4-bromonitrobenzene are 41 °C and 125 °C, respectively. If purification by crystallization does not provide complete separation of the two products and you end up with a mixture, how would that affect the melting point?
A. the mixture would have the same melting point as the low-melting isomer, i.e. 41 °C
B. the mixture would have the same melting point as the high-melting isomer, i.e. 125 °C
C. the melting point would be either 41 or 125 °C, depending on whichever isomer’s crystal ends up in the melting capillary tube.
D. we would observe a melting point depression
E. we wouldn’t be able to measure the melting point in this case
I was leaning toward D on this one, but wasn't quite sure.
And second:
In the reaction below, what is the yield of 1-bromo-4-nitrobenzene?
http://webcom7.grtxle.com/Chem627/uploads/easpart%202_q3.JPG
A. 15%
B. 30%
C. 45%
D. 60%
E. 75%
I had C, but would like to get some other perspectives on this.
Any help at all would be greatly appreciated! Thank you all for your time!

[snorkack]

Hey everyone,
I was looking through a few questions over a nitration of bromobenzene reaction and had a few questions.
First:
Melting points of pure 2- and 4-bromonitrobenzene are 41 °C and 125 °C, respectively. If purification by crystallization does not provide complete separation of the two products and you end up with a mixture, how would that affect the melting point?
A. the mixture would have the same melting point as the low-melting isomer, i.e. 41 °C
B. the mixture would have the same melting point as the high-melting isomer, i.e. 125 °C
C. the melting point would be either 41 or 125 °C, depending on whichever isomer’s crystal ends up in the melting capillary tube.
D. we would observe a melting point depression
E. we wouldn’t be able to measure the melting point in this case
I was leaning toward D on this one, but wasn't quite sure.

Leaning towards D, too.
If you manage to crystallize it as large crystals of pure isomers, and just do not know which crystals are which isomers then, yes, C. But if the crystals are small sticking to each other then you would observe melting point depression.

And second:
In the reaction below, what is the yield of 1-bromo-4-nitrobenzene?
http://webcom7.grtxle.com/Chem627/uploads/easpart%202_q3.JPG
A. 15%
B. 30%
C. 45%
D. 60%
E. 75%
I had C, but would like to get some other perspectives on this.
Any help at all would be greatly appreciated! Thank you all for your time!

Certainly C. 2,6 g divided by 5,8 g.

[bjams12]

2. Most certainly NOT C. Think mole ratios. How many moles of reactant did you start with and how many moles of the product of your interest did you yield?

MW of 4-bromonitrobenzene is 202.005 g/mol. MW of bromobenzene is 157.008 g/mol, but you want to convert your initial 4.5-mLs to g, so you need the density of bromobenzene which is 1.50 g/mL. I'm getting B.

[snorkack]

2. Most certainly NOT C. Think mole ratios. How many moles of reactant did you start with and how many moles of the product of your interest did you yield?

Sorry. My mistake.

I looked at the total of the 2 named products, and forgot to check the total from the stated amount of starting substance... that gave rather less than 100 % yield of the 2 named products combined.