Simple ochemq i need clarified

[stuw]

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So first, I was working through TPR SWB passage 4 for ochem:

"compound a, which showed an optical rotation of 0...."

q: compound a could be described as...
a.) meso
b.) racemic mixture
c.)equal mixture of two diasteroemers
d.) optically pure compound


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answer:b
So I narrowed it down to a and b and ended up choosing a. I guess what threw me off was that I figured "compound a" was going to be ONE compound...and seeing the word "mixture" made me think this was a trick. I understand that meso compounds can have stereocenters but they are NOT optically active due to symmetry...is that not the same as saying it has an optical rotation of 0? Or is it just that there is NO rotation whatsoever with mesocompounds?

And second question:

Could someone explain the core difference Ishould know between heat of combustion and heatofhydrogenation? I'm getting them mixed up constantly.


EDIT*
3rd --> do endothermic reaction typically have a slower rater than exothermic reactions?
If someone has the TPR SWB, it's page 818, #4 of passage 10
Thanks guys!

 

[Cmdr_Shepard]

Well for question 1, racemic mixtures do technically contain 2 physically different compounds, since they can behave differently in biological systems and they have a different arrangement of bonds connecting the atoms together, but for the purposes of o-chem you can consider mirror compounds as one kind of compound. I believe an optical rotation of 0 means no optical activity, unless you want to say that half the meso compound turns the light one way, and the other half turns it the other way! I would probably guess that somewhere in the passage there was information that excluded meso as an answer choice.

For question 2, I think the main difference is that combustion will involve oxygen, and hydrogenation involves hydrogen. More than likely, combustion will also have a larger negative delta H since the fully oxidized compound will be stable and low energy.

For question 3, don't conflate thermodynamics with kinetics. Endo and Exo terminology are referring to enthalpy changes in reactions which is about thermodynamics (stability of reactants and products), and not about the speed of the reaction.

The rate of a reaction typically will depend more on the activation energy and temperature rather than stability and/or entropy of reactants and products.

That's a brief overview, hope that helps.