Orgo Destroyer, need help!

[Bigbirdo]

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1. For question 2, why can’t you add the bromine to the double bond indicated in the picture?

2. For question 7, the answer says that the compounds in the question can be isolated by crystallization, and the para-isomer will crystallize out of solution, but the ortho isomer will not.
How can you tell which one precipitate out based on the given structure?

3. For question 24, why being more branched give the lowest heat of combustion?

 

[SafetyDog]

1. No hydrogen on the carbon. Also, you can't do a regular alkene reaction because remember the double bonds aren't there permanently since they are in resonance.

2. I think it's because the para compound is more on the non-polar side and the mixture is dissolved in ethanol which is polar. Ortho is polar and remains soluble.

3. More branched equals less surface area which means a lower heat of combustion and boiling point. Melting point / Freezing point increase with a branched molecule.

Someone correct me if I'm wrong please. Thanks!

 

[Bigbirdo]

1. No hydrogen on the carbon.

2. I think it's because the para compound is more on the non-polar side and the mixture is dissolved in ethanol which is polar. Ortho is polar and remains soluble.

3. More branched equals less surface area which means a lower heat of combustion and boiling point. Melting point / Freezing point increase with a branched molecule.

For #3, why does less surface area means a lower heat of combustion? Do I just remember this as a fact?

 

[SafetyDog]

I would just remember it as a fact.

If there's less surface area, there is less area to create bonds with other molecules I think.

 

[denttal]

Agree with most of SafetyDog (funny name). Another thing to add with branched alkanes vs. non-branched alkanes: the reason branched alkanes boil at a lower temp. is b/c there is less surface area/less area for intermolecular forces. Therefore, decreasing IM forces decreases the Boiling Point.

However, to get an object to melt you need to "break the crystalline structure" and branched molecules make more compact 3D structures so branching increases MP.

Overall, given the same number of carbons, Branching decreases BP(less IM forces) and increases MP (better compact structure).

***#3) So in combustion you are going to end up with the same products (usually CO2 and H2O are released). The "heat of combustion" is the energy given off when you go from the reactant to the products. Branched alkanes are MORE STABLE than their non-branched counterparts. Therefore, going from a more stable reactant to the product releases LESS energy. This is why branched alkanes have a smaller heat of combustion than non-stable/non-branched alkanes.

 

[Eri3]

For #3, why does less surface area means a lower heat of combustion? Do I just remember this as a fact?

It has to do with Intermolecular forces - specifically London Dispersion.
Remember everything has london dispersion- which is an instantaneous dipole due to the circling of e-s around a nucleus.

Therefore, the larger the surface area of a molecule (less branching) the more london dispersion forces it can form thus it has a higher heat of combustion b/c it take more energy to break the increase in intermolecular forces.