Org chem/physics Questions

[oohpsjin]

Last time I posted lots of questions and the responses were amazing. Hopefully I can get these questions in org and physics that left me puzzled sorted out too!

 

Organic Chemistry

1. How does a base catalyze certain reactions such as hydrolysis of peptide bonds or the reaction between aromatic acid chlorides and alcohol to produce esters? Many reactions occur with the help of aq. acid or aq. base - I understand how acids may protonate atoms to make their bonds unstable, but how does aqueous base (OH/H2O) catalyze reactions?

 

2. Given methyl acetate (methyl ethanoate) and butanone (methyl ethyl ketone), shouldn't the ester be more reactive than the ketone, because the ester has two oxygens that both cause dipole moment in the carbonyl carbon? But how is the ketone more stable?

 

3. There are three intermolecular forces I know of: H-bond, dipole-dipole, and van der Waals. However, in wikipedia http://en.wikipedia.org/wiki/Van_der_Waals_force dipole-dipole, hydrogen bonding, london, and induced dipole-dipole are all considered as van der Waals forces. Which is right? How many distinct intermolecular forces are there and what are they?

 

4. Cellulose, a 1,4'-beta-D-glucose polymer, is comprised of D-glucose alternatingly facing upwards and downwards. Why the alteration? Why wouldn't cellulose have monosaccharide subunits all in the same orientation?

 

Physics

5. Static Friction Fs is the force that must be overcome to set an object in motion. Its equation is 0<Fs<usN where us is the coefficient for static friction and N is the normal force. Why does static friction have a maximum and a minimum instead of an absolute number, and what does maximum usN mean?

 

6. The moon is traveling in a uniform circular motion around Earth, and thus has acceleration that is always directed towards the centre (centripetal force). However, then, why doesn't the moon just crash towards Earth? What other force is acting outwards to push the moon away from earth?

 

7. For reversible isothermal processes, the change in entropy of the system or of the environment can be found from: delta S = delta Q/T. But why is this only for reversible isothermal processes?

 

8. What determines that a positive charge will form on a glass rod when it is rubbed with silk cloth? In other words, is there some kind of quantitative property intrinsic to distinct materials that decides the transfer of charge?

 

9. What in the world is permitivity of free space??? Can anyone please explain this in a simpler manner than in wikipedia?

 

10. Current flows from higher potential (+ terminal) to lower potential (- terminal) because by convention, current is the flow of positive charges. But in a conductor, only negatively charged electrons are free to move. So this means that the electrons are flowing from low to high potential. Are they able to do this "work" because of the voltage difference set up by batteries?

 

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics.

 

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

 

13. Could anyone clarify the physics of a laboratory centrifuge? How do the spinning motion result in denser particles accumulating to the bottom of the centrifuge tube? This must mean that the centrifuge is not spinning in uniform circular motion, since the centripetal force would have particles accelerate towards the centre?

 

oops forgot...one more org chem Q!

14. Nitrogen lone pair electrons in rings often contribute to conjugation. This is a basic concept that confuses me, but how do we know that the lone pair is in the p orbital (and thus contribute to aromaticity) and not any other orbital, such as s, d, or f?

 

Again, thanks in advance for everyone who could help out with any one of these questions.

[AndyDude]

Last time I posted lots of questions and the responses were amazing. But I have more questions in org and physics that left me puzzled:

 

Organic Chemistry

1. How does base catalyze certain reactions such as hydrolysis of peptide bonds or the reaction between aromatic acid chlorides and alcohol to produce esters? Many reactions occur with the help of aq. acid or aq. base - I understand how acids may protonate atoms to make their bonds unstable, but how does aqueous base (OH/H2O) catalyze reactions?

 

[Peptide bonds: Esters are produced by cleavage of amide bond. Hydrolysis of the N renders it more electrophilic (or positively charged) and OH attack on partial positive carbonyl carbon causes C-N to cleave

 

Aromatic acid chlorides + ROH = Ester?

 

2. Between methyl acetate (methyl ethanoate) and butanone (methyl ethyl ketone), shouldn't the ester be more reactive than the ketone, because the ester has two oxygens that cause dipole moment in the carbonyl carbon? How is the ketone more stable?

 

Ketones are more stable due to keto-enol tautomerization.

3. There are three intermolecular forces I know of: H-bond, dipole-dipole, and van der Waals. However, in wikipedia http://en.wikipedia.org/wiki/Van_der_Waals_force dipole-dipole, hydrogen bonding, london, and induced dipole-dipole are all under van der Waals forces. Which is right? How many distinct intermolecular forces are there and what are they?

 

All you mentioned are intermolecular forces but H bonding has greater strength than the rest of London disperson forces

 

4. Cellulose, a 1,4'-beta-D-glucose polymer, is comprised of D-glucose alternatingly facing upwards and downwards. Why is this so? Why wouldn't cellolose have monosaccharide subunits all in the same orientation?

 

My best guess is to minimize steric hindrance and ring strain.

 

Physics

5. Static Friction Fs is the force that must be overcome to set an object in motion. Its equation is0<Fs<usN

 

Right, but remember everything is considered static friction unless it is strictly said to be sliding where it is kinetic friction. i.e. the rolling tires are experiencing static friction not kinetic

 

Now someone can correct me. I am not quite sure about my response to the orgo questions but hopefully someone will take it from there.

[dan0105]

Here are some physics answers.

 

5. Static Friction

Static friction is just the force that is caused by the rough surface of the ground the block is trying to move on, if you think about it though, if you keep pushing the block harder and harder, the static friction will always just equal the force you apply so that it stays stationary, but after a given limit, you will push so hard that static friction can't compete, (this is the usN part) and thus it is no longer static and moves.

 

The moon

It stays in orbit because it also has a big velocity perpendicular to the direction the acceleration is pushing it, thus, as the moon flys through space, it gets pulled a bit in, and then a bit more and its direction keeps changing, but because it is moving so fast it never just falls into the earth, it just keeps turning towards it but overshooting (and thus stays in orbit).

 

Ok I got bored and don't feel like answering more now. maybe later...

 

As for the nitrogen lone pair of electrons in conjugated systems, its tricky. Sometimes lone pairs are contributed to the ring and sometimes they aren't, in general, if they can make the huckel rule work, then they will! And as for which orbital, only p orbitals can make nice little double bonds, the others get a little weird and don't overlap/share well.

 

Check out, http://mymcat.com/w/index.php?title=Aromaticity_Introduction

it explains all that stuff about ring conjugation for the mcat.

[gamma]

7. For reversible isothermal processes, the change in entropy of the system or of the environment can be found from: delta S = delta Q/T. But why is this only for reversible isothermal processes?

 

sorry what's a reversible isothermal process?

 

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics. which book has this in physics section? this is more in gchem

 

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

R = 0.0824 L atm/ mol K, R = 8.31 J/mol K, k=9x10^9 (sorry i forget units! it's the grav. constant), electric charge = 1.6 * 10^-19 C

that's all i know for now

[oohpsjin]

Gamma: in response to your #11 it was in chap 11 Atomic Phenomena, Physics, Kaplan Premier Program 08/09.

[S B]

14. Nitrogen lone pair electrons in rings often contribute to conjugation. This is a basic concept that confuses me, but how do we know that the lone pair is in the p orbital (and thus contribute to aromaticity) and not any other orbital, such as s, d, or f?

First of all if you are talking about lone pairs you don't consider s-ordital as it having a lone pair is not possible. None of the electrons in nitrogen enter d or f orbitals. So, the only two possibilities are the p-orbital or the sp2/sp3 hybridised orbital. Consider pyrrole and pyrimidine. In case of pyrimidine there are three double bonds present in conjugation to each other therefore the lone pair on nitrogen cannot be in the p-orbital (as the p-orbital is already sharing electrons to the pi-electron cloud), so it is present in the sp2 hybridised orbital. In case of pyrrole the lone pair is present in p-orbital as it can resonate with the double bonds leading to a more stable structure. The nitrogen in this case is also sp2 hybridised but the lone pair is part of the p-orbital.

 

Hope this is not too confusing!

[gusdnr123]

Last time I posted lots of questions and the responses were amazing. Hopefully I can get these questions in org and physics that left me puzzled sorted out too!

 

Organic Chemistry

3. There are three intermolecular forces I know of: H-bond, dipole-dipole, and van der Waals. However, in wikipedia http://en.wikipedia.org/wiki/Van_der_Waals_force dipole-dipole, hydrogen bonding, london, and induced dipole-dipole are all considered as van der Waals forces. Which is right? How many distinct intermolecular forces are there and what are they?

I would consider Van der Waal's as something that includes dipole-dipole AND london forces (H-bond is part of dipole-dipole forces, it jsut has a special name because of the enormous strenght in its bonds)

 

Physics

9. What in the world is permitivity of free space??? Can anyone please explain this in a simpler manner than in wikipedia?

I highly doubt that you'll have to know exactly what that is.. from what I remember thats just part of Capacitance equation right? Just know that its used in finding out capacitance value... at least I've never came across a question asking/concerning permitivity of free space.

 

10. Current flows from higher potential (+ terminal) to lower potential (- terminal) because by convention, current is the flow of positive charges. But in a conductor, only negatively charged electrons are free to move. So this means that the electrons are flowing from low to high potential. Are they able to do this "work" because of the voltage difference set up by batteries?

Yes

 

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics.

I doubt you have to know this, because it's not in Princeton books.

 

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

Best to know everything

 

you should try going through practice tests. Solutions tend to be helpful in terms of explaining concepts that you may have missed out...

[Mashmetoo]

Hi guys, I have a question regarding electric conductors.

 

The questions went like this: a electric conductor in the shape of a sphere (with its base insulated so the electrons won't go to ground) is given a charge of 3 coulombs. In an isolated room with a vacuum, there are 3 more spherical conductors like the first one, but the 3 are neutral. Now, the first charged conductor with its 3 coulombs touches the second conductor, then the second conductor touches the 3rd conductor, and the 3rd conductor touches the last (4th) conductor, what is the charge of the last conductor?

 

Similarly, we have 2 conductors in a vacuum, each with 3 coulombs of charge, and they briefly touch. After their separation, what are the charges on these 2 conductors now?

 

Any guesses to these questions?

[oohpsjin]

First of all, Thanks to everyone who helped out!!

 

sorry Mashimetto..that question just beats me.

 

I had a separate quirk that bothers me -

in order for a compound to be opticaly active, it doesn't have to have a chiral CARBON right? like a nitrogen center would still make one optically active.

And does a free electron pair count as a distinct substituent on a chiral center? E.g. if a carbon is attached to ehtane, methane, lone electron pair, and hydroxyl group, would it be considered as optically active?

thankssssss

[dan0105]

I believe that yes the lone pair in examples like nitrogen (Tertiary amines) does count, thus it is a stereocenter, BUT the energy barrier for the inversion of the stereocenter is really small so they interconvert and always form racemixtures.

 

The carbon example you gave however, from my understanding would probably never happen. You are basically saying that it has 5 electrons around it (3 bonds and 2 electrons in the lonepair) thus its negatively charged, and to stabilize it, the lonepair would move towards making a double-bond with the oxygen and knockout the hydrogen in the OH.