Organic Chemistry Acids and Bases #1

Put the following organic compounds in the order of decreasing acidity:

A.  1, 2, 4, 3
B.  3, 1, 2, 4
C.  2, 3, 4, 1
D.  3, 1, 4, 2

Answer: D


SHORT-AND-SWEET:
Two of the compounds are carboxylic acids (1 is propionic acid, 3 is trichloroacetic acid), and two are alcohols (n-hexanol and phenol).  Clearly, one of the carboxylic ACIDS (hint, hint) will be the most acidic of the four compounds (which means that we can eliminate answer C).

When an acid gives away its proton, the result is a conjugate base, which is an anion.  Nature likes things that are stable, which is NOT charged molecules.  However, if negative charge can be somehow "stabilized" nature likes that.  What will stabilize an anion and its negative charge?

1.  The atom carrying the charge.  The more electronegative (e.g. halogen) and larger (more space for negative charge to spread) the element, more stable the charge.  Also more s character to a hybridized orbital means electrons hanging out closer to the nucleus, which confers stability.
2.  The neighborhood.  If there are electronegative elements in the vicinity or several resonant forms, the charge will be spread over the entire molecule, which makes it more stable.

Trichloroacetic acid has not one, but three chlorine atoms, which will help carry the negative charge, and is therefore more acidic than the propionic acid.  What about the alcohols?  After deprotonation, phenoxide will be able to spread the negative charge over the entire ring, which will stabilize it, making phenol more acidic then n-hexanol (answer D).



THE WHOLE STORY:
Most organic chemistry reactions involve acid-base chemistry, which is why a good understanding of this topic will be your secret weapon for doing well on MCAT organic chemistry questions.

As if the topic were not confusing enough, someone had to come up with THREE different definitions of acids and bases: Bronsted-Lowry, Lewis, and Arrhenius.  Before we get to the actual definition, we would like to point out that for the purposes of MCAT you can accomplish a lot with solid understanding of just Bronsted-Lowry acids and bases.  Lewis comes in handy in certain situations, but Arrhenius you can completely discard because it is useless (sorry, Arrhenius).

Like we said, the main definition to take away is Bronsted-Lowry's, which defines an acid as molecule that donates a proton (H+) to a base, and a base as a molecule which accepts the proton.  After the acid donates its proton, it becomes the conjugate base, and similarly, after a base accepts a proton, it becomes the conjugate acid.

Lewis acids and bases are defined based on electron transfer.  Lewis acids accept electrons, and due to their "love" of electrons are also called electrophiles.  Lewis bases donate electrons, and are called nucleophile.  Many organic reactions are essentially interactions between electrophiles and nucleophiles, and an understanding of Lewis acids and bases comes in handy there.

Arrhenius defines acids and bases based on their dissociation in water and whether they produce hydronium ion, H3O+ (acids), or hydroxide ions, OH- (bases).  Now, forget the last sentence.  Moving on to stuff that you will actually use.

REMEMBER:  Most things in chemistry are relative.  

What does that mean?  It is similar to how things actually work in the real world.  Some of you (and us) were athletes in high school.  Remember being on the varsity tennis (or any other sport) team -- you rocked it!  But let's say that we sent you off to the pro tour, where you'd have to face Serena Williams or Roger Federer.  Compared to theirs, your tennis skills (and ours, too) would seem....well....sadly, not as amazing!

Similarly, in chemistry if you have reagent X, this reagent will act one way in the presence of reagent Y, and might do something completely different in the presence of the third reagent Z.

Same thing with acidity.  How acidic a particular compound will be depends on which molecule it is interacting with.  Therefore, in the right environment more or less every compound can act both as an acid and a base.

However, if you compare all of these compounds to water you can calculate their acidity.  This is expressed as acid dissociation constant Ka or as a negative log of Ka, which is pKa.

REMEMBER:  The strongest acids will have the highest dissociation constants Ka and the lowest (even negative) pKa.

From the expressions above, you can see that when discussing relative acidity we talk about Bronsted-Lowry acids.  In this sense the acidity of a molecule tells you how easily a molecule will get rid of its proton.

The result of the proton dissociation is a negatively charged conjugate base.  One of the big concepts in chemistry is that charge is annoying, so nature will tolerate it only if it is stabilized in some way.  This will apply to the conjugate base as well.

REMEMBER:  The stability of the acid's conjugate base will determine how strong the acid is.  The more stable the conjugate base, the stronger the acid.

What determines stability of conjugate base (or any anion for that matter)?

1.  The atom carrying the negative charge.
- Electronegativity:  there are some atoms which just loooooove electrons.  They are considered very electronegative, and the most electronegative of them you will find as you go to the right and up on the periodic table (fluorine is the most electronegative).  The more electronegative the atom carrying negative charge is, the more stability it confers to the whole molecule.  For example, a negatively charged oxygen will be more stable than if it were on a nitrogen or carbon.

- Size:  the larger the atom (which happens as you go down and to the right in the periodic table), the more stable the negative charge, because the charge is delocalized over a larger space.

- Hybridization state:  hybrid orbitals that have more s character are closer to the positively-charged nucleus, which stabilizes electrons.  This means electrons in orbitals with more s character make anion more stable.  sp orbitals (50% s character) are more stable than sp2 (33% s character) which are more stable than sp3 (25% s character).

2.  The neighborhood.
- Electronegative neighbors:  if electronegative atoms (such as halogens) live close to the atom carrying negative charge they will pull some of the electron density away from that atom (inductive effect), spreading the charge over a larger area, which would stabilize the conjugate base.

- Resonance effects:  resonance is the delocalization of electrons in a molecule such that the bonding cannot be expressed by one single Lewis formula.  This is one of the main effects that make carboxylic acids so acidic compared to the other organic compounds.


........Now, back to our question.  Because the question asks you to put the above molecules in order of decreasing acidity, we suggest you start by identifying the most acidic molecule.

Two of the molecules listed are in fact carboxylic ACIDS (propionic and trichloroacetic acid), which are the most acidic of organic compounds (though still weak acids compared to some of the inorganic acids).  Among the four compounds the most acidic one will probably be one of these two, and our correct answer would start with either 1 or 3 (which eliminates answer C).

How do you figure out which one is the most acidic?  Take a better look at the two acids, and ask yourself which one of them will have a more stable conjugate base.  For both of them the negative charge will rest on oxygen and both will have a stabilizing resonance effect.  However, notice the three chlorine atoms in the trichloroacetic acid.  Chlorine, a halogen, is very electronegative, and will "share" the negative charge with oxygen, stabilizing the molecule.  In fact, the pKa of this acid is 0.6, which means it is more than 10,000 times more acidic than the propionic acid, whose pKa is close to 5 (which eliminates answer A).

We are left now with the two alcohols, n-hexanol and phenol.  You probably know that alcohols and their hydroxyl proton (-OH) are not very acidic.  But how do these two compare to each other in this aspect?

What happens when n-hexanol donates its proton?  The molecule is left with a negative charge on the oxygen, which is alright.  Is there any help from its neighboring atoms?  Not really -- carbon atoms are not particularly eager to help carry the negative charge.

What about phenol?  Upon losing the proton, the negative charge is still on the oxygen, BUT the rest of the ring will help out.  The charge will delocalize over the ring through resonance stabilization, which will make the phenoxide anion more stable, therefore making the phenol (pKa 10) a better acid than n-hexanol (pKa 16).

The correct answer is D:  trichloroacetic acid (3) > propionic acid (1) > phenol (4) > n-hexanol (2).


BIG PICTURE:

1.  Given the appropriate environment, every compound can act as an acid or as a base.  Ka and pKa tell you how acidic a compound is compared to water.

2.  A stable conjugate base means strong acid.  (And vice versa - stable conjugate acid means a strong base.)

3.  Which atom carries a negative charge and who its neighbors are determine anion stability.  Electronegative, large atom with optimal hybridization state (more s character = better), that has neighbors who are equally electron-loving equals stable anion.


~The MCAT POD Team~

Electrochemistry #1

In a galvanic cell the voltage (emf) will change with all of the following, EXCEPT:


    A. the chemical reactions occurring in the half-cells
    B. the length of the wire connecting the half-cells
    C. the concentration of the solutions in the half-cells
    D. the temperature of the solutions in the half-cells


ANSWER:  B


SHORT-AND-SWEET:


Galvanic cells convert the chemical energy of a spontaneous redox reaction into electrical energy, which enables the cell to do work.  You can imagine the voltage of a galvanic cell as a measure of galvanic cell "strength".  What makes a galvanic cell "strong"? 


1. The redox reaction which is occurring in the cell:  different redox reactions will have different intrinsic "desires" to occur.  Those that want to occur spontaneously will have a lot of chemical energy that can be converted into electric energy.  The more chemical energy a reaction has, the higher the voltage produced in the galvanic cell.
2. How concentrated the solutions in the half-cells are:  This tells you how many charged units there are in the galvanic cell;  the more charge that there is, the stronger the galvanic cell!?
3. The temperature of the half-cell solutions:  in order to understand this one, we have to recall that the spontaneity of a reaction is partially determined by the temperature in the system (ΔG = ΔH - TΔS; G-free Gibbs free energy, H-enthalpy, T-temperature, S-entropy).  If we look back to 1., the more spontaneous reactions have more free chemical energy, which corresponds to higher voltage in the galvanic cell.


This leaves us with the length of the wire connecting the half-cells as the correct answer (answer choice B).  This variable will not affect the voltage of the galvanic cell.


THE WHOLE STORY:


"Ugh, I wish I could remember that stupid Nernst equation!"  If this is your first response after reading this question, do not despair.  A good understanding of galvanic cells will enable you to answer this question without memorizing that dreaded equation.  

In fact, the MCAT will frequently test your understanding of topics that are represented by complicated equations -- which does not mean that you have to memorize these equations.  This question is a perfect example.  Instead of forcing yourself to memorize this and a bunch of other complicated equations, what you should do is spend this time understanding what these equations mean.  

Let's see how we can apply this principle to our question.  Before thinking about the voltage of a galvanic cell, it is crucial to understand the galvanic cell itself.  

Galvanic cell uses chemical energy intrinsic to a redox reaction to produce electrical energy, which enables it to do useful electrical work.  The key is that galvanic cell requires a spontaneous redox reaction, because the spontaneity of the redox reaction is where the chemical energy comes from.  An example of a spontaneous chemical reaction used to make a galvanic cell is between zinc and copper:  


Cu²⁺ + Zn    Cu + Zn²⁺


Galvanic cell will transform the chemical energy of a spontaneous redox reaction into electric energy, which manifests as the electric current.

Now, if the chemical energy comes from the spontaneity of a redox reaction, you can guess that the redox reactions that are most spontaneous will produce the most energy, therefore generating the greatest voltage (emf) in the galvanic cell.  This means that if we change which chemical redox reactions are occurring in the galvanic cell, the voltage will change (which eliminates answer A).


The voltage of a particular reaction can be calculated from standard potentials (E°) of each of the oxidation and reduction half-reactions.  What does "standard" mean?  Remember that "standard" always refers to some "standard" conditions.  In this particular case, the standard potential of a reaction is measured at standard CONCENTRATION, which is 1M, and at the standard TEMPERATURE, which is 25°C.  


Obviously, if any of these conditions were to change, the potential of the reaction would change (which eliminates answers C and D).  


This leaves answer B.  The length of the wire between the two half-cells has nothing to do with the emf of the galvanic cell.

..........

Like we mentioned in the beginning, the other way of answering this question is to memorize the equation for voltage of a galvanic cell.  This is the dreaded Nernst equation: 

E = E °  - (RT / nF)  ln (Q)

    E = voltage of a redox reaction (under non-standard conditions)

    E°  = standard voltage of a redox reaction (under standard conditions)

    R =  universal gas constant: R = 8.314 J/Kmol

    T = absolute temperature

    n = number of moles of electrons transferred in the cell reaction or half-reaction

    F = Faraday constant, which is the number of coulombs per mole of electrons: F = 96,500 C/mol

    Q = reaction quotient, determined by the concentration of reactants and products in the chemical reaction

Nernst equation looks intimidating because it has so many components, but what it shows, in the simplest terms, is that E (emf) of the galvanic cell depends on:
    -standard potential E°, which is specific to the chemical species reacting in the galvanic cell,
    -temperature T in the system, and 
    -reaction quotient Q, which depends on the concentration of chemicals in the galvanic cell.




BIG PICTURE: 


1.  When you run into a complicated-looking equation (e.g. Nernst equation), don't waste time trying to memorize it.  Look at the variables in the equation, and how they relate to each other.  Finally, practice by explaining these relations in words.


2.  Galvanic cells require a spontaneous redox reaction, whose intrinsic chemical energy the galvanic cell transforms into electric energy and electric work, i.e. current.  


3.  What redox reaction is occurring, and at which reactant concentration, as well as the temperature of the system (one of the determinants of the spontaneity of the reaction) determine what voltage (emf) the galvanic cell will produce. 


~The MCAT POD Team~

Female Reproductive System #1

Which one of the following is associated with the shedding of endometrium during menstruation?


A. Formation of the corpus luteum
B. Rise in estrogen and progesterone
C. Rise in hCG
D. Low LH level

ANSWER: 
D. Low LH level 


SHORT-AND-SWEET:


Menstruation is usually considered to be the beginning of the menstrual cycle, but we suggest you consider it the last phase, which happens if there is no conception.  
No conception means no placenta, which subsequently means no hCG (which eliminates C).  The formation of corpus luteum is associated with the luteal phase (as the name suggests), which occurs after the ovulation and before menstruation (which eliminates A).  Estrogen and progesterone are after ovulation produced by the corpus luteum, so when the corpus luteum dies towards the end of the cycle, levels of estrogen and progesterone drop, which leads to the endometrial shedding (this eliminates B).  
We are left with answer choice D, which is the correct answer.  Remember that at the time of menstruation levels of all hormones are low.


THE WHOLE STORY:  


Female reproductive system is one of the most dreaded topics in the MCAT Biological Sciences section, which is exactly why we included it in our blog.  While others are wasting their time in needless memorization, we will help you understand the female reproductive system.  And there will be no diagrams -- we will think of it as a story.

We will start off with the big picture:  The purpose of the female reproductive system is to enable reproduction!!!  In order for this to happen, what does this system need to provide?

1. A mature egg (oocyte)

2. A way to deliver the egg to a location where sperm can fertilize it (ovulation)

3. A nice home for an embryo (in the uterus)

Think of the uterus, ovaries, and the anterior pituitary as the key players in the female reproductive system, and in particular, think of the anterior pituitary as the brains of the operation.  What do we need to memorize about them?  (We will ask you to memorize only two things in this topics, and this is one of them.) 
   -Non-gravid uterus does not produce hormones.  
   -Ovaries produce estrogen and progesterone.  
   -The anterior pituitary produces FSH (follicle-stimulating hormone) and LH (luteinizing hormone).  


Now that we know the primary purpose of the female reproductive system, the key players, and what they produce, we can start understanding the logic behind the menstrual cycle. 


Let's start by thinking about what happens after the menstruation is over.  The uterus lining is completely bare.  The ovaries, with their numerous immature follicles, are resting, taking a nice little nap.  The anterior pituitary, the brain of the operation, is resting as well.  For now.  


The brain knows that the purpose of this system is to reproduce, and BAM! -- the brain wakes up!  It starts producing FSH to wake up the ovarian follicles, stimulating them to mature.  As the ovarian follicles mature, they produce more and more estrogen.  Estrogen in turn acts on the uterus, causing the proliferation of the endometrial lining.  

Because of the endometrial proliferation and follicular maturation, which occur during this phase, this phase is called the proliferative or follicular phase.  

The product of this phase: 1. MATURE EGG. 

At the same time estrogen, which is produced by the ovaries, exerts negative feedback on the anterior pituitary, inhibiting the secretion of FSH and LH (mechanism seen in the other endocrine systems).  However, when estrogen reaches critically high level, instead of inhibiting LH production, it causes transient LH surge (positive feedback), which subsequently causes follicle rupture, or ovulation.  The LH surge tires the anterior pituitary, which now wants to rest, leading to low LH and FSH levels.

The product of this phase:  2. MATURE EGG DELIVERED TO THE FALLOPIAN TUBE, WHERE IT CAN BE FERTILIZED BY SPERM.

Now that the oocyte is delivered to the fallopian tube, waiting to be fertilized by a sperm, all eyes turn to the uterine endometrium, which now needs to make sure to provide a suitable home for a possible embryo.  How does this happen?


This happens in coordination with the ovaries.  The cells that were surrounding the oocyte in the follicle remain in the form of corpus luteum, and they continue to produce estrogen and progesterone (as they did during the proliferative (follicular) phase).  It is the progesterone (and to a lesser degree, estrogen) secreted by corpus luteum that acts on the endometrium, causing it to develop a thick juicy lining, called the secretory lining.  The secretory lining makes for a perfect embryo home.  

Because of the secretory endometrium and the formation of corpus luteum, this phase is called secretory or luteal phase.  

The product of this phase:  3. NICE HOME FOR A POSSIBLE EMBRYO.

But what happens if there is no fertilization?  
Here is that second thing to memorize: Corpus luteum has an internal "timer", and without hormonal stimulation it will die on its own after 14 days.  As it dies, estrogen and progesterone levels drop, and without these hormones the thick endometrial lining sheds, causing menstrual bleeding.


This brings us back to our initial question about what happens during menstruation.  Like we just said, the menstruation is what occurs when corpus luteum dies, which eliminates A.  As the corpus luteum dies, estrogen and progesterone will decrease, which eliminates B.  hCG (human chorionic gonadotropin) is a hormone produced by the placenta, which forms at the start of pregnancy.  There is no menstruation during pregnancy, which eliminates C. 


Answer D is the correct answer.  Like we said, during menstruation and immediately afterwards, the female reproductive system is "resting".  This includes the pituitary, which secretes minimal amounts of LH and FSH.  It is when this brain of the operation wakes up that the cycle starts anew.     


BIG PICTURE:  

The purpose of the female reproductive system is to enable reproduction!!!  In order for this to happen, the system needs to provide:

1. A mature egg (oocyte) - oocyte maturation is stimulated by FSH (follicle-stimulating hormone).

2. A way to deliver the egg to a location where sperm can fertilize it (think ovulation) - triggered by LH surge.

3. A nice home for an embryo (in the uterus) - stimulated initially by estrogen produced by the ovarian follicle, and then by progesterone produced by corpus luterum, the ovarian follicle remnant.

~The MCAT POD Team~

Fluids #1

An open container filled with ideal fluid starts draining through a spigot, as shown in the image.

Which of these graphs best depicts how height H changes with the fluid velocity v at point 2?

ANSWER:  B.

SHORT-AND-SWEET:

This question requires as little as 20 seconds to answer.  Let's see how.  

What happens as the fluid starts draining?  The height H starts dropping.  What happens to the velocity v of fluid at point 2 as the fluid is draining?  It will decrease as well.  Therefore, H and v track together -- as one goes down, so does the other.  Look at the answer choices.  Answer choices A and C suggest the opposite, and answer choice D suggests that H is constant.  Only answer B suggests the correct relationship between H and v.  Because of conversion of potential energy (static conditions before the spigot opens) to kinetic energy (dynamic conditions once the spigot opens) the actual relationship will be: H = v^2 / 2g.

THE WHOLE STORY:

We start off with an ideal fluid inside an open container.  When the spigot opens the fluid begins draining at a certain velocity, and we can all agree that with that the height (H) of the fluid column starts decreasing.

The question asks about what happens to the height (H) of the fluid as the velocity of the fluid at the spigot (v) changes.

STOP!!!  What I will say next is one of the keys to success on the MCAT, and applies without exception, to every single MCAT section. 

After reading the question, take ten seconds to think about the problem intuitively.  

Chances are that by thinking about the question first, you will be able to guess what the answer should look like.  Then quickly scan through the answer choices, and eliminate those that make no sense.  

The MCAT (and medicine) is as much about picking the right answers, as it is about eliminating the wrong ones.  The reward for this strategy is more precious time left over for questions that actually require calculations.  In fact, most people who end up running out of time in the Physical Sciences section do so because they spend too much time "plugging and chugging" through questions unnecessarily. 

Let's go back to our question and see how we can apply this strategy!

We know that as the fluid is draining the fluid height H will be decreasing.  Initially the fluid velocity v will have some value.  But what happens to the fluid velocity as the container is actively emptying?  With less remaining fluid in the container, the fluid will be draining more slowly (decreased velocity).  

If this doesn't seem right, get a balloon, fill it with water, and punch a whole in the bottom.  Just don't tell your roommates that we told you to do this.

Back to our question.  We have concluded that as the fluid height decreases, the fluid velocity will decrease as well. 

By knowing this, and by looking at the answer choices, we can try eliminating some of the choices.  Choices A. and C. suggest an inverse relationship between fluid height and fluid velocity, which, as we established, is not the case.  Choice D. suggests that fluid height does not change with changes in velocity, which we also know is not the case.  

Therefore, the correct answer is B. 

..........Now, if the answer choices were such that we could not easily eliminate all of the wrong answers, here is the mathematical way to solve this problem. 

What you need to realize is that this is essentially a conservation of energy question.  How?  Before the spigot opens, the system (container with the fluid) is static, because there is no fluid motion.  Once the spigot opens, the fluid will start moving, transforming this into a dynamic system.  

STOP!!!  You should learn that whenever you encounter a question in which you have the same system initially at rest and then in motion, it is very likely that the underlying concept is conservation of energy, focusing on the exchange of potential energy and kinetic energy.  

Let's see how this principle applies here.  For any given fluid molecule its total energy will be the sum of its potential and kinetic energy.  If we take a molecule at the fluid surface (point 1, compared to point 2), given that the molecule is effectively not in motion, its total energy will be its potential energy:  Etotal = Epotential = mgH.

Once the fluid starts draining, by the principle of conservation of energy, each molecule's potential energy will transform into kinetic energy.  At point 2, the molecule will have only kinetic energy, so its total energy will be equal to its kinetic energy:  Etotal = Ekinetic = 1/2 mv^2.

Because total energy is conserved, potential energy at point 1 and kinetic energy at point 2 will be equal:  mgH = 1/2 mv^2

If we divide the whole equation by mass m, we are left with an equation which tells us about the relationship between the height H of the fluid column and the fluid velocity at the spigot v:  

H = v^2 / 2g

This means that the height H of the fluid column is proportional to the square of v.  

You should be able to recognize that what we have in this question is a (parabolic) square function, which is depicted in answer B.

BIG PICTURE:  

1.  Think first, calculate second!  Or, more accurately, think (and eliminate) first, calculate (if needed) second.

2.  Recognize when a question asks you to recognize static ("resting") and dynamic ("moving") conditions within a system.  When they are both in the question, think POTENTIAL and KINETIC ENERGY, and CONSERVATION OF ENERGY!

3.  Know how to interpret graphs, and we mean KNOW IT!

Welcome to MCAT Problem of the Day!

As this is our first post, we're going to provide a roadmap for where we're going and what medical school applicants might hope to get from our site.  First, a little about who we are.

This site is the collaborative effort of a group of Harvard Medical students dedicated to passing on what helped us get into medical school.  All of us have been involved in teaching undergraduate and MCAT prep courses, but we have never seen a resource that provides ALL of the details you need to get into the top medical schools.

Several of us have been part of the admission process at HMS, and have seen why some applicants get admitted and some don't.  There is no mystery or magical formula, it's the same things you've undoubtedly heard about: grades, MCAT scores, personal statement, and letters of recommendation.  It's the little details that make the difference, and we're going to go through all of them.

This is no small task, but if you stick with this, we'll show you how to ace the MCAT, how to write a personal statement that captures the admissions committee's attention, and how to navigate the gauntlet that is the application process.

We will organize this into several pieces:

1. MCAT Problems.  We'll have five MCAT problems per week (one from each of the science areas, and one from verbal reasoning).  The goal is not only to solidify your knowledge of these areas, but to teach you the strategies necessary to get the score you want.  If you're still months or years from applying, it's never too early or too late to start preparing for the MCAT!  Strong performance on the MCAT can put you at the head of the pack going into interviews.
2. Admission Cycle Calendar.  We are about to release a Google Calendar that you can join, which will give you all the key dates related to the admission cycle.  It's a complicated process, and the key is to stay organized.  Lucky for you, our calendar covers not only deadlines, but also key milestones in terms of preparing your application: this includes studying for and taking the MCAT, writing a first draft of your personal statement, approaching recommenders about letters, and starting to fill out your AMCAS application.  Email us if you are interested in joining, and we will add you as soon as it goes live.
3. Personal Statement.  Though we can give general details on what makes for a strong personal statement, and what pitfalls to avoid, the specifics of your personal statement are just as important.  For that reason, we are offering a service to help you write your personal statement from start to finish.  Contact us about more details.

We hope you benefit from our site, and we wish you the best of luck in applying to medical school and a career in medicine!

 

MCAT POD Team