[Opalescent]

Hello folks! This is my first post here so I'll do a quick introduction I guess. I'm a sophomore in college and am double majoring in chemistry and math, planning to teach at the high school level. I have always been an excellent student, especially in these subjects, but am finding myself struggling in organic chemistry due to the teaching style. Let me give some explanation so you can understand the questions I have. If you just want to skip all this to my questions, skip to the row of asterisks below, but you'll miss some context and won't be able to help me as well. My questions are MOSTLY just confirming that the way I'm thinking is right, but I do have some legit "I HAVE NO CLUE" questions.

Our professor wrote our textbook (read: it's self-published) and it does not take any approach that any of my friends at other universities have seen and can help me with. Here is my textbook, which you can access it entirely by registering, save for the questions which are in a separate book. http://ochem4free.info/node/128

We have a test Thursday over chapters 4-6. Our professor moved it then because nobody in my class understands the material at all and we already had a disproportionately low first test grade. Usually around 75 or so, our class average was 67... I got 92. However I've been getting around 75% on the last two weeks' assignments and I want to understand this stuff before the test. Quick overview:

 Chapter 4 is on comparing melting points, boiling points, density, solubility, etc., based only off of a molecular structure. I mostly get that, but there are a few questions I have.

Chapter 5 is on acids and bases. I know the difference between Arrhenius, Bronsted-Lowry, and Lewis acids and bases, but this chapter's on more than that. It's on comparing acidity and basicity, and gives us some rules to go off of. (again, only looking at molecular structure) I have more questions on this than on chapter 4.

Chapter 6, finally, is on reaction mechanisms. There are very few rules to go off of. The main rule seems to be: "Find the strongest acid and base and determine how they will react to form weaker acids and bases. Each of the steps will look like one of 6 mechanisms, but you have no clue how many steps it will take or which mechanism I'll want you to use, and you'll end up going off a vague hunch every time. You should be able to figure it out just by looking at it." As you can tell, I have the most questions on this section.

As you can see, I'm somewhat frustrated. Our professor is very anti memorization and very pro conceptual learning. I support that, because critical thinking is essential, but not the way he's going about teaching it, because he won't let us know when one rule or concept is more important than the other, and doesn't exactly teach anything, he expects us to know it already.

I will be posting any and all questions in this thread for the time being, unless asked to otherwise.

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So! Finally on to my actual questions. I apologize for the book I'm writing here.

Chapter 4 - Physical properties of Organic Compounds.

I understand that the factors that affect melting point are symmetry, polarity, hydrogen bonding, and molecular weight. However, I'm not sure what is most important, because the book doesn't really elaborate. Unfortunately while the examples are simple and only have one thing to go by, the questions we get on homework require knowing what's more important. From what I can tell, symmetry matters when everything else is similar, and the fewer conformations a molecule can make/the closer to a sphere the molecule is, the easier it packs and so the lower melting point. It seems higher molecular weight means higher melting point, but this makes the biggest difference a)when things are significantly different, like CH₃I vs. CH₃Cl and b)in homologous series of compounds, like methane vs. ethane. Polarity and hydrogen bonding seem to be the things that affect melting point most, with hydrogen bonding taking the cake. However if a molecule can form an intramolecular hydrogen bond, some of the intermolecular forces are made lower. So I guess what I'm asking is if I'm missing any factors or have stated something here incorrectly or incompletely.

I'd like to ask similar questions about boiling points (affected by van der waals forces slightly, dipolar attractions greater, hydrogen bonds greatest?), solubility (like dissolves like, meaning the more polar the better it dissolves in water, etc), and density (this one's confusing. High molecular weight is important, but why is the ratio of heavy atoms to carbon atoms more important? Is it less important for liquids? Also, at what point should I be worried about how well the molecules pack because that is considered a factor. Is that only when everything else is the same?)

Chapter 5 - Acid-base Theory

As said earlier, I know the difference between Arrhenius, Bronsted-Lowry, and Lewis acids and bases, and that the Lewis ones are also referred to as electrophiles and nucleophiles. However, everything beyond this is somewhat muddled. We're supposed to be able to compare acid and base strength. I get K_a, I aced that quarter in gen chem, so no questions on what that means. I suppose my questions are more on how to tell what is a strong vs weak acid or base by looking at a molecule. From what I can tell, good acids can lose a proton easily, or desire electrons. Good bases have a free pair of electrons and the looser those electrons are held the better. If it's got a negative charge better still. However, I have a hard time telling which acids are stronger when I'm given two or three that are very similar and trying to decide.  (same with bases). Any tips? Also, what on earth does hardness and softness mean qualitatively? I get that soft acids want to bond with soft bases and hard acids want to bond with hard bases. But When is that significant enough to affect acid/base strength? Finally, when do I want to look out for resonance as it factors in to the strength of an acid/base? I know this is an important factor in carboxylic acid's acidity.

Chapter 6 - Mechanisms
I got the equilibria, rates, and delta G stuff here. All gen chem stuff. No problem understanding the terminology in this chapter either. My problems start around 6.8. I feel like he gives us those six mechanisms and expects us to just guess which ones to use based off how we feel. (well, there's 5 in the book, he gave us another in class). I know that 6.9-6.11 give some examples of reactions of various types but even after scouring examples of substitution, addition, and elimination I never seem to guess the steps correctly. This is the stuff I need the most help on, and the stuff I fail the most at asking for help on. I have no clue what factors make the reaction proceed in one step vs. two, or why two similar reactants will react completely differently with the same substrate. I can say that I understand the basic concept behind each mechanism (heterogenic bond formation, heterolytic bond breaking, 1,3-electron displacement, 1,3-electron abstraction, 1,5- displacement and abstraction), however when they show up as something that looks different than in the book I get confused (e.g when they break a bond in a different spot than is shown in the book I wonder if it can still happen in one step or not.)

ANYWAY. I think that's enough of a book for now. As you can see, I'm not just asking for last minute stuff, I genuinely desire to understand this, and I have read my textbook carefully. I would love to get answers, even ones that go beyond my current level of understanding or ones that address issues that my textbook hasn't yet. Anything to enhance my current level of understanding would be great.

Many thanks to ANYONE who has taken the time to read and respond,
"Opal"

[orgopete]

I think you would get better help if you posted a problem. Otherwise, this is general and I don't know what to suggest.

[Opalescent]

Okay. I am sorry I posted a book. This afternoon I'll post some problems I have. I'll try and come up with a sampling of the types of problems that have confused me. Still, anybody who can help me understand even one of my previous conceptual questions, I would be very grateful.