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Alkane Formula [with free study guide]

 

The Alkane Formula

 

Before we can answer the question of what is the alkane formula, we have to ask ourselves what is an alkane. An alkane is a simple hydrocarbon containing carbon and hydrogen single bonded to each other, with a carbon backbone. Any molecule with this structure is going to have the formula CnHn+2, where n is any integer. Let’s look at some examples of this:

 

 

 

free organic chem study guide

 

Please note, that this formula doesn’t tell us anything about the structure of the alkane, it only tells us that is fully unsaturated. For example, both pentane and 2,2-dimethylpropane will both be fully unsaturated and have the same molecular formula, but completely different structures and physical properties therefore we have to be very careful not to make assumptions about the structure of an alkane just based on its formula.

It’s a nice trick to know that any time you see a molecule with the formula CnHn+2, you know it is a fully saturated hydrocarbon, meaning that there are no double or triple bonds only single bonds. The degree of unsaturation, or “some of double bonds and rings” (SODAR) can be expressed using a similar formula. Here is a great post discussing the SODAR formula.

 

A quick rule of thumb to be able to determine if a molecule is saturated or not is just to ask if there are more than double the number of hydrogens as compared to carbon in the molecule.  For example, if you have a hydrocarbon with the formula C5H12, we know that there are more than double the number of hydrogen therefore this is saturated. If we have a structure with the formula C5H10, there are only double the number of hydrogens therefore this is unsaturated at some point in the molecule. What this little rule of thumb doesn’t tell us is where that unsaturation is or what type of unsaturation it is, a double bond or a ring.

Want some more background on alkanes?  Here is a good alkane post.

 

 

Check out this model kit and DVD

 

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Organic Chemistry Klein Textbook Review [Bonus Free Study Guide]

Organic Chemistry Klein

 

Today, we take a look at the organic chemistry Klein textbook, one of the most popular organic chemistry textbooks on the market right now. Written by David Klein, the book is now widely used in many undergraduate organic chemistry courses and is the number one textbook for organic chemistry on Amazon right now. While the 3rd edition is the most recent, the 2nd edition is still wildly popular.  See it on Amazon here.

But why is this book so widely used? What is so great about it? Let’s take a look.

 

The pros:

1) This textbook has a lot of practice problems. And when I say a lot I really mean a lot. As we noted in our survey of organic chemistry professors, they said the number one way to study was using practice problems and practice exams. The Klein book definitely gets this right. Further, the practice problems are placed after each concept is presented. This makes the structure of the textbook really effective, as it reinforces the topic immediately after it’s presented. Kudos to Professor Kline for constructing a textbook with this in mind.

2) Professor Klein has a great writing style. Many students enjoy the way his books are written and have described it as semi conversational, and very understandable. As organic chemistry can be a very confusing concept, the ability to write in this manner is highly commendable. This is definitely one of the main features of this book. I don’t know if there is a more understandable textbook on the market.

3) The “medically speaking” sections are a nice little bonus. Not only do they make organic chemistry more interesting and demonstrate its applicability in the real world, it keeps our med school students and biology friends interested in our science.

 

free organic chem study guide

 

 

Cons:

1) I have seen complaints online that there are errors in the textbook. While errors are not completely uncommon in organic chemistry textbooks, I have not personally found any of these errors myself in the Klein textbook. That doesn’t mean they don’t exist, it just means I didn’t see them when I read the textbook. Some of these complaints may be from students who don’t understand the concept completely, or got bad information from a tutor. I don’t even know if I should call this a con, but I wanted to add it in there since there were so many complaints about this online.

2) A big drawback is that you also need to purchase the solutions manual if you want to get all the answers to all the practice problems. The textbook itself is expensive enough, but then to throw on the expense of a solution manual is definitely another drawback.

3) Finally, some students have complained that the textbook is not as comprehensive as it could be. Some students actually say the Solomon organic chemistry textbook is a nice companion to this book as Solomon is more comprehensive. (For the record, I would not suggest buying two organic chemistry textbooks to complement one another). Some students don’t actually think the fact that Kline is less comprehensive his actually a problem. I can go either way on this. If indeed you don’t think Klein is comprehensive enough, there are plenty of other resources on the internet to complement what you learn in the Klein book (see below for an example). I don’t think any student should ever rely solely on one source of information in order to learn a very complex topic, such as organic chemistry. We always think it is better to buy some sort of supplemental organic chemistry book (usually at a far cheaper price) to round out your organic chemistry education.

Overall, we really like organic chemistry Klein, and give it our highest recommendation.

 

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What is a constitutional isomer? [with a nice chart]

What is a constitutional isomer?

HINT: It is the same thing as a structural isomer and configurational isomer.

 

 

Are you wondering if you have a constitutional isomer? There are two questions you need to ask yourself before we can answer this question. The first question: do the compounds have the same molecular formula? If they do not have the same molecular formula, then they are different compounds and not isomers. If they do have the same molecular formula, then they are isomers. The second question: what type of isomer are they? If all of the bonds and atoms of the two compounds you were looking at are in the same order, then they are stereoisomers. (There are all sorts of different types of stereoisomers, but we’ll deal with that later).  If they have different connectivity, meaning that the atoms and bonds are in a different order from each other, then you have constitutional isomers.

 

 

 

 

 

So, to answer the question what are constitutional isomers? They are compounds with the same molecular formula BUT different connectivity.  Let’s look at some examples of constitutional isomers.  Here is a typical problem you might see on an exam: draw all the constitutional isomers of C4H11Cl.

constitutional isomers

You can see here that all we are doing is putting different parts in a different arrangement.  First, we start with straight chain butane and put the chlorine in two different positions. Next, we start rearranging the carbon backbone, after each change pausing to move the chlorine around.  In this example, there is only one carbon backbone change that can take place, so we make that change, move the chlorine and have the right answer.  BEWARE: because this is a short chain, chlorine can only be in the 1 or 2 position….there is no 3 or 4 position since we also have to name it to have the lowest number possible.

free organic chem study guide

This is 1-chloro, 2-methylpropane NOT 2-methyl, 3-chloroproane.

Professor Trick: are these compounds constitutional isomers?

The answer is no because they are the same molecule.

 

Professor Trick: Are these compounds constitutional isomers?

The answer is no because they have different formulas, which makes them different molecules.

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How many resonance structures can be drawn for ozone O3 [with diagrams]

How many resonance structures can be drawn for ozone O3?

How many resonance structures can be drawn for ozone O3? The answer is two.  Briefly, if you follow the octet rule, you will see the oxygen atoms are connected linearly, where the central oxygen atom is always positively charged, and the terminal oxygen atoms are either negatively charged with a single bond or neutral with a double bond. We will go more in-depth into this explanation below.

If you would like a brief refresher, please see this post on the what is resonance.

The “normal” or neutral state of an oxygen atom is one in which it has two bonds and two lone pairs, which complete an octet.  However, with ozone you’ll see that all three oxygen atoms cannot have this configuration. To stay consistent with the octet rule, we need to have at least one charged oxygen atom in the structure.

The first resonance structure shown below has one positively charged oxygen, one negatively charged oxygen and one neutral oxygen with two bonds. The overall charge of the molecule is zero, and the octet rule is obeyed for all three atoms.  Randomly, we’ve chosen the blue oxygen atom to take the negative charge in the first resonance structure.

resonance of ozone

The second resonance structure of ozone is very similar, as it has one positively charged oxygen, one negatively charged oxygen, and one neutral oxygen with two bonds again. However, in this structure, the right oxygen atom has the negative charge and the left oxygen atom is neutral. Of course, in reality we can’t tell the difference between these two atoms, but for simplicity’s sake we will show them as blue and red oxygen atoms.  As with the below resonance structure, the octet rule is obeyed.

resonance of ozone

However, we know that electrons are never localized like we show here, and these two structures are constantly going back and forth between each other. This means we can draw a third structure, where both the red and the blue oxygen atom have a partial, or dashed, second bond. While this isn’t a true resonance structure, it is a good representation to show how electrons are constantly moving between atoms, a term called delocalization.

electron delocalization

Ozone is a good demonstration of several rules of resonance you should know:

  • Overall charge does not change between resonance structures. It can change on individual atoms, but not for the overall molecule.
  • Check yourself and be sure that you are sticking with the octet rule for MOST atoms. (Not all follow it all of the time, but most will)
  • Oxygen has different charge depending on its electrons and bonds:
    1. 2 bonds + 2 lone pairs = neutral oxygen atom
    2. 1 bond + 3 lone pairs = negatively charged oxygen atom
    3. 3 bonds and 1 lone pair = positively charged oxygen

So, in summary, if a professors asks you how many resonance structures can be drawn for ozone O3, your answer should be a (confident) two.

 

free organic chem study guide

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What is resonance? [7 rules to master it] – Organic chemistry help

What is resonance?

What is resonance (in organic chemistry)? In one sentence, resonance is the concept where electrons (bonds) are delocalized over three or more atoms which cannot be depicted with one simple Lewis structure.

Resonance is one of those issues that you will have to deal with for both semester I & II of organic chemistry.  It is much better to have a solid understanding of it now, rather than have to worry about it later.  The basic goal of resonance structures is to show that molecules can move electrons and charges onto different atoms on the molecule.  Resonance generally makes a molecule more stable because the charge (or bond) is now delocalized and not “forced” onto an atom that might not want it.

Below are some handy rules of resonance.  If you learn these and think about them when tackling different resonance problems, you will be able to handle whatever is thrown at you.

1) Know each atom’s “natural state”. We talk about this in a different post on atoms’ natural state. You need to recognize what each atom we deal with generally looks like, in an uncharged state.  This will help you to construct the Lewis Dot structure on which you will base your resonance structures.  Remember that halogens and hydrogens are always terminal, meaning that are at the end of the molecule and only have one bond, and therefore, they will not (generally) participate in resonance.

2) Atom positions will not change.  Once you have determined that an atom is bonded to another atom, that order will not change in a resonance structure.  If they do change, it is no longer a resonance structure, but is now a constitutional isomer or a tautomer.

3) Check the structure you have created to make sure that it follows the octet rule.  This will become much easier once you have a better handle on the “natural state” of atoms.  If you violate the octet rule, you need to go back and check to make sure you didn’t make a mistake.

4) When two or more resonance structures can be drawn, the one with the fewest total charges is the most stable.  In the example below, A is more stable than B.

resonance in organic chemistry

5) When two or more resonance structures can be drawn, the more stable has the negative charge on the more electronegative atom.  In the example below, A is more stable than B.

resonance in organic chemistry

6) In the end, each resonance structure should have the same overall charge and total number of electrons (bonds + lone pairs) as when you started.  If it does not, you most likely made a mistake somewhere.

7) Resonance affect the length of a bond between two atoms.  We have another post on this topic, which we called resonance in organic chemistry.

 

Take Home Message:  What is resonance? Resonance is like telemarketers.  They are never going to go away, so you need to learn them well.

 

free organic chem study guide

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Know the “normal” state for common organic atoms [3 rules to live by]

 

Structures of molecules can be difficult to piece together at first when you are just starting in an organic chemistry class. Hopefully you retained some of this knowledge from general chemistry. If not, one of the tricks that can greatly help with this is to know the uncharged or “normal” state for atoms that are commonly found in organic molecules.   Here is a table of the most common of those:

      – C has four bonds and no lone pairs

       – N has three bonds and one lone pair

       – Halogens (F, Cl, Br, I) have one bond and three lone pairs. 

       – O has two bonds and two lone pairs

       – H has one bond and no lone pairs

 

Three more rules:

–          C, N, O are central atoms, meaning that they will always be in the middle of your molecule.

–          H and halogens are terminal atoms, meaning that they will only have one bond and be at the ends of molecules.

–          With the exception of H, atoms in group I & group II are only counterions (+1 or +2 and not involved in resonance).

 

Remember, these rules are for when the atom is uncharged; this does not apply to charged atoms.  For example, a carbocation (a positively charged carbon atom) will have only three bonds with no lone pairs while a carbanion (a negatively charged carbon atom) wlll have three bonds with one lone pair, and a carbene will have two bonds with two lone pairs.

Notice that all of these carbons still follow the octet rule.  However, beware of atoms that do not follow the octet rule, as phosphorus is an example of an atom that can have more than an octet of electrons.  Shown below is triphenylphosphine oxide, a byproduct of the Wittig reaction.

Elements with open d-subshells, like phosphorous and sulfur, do not always follow the octet rule.  More examples of this are SF6 and PCl5.  However, carbon, nitrogen and oxygen will follow the octet rule.

 

free organic chem study guide

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Know your strong nucleophiles [with chart]

What are strong nucleophiles?  

Strong nucleophiles: This is VERY important throughout organic chemistry, but will be especially important when trying to determine the products of elimination and substitution (E1, E2, SN1, SN2)reactions.  In fact, there is not a more important part of an organic chemistry reaction than the nucleophile and the electrophile.   So, let’s look at what makes strong nucleophiles.

There are generally three trends to remember when discussing how nucleophilic a reactant is:

1)      Size – Generally, the more linear and/or smaller the nucleophile, the more nucleophilic it will be.  This is because it can react at more sites and will not be sterically hindered if it is smaller or linear.

2)       Electronegativity– The more electronegative an atom is, the less nucleophilic it will be.   This is because more electronegative atoms will hold electron density closer, and therefore will be less likely to let that electron density participate in a reaction.  We see this in calculations and experiments that show nucleophilicity decreases as you get closer to fluorine on the periodic table (C > N > O > F)

3)      Polarizability– The more polarizable an atom is, the more nucleophilic it will be.   Polarizability is defined as the ability to distort the electron cloud of an atom, which allows it interact with a reaction site more easily.  Generally, polarizability increases as you travel down a column of the periodic table (I > Br > Cl > F)

Below is a table of relative nucleophilic strength.  This is relative because nucleophilic strength is also dependent on other factors in the reaction, such as solvent.

 

VERY Good nucleophiles

HS, I, RS

Good nucleophiles

Br, HO, RO, CN, N3

Fair nucleophiles

NH3, Cl, F, RCO2

Weak nucleophiles

H2O, ROH

VERY weak nucleophiles

RCO2H

 

 

free organic chem study guide

 

As shown above, as a general rule, the anion of a reactant will be a better nucleophile than the neutral form.  (i.e. RCO2 is a better nucleophile than RCO2H)

Step 2 is learning about the electrophiles.  Please visit our recent post on this topic –> electrophiles

For more information on this and other topics of organic chemistry interest, please visit organic chemistry

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Is it E1, E2, SN1, or SN2? [with printable chart]

Is it E1, E2, SN1, SN2??

This is such a common question, not only for students but on exams too.  How the heck do you tell the difference between an E1, E2, SN1, SN2 reaction?  Check out the chart below to start.

E1, e2, sn1, sn2

 

The factors that will decide:

1) Do you have a strong nucleophile?  If you do, it would favor an SN2 reaction.  If it mediocre, it will favor an SN1 reaction.

2) Does your nucleophile double as a base? If yes, it is going to favor elimination (E1/E2) over substitution (SN1/SN2).

3) How good is your leaving group?  If it is awesome, it is more likely to be a carbocation intermediate, ie E1 or SN1 reaction.  If the leaving group is only OK, that means it has to be forced off and is more likely to be a concerted reaction mechanism like SN2 or E2.

4) What is your solvent? Polar protic solvents will stabilize a carbocation better, therefore promote an E1 or SN1 reaction.  Polar aprotic solvents favor SN2 and E2.

5) What kind of substrate do you have?  If your starting material is a tertiary substrate, you are definitely E1 or SN1.  If it is a primary substrate,  you are definitely SN2 or E2. If it is a secondary substrate, it could go any one of the ways.

 

free organic chem study guide

 

Let’s look at an example.

SN1 reaction

 

 

 

 

This is an easier example, but let’s start with it.  Here is the most important thing to see: The product has OTf substituted, NOT eliminated.  Just by looking at the product, we know it has to be an SN1 or SN2 reaction NOT an E1 or E2 reaction.  Therefore, when we look at the different factors below, we are going to ignore E1 and E2.

1) Nucleophile: Cl is good but not great. Mediocre Nu = SN1

2) Basic: NaCl is not basic.  No base = SN1/SN2

3) Leaving group: OTf is a dynamite leaving group.  Awesome LG = SN1

4) Solvent: tBuOH is a polar protic solvent = SN1

5) Substrate: It’s tertiary at the leaving group = SN1

All of the factors point to an SN1 reaction, therefore I feel comfortable saying it is an SN1 reaction.

 

How about this one:

an SN2 reaction

 

 

 

 

This is still clearly a substitution, but it’s on a secondary substrate, so it could go either SN1 or SN2.  Here are the factors:

1) Nucleophile: CN is a great nucleophile.  Great Nu = SN2

2) Basic: NaCN is not basic.  No base = SN1/SN2, but we already knew that.

3) Leaving group: Cl is a decent leaving group. Decent LG = SN2

4) Solvent: acetone is a polar aprotic solvent = SN2

5) Substrate: It’s secondary at the leaving group = SN1 OR SN2

Almost all of the factors point to an SN2 reaction, with the notable exception of the type of substrate.  I still feel comfortable saying it is an SN1 reaction.

 

What do you think?  What is the most difficult substitution/elimination problem you have seen on an exam or in class?

 

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Alkene reactions [with useful chart]

 

Alkene reactions

 

Alkene reactions are great because a double bond in organic chemistry gives you all sorts of way to add on to the molecule and create a bunch of different products.  The chemistry works because there is a ton of electron density within the pi bonds of the double bond.  Since sigma bonds are stronger than pi bonds, double bonds tend to react to convert the double bond into sigma bonds.  In many cases, the mechanisms of these reactions will proceed through a carbocation mechanism, which means we should have a discussion about Markovnikov’s rule.  But that is another post you should look at.

Of note, most of these alkene reactions are second semester organic chemistry reactions. If you are in first semester and have not seen these reactions before, do not panic.  Remember that first semester organic chemistry is heavy on concepts but light on reactions. It should not come as a surprise if you have not seen these before. First semester students should look over the chart below if they are interested, but you probably won’t see most of these on an exam anytime soon.

We also found a guide online that looks pretty good, click here to download it—> alkene guide

Here are some of our favorite ones below, give it a look.

alkene reactions-part 1

free organic chem study guide

 

alkene reactions -part 2

 

 

 

 

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What is a Meso Compound: Surprisingly Not Chiral [with examples]

Q: What is a meso compound?

Q: Why should we care about them in our organic chemistry class?

We care because this is an easy place for a professor to try to trip you up.

What is a meso compound? Meso compounds are molecules that have multiple stereocenters but are superimposable on their mirror images. Meso compounds do not have an enantiomer but do have a plane of symmetry. Meso compounds are not optically active but are three-dimensionally shaped.  They have dashes/wedges on stereocenters, but they are not chiral.

Test your molecule: flip all of the chiral centers and check to see if it is the same molecule as you had before you flipped the centers. If the molecule is the same, it is meso.  If it is not superimposible on its mirror image, it is chiral. Below is a good example of this.

Question: How many forms of tartaric acid are there?

If you use the general rule of 2(pronounced “2 to the n-th power”) for number of diastereomers, where N is the number of stereocenters, you would be wrong in this case.  Here is why: there are three forms of tartaric acid, (R,R), (S,S) and meso tartaric acid. Close examination shows that (R,S) and (S,R) tartaric acid are the same compound, making it meso.

Meso compounds tartaric acid
(R,R) and (S,S) tartaric acid are on top. Meso tartaric acid is below

 

 

You can also look at it as a Fischer projection, which is an even better way to see what is going on here. (Another reason why it pays to know all of the different types of structural representations)

Fischer projections of tartaric acid
Again, meso tartaric acid has no enantiomer.

This is a trick professors will try to pull in first semester organic chemistry, don’t get caught by it. Be sure to look for superimposable mirror images (meso compounds) when doing diastereomer problems on exams. Another place this trick will surface is in NMR problems and calculating the number of unique protons, but that is a post for another time.

For more help with meso compounds, or to take advantage of our testbank or any of our other free resources, please visit us at organic chemistry.

 

free organic chem study guide

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