Browsing: o-chem help

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.



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.



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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.



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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


Good nucleophiles

Br, HO, RO, CN, N3

Fair nucleophiles

NH3, Cl, F, RCO2

Weak nucleophiles


VERY weak nucleophiles




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


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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Resonance between equivalent atoms in organic chemistry means equal bond lengths.

Let’s talk resonance in organic chemistry.  

Resonance in organic chemistry is one of the most fundamental and useful concepts you will learn in this class. Once most students hear this tip, it makes perfect sense to them, but it isn’t one that you might think of on your own.  Take a look at the structure below, and ask yourself: are the two N-O bonds in this molecule the same length?

resonance in organic chemistry

Since freshman chemistry, we have been told that double bonds between two atoms are shorter than a single bond between the same two atoms.  Hence, the N-O double bond should be shorter than the N-O single bond.  Spoiler: it is not.  But before we get into that, let’s look at some resonance forms of the nitro group at the end of this hydrocarbon:

resonance and bond length

Here, we can more clearly see that the nitro group is interconverting between the three resonance structures shown above.  Structure 3, where the charge is spread evenly between the two oxygens is a valid structure and shows that the bond two oxygen atoms in the molecule are equivalent and have the same bond length (124 pm).  This is shown here using the dashed bond, which you can think of as “half of a bond” for lack of a better term.


We care even more about this principle when it can be applied to more complex organic molecules where it is not obvious that the bonds are equivalent.  For example, the cyclopentadiene anion:

At first glance, this appears to have three different carbon atoms.  However, once you start looking at resonance structures, you can see that the anion can be moved to any of the carbons in the ring.  This makes them all equivalent, via resonance.  This is confirmed through analytical studies which show that all C-C bonds are approximately 137pm long.  Additionally, as this fits Huckel’s rule of 4N+2, the molecule is also aromatic.


Take Home Message: If you see symmetry or aromaticity, think equivalent bond lengths

 For more help with resonance, please see our homepage at organic chemistry



free organic chem study guide

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Steps of a Free Radical Reaction [simplified – with a great diagram]

The steps of free radical reactions


This is one of the best depictions of the steps of free radical reactions I have seen.   It shows what can go on in this reactions and how we get from starting material to desired final product.

Steps of a free radical reaction


Initiation = 1 neutral provides two radicals.  This is what starts the entire reaction.  This is also the only initiation step that can occur, as CH4 is not going to break off an H*.

Propagation = 1 neutral + 1 radical provides a different neutral and a different radical.  In this reaction, the most likely propagation is chlorine abstracting a proton from methane to give HCl and the methyl radical.  The next step is where the methyl radical breaks up two Cl atoms.  What I really like about this depiction is that it shows that the Cl* from reaction 3 can be recycled back into step 2.  This means that the reaction is self-propagating.  This also means that IN THEORY you could have one initiation reaction, followed by a bunch of propagation, ending with one termination reaction.  Of course, in real life, for many reasons, this does not happen as there are lots of initiation reactions.

Termination = 2 radicals providing one neutral.  The part to remember here is that any two radicals can get together to terminate the reaction and form a neutral species.  Since we have 2 types of radicals in the reaction (Cl* and CH3*) , there are three combinations of potential termination steps.  Reaction 4 gives us back starting material, so it is fine.  Reaction 6 gives us product, so it is also fine.  Reaction 5 give us a byproduct, which strangely enough can replace methane in the propagation step and give us another by-product.

Think about this picture and figure out all of the side reactions that might occur to fowl up the reaction.  Then, (for you advanced students) think about what ways exist that you can minimize those side reactions.

Here is the quick summary of radical reactions:

  1. Initiation = 1 neutral provides two radicals.
  2. Propagation = 1 neutral + 1 radical provides a different neutral and a different radical.
  3. Termination = 2 radicals providing one neutral.

Hope this was helpful to you all, and as always, happy reacting.


free organic chem study guide






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Functional Groups in Organic Chemistry [with diagrams]

Functional Groups in Organic Chemistry

Welcome back.  Let’s not beat around the bush on this one: functional groups in organic chemistry are why we can do any organic chemistry in the first place. Functional groups are the basis of why molecules can and will react with each other. Without functional groups, everything would be straight chain alkanes and other boring hydrocarbons. So it’s important to learn functional groups, and how they will interact with nucleophiles and electrophiles to react to form new organic molecules.

Major Disclaimer:  This is not meant to be a comprehensive review of all of the functional groups out there, however it’ll be a nice start and a good reference for you.

Hopefully you understand why they are important, now we just have to determine what some of the different types are.


functional groups in organic chemistry

What to learn about nucleophiles?  Click on the link to check it out



Hydrocarbons: these are simply composed of carbon and hydrogen. This group is alkanes, cycloalkanes, alkenes, and alkynes.  Don’t forget about conjugated alkenes too, as they are important in many organic processes such as the Diels-Alder reaction.  While alkanes and cycloalkanes are not particularly reactive, alkenes and alkynes definitely are.

Carbonyls: a “carbon double bond oxygen” is a carbonyl.  It is one of the more important electrophiles you will see in this course.  While there are different variations which can make the carbonyl more or less reactive, the basic functional group is still the same.  The important point here is to know which types of carbonyls are more electrophilic and which ones are less. Generally speaking, if there is an electron withdrawing group attached to the carbonyl carbon, that carbonyl will be more electrophilic and more reactive.

Alkyl Halides: alkanes which are connected to a halogen atom (F, Cl, I, and Br) are good electrophiles.  These can participate in nucleophilic substitution reactions and elimination reactions.  They reactivity depends on the type of alkyl halide (F, Cl, I, Br), its substitution (primary, secondary, tertiary) and the desired reaction (SN1, SN2, E1, E2).

Alcohols, Amines, and Thiols: these are generally very good nucleophiles, as the heteroatoms have lone pairs which will attack an electrophile.

Ethers: do not undergo many organic reactions themselves, but sometimes can be the product of a reaction.  Some chemists refer to ethers as “dead molecules” because of their low reactivity.



And now for some crazy functional groups….







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