Did you know that not all molecules are reactive? Only certain types of molecules will want to react in a mechanism. Let’s dig a little deeper into this...
Stability and reactivity generally have an inverse relationship. If a molecule is unstable in some way, it will want to react! Here are the 4 signs we can look for that determine reactivity:
Concept #1: How to tell if a molecule will be reactive or not.
These reactive trends ARE in order of strength (i.e. a formal charge will typically be more reactive than a dipole).
Using those indicators, let's see if the following molecules are reactive:
Example #1: Are any of the following 4 molecules reactive?
Example #2: Are any of the following 4 molecules reactive?
Again, we are literally just matching these molecules to the 4 patterns discussed above.
Concept #2: How to tell if charged molecules will react as nucleophiles or electrophiles.
Now that we know how to determine if molecules are reactive, we still don’t know HOW they will react! There are two major subtypes of reactivity that we’ll often use in Orgo 1 and 2:
Note that a molecule doesn’t require a negative charge to be a nucleophile, but it needs to have similar properties (i.e. a source of electrons).
That said, try to identify if the following three molecules are nucleophilic or electrophilic.
Example #3: Are the following 3 reactive molecules nucleophiles or electrophiles?
Concept #3: How to tell if uncharged molecules will react as nucleophiles or electrophiles.
So that wasn’t so hard, but those were the easy cases. What if you have nucleophilic AND electrophilic regions on the same molecule? Is it possible to determine how it will react? Yes it is!
Rule: The side of the dipole with the highest bonding preference (the atom that wants to make the most bonds) will determine how the molecule reacts.
Example #4: Are the following 3 reactive molecules nucleophiles or electrophiles?
Now we understand which molecules will want to react, and we are getting better at determining If they are nucleophiles or electrophiles, but how to they actually attack other molecules?
Reactive molecules share electrons to become more stable. Arrows are used to show which direction they are going.
Summary: Molecules with lots of electrons will attack (draw an arrow to) molecules with a positive charge. Let’s get drawing!
Concept #4: Learning the rules of electron movement
Using those rules, let's draw the mechanisms for the following reactions
Example #5: Draw the first arrow of the following mechanism.
Example #6: Draw the first arrow of the following mechanism
Example #7: Draw the first arrow of the following mechanism
So now we know how to make bonds. Do we ever have to break bonds? How do we know if we do or we don’t?
Concept #5: Why we need to break bonds sometimes.
Bond breaking is sometimes required in mechanisms, but only when it is necessary to preserve octets.
Concept #6: The two ways to break bonds.
Out of these two different ways, we will stick to heterolytic cleavage for the foreseeable future (we won’t discuss radicals for a few more chapters).
Identify if the following reactions require bonds to be broken. Draw the products.
Example #8: Identify if the following reaction requires a bond to be broken. Draw the products.
Example #9: Identify if the following reaction requires a bond to be broken. Draw the products.
Example #10: Identify if the following reaction requires a bond to be broken. Draw the products.