Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

3. Acids and Bases

Reaction Mechanism

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Reactivity

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:

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How to tell if a molecule will be reactive or not.

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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:

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Reactivity of Molecules

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Reactivity of Molecules

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Again, we are literally just matching these molecules to the 4 patterns discussed above. 

Nucleophiles vs. Electrophiles

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How to tell if charged molecules will react as nucleophiles or electrophiles.

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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: 

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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. 

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Nucleophile or Electrophile

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How to tell if uncharged molecules will react as nucleophiles or electrophiles.

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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. 

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Nucleophile or Electrophile

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Bond Making

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.

  • Arrows always move from regions of high electron density to low electron density
  • By that logic, nucleophiles must always attack electrophiles.
  • Each attacking arrow represents two electrons being shared.
    • After the reaction is complete, replace that arrow with a new σ -bond

Summary:Molecules with lots of electrons will attack (draw an arrow to) molecules with a positive charge. Let’s get drawing!

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Learning the rules of electron movement

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Using those rules, let's draw the mechanisms for the following reactions

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Drawing Electron Movement

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Drawing Electron Movement

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Drawing Electron Movement

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Bond Breaking

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?

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Why we need to break bonds sometimes.

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Bond breaking is sometimes required in mechanisms, but only when it is necessary to preserve octets.

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The two ways to break bonds.

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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). 

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Identify if the following reactions require bonds to be broken. Draw the products.

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Identifying Bond Breaking

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Identifying Bond Breaking

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Identifying Bond Breaking

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