Drawing Reaction Mechanisms

An Arrow-Pushing Dilemma In Concerted Reactions

April 5, 2013

Up to now, drawing out reaction mechanisms using the curved arrow formalism has been fairly straightforward. Yes, sometimes there is some ambiguity with respect to which carbon of a C-C π bond is forming a new bond to an electrophile, but that can be readily solved by adding a few guidelines. Electrons flow from areas of [...]

Tagged as: arrow pushing, arrows, br2, bromination, curved arrows, stereochemistry, transition states

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Rearrangements in Alkene Addition Reactions

February 26, 2013

In exploring reactions that proceed along the carbocation pathway, every once in awhile you might see an example of an addition reaction that looks a little… strange. The alkene is gone, two new bonds have formed, but the positions of the new bonds is a little out of the ordinary. Like in this example! If [...]

Tagged as: 1, 2-shift, addition, alkyl shift, carbocations, hydride shift, rearrangements, wagner meerwein

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Comparing the E1 and E2 Reactions

October 10, 2012

Now that we’ve gone through the mechanisms of the E1 and E2 reactions, let’s take a moment to look at them side by side and compare them. Here’s how each of them work: Here’s what each of these two reactions has in common: in both cases, we form a new C-C π bond, and break a [...]

Tagged as: anti, base, carbocation, cyclohexane, deprotonation, e1, e2, elimination, stereochemistry

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The SN2 Mechanism

July 4, 2012

Having gone through the two different types of substitution reactions, and talked about nucleophiles and electrophiles, we’re finally in a position to reveal the mechanism for one of the most important reactions in organic chemistry. It’s called the SN2 reaction, and it’s going to be super useful for us going forward. Let’s look at the [...]

Tagged as: backside attack, inversion, leaving groups, nucleophiles, nucleophilic substitution, SN2, stereochemistry, substitution

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Acid Base Reactions Are Fast

May 21, 2012

Common dilemma in organic chemistry as you move through the latter parts of Org 1 and then into Org 2: When more than one reaction is possible, how do you know which one will happen? In a steel cage match between an acid base reaction and other types of reactions, which wins? Here’s a good [...]

Tagged as: acidity, basicity, conjugate acid, conjugate base, nucleophile, nucleophilicity, pka, trick questions

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Curved Arrows (2): Initial Tails and Final Heads

February 29, 2012

Here’s a handy little trick for accounting for charges when you draw curved arrows. You know by now that curved arrows are our accounting system for showing how electrons move, from the tail (electron donor, source of electrons) to the head (electron accepting, destination of electrons). There are only three moves you can do: lone pair [...]

Tagged as: curved arrows, drawing, electron accounting, formal charge, how electrons move, mechanisms, Reaction Mechanisms

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The Third Most Important Question to Ask When Learning A New Reaction

February 20, 2012

When learning any new reaction, I think you always have to start with the “what”. What bonds are forming, and what bonds are breaking. After you answer “what”, then you can start asking “where” – as in , “where are the electrons of the reactants?” What areas are electron rich? What areas are electron poor? [...]

Tagged as: curved arrows, formal charge, lone pairs, mechanisms, pi bonds, sigma bonds

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Curved Arrows (for reactions)

February 15, 2012

If you think of electrons as the currency of chemistry, reactions are transactions of electrons between atoms. Just like double entry book keeping was developed to formalize how financial transactions are recorded, chemists have developed their own convention for showing transactions of electrons between atoms. It’s called the curved arrow formalism. Previously I covered how [...]

Tagged as: curved arrows, drawing mechanisms, electron accounting, electron flow, formal charge

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How to apply electronegativity and resonance to understand reactivity

January 17, 2012

One thing has been missing from the discussion of resonance. What’s the point? Who cares if we can write out resonance structures? What does it matter if we can figure out the two or three most stable resonance structures? So what? Here’s the point: we can apply resonance (and electronegativity) to figure out the electron [...]

Tagged as: charges, electronegativity, formal charge, opposite charges attract, resonance

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Grossman’s Rule

October 6, 2011

A few months ago after putting up this post on “Hidden Hydrogens, Hidden Lone Pairs, and Hidden Counterions”, commenter Stewie Griffin made me aware of this little gem: With regards to hidden hydrogens, don’t forget “Grossman’s Rule” (named by Robert Grossman in his “Art of Writing Reasonable Organic Reaction Mechanisms” book) which basically says “When [...]

Tagged as: addition, elimination, grossman's rule, hidden hydrogens, leaving groups, study tips, substitution

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