Understanding Electron Flow

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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Alkene Addition Pattern #2: The “Three-Membered Ring” Pathway

Alkene Addition Pattern #2: The “Three-Membered Ring” Pathway

March 20, 2013

The “Three-Membered Ring” pathway In the last post we walked through a proposal for how the bromination of alkenes works and showed that it adequately explains many of the experimental observations made for this reaction. Namely, the reaction proceeds with anti addition of substituents across the alkene, and (where relevant) the reaction proceeds with “Markovnikov“ [...]

Tagged as: addition, alkenes, anti, br2, bromination, bromonium, chlorination, Cl2, i2, iodination, markovnikoff, markovnikov, oxymercuration, syn

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Markovnikov’s Rule (2) – Why It Works

February 11, 2013

Understanding Why Markovnikov’s Rule Works Let’s assemble all the facts we know about the reactions of alkenes with an acid like HCl so far. Regiochemistry: as we saw in the last post, reactions of alkenes with acids like HCl follow Markovnikov’s rule: the major product formed is that where the hydrogen adds to the carbon [...]

Tagged as: addition reactions, alkenes, hcl, markovnikoff, markovnikov, rearrangements, regiochemistry, stereochemistry

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The E1 Reaction

September 19, 2012

Last time in this walkthrough on elimination reactions, we talked about two types of elimination reactions. In this post, we’re going to dig a little bit deeper on one type of elimination reaction, and based on what experiments tell us, come up with a hypothesis for how it works. Here’s the reaction. First, look at the [...]

Tagged as: alkenes, alkyl halides, base, carbocation stability, e1, elimination, mechanism, primary, rate laws, secondary, tertiary

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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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Two Types of Substitution Reactions

June 27, 2012

Chemistry is an experimental science. There is no great Ramanujan of our discipline, who, starting with a simple set of premises, could derive and predict all of the depth and variety of modern chemistry. No, it is much messier than that. Chemists have to actively interrogate Nature to learn her secrets. We add substances together [...]

Tagged as: alkyl halide, leaving group, nucleophile, primary, rate law, sn1, SN2, substitution, tertiary

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Leaving Groups Are Nucleophiles Acting In Reverse

June 25, 2012

We’ve covered “what makes a good leaving group” before – the weaker the base, the better the leaving group. In this short post, I just want to make an observation. Note how similar nucleophiles are to leaving groups: A nucleophile donates a pair of electrons A leaving group accepts a pair of electrons Both contain [...]

Tagged as: basicity, conjugate base, equilibrium, finkelstein, leaving groups, nucleophiles, substitution reactions

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What Makes A Good Nucleophile?

June 18, 2012

If you read the last post, you’ll recall that a nucleophile is a species that donates a pair of electrons to form a new covalent bond. Nucleophilicity is measured by comparing reaction rates; the faster the reaction, the better (or, “stronger”) the nucleophile. When discussing nucleophilicity we’re specifically talking about donating a pair of electrons [...]

Tagged as: base, charge, electron rich, electronegativity, leaving groups, lewis bases, nucleophiles, nucleophilicity, solvent, steric hindrance

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Nucleophilicity vs. Basicity

June 6, 2012

Following up on Nucleophiles and Electrophiles, here’s a common question students have about nucleophilicity: What’s the difference between nucleophicity and basicity? Great, great question. First of all, remember that basicity is a subset of nucleophilicity. All nucleophiles are Lewis bases; they donate a lone pair of electrons. A “base” (or, “Brønsted base”) is just the [...]

Tagged as: acids, bases, basicity, electrophiles, kinetic, nucleophiles, nucleophilicity, rates, thermodynamic

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Nucleophiles and Electrophiles

June 5, 2012

All through the series on understanding where electrons are, and how they flow, we’ve been talking about how the basis of chemistry is that opposite charges attract and like charges repel, and that in reactions, electrons flow from “electron rich” areas to “electron poor” areas. Today, we’ll officially give a name to the types of [...]

Tagged as: acids, bases, electrophiles, electrophilicity, lewis acids, lewis bases, nucleophiles, nucleophilicity

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