Alkyl Halides

Alkynes Via Elimination Reactions

June 11, 2013

We’ve gone through elimination reactions before – treatment of alkyl halides with base gives alkenes.  Some time ago we discussed elimination reactions here. Elimination reactions follow the general pattern below, where two adjacent bonds to carbon are broken – usually C-H and C-X, where X is a leaving group – and in place we form [...]

Tagged as: alkyne, dihalides, e2, elimination, nanh2, synthesis

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The 2 Most Important Reactions of Alkynes

May 1, 2013

With the series of posts on alkenes in the can, let’s move on and talk about a closely related functional group that shares many reactions in common with alkenes: acetylenes, or as they are more commonly referred to, “alkynes”. Alkynes, as we shall see, do share many reactions in common with alkenes. However there are [...]

Tagged as: acidity, alkynes, SN2, sp, substitution, synthesis

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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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Bromination of Alkenes: The Mechanism

Bromination of Alkenes: The Mechanism

March 15, 2013

In the last post of this series we showed how the mechanism for bromination of alkenes can’t possibly go through a carbocation intermediate. Why not? We have at least 3 good reasons: The two atoms that form new bonds to carbon add to opposite faces of the alkene (“anti” stereoselectivity) The reaction is stereospecific – (Z)-2-butene [...]

Tagged as: alkenes, anti, bromination, bromonium, chlorination, halogenation, halonium, markovnikov, mechanisms, syn

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Markovnikov’s Rule (1)

February 8, 2013

Onward with addition reactions! Quiz time: let’s see if you can recognize the patterns in the following 3 reactions. Look carefully. What do each of the major products have in common? Hopefully you can see that in each case, we’re breaking C-C (π) and forming a new C-H and C-Cl bond. But there’s more. The [...]

Tagged as: addition reactions, alkenes, alkyl halides, markovnikoff, markovnikov, most substituted

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Deciding SN1/SN2/E1/E2 (2) – The Nucleophile/Base

November 30, 2012

Last time I talked about the process of deciding if a reaction goes through SN1, SN2, E1, or E2 as asking a series of questions. I call it The Quick N’ Dirty Guide To SN1/SN2/E1/E2. This is the second instalment. Once we’ve looked at a reaction and recognized that it has the potential for proceeding through SN1/SN2/E1/E2 – that [...]

Tagged as: bulky base, e1, e2, elimination, leaving group, nucleophile, sn1, SN2, substitution

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Elimination Reactions With Rearrangments

November 9, 2012

One last (weird) reaction to show you with respect to elimination reactions. Can you see what’s weird about it? How did that double bond get over there? Normally when elimination occurs, we remove a hydrogen from the carbon adjacent to the leaving group. But here, something extra has taken place. Let’s look at all the bonds that [...]

Tagged as: alkyl shift, e1, eliminations, hydride shift, rearrangement, sn1

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Comparing the E1 and SN1 Reactions

November 8, 2012

Imagine you’re starting with the alcohol on the left and you’d like to get to the alkene on the right. What bonds are formed and broken here? We’re forming C-C (π), we’re breaking C-H, and we’re breaking C-OH. It’s an elimination reaction. Notice a problem here? We need to have HO(-) as a leaving group. [...]

Tagged as: base, conjugate acid, e1, e2, elimination, leaving groups, nucleophile

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Bulky Bases in Elimination Reactions

October 24, 2012

We’ve recently talked about Zaitsev’s rule in elimination reactions, and how the transition state leading to the more substituted alkene is lower in energy. The bottom line in elimination reactions is that you should expect to form the “more substituted” alkene if at all possible. Like in the elimination reaction below, for instance, we get [...]

Tagged as: alkenes, base, bulky base, butoxide, e1, e2, elimination, steric hindrance, zaitsev

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The E2 Reaction and Cyclohexane Rings

October 18, 2012

Last time we compared the E1 and E2 reactions and mentioned one of the key differences was the stereochemistry of the E2 reaction. Remember that in the E2, the leaving group is always “anti” to the hydrogen that is removed on the adjacent carbon. [That means that they're directly opposed to each other, or 180°; [...]

Tagged as: anti, base, chair, cyclohexane, e1, e2, elimination, strong base

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