Alkenes

Alkyne Reaction Patterns – The Carbocation Pathway

May 24, 2013

In the previous three posts on alkynes we’ve introduced some new reactions that are specific to alkynes (versus alkenes): deprotonation (and subsequent substitution), partial reduction to alkenes, and the formation of aldehydes and ketones through net “hydration”. With all the focus on the ways in which alkyne chemistry can differ from alkene chemistry, it’s helpful [...]

Tagged as: addition, carbocation, geminal dihalide, hbr, hcl, hi, markovnikov, vinyl hallide

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Hydroboration and Oxymercuration of Alkynes

May 14, 2013

Alkyne chemistry bears many resemblances to alkene chemistry, but in these first few posts on the subject, the purpose is to illustrate how one seemingly minor change – an extra π bond – can lead to significant differences in chemical behavior. Previously, we saw that the sp hybridization of alkynes leads to increased acidity, and the [...]

Tagged as: alkenes, alkynes, enol, hydroboration, markovnikov, mercuration, oxymercuration, tautomerism

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Partial Reduction of Alkynes

May 8, 2013

Alkynes bear many similarities to alkenes, but as we have already seen, their chemistry can differ in subtle and interesting ways. Today’s post is another case in point. The reduction of alkenes by hydrogen in the presence of a metal catalyst (“catalytic hydrogenation”) is a time-honoured reaction recognized by Sabatier’s receipt of the Nobel Prize for [...]

Tagged as: alkyne reduction, cis alkene, lindlar, na/nh3, sodium, trans alkene

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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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Summary: Alkene Reaction Pathways

April 25, 2013

In this post we’ll do a final review of alkene addition reactions and sum up the three major pathways (and two minor classes of reactions worth paying attention to). Reaction Pathway #1 – The Carbocation Pathway In the Carbocation Pathway, the alkene acts as a nucleophile and attacks an electrophile, resulting in the formation of [...]

Tagged as: alkenes, bromination, carbocation, hydroboration, markovnikoff, markovnikov, patterns, stereochemistry

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Alkene Reactions: Ozonolysis

April 23, 2013

Today’s post represents not so much a pattern in alkene reactions, so much as it does a very common reaction that bears mentioning along with the rest. What makes this reaction special is that it does not simply break the carbon-carbon π bond, as we have been accustomed to seeing, but additionally breaks the C-C σ bond as [...]

Tagged as: carbonyl, carboxylic acid, kmno4, o3, oxidation, oxidative cleavage, ozone, ozonolysis

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A Fourth Alkene Addition Pattern – Free Radical Addition

April 12, 2013

I’ve written that there are three major alkene reactivity patterns [carbocation, three membered ring, and concerted], but there are two minor pathways as well. This post discusses one of them. As discussed previously, alkenes normally react with HBr to give products of “Markovnikov” addition; the bromine ends up on the most substituted carbon of the [...]

Tagged as: addition, alkenes, free radicals, hbr, markovnikov, regiochemistry, stereochemistry

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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 #3: The “Concerted” Pathway

April 2, 2013

In contrast to alkene addition reactions in the Carbocation Pathway and the 3-Membered Ring Pathway, we saw in the last two posts that hydroboration of alkenes is anomalous. The regioselectivity of the reaction is “anti-Markovnikov” and the stereochemistry of the addition is “syn“: We also saw that the “syn” stereochemistry is due to the concerted [...]

Tagged as: addition, ch2i2, cyclopropanation, dichlorocarbene, dihydroxylation, epoxidation, hydroboration, hydrogenation, mCPBA, osmylation, oso4, simmons smith, zn-cu

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Hydroboration of Alkenes: The Mechanism

March 28, 2013

In the last post we saw that the results of hydroboration of alkenes are not in accord with any of the two families of mechanisms we’ve previously seen (carbocation pathway, 3-membered ring pathway). With hydroboration, we observe that the regiochemistry is “anti-Markovnikov” (H ends up bound to the most  substituted carbon) and the stereochemistry of [...]

Tagged as: anti-markovnikoff, anti-markovnikov, bh3, concerted, h2o2, hydroboration, organoborane, oxidation, syn

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