Stereochemistry

Cycloalkanes – Dashes And Wedges

Cycloalkanes – Dashes And Wedges

March 20, 2014

In the last post, we mentioned that one of the consequences of the fact that carbon can form rings is that small rings (less than 8 carbons) are so rigid that they can’t be turned inside out [video]. One of the important consequences of this, as we’ll see today, is that it leads to the [...]

Read the full article →
Partial Reduction of Alkynes

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 [...]

Read the full article →

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 [...]

Read the full article →

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 [...]

Read the full article →

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 [...]

Read the full article →

Bromination of Alkenes – How Does It Work?

March 6, 2013

In a previous post we went through the key reactions of the carbocation pathway. It’s a family of reactions which proceed through 1) attack of an alkene upon an acid, forming a free carbocation, and 2) attack of a nucleophile upon the carbocation. Although we saw that several key reactions of alkenes were consistent with [...]

Read the full article →

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 [...]

Read the full article →

Addition Reactions: Stereochemistry

January 30, 2013

In the last post on alkene addition reactions, we discussed one of the two key themes to look for in addition reactions: regiochemistry (in other words – what “region” of the alkene do different atoms end up forming bonds to?). This post is about the second key theme in addition reactions of alkenes: stereochemistry.  We’re going [...]

Read the full article →

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 [...]

Read the full article →

The E2 Mechanism

September 27, 2012

Having gone through the E1 mechanism for elimination reactions, we’ve accounted for one way in which elimination reactions can occur. However, there’s still another set of data that describes some elimination reactions that we haven’t adequately explained yet. Here’s an example of the reaction I’m talking about: What’s interesting about this reaction is that it [...]

Read the full article →