Reactions and Reaction Mechanisms A reaction mechanism shows the actual flow of electrons and movement of the atoms during the reaction. If one can understand a reaction mechanism one can predict the course of other reactions.

Attack of a Lewis Base on a Lewis Acid

It is also useful to correlate changes in energy with the movement of atoms. To do this one creates an energy profile of the reaction showing how the energy changes as the atoms move.

Energy Profile of a Exothermic Reaction Potential Energy

Transition State

But if you mix propene and H 2 nothing happens! Molecules repel each other. No good mechanism for the reaction.

? E a too high thus no reaction Try to find a new mechanism with a lower E a How do you do that? Use a catalyst.

Palladium metal can be used as a catalyst for the hydrogenation of alkenes. H 2 will actually dissolve in Pd metal. The H atoms dissociate and go into the octahedral holes. The  bond of an alkene will bond to the surface atom of the Pd metal. This is all we need!

Palladium metal is a catalyst for the hydrogenation of alkenes.

Addition of HBr to ethylene gives bromoethane But what does HBr give when it reacts with 2-methylpropene?

But what does HBr give when it reacts with 2-methylpropene? For this we need to know the mechansism. The reaction involves a carbocation intermediate.

Which carbocation is formed?

Alkyl groups stabilize carbocations via Hyperconjugation. It involves “no bond” resonance structures.

Empty p orbital Filled sp 3 hybrid orbital Electrons from the C-H bond help stabilize the empty p orbital

Order of Carbocation Stability Least Stable Most Stable primary secondarytertiary

Which carbocation is formed? Tertiary most stable

How does a carbocation react? Since the carbocation is a Lewis acid It can react with a Lewis base. The electron pair on the base attacks the electron deficient center

General Reaction: The addition of a hydrogen halide, HCl, HBr or HI to an alkene gives an alkyl halide. The regiochemistry is determined by which carbocation is the most stable.

more stable

Both carboncation intermediates would be secondary, little selectivity

What happens if there is no halide? Hydration of alkenes to give alcohols. H 2 SO 4 is a catalyst

What is the regiochemistry of the reaction? What is the mechanism of the reaction?

The H 2 SO 4 dissociates to give hydronium ion.

What is the regiochemistry of the reaction? The regiochemistry is determined by the relative stability of the intermediate carbocation.

Reactions of alkenes so far:

One more reaction for the sake of completeness Bromine or chlorine easily add to a double bond to give dibromo or dichloro compounds.

Reactions of alkenes so far:

Enough of this simple stuff. The acid catalyzed addition of water to an alkene gives an alcohol. The regiochemistry is determined by the relative stability of the intermediate carbocation. But what if you want 1-propyl alcohol? What could you do to “trick” the regiochemistry?

The Trick The observed regiochemistry is determined by the first step of the reaction. But we want an OH - group to add to the primary carbon. How do we do that? Add some other Lewis Acid instead of H + then convert it to an OH group.

The Trick tripropylborane

But what good is tripropylborane? We are tryingto make 1-propanol? It turns out that oxidation of alkyl boranes will give boric acid plus alcohols.

Net Reaction This is called hydroboration

Thus we have a choice in regiochemistry.

Oxidation states in carbon chemistry. Oxidation and reduction are important chemical processes. In organic chemistry it is not always obvious that you are carrying out an oxidation or a reduction.

Consider average oxidation states of the C atoms in each of the following molecules. Remember that H is counted as +1 and O as –2.

Adding H 2 to a double bond is a reduction. The reverse reaction ( removing H 2 ) would be an oxidation.

Adding H 2 O is not an oxidation. Removing H 2 O is not a reduction.

Adding H 2 O is not an oxidation. Removing H 2 O is not a reduction.

To go from an alcohol to an aldehyde (or a ketone ) is an oxidation. Going from an aldehyde on to a carboxylic acid is a further oxidation.

Primary alcohols can be oxidized to carboxylic acids using CrO 3. Chromium (VI) is a strong oxidizing agent. It is used with an acid catalyst.

Alcohols can be oxidized to aldehydes or ketones using a modified form of CrO 3 called PCC for pyridinium chlorochromate, (C 5 H 6 NCrO 3 Cl). It is a milder reagent and if you use it carefully you can stop the reaction at the intermediate aldehyde step.

Going Backwards: Reduction of aldehydes or ketones to an alcohol. To reduce a carbonyl group, C=O, to an alcohol one could use H 2 gas, but a simpler way is to use a hydride as a source of H - and a acid as a source of H +. The reagent of choice is NaBH 4, followed by some acid.

Starting with an alkene how would you make?

Can you explain an unexpected result? 82% + ? 18%

and 82% ? 18%