Why Addition Reactions Proceed: Thermodynamic Feasibility
12-1
Because the C-C bond is relatively weak, alkene chemistry is dominated by its reactions.
The addition of a reagent, A-B, to give a saturated compound is the most common transformation of an alkene.
Ho for the above reaction can be estimated from the relevant bond energies:
Ho = (DHo bond + DHo
A-B) – (DHoC-A + DHo
C-B)
Most additions to alkenes should proceed to products with the release of energy.
Catalytic Hydrogenation12-2
Hydrogenation takes place on the surface of a heterogeneous catalyst.In the absence of a catalyst, hydrogenations of alkenes, although exothermic, do not spontaneously occur, even at high temperatures.
In the presence of a catalyst, the same hydrogenations proceed at a steady rate, even at room temperature.
The most frequently used catalysts for hydrogenation reactions are:
• Palladium dispersed on carbon (Pd-C)
• Collodial platinum (Adam’s catalyst, PtO2)
• Nickel (Raney nickel, Ra-Ni)
The primary function of a catalyst in hydrogenation reactions is to provide metal-bound hydrogen atoms on the catalyst surface.
Common solvents used for hydrogenations include methanol, ethanol, acetic acid, and ethyl acetate.
Hydrogenation is stereospecific.During a hydrogenation reaction, both atoms of hydrogen are added to the same face of the double bond (syn addition).
In the absence of steric hindrance, addition to either face of the double bond can occur with equal probability which results in a racemic mixture of products.
Nucleophilic Character of the Pi Bond: Electrophilic Addition of Hydrogen Halides
12-3
The electrons of a double bond are more loosely held than those of the bond.
As a result, the electrons, which extend above and below the molecular plane of the alkene, can act as a nucleophile in a manner similar to that of more typical Lewis bases.
2,3-dimethylbutene
The electrophilic addition reactions of alkenes can be both regioselective and stereospecific.
Electrophilic attack by protons gives carbocations.A strong acid may add a proton to a double bond to give a carbocation.
This reaction is simply the reverse of the last step in an E1 elimination reaction and has the same transition state.
At low temperatures and with a good nucleophile, an electrophilic addition product is formed.
Typically, the gaseous HX (HCl, HBr, or HI) is bubbled through the pure or dissolved alkene. The reaction can also be carried out in a solvent such as acetic acid.
The Markovnikov rule predicts regioselectivity in electrophilic additions.
The only product formed during the reaction of propene with HCl is 2-chloropropane:
Other addition reactions show similar results:
If the carbon atoms participating in the double bond are not equally substituted, the proton from the hydrogen halide attaches itself to the less substituted carbon.
As a result, the halogen attaches to the more substituted carbon.
This result is known as “Markovnikov’s rule” and is based on the stability of the carbocation formed by the addition of the proton.
Markovnikov’s rule can also be stated:
HX adds to unsymmetric alkenes in a way that the initial protonation gives the more stable carbocation.
Product mixtures will be formed from alkenes that are similarly substituted at both sp2 carbon atoms.
If addition to an achiral alkene generates a chiral product, a racemic mixture will be obtained.
Carbocation rearrangements may follow electrophilic addition.In the absence of a good nucleophile, a rearrangement of the carbocation may occur prior to the addition of the nucleophile.
An example of such a rearrangement is the addition of trifluoroacetic acid to 3-methyl-1-butene, where a hydride shift converts a secondary carbocation into a more stable tertiary carbocation:
The extent of carbocation rearrangement depends upon:• alkene structure
• solvent
• strength and concentration of nucleophile
• temperature
Rearrangements are generally favored under strongly acidic, nucleophile-deficient conditions.
