hydrogenation
the addition of H2 to a multiple bond
unsaturated hydrocarbons
alkenes that have the capacity to react with substances that add to them
heat of hydrogenation
hydrogenation of all alkenes is exothermic
platinum
hydrogenation catalyst most often used
-palladium, nickel, and rhodium are also used
metal-catalyzed addition of H2
is normally rapid at room temp and alkane yield is very high (usually only product)
heterogeneous reactions
reactions involving a substance in one phase with a different substance in a second phase
decreasing heat of hydrogenation
increasing stability of the double bond
ethylene>monosubstituted>disubstituted>tri>tetra
H2C=CH2>RCH=CH2>RCH=CHR>R2C=CHR>R2C=CR2
Mechanism
Hydrogenation of alkenes
Step 1
- H molecules react w/metal atoms at the catalyst surface. The relatively strong H-H sigma bond is broken and replaced by 2 weak metal-H bonds.
Mechanism
hydrogenation of alkenes
Step 2
- Alkene reacts w/metal catalyst. The pi component of the = bond between 2 C’s is replaced by 2 relatively weak C-metal sigma bond
Mechanism
hydrogenation of alkenes
Step 3
- H atom is transferred from the catalyst surface to 1 of the C’s of the = bond
Mechanism
hydrogenation of alkenes
step 4
- Second H atom is transferred, forming the alkane. Sites on catalyst surface at which the reaction occurred are free to acept additional H’s and alkene molecules
stereochemistry
syn addition
H atoms are added to the same face of the double bond (same side)
-start with a molecule and add H2, the result is a product with H’s that are cis
stereoselectivity
stereoselective reaction starts with a single reactant and give two or more stereoisometric products but yields one of them in greater amounts
Electrophilic addition of Hydrogen halides to alkenes
- addition of polar molecule (H2 is nonpolar)
- addition happens rapidly
- two step mechanism
Mechanism
electrophilic addition of a hydrogen halide to an alkene
- hydrogen halide donates proton to alkene, forming a carbocation (slow)
- carbocation-anion combination (fast)
increasing reactivity of hydrogen halides in addition to alkenes
HF«HI
Protonation of the C-C double bond
- pi electrons are in the double bond
- are only interested in something with a + charge, especially H+
electrophilic
positively polarized
-H+
Markovnikov’s rule
when an unsymmetrically substituted alkene reacts with a hydrogen halide, the H adds to the C that has the greater numbers of H’s, and the halogen adds to the C having fewer H’s
alkyl groups
- are electron-releasing
- the more electron rich a double bond, the better it can share its pi electrons with an electophile
hydroboration/oxidation reactions
- results in primary alcohol
- does not form carbocations
- syn addition (H and OH are cis)
- makes one product exclusively
- regioselectivity and stereochemistry control
hydration of alkenes
by using sulfuric acid and then H2O is the reversible dehydration of alcohol reaction
-it is limited to mono and di substituted alkenes
relative rates of acid-catalyzed hydration of alkenes
ethylene (H2C=CH2)<2-Methylpropene ((CH3)2C=CH2)
hydroboration-oxidation of alkenes
- carry out step 1 (hydroboration) by adding Boron
- then carry out step 2 (oxidation) by adding a base (H2O2, NaOH, H2O)
- reaction must be carried out in dry conditions (no H2O)
