Alkene and Alkyne Mechanisms Flashcards
Name this Alkene mechanism
Halogenation (X2 addition)
Name this Alkene overall transformation
Halohydrin formation (X2+H2O/ROH)
Name this Alkene overall transformation
Acid-Catalyzed Addition of Water
Name this Alkene overall transformation
Oxymercuration/Reduction
Name this Alkene overall transformation
Hydroboration/Oxidation
Name this Alkene overall transformation
Catalytic Hydrogenation. Stereospecific. Reduction of alkene to alkane
Name this Alkene overall reaction
Epoxidation
Name this Alkene overall transformation
Ozonolysis
Name this Alkene overall transformation
Cycloproponation
Name this Alkene overall transformation
Hydroxylation
Name this Alkene overall transformation
Oxidative Cleavage of syn-Glycols with HIO4
Alkene is a loose sea of electrons that reacts with a strong acid to break the bond from halide
H-X Addition
What is DCM?
Organic solvent dichloromethane (or methylene chloride) that does not participate in the rxn but gets the compounds into a reactive medium
If Cl and Cl or Br and Br atoms join, what happens to the electrons?
The partial (+) and partial (-) charges (dipole interactions) on both of the highly electronegative atoms will drive the electrons to one side and cause a chemical reaction
What makes this bromonium ring reactive, yet “happy”?
It wants to be broken open The bromonium ring structure is happy because it has a full octet, but it is highly reactive due to the 3-angle ring strain (bond angle about 60 degrees) and all electrons are eclipsed by each other. Also, bromine has a (+) charge. Since bromine is electronegative, so it wants electrons.
Does a bond become a dash or a wedge when you attack the back side of a wedge?
dash
Name this Alkene overall transformation
Halogenation (X2 addition)
What is regioselectivity?
rxn where one constitutional isomer is favored over the other
What is stereoselective rxn?
rxn where one stereoisomer is formed in preference (R/S)
Stereospecific rxn
What time of rxn results in one stereoisomer forming based on the mechanism alone (always san/anti or inversion of stereochemistry
Name this Alkene mechanism
Halogenation (X2 addition)
Halogenation (X2 addition) mechanisms produce a mixture of enantiomers. Why?
The molecule has 2 sigma* bonds (anti) which are empty space that can accept electrons.
Nucleophiles such as Br2 or Cl2 attack with filled MO. To break the sigma bond, you must attack the empty version of the sigma bond which is the sigma* bond. Then, put electrons in empty MO (sigma*).
Why don’t we use F2 or I2 in Halogenation (X2 addition)?
Both work, but F2 is explosive and I2 produces unstable products that decompose readily
Name this Alkene mechanism
Halohydrin formation (X2+H2O/ROH)
In a regioselective rxn, why does the nucleophile attack the more substituted carbon?
Nucleophile attacks the longer, weaker bond. The carbon on the weaker bond can handle the partial charge better because it is the most stable with more substitution and hyperconjugation.
Name this Alkene mechanism
Acid-Catalyzed Addition of Water
When a strong acid and water are mixed completely with an alkene, what is the product?
An alcohol
Properties of Acid-Catalyzed Addition of Water Alkene Mechanisms
- Markov. In the RDS: Acid being added to an alkene (Markov. - proton goes on the carbon with more protons) forming a carbocation
- Intermediate is a carbocation. *Look for carbocation rearrangements (1,2-methyl shift, 1,2- hydride shift, and ring expansion)
- Add water and remove the proton with solvent. Rxn is catalytic in hydronium or proton (does not officially get consumed in the reaction, regenerated in the product). There is a racemic mixture of products if there is stereochemistry on your substrate
H2SO4 + H2O dissociates completely to form
H3O+ (does the chemistry) and HSO4- (spectator)
What is the product of this Acid-Catalyzed Addition of Water Alkene mechanism?
tertiary alcohol
What are the 3 types of Alkene hydration (adding water across an alkene) mechanisms?
- *1. Acid-Catalyzed Addition of Water** - Catalytic. Goes through a carbocation intermediate. First step is an acid addition, Markov.
- *2. Oxymercuration/Reduction** - Goes through mercuronium ring, water attacks the more substituted carbon. No rearrangements. Alcohol ends up where starting alkene was, Markov. Mixture of enantiomers.
- *3. Hydroboration/Oxidation** - Enantiomer of Syn addition of water, anti-Markov. Only + enant if there is stereochemistry
Name this Alkene mechanism
Oxymercuration/Reduction
What two mechanisms are being compared in this transformation?
oxymerization/reduction and acid-catalyzed addition of H2O
Properties of Oxymercuration/Reduction
Properties:
- Markov-addition of water. H goes on carbon with more H’s (protons) and OH went on the carbon with less protons
- Goes through a Hg(+) mercuronium ring intermediate. Origin of regioselectivity when H2O attacks the more substituted carbon.
- Distereoselective= anti addiction of water. When the ring pops open after adding water, the thing added and the thing that popped open will be anti to each other.
- No carbocations, so no rearrangements.
What two mechanisms are being compared in this transformation?
oxymerization/reduction and acid-catalyzed addition of H2O
Name this Alkene mechanism
Hydroboration.
Boron lines up on carbon with more hydrogens and the hydride will line up with the carbon with less hydrogens. Syn insertion reaction because boron and hydrogen are being added to the same side of the alkene.
Define Anti-Markov
when a proton gets added to the carbon with less hydrogens on it
What is syn addition of water?
The water (H and OH) get added on the same side (dash and dash). Opposite of anti where the addition of water are opposites (one on wedge and one on dash).
In hydroboration, what are the 2 reasons boron chooses carbon with less hydrogens:
- Electronics - B is partial (-) and carbon with less hydrogens is partial (+)
- Sterics - boron will try to be as far away from a group as possible to avoid bumping electron clouds. The more sterically hindered alkene, the higher selectivity for anti-markov
What is a concerted rxn?
all breaking/forming bonds happen at the same time (no intermediate step)
3 ways of adding H2O and their transformations
- Acid-Catalyzed Addition of Water
- Oxymercuration/Reduction
- Hydroboration/Oxidation
Name this Alkene reaction
Catalytic Hydrogenation.
Is Benzene an alkene?
No, leave it alone.
Drawing that represents the alkene interacting with a metal surface in catalytic hydrogenation
the metal catalyst (Pd) is a surface of atoms and the alkene can only interact with one side of the surface at a time which is the origin of diastereoselectivity.
Properties of Catalytic hydrogenation reactions
- Syn addition of H2
- Common Catalysts: Pd/C, PtO2
- Using gas (H2) - typically reduces all alkenes (using an excess amount of hydrogen)
- Sensitive to sterics: Reduce the side of the alkene that is less blocked by sterics
Define chemoselectivity
Reducing selectively one pi system over all others. Just reduces alkene.
For surface rxns, such as catalytic hydrogenations, we reduce the side of the alkene that is less
sterically hindered
Define catalyst
speeds up the reaction by lowering the activation energy/ barrier but is not consumed
What is reduction/oxidation in General Chemistry?
oxidation is loss of electrons and reduction is the gain of electrons.
What is reduction/oxidation in Organic Chemistry?
Reduction is a gain in hydrogens and oxidation is a gain in oxygens.
Name this Alkene mechanism
Epoxidation
Properties of epoxidation reactions
- Concerted (no carbocations, no intermediates)
- Substituents on alkene remain pointing the same way
What are concerted reactions?
all breaking/forming bonds happen at the same time.
Substituents stay pointing in the same direction that they already were, or they will stay pointing in opposite directions if they already were.
DRAW ALL AT THE SAME TIME.
What is MCPBA and what family does it belong to?
meta-chloroperoxybenzoic acid belonging to the peroxy acid family
What causes the reaction to happen in a concerted epoxidation reaction?
In a concerted rxn, there is no time to equilibrate. MCPBA is what causes the concerted rxn, not structure. Cis-Alkenes to cis Alkenes or trans Alkenes to trans Alkenes
Name this Alkene overall transformation
Ozonolysis
Name this Alkene mechanism
Ozonolysis
Examples of Ozonolysis
cut and oxidize the alkene bonds to make carbonyls
Why is -78 degrees C a common temperature to perform reactions under in organic chemistry?
Cold temperature allows chemists to control the reaction and avoid overheating the reaction and potentially dissolving a solvent or exploding.
Equilibrium temp of ice in water is 0 degrees C.
The equilibrium temperature of dry ice in isopropanol (IPA) is -78 degrees Celsius.
What does ozone O3 look like? How many resonance structures does it have?
2 resonance structures
What is contained in ozonolysis steps 1&2?
Ozonolysis step 1 contains O3, MeOH, and it is at -78 degrees C
Ozonolysis step 2 contains Zn, AcOH, H2O OR DMS (CH3SCH3)
What is the Simmons-Smith reaction?
What type of formation is this?
Carbene formation
Name this Alkene mechanism
Cyclopropanation
What is a carbene?
A highly unstable intermediate because carbon only has 6 electrons, not a full octet. Carbenes are Sp2 hybridized.
Carbenes plus an alkene make
3 membered rings or cyclopropane
Name some properties of hydroxylation
- No rearrangements, every step is concerted
- Stereochemistry is retained (starts cis ends cis/starts trans ends trans)
- OsO4 very toxic reagent (chemists use a catalytic amount which is way less of an amount to endorse green chemistry).
What is another way of naming glycol?
1,2 - diol
What type of ring is necessary to make in oxidative cleavage of syn-glycols?
We must be able to make a 5 membered ring. The only way we can make a 5-membered ring is if the OH’s are pointing on the same side of the compound. Then, the ring decomposes and we get our product. Diols (glycols) pointing in opposite directions don’t react.
Example of why it is geometrically impossible for anti-OH’s to make a 5-membered ring and undergo oxidative cleavage.
periodic acid is a way to take 2 OH and cut the bond in between and made a carbonyl. Have to be cis and adjacent
Name this Alkene reaction
Oxidative Cleavage of syn-Glycols
Rules for Alkyne Nomenclature
- Alkynes demand priority: Gets lowest number Carbon. Priority demanding group regardless of any other substituents 2. Name the longest chain that contains the Alkyne, drop “-and” and replace with the “-yne”
Name this Alkyne overall transformation
Alkyne H-X Addition
When a compound has both an Alkene and Alkyne, who gets priority?
Alkene priority = Alkyne priority, except in a tire alkene wins
Hammond’s postulate states that TS is going to be most similar to the things closest in energy. If I know the relative energy of intermediates then I know
the relative rates of formation of those intermediates. TS is often closets in energy to the intermediate.
Who is more thermodynamically stable, Alkene or Alkyne?
Alkene
Just because an Alkyne has more internal energy than an alkene does not mean that it will react quicker. Instead, what do we look at to determine the relative reaction rates?
Activation barrier or activation energy
Carbocation stability comes from what 2 things?
Hyperconjugation and resonance
When comparing a secondary carbocation and a secondary vinyl carbocation, which is more stable and therefore quicker to form and why?
The secondary carbocation is more stable than the secondary vinyl carbocation because the vinyl carbocation sits in an empty Sp2 orbital with ZERO hyperconjugation. The secondary carbocation rate to form will require less energy and therefore be faster.
Are alkene and alkyne reactions endo or exothermic?
Exothermic; they release heat from losing a pi bond.
Name this Alkyne mechanism
H-X Addition to Alkynes
Does H-X Addition to Alkynes participate in Markov.?
Yes!
