Chapter 8: Alkenes and Alkynes Reactions Part 2 Flashcards
What are Carbenes?
is a class of compound that is highly reactive, and contains divalent carbon atoms. The carbon atom will only contain 2 bonds
What are Electrophiles?
Electrophiles are species that seek for electrons
Ex: Electrophiles can be Bronsted Lowry Acid or Neutral specie such as AlCl and Ag+
What are Regioslective Reactions?
Regioselective reactions refer to reactions in which one direction of the bond formation or breaking predominates over the other direction
Ex: Anti-Marovnikov (OH goes to the least substituted group)
What are Stereospecific Reactions?
Stereospecific is a reaction where the starting material differ only in configuration, and are converted into stereoisomeric products
- A stereospecific process is always a stereoselective, but not all stereoselective processes are stereospecific
What are Stereoselective Reactions?
Stereoselective is a chemical reaction where one or more products produced is predominantly in one form of the stereoisomer, even though the initial reactant is not chiral
What are Hydrohalogenation addition (H-X) Reactions?
- Addition Reactions occur when a double bond is converted to a single bond, which is an exothermic reaction
- The pi bond electrons are exposed thus, make addition reactions possible
- The pi bonds will react with electrophiles - Trans Addition Reaction
- X = I, Br, Cl not F; F does not want to leave
- If the hydrogen halide is added to an unsymmetrical alkene, then 2 products can be generated
- Only with HBr with peroxide present; one must follow anti-markovnikov’s rule; other than that markovnikov’s original rule applies
What are Hydration (Acid- Catalyzed) addition reactions?
- Adding water in the presence of an acid to an alkene (Hydration) can be used to generate alcohols
- This reaction is useful to generate small alcohols - The acid portion is usually a diluted solution of phosphoric or sulfuric acid
- dilute acid was used in the reaction to minimize the effect of acid and maximize the addition of water effect - The first step of the reaction is the slow step(acid attacks acid to be protonated)
- creating carbocation, and water attacks carbocation
- then excess water attacks the water attached to carbocation to deprotonate it
- Acid should be regenerated in the end - This reaction also follows Markovnikov’s rule (rearrangements will occur if it can generate a more stable carbocation; most substituted)
- Trans Addition reaction
What are Hydroboration-Oxidation Addition Reactions?
- This reaction yields an Anti-Markovnikov product
- Reagents such as diborane or solution of borane in tetrahydrofuran are used (BH3:THF)
- The first step is called hydroboration and the second step is an oxidation reaction (H2O2/OH-) where an alcohol is obtained through hydrolysis
- Both anti and syn additions occur, but syn MUST be the MAIN product
- This reaction is an anti-Markovnikov reaction, since the hydrogen will add to the less substituted carbon atom
- Stereospecific
EXACT OPPOSITE OF OXYMERCURATION
What are Halogenation (Br2 or Cl2) Addition Reactions?
- The addition of chlorine and bromine to an alkene will generate trans or anti products
- CCl4 is the most useful solvent, gets dibromide
- Forms Bromium Ion (cyclic) or chloride ion if Cl2 used
- Addition of bromine to an alkene is a stereospecific reaction
- cis alkene will generate a pair of enantiomers, whereas trans alkene will generate meso compounds, and is achiral - Forms equally major products
- Cyclic Bromium ion must open in the opposite direction = Trans ALWAYS
What are Halohydrination (Br2 or Cl2 with H2O) Addition Reactions?
- If the halogenation occurs in aqueous solution with an alkene, a halohydrin is produced instead
- Halohydrin is when a halide and an OH group are added to the carbon atom of the double bond
- An unsymmetrical alkene will generate a product that has the halogen on the carbon that contains the most hydrogen atoms. The water will attack the carbon atom which is the most substituted
- Since water is more abundant, water will act as the nucleophile
- This reaction will also be trans as well
What are Oxymercuration-Demurcuration Addition Reactions?
You Do Not Need to Know the Mechanism for This
1. Generates Alcohol, a 2 Step reaction
2. Mercuric acetate in tetrahydrofuran Hg(OCOCH3)2 is mixed with alkenes and water in the first step to produce (Hydroxylalkyl) mercury compound
3. Step 2 involves the reduction of step 1 using sodium borohydride (NaBH4) to obtain an alcohol. The acetoxymercury group is reduced to replace it by a hydrogen atom
4. Anti-addition for this reaction, not stereospecific
5. Follows Markovnikov’s Rule
NO REARRANGEMENT FOR THIS REACTION
What are Alkene Oxidation (1,2-dihydroxylation Reaction) Addition Reactions?
Generates a Diol, No mechanism need for this Reaction
- Syn Addition Reaction
- The Osmium tetroxide (OsO4, pyridine) catalyzed dihydroxylation of alkenes is developed as highly efficient and enantioselective technique for dihydroxylation of alkenes
- 1,2-diols can be produced using KMnO4 (COLD) or OsO4 (not used often because it is expensive and toxic)
What are Oxidative Cleavage Addition Reactions?
0 Substituents = CO2
1 Substituents = R-C=O-OH (Carboxylic Acid)
2 Substituents = R-C=O-R (Ketone)
1. Permanganate is a strong oxidizing agent that easily oxidizes diol to further products
2. Involves HOT KMnO4, with H3O+
3. Unsubstituted alkenes with two hydrogen atoms are oxidized to carbon dioxide and water
4. Monosubstituted alkenes (one Alkyl group) will oxidatively cleave to generate carboxylic acids
5. Disubstituted alkenes (Two alkyl group) will oxidatively cleave to ketones
What are Ozonolysis Addition Reactions?
Ozonolysis is when an alkene is treated with ozone at low temperature (-78C), in CH2Cl2. Second step involves treating the solution with dimethyl sulfide or zinc with acetic acid
0 Substituents = C=O (Formaldehyde)
1 Substituents = R-CH=O (Aldehyde)
2 Substituents = R2-C=O
1. The double bond breaks and the carbons that were doubly bonded to each other are now doubly bonded to oxygens
2. Creates intermediates called Ozonides which are very unstable and reactive
What is Epoxidation?
Epoxides are a sub-class of ethers, in which they contain a C-O-C group
- No mechanism needed for this Reaction
- Can be generated when an alkene is reacted with peroxy acid (Peracid = RCO3H) or MCPBA (meta-chloroperoxybenzoic acid)
- Epoxidation of an alkene is a syn addition reaction that is stereospecific
- Cyclic structures are extremely strained and are subjected to nucleophilic substitution reactions
