Unit 8: Alkenes and Electrophilic Addition
One pi bond, a dozen different ways to break it: Markovnikov and anti-Markovnikov addition, bromonium ions, epoxides, and a double bond cut clean in two.
12 topics · 42 questions
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Alkene Nomenclature and E/Z Configuration
- How it worksHow do you rank the two groups on each end of a double bond by priority to assign it E or Z?
- Worked exampleFind the E or Z designation for the double bond in 1-bromo-1-chloropropene.
- Common mistakeWhy isn't the Z isomer always the same molecule as the cis isomer once the substituents get more complicated?
Your turn1 of 2
In 1-bromo-1-chloropropene, one carbon of the double bond bears Br and Cl, the other bears H and CH3. If the higher-priority groups on each carbon, Br on one side and CH3 on the other, are on the same side of the double bond, is this the E or Z isomer?Alkene Stability and Heat of Hydrogenation
- How it worksHow does measuring the heat released on hydrogenation reveal which of two isomeric alkenes is more stable?
- Worked exampleFind which alkene is more stable, cis-2-butene releasing 28.6 kcal/mol on hydrogenation or trans-2-butene releasing 27.6.
- Going furtherWhat happens to the stability ranking once a trisubstituted double bond is compared against a disubstituted one?
Your turn1 of 2
Two isomeric alkenes are each hydrogenated to the exact same alkane product. One releases 30 kcal/mol of heat, and the other releases 30 kcal/mol. What is the energy difference in stability between the two alkenes, in kcal/mol?Addition of HX and Markovnikov's Rule
- How it worksHow does the alkene's π bond attack a proton from HBr to generate the more stable of two possible carbocations?
- How it worksHow does bromide then attack that carbocation from either open face to finish the addition?
- Worked exampleFind the major product when HBr adds to 2-methyl-2-butene.
- Common mistakeWhy isn't Markovnikov's rule really about which carbon already has more hydrogens on it?
- Going furtherWhat happens to the product mix when an alkene is unsymmetrical but both possible carbocations are equally stable?
Your turn1 of 2
HBr adds to 2-methyl-2-butene, (CH3)2C=CH-CH3. What is the major product?Carbocation Rearrangements During Electrophilic Addition
- How it worksHow does a hydride shift convert a secondary carbocation into a more stable tertiary one before the nucleophile attacks?
- Worked exampleFind the rearranged major product when HCl adds to 3-methyl-1-butene.
- Common mistakeWhy doesn't every electrophilic addition to an alkene give the product Markovnikov's rule alone would predict?
Your turn1 of 2
HCl adds to 3-methyl-1-butene, CH2=CH-CH(CH3)-CH3. Protonation of the terminal carbon initially forms a secondary carbocation at C2. What happens next, and what is the major product?Acid-Catalyzed Hydration of Alkenes
- How it worksHow does protonating the alkene first generate a carbocation that a water molecule can then attack?
- Worked exampleFind the major product when 1-methylcyclohexene undergoes acid-catalyzed hydration with dilute sulfuric acid.
- Ties togetherHow does hydration's Markovnikov selectivity connect back to the same carbocation logic seen in HX addition?
Your turn1 of 2
1-methylcyclohexene undergoes acid-catalyzed hydration with dilute sulfuric acid. What is the major product?Halogenation and the Bromonium Ion
- How it worksHow does bromine form a three-membered bromonium ion bridging both alkene carbons before any nucleophile attacks?
- Worked exampleFind the stereochemistry of the product, anti or syn, when bromine adds to cyclopentene.
- Common mistakeWhy doesn't halogenation of an alkene go through a free, open carbocation the way HX addition does?
- Going furtherWhat happens to the bromonium ion when the reaction runs in water instead of pure bromine, forming a halohydrin?
Your turn1 of 2
Bromine adds to cyclopentene. What is the stereochemistry of the resulting dibromide product, anti or syn addition?Halohydrin Formation
- How it worksHow does water attack the more substituted carbon of a bromonium ion to open it and place a hydroxyl there?
- Worked exampleFind the major product when propene reacts with bromine water to form a halohydrin.
- Going furtherWhat happens to a halohydrin's regiochemistry once the starting alkene is unsymmetrical instead of symmetrical?
Your turn1 of 2
Propene reacts with bromine water to form a halohydrin. What is the major product?Hydroboration-Oxidation
- How it worksHow does boron and hydrogen add to the same face of a double bond in one concerted step with no carbocation?
- How it worksHow does the oxidation step swap a carbon-boron bond for a carbon-oxygen bond while keeping the same stereochemistry?
- Worked exampleFind the major product when 1-methylcyclopentene undergoes hydroboration-oxidation with BH3 then hydrogen peroxide.
- Common mistakeWhy doesn't hydroboration-oxidation ever give a rearranged product the way acid-catalyzed additions sometimes do?
- Going furtherWhat happens to the product's stereochemistry compared against the product of acid-catalyzed hydration of the same alkene?
Your turn1 of 2
1-methylcyclopentene undergoes hydroboration-oxidation with BH3 followed by hydrogen peroxide. What is the major product?Catalytic Hydrogenation of Alkenes
- How it worksHow does a metal surface hold both the alkene and the hydrogen molecule in place to deliver both hydrogens to one face?
- Worked exampleFind the stereochemistry of the product, syn or anti, when cyclohexene is hydrogenated over a palladium catalyst.
- Going furtherWhat happens to the reaction rate when a double bond gets more substituted and more crowded near the catalyst surface?
Your turn1 of 2
Cyclohexene is hydrogenated over a palladium catalyst. What is the stereochemistry of hydrogen addition, syn or anti?Epoxidation with a Peroxyacid
- How it worksHow does a peroxyacid transfer its oxygen to both alkene carbons at once in a single concerted step?
- Worked exampleFind the product when cis-2-butene reacts with mCPBA to form an epoxide, noting the product's stereochemistry.
- Common mistakeWhy doesn't epoxidation with a peroxyacid ever go through a carbocation intermediate along the way?
Your turn1 of 2
Cis-2-butene reacts with mCPBA to form an epoxide. What is the stereochemistry of the product?Ozonolysis
- How it worksHow does ozone form an unstable ozonide that falls apart into two separate carbonyl fragments during workup?
- Worked exampleFind the two carbonyl products formed when 2-methyl-2-pentene undergoes ozonolysis with a reductive workup.
- Common mistakeWhy doesn't ozonolysis with a reductive workup ever leave a carboxylic acid sitting among its products?
- Ties togetherHow does reading an ozonolysis product backward let a chemist pinpoint exactly where the double bond sat originally?
Your turn1 of 2
2-methyl-2-pentene undergoes ozonolysis with a reductive workup. What are the two carbonyl products?Syn Dihydroxylation
- How it worksHow does osmium tetroxide form a cyclic intermediate that delivers both oxygens to one face in a single step?
- Worked exampleFind the product and its stereochemistry when cyclohexene undergoes dihydroxylation with OsO4.
- Going furtherWhat happens to the product's stereochemistry compared against epoxide ring-opening carried out under basic conditions?
Your turn1 of 2
Cyclohexene undergoes dihydroxylation with OsO4. What is the product and its stereochemistry?