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How does oxymercuration add water across an alkene?

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Oxymercuration adds water in two steps: electrophilic addition of Hg(OAc)₂ and water, followed by reduction with NaBH₄. It results in Markovnikov addition of OH (to the more substituted carbon) and H (to the less substituted carbon).

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What is the regioselectivity of oxymercuration?

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OH adds to the more substituted carbon due to formation of the more stable carbocation-like intermediate.

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26 Terms

1
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How does oxymercuration add water across an alkene?

Oxymercuration adds water in two steps: electrophilic addition of Hg(OAc)₂ and water, followed by reduction with NaBH₄. It results in Markovnikov addition of OH (to the more substituted carbon) and H (to the less substituted carbon).

2
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What is the regioselectivity of oxymercuration?

OH adds to the more substituted carbon due to formation of the more stable carbocation-like intermediate.

3
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What is the stereochemical outcome of hydroboration of alkenes?

Syn addition: both H and B add to the same face of the alkene.

4
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What is the regioselectivity of hydroboration?

Boron adds to the less substituted carbon due to steric and electronic effects.

5
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Describe the mechanism of hydroboration of alkenes.

It is a concerted, asynchronous addition where C–B and C–H bonds form simultaneously.

6
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How are organoboranes converted into alcohols?

By oxidation using H₂O₂ and NaOH, which retains configuration and yields alcohols from organoboranes.

7
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What reagents are used to hydrogenate alkenes?

H₂ gas with a Pd/C (or Pt/Ni) catalyst.

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What is the stereochemical outcome of alkene hydrogenation?

Syn addition of hydrogen; both hydrogens add to the same face.

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What reagents are used to dihydroxylate alkenes?

OsO₄ with H₂O (and often NMO as co-oxidant) in acetone or tert-butanol.

10
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What is the stereochemical outcome of alkene dihydroxylation?

Syn addition of two OH groups forming a cis-1,2-diol.

11
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What are the reagents and conditions for ozonolysis of alkenes?

  1. O₃ in CH₂Cl₂ at –78°C; 2. Reductive workup (e.g. Zn/H₂O, Me₂S, or Ph₃P).
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What products are formed from ozonolysis of alkenes?

Cleavage of the double bond yielding aldehydes or ketones (carboxylic acids under oxidative workup).

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How can alkenes be epoxidised?

Using peracids like mCPBA.

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What is the stereochemical outcome of epoxidation?

Syn addition; epoxide is formed with both atoms on the same face.

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How do epoxides react with nucleophiles?

Via SN2-type attack with inversion of configuration at the attacked carbon.

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Why are epoxides valuable in synthesis?

They can be ring-opened to create trans-1,2-disubstituted compounds with high stereocontrol.

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What is the mechanism for addition of HX to alkynes?

Electrophilic addition forming a vinyl carbocation intermediate.

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Why is the addition of hydrogen halides to alkynes generally poor?

Vinyl carbocations are highly unstable and hard to stabilize.

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How is water added to alkynes via mercury catalysis?

Using Hg(OAc)₂, H₂O, and H₂SO₄. The initial enol tautomerizes to a ketone.

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Describe the tautomerisation of enols.

Acid-catalysed proton transfer and shift of double bond convert enol to ketone.

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What is the stereochemical and regiochemical outcome of alkyne hydroboration?

Boron adds to the less substituted carbon; syn addition gives cis product. Oxidation yields an enol, which tautomerises to an aldehyde.

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What do organoboranes derived from alkynes form upon oxidation?

Enols, which tautomerise to carbonyl compounds (typically aldehydes).

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What is the typical pKa of a terminal alkyne?

Around 25.

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Why are alkynes relatively acidic?

sp-hybridised carbon has more s-character, making it more electronegative and stabilizing the conjugate base.

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How are alkynes synthesized using acetylide anions?

Terminal alkynes are deprotonated with strong bases (e.g. NaNH₂), then reacted with alkyl halides to form new C–C bonds.

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