topic 1

What is oxymercuration and how is it used in alkene chemistry?

Oxymercuration is a two-step reaction that adds water across an alkene using Hg(OAc)₂ and NaBH₄, forming alcohols with Markovnikov regioselectivity.

What is the regioselectivity of oxymercuration?

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

Describe the hydroboration of alkenes.

BH₃ or an organoborane adds across the double bond in a concerted mechanism, giving syn addition of H and B.

What is the mechanism of hydroboration of alkenes?

Concerted asynchronous addition of H and B to the same face of the alkene; arrow-pushing involves alkene π electrons attacking electrophilic boron.

What is the stereochemical outcome of hydroboration of alkenes?

Syn addition (both H and B add to the same face of the double bond).

What is the regiochemistry of hydroboration of alkenes?

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

How are organoboranes converted into alcohols?

Oxidation using H₂O₂ and NaOH retains stereochemistry and converts the C–B bond to a C–OH bond.

What reagents are used to hydrogenate alkenes?

H₂ gas with a metal catalyst such as Pd/C; adds two H atoms across the double bond.

What is the stereochemical outcome of alkene hydrogenation?

Syn addition; both hydrogens add to the same face due to the surface mechanism on the catalyst.

What reagents are used for alkene dihydroxylation?

OsO₄ with H₂O (often with NMO); adds two OH groups across the double bond.

What is the stereochemical outcome of alkene dihydroxylation?

Syn addition (cis diol) via a cyclic osmate ester intermediate.

What reagents cleave alkenes using ozone?

1) O₃, CH₂Cl₂ (–78°C) 2) Reducing agent such as Zn/AcOH or Me₂S.

What are the products of ozonolysis of alkenes?

Aldehydes or ketones, depending on the substitution pattern of the alkene.

What reagent is commonly used for epoxidation of alkenes?

mCPBA (meta-chloroperbenzoic acid), a peracid.

What is the stereochemical outcome of epoxidation?

Syn addition; stereospecific depending on E/Z configuration of the starting alkene.

How are epoxides used to make complex molecules?

Via nucleophilic ring-opening reactions with inversion of configuration; useful in organic synthesis.

What is the mechanism of hydrogen halide addition to alkynes?

Electrophilic addition; the alkyne attacks H⁺ forming a vinyl carbocation, followed by halide attack.

Why is the addition of HX to alkynes inefficient?

Vinyl carbocations are highly unstable due to sp hybridization and poor stabilization.

How is water added to alkynes via mercury-catalysis?

HgSO₄ in H₂SO₄/H₂O adds water across the triple bond; forms an enol that tautomerizes to a ketone.

Describe keto-enol tautomerism in alkyne hydration.

Acid-catalyzed migration of a proton and double bond rearranges the enol to the more stable ketone.

Describe hydroboration of alkynes.

BH₃ adds across the triple bond to the less substituted carbon; oxidation yields an enol, which tautomerizes to an aldehyde.

What is the stereochemical outcome of alkyne hydroboration?

Syn addition forming a cis enol intermediate.

How do organoboranes from alkynes become carbonyl compounds?

Oxidation with H₂O₂/NaOH gives enols that tautomerize to aldehydes or ketones.

What is the typical pKa of a terminal alkyne?

Approximately 25; terminal alkynes are relatively acidic.

Why are terminal alkynes acidic?

Their sp-hybridized carbon has high s-character, stabilizing the negative charge on the conjugate base.

How is alkyne alkylation achieved?

Deprotonate a terminal alkyne with a strong base (e.g., NaNH₂), then react the acetylide anion with a primary alkyl halide.