alkyne reactions

Preparation of alkynes: mechanism

Two eliminations (halogen and LG); aqueous workup to reprotonate alkynide ion

Preparation of alkynes: intermediate

alkynide ion

Preparation of alkynes: reagents

3 equivalents of strong base (NaNH2, NH3)

Preparation of terminal alkynes: reagents

1.) xs NaNH2 / NH3

2.) H2O

Preparation of alkynes: results

Alkyl halide/dihalide → alkyne

alkyl halide

R-X

Catalytic hydrogenation: stereochemistry

syn addition

Catalytic hydrogenation: mechanism

2 equivalents of H2 per pi bond

Catalytic hydrogenation: reagents

H2, Lindlar's catalyst/Ni2B

Catalytic hydrogenation with poisoned catalyst results

Alkyne → cis alkene

Catalytic hydrogenation (without poisoned catalyst) results

Alkyne → alkane

Catalytic hydrogenation: intermediate

alkene that cannot be isolated

Dissolving metal reduction: stereochemistry

anti addition (trans alkene)

Dissolving metal reduction: reagents

Na, NH3

Dissolving metal reduction: results

Internal alkyne → trans alkene

Hydrohalogenation: regiochemistry

Markovnikov

Hydrohalogenation: stereochemistry

racemic; proceeds via more stable, secondary vinylic carbocation OR termolecular

termolecular

hydrohalogenation; no vinylic carbocation; positive charge on more substituted position

Hydrohalogenation: mechanism

1st equ: trp bond attacks H, sigma bond attacks X (PT); halogen anion attacks pos charge on intermediate (NA)

2nd equ: alkene attacks HX; anion attacks pos charge

OR transition state

Hydrohalogenation: reagents

HX

xs HX

Hydrohalogenation: 1 equivalent results

Alkyne → halogen on more substituted C on alkene

Hydrohalogenation: excess reagent results

2 halogen on same C on alkane (geminal dihalide)

Hydrohalogenation: intermediate

vinylic carbocation

Hydrohalogenation: HBr + peroxides regiochemistry

anti-Markovnikov

Hydrohalogenation: HBr + peroxides stereochemistry

E and Z isomers

Hydrohalogenation: HBr + peroxides mechanism

radical

Hydrohalogenation: HBr + peroxides reagents

HBr, ROOR

Hydrohalogenation: HBr + peroxides results

Alkyne → Br terminal position

Acid-catalyzed hydration: regiochemistry

Markovnikov

Acid-catalyzed hydration: stereochemistry

No control

Acid-catalyzed hydration: intermediate

enol

Acid-catalyzed hydration: reagents

H2SO4, H2O, HgSO4

Acid-catalyzed hydration: results

Alkyne → ketone

Acid-catalyzed hydration used only for what?

terminal and symmetrical alkynes

Hydroboration-oxidation / base-catalyzed tautomerization: regiochemistry

anti-Markovnikov

Hydroboration-oxidation / base-catalyzed tautomerization: stereochemistry

syn addition

Hydroboration-oxidation / base-catalyzed tautomerization: intermediate

enolate ion (resonance-stabilized anion; cannot be isolated)

Hydroboration-oxidation / base-catalyzed tautomerization: results

Alkyne → aldehyde

Hydroboration-oxidation / base-catalyzed tautomerization: reagents

1.) BH3·THF or 9-BBN

2.) H2O2, NaOH

halogenation: regiochemistry

markovnikov

halogenation: stereochemistry

anti addition/trans isomer; E isomer with one equivalent; achiral

halogenation: xs reagents

xs X2

halogenation: one equivalent reagents

X2, CCl4

halogenation: two equivalents results

tetrahalide

halogenation: one equivalent results

2 halogen trans alkene (dihalide)

halogenation: XS reagent results

4 halogen on alkane

ozonolysis: reagents

1.) O3

2.) H2O

ozonolysis: internal alkyne results

2 carboxylic acids

ozonolysis: terminal alkyne results

1 carboxylic acid, 1 carbon dioxide

alkylation: mechanism

Forming C-C bonds (alkyl halide electrophile)

1.) terminal alkyne deprotonated by strong base

2.) deprotonated alkyne attacks carbon of methyl or alkyl halide (NA)

3.) sigma bond to halogen (LLG)

alkylation: reagents

1.) NaNH2

2.) XI

alkylation: intermediate

alkynide ion

alkylation: results

Elongated carbon chain; installation of alkyl group; two terminal protons = alkylation twice (separate)

alkylation restriction

methyl or primary alkyl halides only

dialkylation: reagents

1.) NaNH2

2.) Me/EtI

dialkylation: results

Addition of two alkyl groups to each side

mechanisms need to know with arrows

hydrohalogenation (3), alkylation, preparation of alkyne, HBO, ACH

Mechanisms with regiochemistry

ACH, HBO, hydrohalogenation, halogenation

Mechanisms with syn stereochemistry

HBO, catalytic hydrogenation

Mechanisms with anti stereochemistry

halogenation, dissolving metal reduction

stepwise mechanisms

HBO, ozonolysis, preparation of terminal alkyne, dialkylation alkyne, alkylation