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What is the most common preparation of alkenes/alkynes?
- Geminal or vicinal dibromide treated with a strong base (NaNH3 in NH3(l) at -33C).
- This runs what is essentially an E2 rxn and forms the double bond (or triple bond if run again).
-
Alkynes as acids
- R-C=C-H
- H can be depronated by a strong base (stronger than oxybase eg NaNH3/NH3(l)) to beccome the acetylide ion R-C=C:
- Acetylide ion is fantastic base!
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Hydrogenation [alkyne]
H2
----->
Pt
- Alkyne -> Alkene -> Alkane
- Reacts too quickly to isolate alkene
- Requires 2 equivalents of H2
-
Hydrogenation [alkyne]
H2
-------->
Lindlar's catalyst
- Alkyne -> Cis Alkene
- Syn addition
- Cis alkene
- NOTE: can also use Ni2B
-
Dissolving Metal reduction [alkyne]
Na
----->
NH3 (l)
- Alkyne -> trans alkene
- Anti addition
- Trans alkene only
- NOTE - easy cleanup by evaporating solvent
-
Hydrohalogenation [alkyne]
HBr
----->
ROOR
- Addition of H and BR across 3x bond
- Anti-markovnikov
- Both E and Z products
- MECHANISM: radical
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Acid-catalyzed Hydration [alkyne]
H2SO4, H2O
--------------->
HgSO4
- Addition of OH and H across 3x bond
- Markovnikov
- Enol -> keto tautomerization (C=O not C=C)
- No stereochemistry, typically used for terminal alkenes
- NOTE - ideal conditions differ from alkene
-
Hydroboration-oxidation [alkyne]
1)BH3*THF
------------->
2) HOOH, NaOH
- Addition of OH and H across 3x bond
- Anti-markovnikov
- Enol -> keto tautomerization (C=O not C=C)
- NOTE - terminal alkynes are too reactive for BH3 so (sia)2BH or 9-BBH must be used. This converts the terminal alkynes to aldehydes.
-
Halogenation [alkyne]
Br2
------->
CH3COOH, LiBr
- Addition of Br and Br across 3x bond
- Anti addition
- Trans product
- MECHANISM: Bromonium ion
- NOTE - Cl will also work
- NOTE - can be performed again on 2x bond
- NOTE - CH3COOH+LiBr could also be CCl4
-
Hydrohalogenation [alkyne]
HBr
----->
- Addition of Br and H across 3x bond
- Markovnikov
- MECHANISM: carbocation intermediate
- NOTE - Cl will also work
- NOTE - can be performed again on 2x bond
-
Ozonolysis [alkyne]
1) O3
----->
2) H2O
- INTERNAL - Cleavage of triple bond, yields two carboxylic acids
- TERMINAL - Cleavage of triple bond, yields carboxylic acid and CO2
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Alkylation [alkyne]
1) NaNH2
------->
2) RX
Replacement of H with R group
- MECHANISM: removal of acidic H to create acetylide ion. Ion acts as base in SN2 rxn.
- NOTE- immensely important because it can create new c-c bonds!
- NOTE- should be primary alkyl halide (SN2 reaction)
- NOTE- acetylene can do this twice
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