Preparation of Alkynes: MDCAT Chemistry notes
Preparation of Alkynes for MDCAT: dehydrohalogenation of vicinal and geminal dihalides with alc. KOH and NaNH2, dehalogenation, and CaC2 route.
Main idea
An alkyne has a $\mathrm{C\equiv C}$ triple bond. To make one from a saturated compound we must remove two pairs of atoms from neighbouring carbons. So every laboratory method of preparing alkynes is an elimination reaction, never an addition.
1. Dehydrohalogenation of dihalides
A vicinal dihalide (halogens on adjacent carbons) or a geminal dihalide (both halogens on one carbon) loses two molecules of HX when heated with a strong base.
- Step 1: alcoholic KOH removes the first HX and gives a vinyl halide (haloalkene).
- Step 2: the vinyl halide is less reactive, so a stronger base, sodamide ($\mathrm{NaNH_2}$), removes the second HX.
$$\mathrm{CH_2Br{-}CH_2Br \xrightarrow{alc.\ KOH} CH_2{=}CHBr \xrightarrow{NaNH_2} HC\equiv CH}$$
Key conditions: a strong base and heat. Water-based (aqueous) KOH would instead substitute the halogen with OH.
2. Dehalogenation of tetrahalides
A 1,1,2,2-tetrahaloalkane heated with zinc dust in alcohol loses two molecules of halogen:
$$\mathrm{CHBr_2{-}CHBr_2 + 2Zn \rightarrow HC\equiv CH + 2ZnBr_2}$$
Note the difference: zinc removing halogen from a vicinal dihalide gives only an alkene, because only one bond is formed.
3. Other routes
- Calcium carbide and water (industrial source of acetylene): $\mathrm{CaC_2 + 2H_2O \rightarrow HC\equiv CH + Ca(OH)_2}$.
- Kolbe's electrolysis of the potassium salt of maleic or fumaric acid (a dicarboxylic acid) gives ethyne.
- Higher alkynes: sodium acetylide reacts with an alkyl halide, e.g. $\mathrm{HC\equiv CNa + CH_3I \rightarrow HC\equiv C{-}CH_3 + NaI}$.
Which method gives which product?
| Reaction | Product |
|---|---|
| Vicinal or geminal dihalide + alc. KOH / $\mathrm{NaNH_2}$, heat | Alkyne |
| Tetrahalide + Zn dust | Alkyne |
| Vicinal dihalide + Zn dust | Alkene |
| Alcohol + $\mathrm{Al_2O_3}$ at about 350–450 °C, or conc. $\mathrm{H_2SO_4}$ | Alkene (dehydration) |
| Kolbe electrolysis of a monocarboxylic acid salt, e.g. sodium ethanoate | Alkane |
| Alkyl halide + alc. KOH | Alkene |
Common MDCAT traps
- "Addition" is never part of converting a dihaloalkane to an alkyne; base, heat and elimination are.
- Dehydration of alcohols over $\mathrm{Al_2O_3}$ gives alkenes, not alkynes.
- Kolbe's electrolysis of ordinary carboxylate salts gives alkanes; only unsaturated dicarboxylate salts give ethyne.
- Dehalogenation (Zn) of a vicinal dihalide stops at the alkene; you need a tetrahalide for an alkyne.
- Reducing agents ($\mathrm{LiAlH_4}$, $\mathrm{H_2}$/catalyst) and oxidants ($\mathrm{KMnO_4}$) do not create triple bonds.
Quick revision
- Alkyne preparation = double elimination.
- Reagents: alcoholic KOH, then $\mathrm{NaNH_2}$, with heat.
- Two HX molecules are removed from a dihalide.
- Tetrahalide + Zn gives alkyne; dihalide + Zn gives alkene.
- $\mathrm{CaC_2 + H_2O}$ gives acetylene.