Acidity of Alkynes: MDCAT Chemistry notes
Acidity of Alkynes for MDCAT: why terminal alkynes are acidic, sp hybridization, silver and copper acetylide tests, and telling 1-alkynes from 2-alkynes.
Why a terminal alkyne is acidic
A hydrogen attached to a triply bonded carbon ($\equiv$C–H) is weakly acidic. Only terminal alkynes (1-alkynes such as ethyne, propyne, 1-butyne, 1-pentyne) have this hydrogen. Internal alkynes such as 2-butyne and 2-pentyne have no H on an sp carbon, so they show no acidic reactions.
The reason is hybridization. The more s-character an orbital has, the closer its electrons are held to the nucleus. An sp carbon is therefore the most electronegative kind of carbon. It pulls the C–H bonding pair towards itself, the bond becomes polar, and the hydrogen can be removed as $\mathrm{H^+}$. The resulting acetylide ion ($\mathrm{RC\equiv C^-}$) is fairly stable because its lone pair sits in an sp orbital close to the nucleus.
| Hydrocarbon | Carbon hybridization | % s-character | Acidity of C–H |
|---|---|---|---|
| Alkane (ethane), cycloalkane | sp$^3$ | 25% | Lowest |
| Alkene (ethene) | sp$^2$ | 33% | Intermediate |
| Terminal alkyne (ethyne) | sp | 50% | Highest |
So the acidity order is alkyne > alkene > alkane. Even so, ethyne is a very weak acid, weaker than water and alcohols. It turns no litmus red and needs a strong base or a reactive metal to lose its proton.
Reactions that show the acidity
With sodium or sodamide
Ethyne reacts with sodium metal to give sodium acetylide and hydrogen gas, and with sodamide ($\mathrm{NaNH_2}$) in liquid ammonia to give sodium acetylide and ammonia:
$$\mathrm{HC\equiv CH + NaNH_2 \rightarrow HC\equiv C^-Na^+ + NH_3}$$
Sodium acetylides react with alkyl halides to make higher alkynes, which is a way of lengthening the carbon chain.
With ammoniacal silver nitrate
When ethyne is passed through ammoniacal silver nitrate ($\mathrm{AgNO_3 + NH_4OH}$, i.e. Tollens' reagent), a white precipitate of silver acetylide forms:
$$\mathrm{HC\equiv CH + 2[Ag(NH_3)_2]^+ \rightarrow AgC\equiv CAg\downarrow + 2NH_4^+ + 2NH_3}$$
With ammoniacal cuprous chloride
Ammoniacal $\mathrm{Cu_2Cl_2}$ gives a red (red-brown) precipitate of copper(I) acetylide, $\mathrm{CuC\equiv CCu}$.
These metal acetylides are explosive when dry. Warming them with dilute mineral acid gives back the alkyne, so the tests are also used to purify terminal alkynes.
Using the tests to distinguish compounds
- 1-butyne vs 2-butyne, 1-pentyne vs 2-pentyne: only the 1-alkyne gives a precipitate with ammoniacal $\mathrm{AgNO_3}$ or $\mathrm{Cu_2Cl_2}$.
- Alkyne vs alkene: ethene and butenes have no acidic hydrogen, so they give no precipitate.
- Bromine water, $\mathrm{Br_2/CCl_4}$ and cold alkaline $\mathrm{KMnO_4}$ (Baeyer's reagent) react with all alkenes and alkynes, so they cannot tell a terminal alkyne from an internal one. Acidified dichromate cannot either.
Common MDCAT traps
- "Terminal alkynes are acidic because of hydrogen bonding" is wrong; the cause is the sp hybridized carbon.
- Silver acetylide is white; copper(I) acetylide is red. Do not swap the colours.
- 2-butyne and 2-pentyne are alkynes but give no acetylide, because they lack a $\equiv$C–H.
- Cycloalkanes behave like alkanes (sp$^3$ carbon), so they are not more acidic than alkynes.
- Ethylene, "methylene" and "vinylene" give no precipitate; only acetylene (ethyne) does.
Quick revision
- s-character: sp 50%, sp$^2$ 33%, sp$^3$ 25%.
- Acidity of C–H: alkyne > alkene > alkane.
- Only terminal alkynes (R–C$\equiv$C–H) are acidic.
- Ammoniacal $\mathrm{AgNO_3}$ gives white $\mathrm{AgC\equiv CAg}$.
- Ammoniacal $\mathrm{Cu_2Cl_2}$ gives red $\mathrm{CuC\equiv CCu}$.
- Ethyne + $\mathrm{NaNH_2}$ gives sodium acetylide + $\mathrm{NH_3}$.