Chapter 9 · Hydrocarbons

9.3 Alkynes

Alkynes, $\mathrm{C_{n}H_{2n-2}}$, contain a C≡C triple bond — one σ and two π bonds on a straight line. How they are named and made (from calcium carbide and from vicinal dihalides), why the hydrogen of a terminal alkyne is acidic, how alkynes add reagents twice, and how three ethyne molecules join into benzene.

Drop a lump of grey calcium carbide into water and it fizzes, giving off a gas with a garlic-like smell (from impurities) that burns with a brilliant, sooty flame. The gas is ethyne — still widely called acetylene. Miners' and fishermen's lamps once burned it; welders still do, because ethyne burning in pure oxygen reaches about $3100\ ^{\circ}\mathrm{C}$, hot enough to cut steel. The same reaction has a darker use: carbide is sometimes illegally put among mangoes and bananas to ripen them, and the arsenic and phosphorus compounds it contains make that practice dangerous enough to be banned under India's food-safety rules.

Ethyne is the simplest alkyne. Its triple bond is shorter and stronger than a double bond, yet even more reactive, because it holds two exposed π bonds. This section covers the structure, naming and isomerism of alkynes; their preparation from calcium carbide and from vicinal dihalides; the unusual acidity of a hydrogen atom on a triple-bonded carbon; the addition of hydrogen, halogens, hydrogen halides and water — usually twice; and the cyclic polymerisation of ethyne to benzene, which leads into Section 9.4.

Structure, Naming and Isomerism

📘 Definition 9.11 — Alkynes and the C≡C bond
An alkyne is an unsaturated hydrocarbon containing a carbon–carbon triple bond; with one triple bond and no ring the general formula is $\mathrm{C_{n}H_{2n-2}}$ (ethyne $\mathrm{C_{2}H_{2}}$, propyne $\mathrm{C_{3}H_{4}}$, …). Each triple-bonded carbon is sp hybridised: its two sp orbitals point in opposite directions and form two σ bonds at $180^{\circ}$, so the two carbons and the atoms attached to them lie on one straight line. The two unhybridised p orbitals on each carbon ($\mathrm{p_{y}}$ and $\mathrm{p_{z}}$) overlap sideways to form two π bonds at right angles to each other; together their electron clouds form a cylinder around the C–C axis. So C≡C is one σ + two π bonds, $120\ \mathrm{pm}$ long, with bond enthalpy $839\ \mathrm{kJ\,mol^{-1}}$ (Section 4.1) — shorter and stronger than C=C ($134\ \mathrm{pm}$, $614$) and C–C ($154\ \mathrm{pm}$, $348$).

Alkynes are named with the ending -yne, numbering the chain to give the triple bond the lowest locant: $\mathrm{HC{\equiv}CCH_{2}CH_{3}}$ but-1-yne, $\mathrm{CH_{3}C{\equiv}CCH_{3}}$ but-2-yne. They show position isomerism (but-1-yne and but-2-yne) and, from five carbons, chain isomerism (pent-1-yne and 3-methylbut-1-yne). Because the groups on a triple bond lie on a straight line, there is no "same side" or "opposite side": alkynes cannot show geometrical isomerism. An alkyne with the H atom on a triple-bonded carbon ($\mathrm{R{-}C{\equiv}C{-}H}$) is a terminal alkyne; one with the triple bond inside the chain ($\mathrm{R{-}C{\equiv}C{-}R'}$) is an internal alkyne.

🧭 Reading unsaturation from a molecular formula
An open-chain saturated hydrocarbon has $2n+2$ H atoms. Every ring or C=C bond removes 2 H atoms, and every C≡C removes 4. So for a hydrocarbon $\mathrm{C_{n}H_{m}}$ count the "missing pairs" $\tfrac12(2n+2-m)$: $\mathrm{C_{4}H_{6}}$ is missing $\tfrac12(10-6)=2$ pairs — one C≡C (but-1-yne, but-2-yne), or two C=C (buta-1,3-diene), or a ring and a C=C (cyclobutene). Alkynes are therefore isomeric with dienes and cycloalkenes (functional isomers).
✏️ Example 1 — Isomers of alkynes
(a) Draw and name the alkynes of formula $\mathrm{C_{5}H_{8}}$. Which are terminal alkynes? (b) How many alkynes have the formula $\mathrm{C_{6}H_{10}}$? Name them. (c) Explain why hex-3-yne has no cis–trans isomers although hex-3-ene does.
Rule A triple-bonded carbon can carry only one more group, so the triple bond must join two carbons each with at most one other carbon neighbour — never at a branch point.
(a) $\mathrm{HC{\equiv}CCH_{2}CH_{2}CH_{3}}$ pent-1-yne; $\mathrm{CH_{3}C{\equiv}CCH_{2}CH_{3}}$ pent-2-yne; $\mathrm{HC{\equiv}CCH(CH_{3})_{2}}$ 3-methylbut-1-yne. Terminal: pent-1-yne and 3-methylbut-1-yne. (On the 2,2-dimethylpropane skeleton no triple bond is possible.)
(b) 7: hex-1-yne, hex-2-yne, hex-3-yne, 4-methylpent-1-yne, 3-methylpent-1-yne, 4-methylpent-2-yne and 3,3-dimethylbut-1-yne.
(c) Around the triple bond the arrangement $\mathrm{C{-}C{\equiv}C{-}C}$ is linear; each triple-bonded carbon carries only one group, so there is no second group to place on the same or the opposite side.

Preparing Alkynes

📘 Definition 9.12 — Two preparations of alkynes
  • From calcium carbide (industrial ethyne). Limestone is heated to quicklime, which is heated with coke in an electric furnace to give calcium carbide; water then hydrolyses the carbide: $$\mathrm{CaCO_{3}(s)\xrightarrow{\Delta}CaO(s)+CO_{2}(g)}$$ $$\mathrm{CaO(s)+3C(s)\xrightarrow{\Delta}CaC_{2}(s)+CO(g)}$$ $$\begin{aligned}&\mathrm{CaC_{2}(s)+2H_{2}O(l)}\\&\quad\mathrm{\rightarrow Ca(OH)_{2}(aq)+C_{2}H_{2}(g)}\end{aligned}$$ Calcium carbide is ionic, $\mathrm{Ca^{2+}[C{\equiv}C]^{2-}}$; the carbide ion takes two protons from water.
  • From vicinal dihalides by a double dehydrohalogenation. Alcoholic KOH removes the first molecule of HX, giving a vinylic halide (halogen on a C=C carbon); a stronger base, sodamide $\mathrm{NaNH_{2}}$, is needed to remove the second: $$\begin{aligned}&\mathrm{BrCH_{2}CH_{2}Br+KOH(alc)}\\&\quad\mathrm{\rightarrow CH_{2}{=}CHBr+KBr+H_{2}O}\end{aligned}$$ $$\begin{aligned}&\mathrm{CH_{2}{=}CHBr+NaNH_{2}}\\&\quad\mathrm{\rightarrow HC{\equiv}CH+NaBr+NH_{3}}\end{aligned}$$
✏️ Example 2 — Ethyne from carbide and from a dihalide
(a) A $1.00\ \mathrm{kg}$ sample of commercial calcium carbide contains $80.0\%$ $\mathrm{CaC_{2}}$ by mass. What mass of ethyne can it give with excess water? (b) What mass of coke (carbon) is consumed in making $1.00\ \mathrm{kg}$ of pure $\mathrm{CaC_{2}}$? (c) Name the alkyne formed when 1,2-dibromopropane is treated first with alcoholic KOH and then with sodamide, and name the intermediate(s). ($M$: $\mathrm{CaC_{2}}$ $64.10$, $\mathrm{C_{2}H_{2}}$ $26.04$, C $12.01\ \mathrm{g\,mol^{-1}}$)
(a) $m(\mathrm{CaC_{2}})=800\ \mathrm{g}$; $n=800/64.10=12.48\ \mathrm{mol}$. One mole of carbide gives one mole of ethyne: $12.48\times26.04=325\ \mathrm{g}$.
(b) $n(\mathrm{CaC_{2}})=1000/64.10=15.60\ \mathrm{mol}$; $\mathrm{CaO+3C\rightarrow CaC_{2}+CO}$ needs 3 mol C per mol: $46.80\ \mathrm{mol}\times12.01=562\ \mathrm{g}$ of carbon (two-thirds of it ends up in the carbide, one-third leaves as CO).
(c) $\mathrm{CH_{3}CHBrCH_{2}Br}$ loses HBr to give a bromopropene — 1-bromoprop-1-ene $\mathrm{CH_{3}CH{=}CHBr}$ or 2-bromopropene $\mathrm{CH_{3}CBr{=}CH_{2}}$ — and $\mathrm{NaNH_{2}}$ then removes a second HBr from either to give propyne, $\mathrm{CH_{3}C{\equiv}CH}$.
Check Atom balance of (a): $\mathrm{CaC_{2}+2H_{2}O}$: Ca 1, C 2, H 4, O 2 = $\mathrm{Ca(OH)_{2}+C_{2}H_{2}}$ ✓.

Acidic Character of Terminal Alkynes

Physical properties. Alkynes are non-polar, so like alkanes and alkenes they are insoluble in water and soluble in organic solvents; ethyne, propyne and but-1-yne are gases at room temperature, and boiling points rise with molar mass. Ethyne itself has a faint, pleasant smell when pure; the smell of carbide gas comes from impurities.

One property sets terminal alkynes apart from all other hydrocarbons: the hydrogen on a triple-bonded carbon can be removed as $\mathrm{H^{+}}$ by a strong base.

📐 Rule 9.9 — Acidity of terminal alkynes
A hydrogen atom attached to an sp carbon is weakly acidic. An sp orbital has 50% s character (sp² 33%, sp³ 25%); s electrons are held closer to the nucleus, so an sp carbon is the most electronegative of the three. It pulls the C–H bonding pair towards itself, and the carbanion left after losing $\mathrm{H^{+}}$ keeps its lone pair in a low-energy orbital close to the nucleus. Hence $$\mathrm{HC{\equiv}CH\gt H_{2}C{=}CH_{2}\gt H_{3}C{-}CH_{3}}$$ (order of acid strength). Terminal alkynes react with sodium metal or sodamide to form sodium alkynides (acetylides), with $\mathrm{H_{2}}$ or $\mathrm{NH_{3}}$: $$\mathrm{HC{\equiv}CH+Na\rightarrow HC{\equiv}C^{-}Na^{+}+\tfrac12H_{2}}$$ $$\begin{aligned}&\mathrm{CH_{3}C{\equiv}CH+NaNH_{2}}\\&\quad\mathrm{\rightarrow CH_{3}C{\equiv}C^{-}Na^{+}+NH_{3}}\end{aligned}$$ Alkenes and alkanes do not react, and neither do internal alkynes such as but-2-yne, which have no H on an sp carbon. Ethyne is still a much weaker acid than water: alkynides are decomposed by water, $\mathrm{HC{\equiv}C^{-}Na^{+}+H_{2}O\rightarrow HC{\equiv}CH+NaOH}$, and ethyne does not turn litmus red.
✏️ Example 3 — Which compounds react with sodamide?
(a) Which of these react with sodamide: ethyne, ethene, propyne, but-2-yne, but-1-yne? Write equations for those that do. (b) Arrange ethane, ethene and ethyne in order of increasing acidity and explain with the s character of carbon. (c) Why can sodium ethynide not be prepared in aqueous solution?
(a) Only terminal alkynes: ethyne, propyne and but-1-yne. $\mathrm{HC{\equiv}CH+NaNH_{2}\rightarrow HC{\equiv}CNa+NH_{3}}$; $\mathrm{CH_{3}C{\equiv}CH+NaNH_{2}\rightarrow CH_{3}C{\equiv}CNa+NH_{3}}$; $\mathrm{CH_{3}CH_{2}C{\equiv}CH+NaNH_{2}\rightarrow CH_{3}CH_{2}C{\equiv}CNa+NH_{3}}$. Ethene (sp² C–H) and but-2-yne (no ≡C–H) do not react.
(b) Ethane (sp³, 25% s) $\lt$ ethene (sp², 33%) $\lt$ ethyne (sp, 50%). More s character → more electronegative carbon → the C–H pair is held more tightly by carbon and the anion formed is more stable.
(c) Water is a stronger acid than ethyne, so it gives $\mathrm{H^{+}}$ back to the ethynide ion: $\mathrm{HC{\equiv}C^{-}+H_{2}O\rightarrow HC{\equiv}CH+OH^{-}}$. The salt must be made with a base stronger than $\mathrm{OH^{-}}$ (such as $\mathrm{NH_{2}^{-}}$) in the absence of water, e.g. in liquid ammonia.
Interactive 1 — s Character and the Acidity of C–H Bonds

Choose a hydrocarbon and a base, or press ▶ Play. The left panel shows the hybrid orbital that holds the C–H bond, its share of s character and how close its electron pair sits to the carbon nucleus. The right panel is an acidity ladder of approximate $\mathrm{p}K_{a}$ values (lower = stronger acid): a base can remove $\mathrm{H^{+}}$ from an acid only if the acid lies below the base's own conjugate acid on the ladder.

C–H carbon—
approx. pKa—
base's conjugate acid—
reaction?—

Addition Reactions and Cyclic Polymerisation

A triple bond can add two molecules of a reagent, one for each π bond. The first addition gives a substituted alkene; the second gives a saturated product. The π electrons of an alkyne are held more tightly by the sp carbons than those of an alkene, so alkynes are in fact somewhat less reactive than alkenes towards electrophiles, though both stages occur readily.

📘 Definition 9.13 — Addition reactions of alkynes
  • Hydrogen: over Pt, Pd or Ni, two moles add, $\mathrm{HC{\equiv}CH+2H_{2}\rightarrow CH_{3}CH_{3}}$; with Lindlar's catalyst the reaction stops at the cis alkene (Section 9.2).
  • Halogens: bromine adds twice, and its reddish-orange colour disappears (test for unsaturation): $$\begin{aligned}&\mathrm{HC{\equiv}CH\xrightarrow{Br_{2}}CHBr{=}CHBr}\\&\quad\mathrm{\xrightarrow{Br_{2}}CHBr_{2}CHBr_{2}}\end{aligned}$$ (1,2-dibromoethene, then 1,1,2,2-tetrabromoethane).
  • Hydrogen halides (order $\mathrm{HI\gt HBr\gt HCl}$) add twice, both times by Markovnikov's rule, so both halogen atoms end up on the same carbon (a geminal dihalide): $$\begin{aligned}&\mathrm{HC{\equiv}CH\xrightarrow{HBr}CH_{2}{=}CHBr}\\&\quad\mathrm{\xrightarrow{HBr}CH_{3}CHBr_{2}}\end{aligned}$$ $$\begin{aligned}&\mathrm{CH_{3}C{\equiv}CH\xrightarrow{HBr}CH_{3}CBr{=}CH_{2}}\\&\quad\mathrm{\xrightarrow{HBr}CH_{3}CBr_{2}CH_{3}}\end{aligned}$$
  • Water adds once, when the alkyne is warmed with dilute sulphuric acid and mercury(II) sulphate at about $333\ \mathrm{K}$. The first product is an enol (OH on a C=C carbon), which immediately rearranges to the carbonyl compound: $$\begin{aligned}&\mathrm{HC{\equiv}CH+H_{2}O\xrightarrow[333\,\text{K}]{\mathrm{Hg^{2+}/H^{+}}}[CH_{2}{=}CHOH]}\\&\quad\mathrm{\rightarrow CH_{3}CHO}\end{aligned}$$ Ethyne gives ethanal; other alkynes give ketones, e.g. propyne → propanone ($\mathrm{CH_{3}COCH_{3}}$), by Markovnikov addition.
📘 Definition 9.14 — Cyclic polymerisation of ethyne
When ethyne is passed through a red-hot iron tube at about $873\ \mathrm{K}$, three molecules join into a six-membered ring — benzene: $$\mathrm{3HC{\equiv}CH\xrightarrow[873\,\text{K}]{\text{red-hot Fe tube}}C_{6}H_{6}}$$ This is how an aliphatic compound is converted into an aromatic one (Section 9.4). Ethyne also burns, $\mathrm{2C_{2}H_{2}(g)+5O_{2}(g)\rightarrow 4CO_{2}(g)+2H_{2}O(l)}$, with $\Delta_{c}H^{\circ}=-1299.5\ \mathrm{kJ\,mol^{-1}}$ (Section 5.3); in oxygen this gives the very hot oxy-acetylene flame.
✏️ Example 4 — Products of addition
Give the product(s) and name them: (a) propyne + 2 mol HBr; (b) propyne + water, $\mathrm{Hg^{2+}/H^{+}}$; (c) but-2-yne + water, $\mathrm{Hg^{2+}/H^{+}}$; (d) propyne + 2 mol $\mathrm{Br_{2}}$; (e) ethyne + 1 mol HCl (the monomer of PVC); (f) but-2-yne + $\mathrm{H_{2}}$/Lindlar, then $\mathrm{Br_{2}}$.
(a) Markovnikov twice: $\mathrm{CH_{3}CBr{=}CH_{2}}$ (2-bromopropene), then $\mathrm{CH_{3}CBr_{2}CH_{3}}$, 2,2-dibromopropane.
(b) Enol $\mathrm{CH_{3}C(OH){=}CH_{2}}$ → $\mathrm{CH_{3}COCH_{3}}$, propanone.
(c) Symmetrical alkyne: $\mathrm{CH_{3}C(OH){=}CHCH_{3}}$ → $\mathrm{CH_{3}COCH_{2}CH_{3}}$, butanone.
(d) $\mathrm{CH_{3}CBr{=}CHBr}$ (1,2-dibromopropene), then $\mathrm{CH_{3}CBr_{2}CHBr_{2}}$, 1,1,2,2-tetrabromopropane.
(e) $\mathrm{HC{\equiv}CH+HCl\rightarrow CH_{2}{=}CHCl}$, chloroethene (vinyl chloride).
(f) cis-But-2-ene, then $\mathrm{CH_{3}CHBrCHBrCH_{3}}$, 2,3-dibromobutane.
✏️ Example 5 — An oxy-acetylene torch
A welding cylinder contains $5.00\ \mathrm{kg}$ of ethyne. (a) Write the balanced equation for its complete combustion and check the atom balance. (b) What mass of oxygen is needed to burn it all? (c) How much heat is released? (d) Why does ethyne burn with a sootier flame in air than ethane does? ($M$: $\mathrm{C_{2}H_{2}}$ $26.04$, $\mathrm{O_{2}}$ $32.00\ \mathrm{g\,mol^{-1}}$; $\Delta_{c}H^{\circ}=-1299.5\ \mathrm{kJ\,mol^{-1}}$)
(a) $\mathrm{2C_{2}H_{2}(g)+5O_{2}(g)\rightarrow 4CO_{2}(g)+2H_{2}O(l)}$: C $4=4$, H $4=4$, O $10=8+2$ ✓.
(b) $n=5000/26.04=192.0\ \mathrm{mol}$; $\mathrm{O_{2}}$: $2.5\times192.0=480.0\ \mathrm{mol}$, $\times32.00=1.54\times10^{4}\ \mathrm{g}=15.4\ \mathrm{kg}$.
(c) $192.0\times1299.5=2.50\times10^{5}\ \mathrm{kJ}$ ($250\ \mathrm{MJ}$; $49.9\ \mathrm{kJ\,g^{-1}}$).
(d) Ethyne has a high carbon content ($92.3\%$ C by mass, against $79.9\%$ for ethane); in a limited supply of air not all the carbon is oxidised, and glowing particles of soot make the flame bright and smoky.
✏️ Example 6 — A route from limestone to benzene and ethanal
Show how ethanal and benzene can each be made starting from limestone, coke and water, naming each reagent. What mass of benzene could, at most, be made from $13.0\ \mathrm{g}$ of ethyne? ($M$: $\mathrm{C_{6}H_{6}}$ $78.11\ \mathrm{g\,mol^{-1}}$)
Ethyne $\mathrm{CaCO_{3}\xrightarrow{\Delta}CaO}$; $\mathrm{CaO+3C\rightarrow CaC_{2}+CO}$ (electric furnace); $\mathrm{CaC_{2}+2H_{2}O\rightarrow C_{2}H_{2}+Ca(OH)_{2}}$.
Ethanal Ethyne + water with dilute $\mathrm{H_{2}SO_{4}}$/$\mathrm{HgSO_{4}}$ at $333\ \mathrm{K}$ → $\mathrm{CH_{3}CHO}$.
Benzene Ethyne through a red-hot iron tube at $873\ \mathrm{K}$: $\mathrm{3C_{2}H_{2}\rightarrow C_{6}H_{6}}$.
Mass $n(\mathrm{C_{2}H_{2}})=13.0/26.04=0.4992\ \mathrm{mol}$ → $0.1664\ \mathrm{mol}$ benzene → $0.1664\times78.11=13.0\ \mathrm{g}$. All the atoms end up in the product, so the mass is unchanged ($3\times26.04=78.12\approx78.11$).
Interactive 2 — One π Bond at a Time: Addition to Alkynes

Choose an alkyne and a reagent and step through the slider (or press ▶ Play). Step 1 uses up the first π bond and gives an alkene; step 2 uses up the second (or, for water, turns the enol into a carbonyl compound). The bar shows how many π bonds remain, and the readouts name each product.

start—
after step 1—
after step 2—
π bonds left—
🚀 Beyond the board syllabus (JEE Advanced · JEE Main · NEET) — tests for terminal alkynes and building longer chains

Not required by the CBSE syllabus. JEE Advanced lists metal acetylides; JEE Main and NEET list the acidic character of alkynes, which these reactions show.

Terminal alkynes give insoluble metal alkynides with ammoniacal silver nitrate (Tollens' reagent) — a white precipitate of $\mathrm{AgC{\equiv}CAg}$ from ethyne — and with ammoniacal copper(I) chloride — a red precipitate of $\mathrm{CuC{\equiv}CCu}$. Internal alkynes such as but-2-yne give neither, so these tests distinguish but-1-yne from but-2-yne. Sodium alkynides are also useful nucleophiles: $\mathrm{HC{\equiv}C^{-}Na^{+}+CH_{3}Br\rightarrow HC{\equiv}CCH_{3}+NaBr}$ adds a carbon chain to ethyne, a standard step in synthesis problems (works well only with primary alkyl halides).

Exercises

I
Structure, naming and isomerism. Atomic masses: C $12.01$, H $1.008$.
  • 1 Which of these formulae could belong to an open-chain alkyne: $\mathrm{C_{3}H_{4}}$, $\mathrm{C_{4}H_{8}}$, $\mathrm{C_{5}H_{8}}$, $\mathrm{C_{6}H_{14}}$, $\mathrm{C_{7}H_{12}}$?
  • 2 Describe the bonding in ethyne: the hybridisation of carbon, the number and type of bonds, and the shape.
  • 3 Name: (a) $\mathrm{CH_{3}C{\equiv}CCH_{2}CH_{3}}$; (b) $\mathrm{HC{\equiv}CCH_{2}CH(CH_{3})_{2}}$; (c) $\mathrm{CH_{3}C{\equiv}CC(CH_{3})_{3}}$; (d) $\mathrm{HC{\equiv}CC(CH_{3})_{3}}$.
  • 4 Write condensed formulae for but-1-yne and 3-methylpent-1-yne. Which carbon atoms are sp hybridised?
  • 5 Why does pent-2-yne not show geometrical isomerism?
  • 6 Arrange C–C, C=C and C≡C in order of increasing bond length and of increasing bond enthalpy, with values.
  • 7 Find the molar mass and the percentage of carbon by mass in hex-1-yne.
  • 8 Write two compounds that are isomers of but-1-yne but are not alkynes.
1 $\mathrm{C_{n}H_{2n-2}}$: $\mathrm{C_{3}H_{4}}$, $\mathrm{C_{5}H_{8}}$, $\mathrm{C_{7}H_{12}}$. ($\mathrm{C_{4}H_{8}}$ is $\mathrm{C_{n}H_{2n}}$; $\mathrm{C_{6}H_{14}}$ is an alkane.)
2 Each C is sp: one sp–s σ bond to H and one sp–sp σ bond to the other C, plus two π bonds from sideways overlap of $\mathrm{p_{y}}$–$\mathrm{p_{y}}$ and $\mathrm{p_{z}}$–$\mathrm{p_{z}}$. Three σ and two π bonds in all; linear molecule, $180^{\circ}$.
3 (a) pent-2-yne; (b) 4-methylpent-1-yne; (c) 4,4-dimethylpent-2-yne; (d) 3,3-dimethylbut-1-yne.
4 $\mathrm{HC{\equiv}CCH_{2}CH_{3}}$; $\mathrm{HC{\equiv}CCH(CH_{3})CH_{2}CH_{3}}$. In each, C1 and C2 (the triple-bonded carbons) are sp; the rest are sp³.
5 $\mathrm{C{-}C{\equiv}C{-}C}$ is linear and each triple-bonded carbon carries only one group, so there is no cis/trans arrangement.
6 Length: C≡C $120$ $\lt$ C=C $134$ $\lt$ C–C $154\ \mathrm{pm}$. Enthalpy: C–C $348$ $\lt$ C=C $614$ $\lt$ C≡C $839\ \mathrm{kJ\,mol^{-1}}$.
7 $\mathrm{C_{6}H_{10}}$: $6(12.01)+10(1.008)=82.14\ \mathrm{g\,mol^{-1}}$; $\%\mathrm{C}=72.06/82.14\times100=87.7\%$.
8 For example buta-1,3-diene $\mathrm{CH_{2}{=}CHCH{=}CH_{2}}$ and cyclobutene (or buta-1,2-diene), all $\mathrm{C_{4}H_{6}}$.
II
Preparation. $M$: $\mathrm{CaC_{2}}$ $64.10$, $\mathrm{C_{2}H_{2}}$ $26.04$, $\mathrm{Ca(OH)_{2}}$ $74.09\ \mathrm{g\,mol^{-1}}$.
  • 1 Write the equation for the action of water on calcium carbide.
  • 2 How is calcium carbide made from quicklime? Write the equation.
  • 3 Give the reagents, in order, for converting 1,2-dibromoethane into ethyne, and write both equations.
  • 4 Name the alkyne obtained from 1,2-dibromobutane by this method.
  • 5 Name the alkyne obtained from 2,3-dibromobutane by this method.
  • 6 What mass of pure calcium carbide is needed to make $52.1\ \mathrm{g}$ of ethyne?
  • 7 Why is sodamide, rather than alcoholic KOH, used for the second elimination?
  • 8 What mass of calcium hydroxide forms when $6.41\ \mathrm{g}$ of $\mathrm{CaC_{2}}$ reacts completely with water?
1 $\mathrm{CaC_{2}(s)+2H_{2}O(l)\rightarrow Ca(OH)_{2}(aq)+C_{2}H_{2}(g)}$.
2 By heating it with coke in an electric furnace: $\mathrm{CaO+3C\rightarrow CaC_{2}+CO}$.
3 Alcoholic KOH, then $\mathrm{NaNH_{2}}$: $\mathrm{BrCH_{2}CH_{2}Br+KOH\rightarrow CH_{2}{=}CHBr+KBr+H_{2}O}$; $\mathrm{CH_{2}{=}CHBr+NaNH_{2}\rightarrow HC{\equiv}CH+NaBr+NH_{3}}$.
4 The two Br are on C1 and C2, so the triple bond forms there: but-1-yne, $\mathrm{HC{\equiv}CCH_{2}CH_{3}}$.
5 Br on C2 and C3: but-2-yne, $\mathrm{CH_{3}C{\equiv}CCH_{3}}$.
6 $n=52.1/26.04=2.001\ \mathrm{mol}$; $\mathrm{CaC_{2}}$: $2.001\times64.10=128\ \mathrm{g}$.
7 A halogen on a C=C carbon (a vinylic halide) is held much more firmly, and removing HX from it needs a much stronger base than $\mathrm{OH^{-}}$; the amide ion $\mathrm{NH_{2}^{-}}$ is such a base.
8 $n=6.41/64.10=0.1000\ \mathrm{mol}$ → $0.1000\ \mathrm{mol}$ $\mathrm{Ca(OH)_{2}}$ → $7.41\ \mathrm{g}$.
III
Acidity of terminal alkynes. $M(\mathrm{H_{2}})=2.016\ \mathrm{g\,mol^{-1}}$.
  • 1 Explain why the hydrogen atoms of ethyne are acidic while those of ethene are not.
  • 2 Arrange ethane, ethene and ethyne in order of increasing acidic strength.
  • 3 Which of these react with sodium metal: but-1-yne, but-2-yne, propene, propyne?
  • 4 Write the equation for the reaction of propyne with sodamide and name the product.
  • 5 What mass of hydrogen is evolved when $2.60\ \mathrm{g}$ of ethyne reacts with excess sodium to give monosodium ethynide?
  • 6 Is ethyne a stronger or weaker acid than water? Give one piece of evidence.
  • 7 Why does greater s character make a carbon atom more electronegative?
  • 8 An alkyne $\mathrm{C_{5}H_{8}}$ reacts with sodamide, and on complete hydrogenation gives 2-methylbutane. Identify it.
1 In ethyne the H is on an sp carbon (50% s character), which is more electronegative than the sp² carbon of ethene (33%); the C–H bond is more polar and the ethynide ion formed is more stable, with its lone pair held close to the nucleus.
2 Ethane $\lt$ ethene $\lt$ ethyne.
3 But-1-yne and propyne (terminal alkynes) only.
4 $\mathrm{CH_{3}C{\equiv}CH+NaNH_{2}\rightarrow CH_{3}C{\equiv}C^{-}Na^{+}+NH_{3}}$: sodium propynide.
5 $\mathrm{HC{\equiv}CH+Na\rightarrow HC{\equiv}CNa+\tfrac12H_{2}}$. $n=2.60/26.04=0.0998\ \mathrm{mol}$ → $0.0499\ \mathrm{mol}$ $\mathrm{H_{2}}$ → $0.101\ \mathrm{g}$.
6 Weaker: water decomposes sodium ethynide, $\mathrm{HC{\equiv}CNa+H_{2}O\rightarrow HC{\equiv}CH+NaOH}$ (the stronger acid gives its proton to the conjugate base of the weaker one). Ethyne also has no effect on litmus.
7 An s orbital is held closer to the nucleus than a p orbital, so electrons in a hybrid with more s character are, on average, nearer the nucleus and more strongly attracted by it.
8 It must be terminal and have the 2-methylbutane skeleton: 3-methylbut-1-yne, $\mathrm{HC{\equiv}CCH(CH_{3})_{2}}$.
IV
Addition reactions and polymerisation. $M$: $\mathrm{C_{2}H_{2}}$ $26.04$, $\mathrm{Br_{2}}$ $159.80\ \mathrm{g\,mol^{-1}}$.
  • 1 Give the product of ethyne with excess hydrogen over nickel.
  • 2 Give the two products formed in turn when ethyne reacts with bromine, and name them.
  • 3 Name the final product of propyne with excess HCl.
  • 4 Give the product of hydration ($\mathrm{Hg^{2+}/H^{+}}$) of (a) ethyne, (b) but-1-yne.
  • 5 Under what conditions does ethyne form benzene? Write the equation.
  • 6 Use Markovnikov's rule to explain why HBr adds to bromoethene to give 1,1-dibromoethane, not 1,2-dibromoethane.
  • 7 What mass of bromine is needed to convert $2.60\ \mathrm{g}$ of ethyne completely into 1,1,2,2-tetrabromoethane?
  • 8 Give one chemical test that distinguishes ethyne from ethene, with the observations.
1 $\mathrm{HC{\equiv}CH+2H_{2}\xrightarrow{\text{Ni}}CH_{3}CH_{3}}$: ethane.
2 $\mathrm{CHBr{=}CHBr}$ (1,2-dibromoethene), then $\mathrm{CHBr_{2}CHBr_{2}}$ (1,1,2,2-tetrabromoethane).
3 $\mathrm{CH_{3}CCl_{2}CH_{3}}$, 2,2-dichloropropane (via 2-chloropropene).
4 (a) Ethanal, $\mathrm{CH_{3}CHO}$. (b) Butanone, $\mathrm{CH_{3}COCH_{2}CH_{3}}$ (OH goes to C2 by Markovnikov's rule; the enol then rearranges).
5 Passed through a red-hot iron tube at $873\ \mathrm{K}$: $\mathrm{3C_{2}H_{2}\rightarrow C_{6}H_{6}}$.
6 In $\mathrm{CH_{2}{=}CHBr}$ the $\mathrm{CH_{2}}$ carbon has more H atoms, so H goes there and Br goes to the carbon that has fewer H — the one already carrying Br: $\mathrm{CH_{3}CHBr_{2}}$.
7 Two moles of $\mathrm{Br_{2}}$ per mole: $0.0998\times2\times159.80=31.9\ \mathrm{g}$.
8 Add sodium metal (or sodamide) in an inert solvent: ethyne reacts (with sodium, bubbles of $\mathrm{H_{2}}$), ethene does not. Both decolourise bromine, so that test does not distinguish them.
🎯 Exam corner BoardJEE MainJEE AdvancedNEET

E1. NEET The most acidic of these hydrocarbons is (A) ethane (B) ethene (C) ethyne (D) propane

Idea Acidity follows the s character of the carbon carrying H: sp (50%) $\gt$ sp² (33%) $\gt$ sp³ (25%).
Answer (C).

E2. JEE Main Propyne is warmed with dilute sulphuric acid containing mercury(II) sulphate. The product is (A) propanal (B) propanone (C) propan-1-ol (D) propan-2-ol

Markovnikov OH adds to C2: enol $\mathrm{CH_{3}C(OH){=}CH_{2}}$.
Rearrangement The enol changes at once into the ketone $\mathrm{CH_{3}COCH_{3}}$. Alcohols are not formed: only one water molecule adds and the product is not reduced.
Answer (B).

E3. Board · 3 marks Describe, with equations, how ethyne is prepared (a) from calcium carbide, (b) from 1,2-dibromoethane. Why is a stronger base needed in the second step of (b)?

1 mark $\mathrm{CaC_{2}(s)+2H_{2}O(l)\rightarrow Ca(OH)_{2}(aq)+C_{2}H_{2}(g)}$.
1 mark $\mathrm{BrCH_{2}CH_{2}Br+KOH(alc)\rightarrow CH_{2}{=}CHBr+KBr+H_{2}O}$; $\mathrm{CH_{2}{=}CHBr+NaNH_{2}\rightarrow HC{\equiv}CH+NaBr+NH_{3}}$.
1 mark The Br in bromoethene is on a C=C carbon and is held firmly, so a very strong base ($\mathrm{NH_{2}^{-}}$) is needed to remove HBr.

E4. JEE Main Numerical answer. $2.60\ \mathrm{g}$ of ethyne is completely hydrogenated to ethane. The mass of hydrogen used, in mg, to the nearest integer, is ____. ($M$: $\mathrm{C_{2}H_{2}}$ $26.04$, $\mathrm{H_{2}}$ $2.016\ \mathrm{g\,mol^{-1}}$)

Equation $\mathrm{C_{2}H_{2}+2H_{2}\rightarrow C_{2}H_{6}}$: two moles of $\mathrm{H_{2}}$ per mole.
Amounts $n(\mathrm{C_{2}H_{2}})=2.60/26.04=0.09985\ \mathrm{mol}$; $\mathrm{H_{2}}$: $0.1997\ \mathrm{mol}\times2.016=0.4026\ \mathrm{g}$.
Answer 403.

E5. JEE Advanced One or more options correct. (A) But-2-yne reacts with sodamide to form a sodium alkynide. (B) Propyne with excess HBr gives 2,2-dibromopropane. (C) But-2-yne with $\mathrm{H_{2}}$ and Lindlar's catalyst gives trans-but-2-ene. (D) The C≡C bond is shorter than the C=C bond.

(A) False: but-2-yne has no H on an sp carbon.
(B) True: Markovnikov twice puts both Br on C2.
(C) False: Lindlar gives the cis alkene; Na in liquid $\mathrm{NH_{3}}$ gives trans.
(D) True: $120$ against $134\ \mathrm{pm}$.
Answer (B), (D).

E6. Board · Case study Some traders ripen mangoes with calcium carbide wrapped in paper; moisture in the air slowly releases a gas. (i) Write the equation for the release of the gas and name it. (1 mark) (ii) The gas is a weak acid while ethene is not. Explain. (2 marks) (iii) Name the product when this gas is passed into warm dilute $\mathrm{H_{2}SO_{4}}$ containing $\mathrm{HgSO_{4}}$. (1 mark)

(i) $\mathrm{CaC_{2}(s)+2H_{2}O(l)\rightarrow Ca(OH)_{2}(s)+C_{2}H_{2}(g)}$: ethyne (acetylene).
(ii) The H atoms of ethyne are on sp carbons with 50% s character (ethene: sp², 33%). The sp carbon is more electronegative, so the C–H bond is more polar and the ion $\mathrm{HC{\equiv}C^{-}}$ is relatively stable; strong bases such as $\mathrm{NaNH_{2}}$ remove $\mathrm{H^{+}}$.
(iii) Ethanal, $\mathrm{CH_{3}CHO}$.
🔬 Research–Teaching NexusResearch-tutored · optional, about 30 minutes

Research question. Why do food regulators in India, Bangladesh, Pakistan and elsewhere ban calcium carbide for ripening fruit, and what safer alternatives exist?

Why it matters. Mangoes and bananas are picked unripe for transport. Fruits ripen naturally in response to ethene; ethyne from carbide imitates it weakly. Industrial carbide also contains calcium phosphide and arsenic compounds, which release toxic phosphine and arsine with water. Ripening chambers that use a controlled trace of ethene are the approved alternative.

What to do.

  1. Write balanced equations for the hydrolysis of $\mathrm{CaC_{2}}$ and of calcium phosphide, $\mathrm{Ca_{3}P_{2}}$, which gives $\mathrm{Ca(OH)_{2}}$ and phosphine $\mathrm{PH_{3}}$. Check the atom balance.
  2. A trader uses $1.00\ \mathrm{kg}$ of carbide that is $85.0\%$ $\mathrm{CaC_{2}}$ and $0.10\%$ $\mathrm{Ca_{3}P_{2}}$ by mass. Calculate the masses of ethyne and of phosphine released.
  3. Find the regulatory limit on carbide use in your country and the recommended ethene concentration (in ppm) in a ripening chamber.
  4. Explain, using the structures of ethene and ethyne, why a plant receptor that binds ethene might respond only weakly to ethyne.

Tools: Definition 9.12, molar masses (Section 1.2), a food-safety authority's guidance on artificial ripening.

Hand in: Balanced equations, the two masses, and a one-page leaflet for fruit sellers explaining the risks and the alternative.

Going further. Ethephon, a liquid that slowly releases ethene in fruit, is the approved ripening agent in many countries. Find its formula and write the equation for its breakdown.

Instructor note: $\mathrm{CaC_{2}+2H_{2}O\rightarrow Ca(OH)_{2}+C_{2}H_{2}}$; $\mathrm{Ca_{3}P_{2}+6H_{2}O\rightarrow 3Ca(OH)_{2}+2PH_{3}}$. $850\ \mathrm{g}/64.10=13.26\ \mathrm{mol}$ → $345\ \mathrm{g}$ ethyne; $1.00\ \mathrm{g}/182.18=5.49\times10^{-3}\ \mathrm{mol}$ $\mathrm{Ca_{3}P_{2}}$ → $1.098\times10^{-2}\ \mathrm{mol}$ $\mathrm{PH_{3}}$ → $0.373\ \mathrm{g}$. Typical ripening-chamber ethene: about $100\ \mathrm{ppm}$ for about $24\ \mathrm{h}$. Ethephon $\mathrm{ClCH_{2}CH_{2}PO_{3}H_{2}}$ breaks down to ethene, chloride and phosphate in the plant.

🧪 Self-check quiz

0/5 correct

Q1. In ethyne, each carbon atom is

Q2. Which compound does not react with sodamide?

Q3. Ethyne warmed with dilute $\mathrm{H_{2}SO_{4}}$ and $\mathrm{HgSO_{4}}$ gives

Q4. Ethyne reacts with excess HBr to give

Q5. Ethyne passed through a red-hot iron tube at about $873\ \mathrm{K}$ gives

← 9.2 Alkenes9.4 Aromatic Hydrocarbons →