Alkynes

Weld a cracked ship hull or cut through a steel girder and the torch roaring at over 3000 degrees Celsius is very likely burning a gas made from grey rock and water. That gas is ethyne, more commonly called acetylene — the smallest member of the alkyne family and the only alkyne named explicitly in the ICSE Class 10 syllabus. Alkynes are hydrocarbons built around a carbon-carbon triple bond, the tightest, most electron-rich bond ordinary organic chemistry has to offer, and that triple bond is what makes ethyne so eager to react. This article stays tightly inside the syllabus scope: what ethyne is, how it is made from calcium carbide, why its general formula has two fewer hydrogens than an alkane, how it adds hydrogen twice instead of once, and the one laboratory test that picks it out from ethene and ethane in a lineup of three gas jars.

The core insight

Ethyne, $HC\equiv CH$ , is the ICSE-prescribed alkyne, made by dropping water onto calcium carbide, and its carbon-carbon triple bond lets it add hydrogen twice (unlike an alkene's single addition) while giving a unique precipitate test with ammoniacal cuprous chloride that alkenes and alkanes do not give.

The core idea

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. Because a triple bond ties up three of the four bonding positions on each carbon, alkynes carry fewer hydrogens per carbon than alkanes or even alkenes, giving the general formula

C_nH_{2n-2}

where: $n$ is the number of carbon atoms in the chain ( $n \geq 2$ , since a triple bond needs two carbons to sit between). The smallest and only ICSE-relevant member is ethyne, $HC\equiv CH$ ( $C_2H_2$ ), a colourless gas that is almost odourless when pure, though the commercial gas often smells of garlic from trace impurities picked up during manufacture; it is only slightly soluble in water. Ethyne is made in the laboratory from calcium carbide and water, a reaction vigorous enough that water must be added dropwise rather than poured:

CaC_2 + 2H_2O \rightarrow C_2H_2\uparrow + Ca(OH)_2

where: $CaC_2$ is calcium carbide (a grey solid), and the products are ethyne gas (collected by downward displacement of water) and calcium hydroxide, better known as slaked lime. This is classed as a hydrolysis, since water is the reagent that breaks the ionic carbide apart. Chemically, ethyne behaves like an alkene turned up to full volume: every reaction is an addition reaction across the triple bond, but because the triple bond has two pi bonds to break instead of one, ethyne can accept two separate additions where ethene accepts only one. It decolourises bromine water even more readily than an alkene does, confirming its unsaturation, and it adds a hydrogen halide such as HBr to give a haloalkene. The reaction that ICSE singles out as diagnostic, though, is not an addition at all — it is a precipitation test. Because ethyne is a terminal alkyne (its triple-bonded carbons each carry a hydrogen right at the end of the two-carbon chain), it reacts with ammoniacal cuprous chloride to form a reddish precipitate of copper acetylide, and with ammoniacal silver nitrate to form a white-to-grey precipitate of silver acetylide. Neither ethene nor ethane — which have no such terminal triple-bond hydrogen — gives this precipitate, so the test is the definitive way to pick ethyne out of a mixed lineup of hydrocarbon gases.

Figure

Three gas jars of ethyne ethene and ethane tested first with bromine water then with ammoniacal cuprous chloride showing only ethyne giving a reddish precipitate

Derivation AFrom bonding pattern to the alkyne general formula

Show that fixing one carbon-carbon triple bond in an open, unbranched hydrocarbon chain of $n$ carbons forces exactly $2n-2$ hydrogen atoms onto the molecule, which is why every alkyne obeys $C_nH_{2n-2}$ and carries two hydrogens fewer than the corresponding alkene and four fewer than the corresponding alkane.

Step 1. Each carbon atom in a neutral organic molecule always forms exactly four bonds, no more and no less. In a straight-chain hydrocarbon of $n$ carbons joined only by single bonds (an alkane), every carbon uses two of its four bonds to link to its neighbours in the chain (except the two end carbons, which use only one bond each for the chain), leaving the rest of each carbon's bonding capacity free for hydrogen.

Step 2. Counting that free capacity across the whole alkane chain gives $2n + 2$ hydrogen atoms in total — the familiar alkane formula $C_nH_{2n+2}$ . This is the reference point: it is what the chain would look like if every carbon-carbon link were a single bond.

Step 3. Now convert one single bond between two adjacent carbons into a triple bond, which is exactly what defines an alkyne. A triple bond uses three bonding positions on each of the two carbons involved instead of one, so each of those two carbons loses two hydrogens it would otherwise have carried (three bonding slots used for the chain instead of one, so two fewer slots remain for hydrogen). Two carbons losing two hydrogens each removes four hydrogens in total from the alkane count.

Step 4. Subtracting those four hydrogens from the alkane formula gives $2n + 2 - 4 = 2n - 2$ , which is exactly the alkyne general formula. This also shows directly why an alkyne has two fewer hydrogens than the corresponding alkene (which loses only two hydrogens for its one double bond) and four fewer than the corresponding alkane, and why the smallest possible alkyne needs $n = 2$ : with $n = 1$ there is only one carbon, and a triple bond cannot exist between a carbon and itself.

Worked example

Question: 5.0 g of calcium carbide is reacted completely with excess water. Calculate the mass of ethyne gas produced, and describe how you would confirm that the gas collected is indeed ethyne. (Molar masses: $CaC_2 = 64\ \text{g/mol}$ , $C_2H_2 = 26\ \text{g/mol}$ .)

Step 1. Write the balanced equation and read off the mole ratio between calcium carbide and ethyne:

CaC_2 + 2H_2O \rightarrow C_2H_2\uparrow + Ca(OH)_2

One mole of $CaC_2$ produces exactly one mole of $C_2H_2$ , so the mole ratio of carbide to ethyne is $1:1$ .

Step 2. Convert the given mass of calcium carbide into moles using its molar mass:

\text{moles of } CaC_2 = \frac{5.0\ \text{g}}{64\ \text{g/mol}} = 0.078\ \text{mol}

Step 3. Apply the $1:1$ mole ratio to get moles of ethyne, then convert to mass using ethyne's molar mass:

\text{moles of } C_2H_2 = 0.078\ \text{mol}, \qquad \text{mass} = 0.078 \times 26 = 2.03\ \text{g}

Step 4. To confirm the gas is ethyne, pass a sample through bromine water — it should decolourise rapidly, confirming unsaturation — and then pass a fresh sample through ammoniacal cuprous chloride solution, where the appearance of a reddish precipitate of copper acetylide confirms specifically that the unsaturated gas is a terminal alkyne, distinguishing it from ethene, which would decolourise bromine water but give no precipitate with ammoniacal cuprous chloride.

The reaction of 5.0 g of calcium carbide with excess water produces approximately 2.03 g of ethyne, confirmed as ethyne (rather than ethene) by the reddish precipitate it forms with ammoniacal cuprous chloride.

Common traps

Using any carbide or hydrocarbon other than calcium carbide as the starting material for ethyne — the ICSE-prescribed preparation is specifically $CaC_2$ plus water.
Thinking bromine water alone can distinguish ethyne from ethene — both are unsaturated and both decolourise it; only the ammoniacal cuprous or silver test picks out the terminal alkyne.
Writing only one step of hydrogen addition for ethyne — the triple bond lets it add hydrogen twice (first to ethene, then on to ethane), unlike an alkene, which adds hydrogen only once.
Naming the calcium carbide reaction's by-product wrongly — it is calcium hydroxide, $Ca(OH)_2$ , not calcium oxide or calcium carbonate.

Ask the tutor: Want to work through more calcium-carbide stoichiometry problems or practice identifying unknown hydrocarbon gases using both tests in sequence? Ask the AI tutor for guided examples.

Why does ethyne react so much more vigorously than ethene if both have multiple bonds

Ethyne's carbon-carbon triple bond packs one sigma bond and two pi bonds into a very short bond length, concentrating a large amount of electron density into a small region between the two carbons. That crowded, electron-rich triple bond is highly attractive to electron-seeking reagents like bromine, so ethyne tends to react faster and can undergo addition twice over, once to give an alkene-like intermediate and again to reach the fully saturated alkane. At ICSE level, the key point to remember is simply that ethyne's reactions are of the same addition type as an alkene's, just doubled.

Is acetylene the same thing as ethyne, and why does the gas smell like garlic sometimes

Yes, acetylene is just the older common name for ethyne; both refer to the same molecule, $HC\equiv CH$ . Pure ethyne is almost odourless, but the acetylene produced industrially from technical-grade calcium carbide usually carries a garlic-like smell because the carbide itself contains trace impurities (such as compounds of phosphorus and sulfur) that react alongside the carbide and contaminate the gas. The smell is therefore a property of the impure commercial gas, not of pure ethyne itself.

Why does the calcium carbide and water reaction need to be done so carefully

The reaction between calcium carbide and water is strongly exothermic and quite fast, so pouring water onto a large lump of carbide all at once can cause the reaction to run away, spattering hot slaked-lime slurry and releasing ethyne gas too quickly to collect safely. Ethyne is also flammable and can form explosive mixtures with air, so keeping the reaction controlled (adding water dropwise, away from flames) matters both for a clean gas collection and for basic laboratory safety.

Alkynes

The core idea

Worked example

Common traps

Before this

Where this leads