Alkenes

What makes a hydrocarbon an alkene. Alkanes are "saturated" — every carbon already holds as many hydrogens as it can. Alkenes are unsaturated: somewhere along the chain, two carbons share a double bond instead of a single one, so each of those two carbons carries one less hydrogen than it would in the saturated chain. That single structural difference is why alkenes have their own general formula, distinct from the alkane series $C_nH_{2n+2}$:

where: $n$ is the number of carbon atoms in the chain ($n \geq 2$, since a double bond needs two carbons to share). The smallest alkene is ethene, $CH_2=CH_2$ (molecular formula $C_2H_4$) — there is no such thing as a one-carbon alkene.

Making ethene in the lab. Two prescribed routes both strip something away from a saturated starting material to create the double bond:

• Dehydration of ethanol — ethanol vapour passed over hot concentrated sulphuric acid (or hot aluminium oxide) loses a water molecule:

Concentrated sulphuric acid here acts purely as a dehydrating agent, pulling $H$ and $OH$ off adjacent carbons.

• Dehydrohalogenation — an alkyl halide such as bromoethane, heated with alcoholic potassium hydroxide, loses a molecule of hydrogen halide:

Why the double bond changes everything. Because it holds extra, exposed electron density, the double bond can "open up" and accept new atoms directly — this is addition, and it is the signature chemistry of every alkene. A saturated alkane has no such weak point, so it can only react by the much harder route of substitution (swapping one atom for another, and usually needing sunlight to do it).

The bromine-water test is the cleanest window into why unsaturation matters. Bromine water is a reddish-brown solution of $Br_2$ dissolved in water. Shake it with ethene and the colour vanishes almost at once, at room temperature, with no catalyst or light needed:

The double bond breaks open, one bond from each carbon reaching out to grab a bromine atom, and the product, 1,2-dibromoethane, is colourless — hence the fading colour. Shake the same bromine water with ethane, a saturated alkane, and nothing happens under these mild conditions: ethane has no double bond to open, so it cannot add $Br_2$ at all. Ethane can eventually react with bromine, but only by substitution under sunlight, a completely different and much slower mechanism, one hydrogen being swapped for one bromine at a time rather than both bromines adding in one step. This contrast — instant decolourisation for an alkene, no reaction in the cold and dark for an alkane — is precisely why bromine water is the standard test for unsaturation, and it is the same reasoning that explains ethene's other addition reactions: with hydrogen (using a nickel catalyst, giving ethane) and with a hydrogen halide such as HBr (giving bromoethane directly). In every case, the double bond is consumed and a single bond takes its place.

Question: A student is handed two colourless gas jars, one containing ethane and the other ethene, with no labels. Describe a simple chemical test to identify which jar is which, giving the relevant equation and explaining why the two gases behave differently.

Step 1. Pass each gas through a test tube of bromine water (a reddish-brown solution) and watch the colour.

Step 2. In the jar containing ethene, the reddish-brown colour disappears rapidly, even at room temperature and without light, because the double bond adds across the bromine molecule:

Step 3. In the jar containing ethane, the bromine water stays reddish-brown under these same mild conditions, because ethane is saturated and has no double bond available for addition; any reaction with bromine would need sunlight and would proceed by substitution instead, which does not happen here.

Step 4. Conclude from the observation. The gas that decolourises bromine water rapidly is ethene; the gas that leaves it unchanged is ethane — the fastest, syllabus-prescribed way to tell a saturated hydrocarbon from an unsaturated one.

Uses of ethene. Beyond being a chemistry-lab curiosity, ethene is genuinely useful: it is the monomer from which polythene is manufactured, it is hydrated back to ethanol as a starting material for other organic chemicals, and it is used commercially as a fruit-ripening agent — the same natural process that softens a banana sealed with a ripe one, now put to deliberate use.

• Calling ethanol-to-ethene a "substitution" — it is a dehydration/elimination: a whole water molecule leaves and a double bond forms, unlike substitution where one atom simply replaces another.
• Trying to write addition reactions (with $H_2$, halogens, or HX) for a saturated alkane like ethane — addition is only possible because of the double bond that alkanes lack.
• Confusing the mild, catalyst-free bromine-water test for alkenes (addition, works instantly in the cold) with the sunlight-requiring halogenation of alkanes (substitution, needs light and heat) — the conditions themselves are the giveaway of the mechanism.
• Assuming the alkene general formula extends to one carbon — a double bond needs two carbon atoms, so there is no "methene."

Ask the tutor: Want to see the mechanism drawn out for why the double bond attacks bromine, or a side-by-side table comparing every alkane versus alkene reaction? Ask the AI tutor to walk you through it.