Chemistry · Organic chemistry · Grade 10

Alkenes and alkadienes

Why does ethene decolourise bromine water and ripen tomatoes?

The situation

If you put a green tomato from the market into a paper bag with one ripe banana, it turns red faster. The reason is a colourless gas given off by the fruit, ethylene $C_{2} H_{4}$ : it is a plant hormone. The same gas is used to make polyethylene at the Shurtan gas-chemical complex in Kashkadarya. If you pass ethylene through bromine water, the yellow colour disappears at once. What kind of bond does the ethylene molecule have that lets it “attach” other substances to itself, while methane does not?

Topic

Alkenes and alkadienes. Shurtan gas-chemical complex: polyethylene from ethene; bromine water decolourised and the gas that ripens tomatoes

Key ideas

C=C: σ + π bond
Addition, Markovnikov's rule
Preparing ethene (practical)

Formulas

C_{n} H_{2 n} (n \geq 2)

The homologous series of alkenes: one C=C double bond (σ + π); alkadienes — $C_{n} H_{2 n - 2}$ .

C_{2} H_{5} OH \overset{\overset{}{H_{2} SO_{4} (conc \cdot), 170^{\circ} C}}{\to} CH_{2} = CH_{2} ↑ + H_{2} O

Preparing ethylene in the laboratory (practical 2.9): dehydration of ethanol.

CH_{2} = CH_{2} + Br_{2} \overset{}{\to} CH_{2} Br - CH_{2} Br

Addition: bromine water is decolourised — the qualitative test for a double bond; $n (C_{2} H_{4}) = n (Br_{2})$ .

CH_{2} = CH - CH_{3} + HBr \overset{}{\to} CH_{3} - CHBr - CH_{3}

Markovnikov's rule: hydrogen adds to the carbon with more hydrogens, the halogen to the carbon with fewer.

2 C_{2} H_{5} OH \overset{\overset{}{Al_{2} O_{3}, ZnO, 425^{\circ} C}}{\to} CH_{2} = CH - CH = CH_{2} + 2 H_{2} O + H_{2} ↑

Lebedev process: buta-1,3-diene from ethanol (raw material for synthetic rubber).

Theory

Alkenes: structure and naming (2.7)

Alkenes are unsaturated hydrocarbons whose molecules contain one $C = C$ double bond, with the general formula $C_{n} H_{2 n}$ : ethene (ethylene) $C_{2} H_{4}$ , propene $C_{3} H_{6}$ , butene $C_{4} H_{8}$ … The carbon atoms of the double bond are sp²-hybridised: three hybrid orbitals form σ bonds in one plane at 120° to each other, while the unhybridised p orbitals overlap side by side above and below the plane to give the π bond. So ethylene is a flat molecule, with a C=C length of 134 pm (154 pm for C–C). The π bond is weaker than the σ bond and its electron cloud is exposed — this is where reactions take place. Naming: the longest chain must contain the double bond, numbering starts from the end nearer the double bond, the “-ane” of the alkane name changes to “-ene”, and the number of the double bond is given: $CH_{2} = CH - CH_{2} - CH_{3}$ is but-1-ene, $CH_{3} - CH = CH - CH_{3}$ is but-2-ene. The $CH_{2} = CH^{-}$ group is vinyl.

Isomerism: why cis and trans do not interconvert

Alkenes show four kinds of isomerism: carbon chain (but-1-ene and 2-methylpropene), position of the double bond (but-1-ene and but-2-ene), geometric (cis–trans) and interclass — with cycloalkanes (butene and cyclobutane — both $C_{4} H_{8}$ ). Geometric isomerism is due to the π bond: to rotate about C=C, the overlap of the p orbitals would have to be destroyed — the π bond would have to be broken, which takes roughly 270 kJ/mol. At room temperature molecules do not have this much energy, so cis-but-2-ene (boiling point 3.7 °C) and trans-but-2-ene (0.9 °C) are separate substances. See this for yourself with the twist slider in “Structure” mode.

Preparation; practical 2.9

In the laboratory ethylene is made by heating ethanol with concentrated sulfuric acid at about 170 °C (dehydration); sand or pieces of porous porcelain are put in the flask to even out the boiling. At about 140 °C the main product is different — diethyl ether $C_{2} H_{5} - O - C_{2} H_{5}$ . We pass the gas through bromine water and through $KMnO_{4}$ solution: both are decolourised, which are the qualitative tests for a double bond; at the end of the tube the gas burns with a bright flame (brighter than methane). Ethylene can also be made by treating haloalkanes with an alcoholic solution of alkali. In industry it is made by pyrolysis (cracking) of ethane from petroleum products and natural gas, and by dehydrogenating alkanes: $C_{2} H_{6} \overset{}{\to} C_{2} H_{4} + H_{2}$ . Safety: work with concentrated $H_{2} SO_{4}$ wearing safety goggles and gloves, and add the acid to the alcohol a little at a time, cooling it; while gas is coming off, take the delivery tube out of the liquid before putting out the lamp (otherwise the liquid will be sucked back); ethylene forms an explosive mixture with air — wait for the air to be driven out before lighting it.

Chemical properties and uses (2.8)

Addition reactions are typical of alkenes: hydrogen (over Ni or Pt catalysts — an alkane), halogens (bromine water is decolourised), hydrogen halides and water (with an acid catalyst — an alcohol). When adding to unsymmetrical alkenes, Markovnikov's rule (1869) applies: the hydrogen atom adds to the carbon of the double bond that has more hydrogens — propene with HBr gives mainly 2-bromopropane, because the intermediate secondary carbocation is more stable than the primary one. A neutral solution of potassium permanganate oxidises an alkene to a diol (a brown precipitate of $MnO_{2}$ forms), and alkenes burn. Many molecules join together to form a polymer: polyethylene, polypropylene. Ethanol, ethylene glycol (antifreeze), vinyl chloride and styrene are made from ethylene; it is also used in stores as the plant hormone that ripens fruit.

Alkadienes (2.10–2.11)

Alkadienes contain two double bonds, with the general formula $C_{n} H_{2 n - 2}$ (interclass isomers of the alkynes). Naming: instead of the “-ane” of the alkane name, “-diene”, with the numbers of both double bonds shown: buta-1,3-diene, penta-1,4-diene, 2-methylbuta-1,3-diene. Isomerism: position of the double bonds (penta-1,3-diene and penta-1,4-diene), carbon chain (penta-1,3-diene and isoprene) and interclass — with alkynes (buta-1,3-diene and but-1-yne — both $C_{4} H_{6}$ ). By the arrangement of the double bonds: cumulated (allene $CH_{2} = C = CH_{2}$ ), conjugated — buta-1,3-diene $CH_{2} = CH - CH = CH_{2}$ and isoprene (2-methylbuta-1,3-diene), and isolated — penta-1,4-diene. In conjugated dienes the four p orbitals overlap side by side to form a single π system: the central C2–C3 bond (146 pm) is shorter than an ordinary single bond. For this reason bromine undergoes 1,2- and 1,4-addition; in 1,4-addition the double bond moves to the middle. Preparation: by S. V. Lebedev's method from ethanol (in industry since 1932), and by dehydrogenating butane and isopentane: $C_{4} H_{10} \overset{}{\to} C_{4} H_{6} + 2 H_{2}$ . Uses: synthetic rubber and elastomers, plastics (ABS), butadiene–styrene rubber. Dienes polymerise to give rubber: natural rubber is cis-1,4-polyisoprene; vulcanising rubber with sulfur gives elastic rubber (in detail — in the “Polymers, rubber and fibres” experiment).

Problems and limits of the model (2.23)

Example: how much bromine will 0.56 L (STP) of ethylene add? $n (C_{2} H_{4}) = 0.56 / 22.4 = 0.025$ mol $= n (Br_{2})$ , $m (Br_{2}) = 0.025 \cdot 160 = 4$ g. If the molar mass of an alkene is 56 g/mol: $14 n = 56$ , $n = 4$ — $C_{4} H_{8}$ . Model: gas volumes are reduced to standard conditions (0 °C, 101.325 kPa, $V_{m}$ = 22.4 L/mol); in the test tubes ethylene is assumed to use up the bromine and the permanganate completely and instantly. In reality, in bromine water part of the product is 2-bromoethanol (water also adds), but 1 mol of ethylene still uses 1 mol of $Br_{2}$ . The mixture in the flask darkens on heating and a little $SO_{2}$ and $CO_{2}$ is given off (in careful work the gas is passed through alkali solution) — the model ignores these impurities. The formation of ether or ethylene depending on temperature is shown schematically. In the molecule models the atoms have the real bond lengths and angles, at a scale of 1 Å ≈ 4.5 cm.

Did you know?

That ethylene is a plant hormone was first shown in 1901 by the Russian botanist Dimitri Neljubow: ethylene in lighting gas changed the growth of pea seedlings. Today green bananas and tomatoes that have been picked are ripened in stores in chambers filled with ethylene.
The Shurtan gas-chemical complex in the Guzar district of Kashkadarya region started operating in 2001; its design capacity is 125 thousand tonnes of polyethylene a year — hydrocarbons separated from natural gas are pyrolysed to give ethylene first.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Chemistry in our body — Grade 7
Organic chemistry: structure — Grade 10
Isomers and nomenclature — Grade 10
Alkanes and cycloalkanes — Grade 10
Polymers, rubber and fibres — Grade 10
Alkynes, benzene and styrene — Grade 10
Natural gas, oil and coal — Grade 10
Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
Acids, esters and soap — Grade 10