Alkenes and alkynes: shape, isomers and preparation

The fruit and the lamp

A green banana in a closed bag ripens sooner, because the fruit gives out a gas, ethene, that speeds up ripening. Miners once carried a lamp that burned a gas made by dropping water on grey lumps of calcium carbide. That gas was ethyne. Both are unsaturated hydrocarbons. This lesson is about their shape and how we make them.

Shape and formula

In chapter 8 you saw the shapes of ethene and ethyne. Here is a short reminder.

• An alkene has a C=C bond. Each carbon of the bond is $\text{sp}^2$ hybridised. The bonds around it are flat, at $120^{\circ}$. The double bond is a sigma bond and a pi bond. It is $134\ \text{pm}$ long, shorter than a C-C single bond at $154\ \text{pm}$. The general formula of an open-chain alkene is $\text{C}_n\text{H}_{2n}$.
• An alkyne has a C≡C bond. Each carbon of the bond is $\text{sp}$ hybridised, and the atoms joined to it lie on a line, at $180^{\circ}$. The triple bond is one sigma and two pi bonds, and it is $120\ \text{pm}$ long. The formula is $\text{C}_n\text{H}_{2n-2}$.

Each pi bond or ring takes away two hydrogen atoms from the alkane formula. The number of rings and pi bonds in a molecule of carbon and hydrogen is

For $\text{C}_{4}\text{H}_{8}$ it is $\dfrac{2 \times 4 + 2 - 8}{2} = 1$: one double bond or one ring.

Geometrical isomerism

A pi bond is made by the sideways overlap of two $p$ orbitals. If one end of the bond turned, the overlap would be lost. So the two ends cannot turn against each other. Groups on the two carbon atoms keep their positions, and this gives geometrical isomers.

Take but-2-ene. In cis-but-2-ene the two $\text{CH}_{3}$ groups are on the same side of the double bond. In trans-but-2-ene they are on opposite sides.

Geometrical isomers can exist only when each carbon of the double bond carries two different groups. But-1-ene, $\text{CH}_{2}\text{=}\text{CHCH}_{2}\text{CH}_{3}$, has no isomers of this kind, because one carbon has two hydrogen atoms.

When the four groups are all different, cis and trans are not enough. The E and Z system, used in A-Level and IB, gives each group a priority by the atomic number of its first atom. If the two groups of higher priority are on the same side, the isomer is Z (from German zusammen, together). If they are on opposite sides, it is E (entgegen, opposite). For $\text{CHCl}\text{=}\text{CHBr}$, bromine beats chlorine, so the isomer with Br and Cl on the same side is Z.

Properties of cis and trans isomers

• Boiling point. The cis isomer is slightly polar, because the two bond dipoles do not cancel. The trans isomer is nearly nonpolar. So cis boils higher: $3.7\ ^{\circ}\text{C}$ against $0.9\ ^{\circ}\text{C}$ for but-2-ene.
• Melting point. The trans isomer is more symmetrical and packs better in a crystal, so it melts higher: $-105.5\ ^{\circ}\text{C}$ against $-138.9\ ^{\circ}\text{C}$.
• Stability. The trans isomer is more stable, because its two methyl groups are far apart. In the cis isomer they crowd each other.

Which alkene is more stable

We can compare alkenes by adding hydrogen to them. Isomeric alkenes that make the same alkane end at the same energy. The one that gives out less heat started lower, so it is more stable.

The more alkyl groups on the C=C carbon atoms, the more stable the alkene. Alkyl groups push electrons into the pi bond, and the C-H bonds next to it can hyperconjugate, as you saw in chapter 8.

Practise with new numbers

Making alkenes

1. Partial hydrogenation of an alkyne. The catalyst decides the shape of the product.

• Hydrogen with Lindlar's catalyst (palladium on calcium carbonate, poisoned with quinoline or a sulphur compound) gives the cis alkene.
• Sodium in liquid ammonia gives the trans alkene.
• Hydrogen with ordinary palladium on carbon does not stop. It goes on to the alkane.

2. Dehydrohalogenation of an alkyl halide. A solution of potassium hydroxide in alcohol, heated with an alkyl halide, removes a hydrogen halide. This is an elimination.

Aqueous potassium hydroxide does something else: it replaces the halogen by OH and makes an alcohol. The solvent decides.

When the halide has more than one kind of neighbouring hydrogen, Saytzeff's rule (also spelled Zaitsev) says the more substituted alkene is the main product. 2-Bromobutane gives mostly but-2-ene, about 80 per cent, and a little but-1-ene.

3. Dehydration of an alcohol. Concentrated sulphuric acid at about $443\ \text{K}$ removes water from ethanol.

At a lower temperature, about $413\ \text{K}$, the same reagents make an ether instead. Tertiary alcohols dehydrate most easily, then secondary, then primary. The Saytzeff rule applies here too.

4. Dehalogenation of a vicinal dihalide. Two halogen atoms on neighbouring carbon atoms are removed by zinc dust.

Making alkynes

1. From calcium carbide. Calcium carbide is made by heating calcium oxide with coke in an electric furnace. Water then gives ethyne.

2. From a vicinal dihalide. Alcoholic potassium hydroxide removes one hydrogen halide and makes a vinyl halide. The second is harder to remove, so a stronger base, sodium amide, $\text{NaNH}_{2}$, is used.

Try it in Python

The program counts the geometrical isomers of open-chain polyenes. It tries every pattern of E and Z, and it counts a pair once when the molecule can be read from either end.

Common mistakes

• Expecting cis-trans isomers in but-1-ene. One carbon of its double bond has two hydrogen atoms.
• Using aqueous potassium hydroxide to make an alkene. It gives an alcohol. The solvent must be alcohol.
• Forgetting which catalyst gives which isomer. Lindlar gives cis. Sodium in ammonia gives trans.
• Assuming the trans isomer boils higher. It usually melts higher. The cis isomer boils higher.
• Writing a dehydration at the wrong temperature. Ethanol gives ethene at $443\ \text{K}$ and an ether at $413\ \text{K}$.

Remember these

• Alkene: $\text{C}_n\text{H}_{2n}$, $\text{sp}^2$, flat. Alkyne: $\text{C}_n\text{H}_{2n-2}$, $\text{sp}$, linear.
• Cis-trans isomers need two different groups on each carbon of the double bond. Trans is more stable.
• More alkyl groups on the double bond means a more stable alkene.
• Alkenes: Lindlar (cis), sodium and ammonia (trans), alcoholic KOH (Saytzeff), $\text{H}_{2}\text{SO}_{4}$ at $443\ \text{K}$, zinc on a vicinal dihalide.
• Ethyne: calcium carbide and water, or two eliminations from a vicinal dihalide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Does pent-2-ene show geometrical isomerism? Does pent-1-ene?

2. Which reagent turns but-2-yne into cis-but-2-ene?

3. What is the main product when 2-bromopentane is heated with alcoholic potassium hydroxide?

4. Why does the cis isomer of but-2-ene boil higher than the trans isomer?

5. How many rings and double bonds does a molecule of $\text{C}_{6}\text{H}_{10}$ have in all?

References

• OpenStax Chemistry 2e, chapter 20, section 20.1 "Hydrocarbons" (alkenes and alkynes).
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons" (alkenes and alkynes: nomenclature, structure, isomerism, preparation).
• OpenStax Organic Chemistry (McMurry), chapter 7, the sections "Alkene Stereochemistry and the E,Z Designation" and "Stability of Alkenes" (heats of hydrogenation).