Oxidation, ozonolysis and the reactions of alkynes

Oxidising an alkene

An alkene reacts with oxidising agents at the double bond. What it gives depends on how strong the agent is.

1. Cold, dilute, alkaline potassium permanganate (Baeyer's reagent). The pi bond is used up and two OH groups are added to the two carbon atoms. A diol forms. Ethene gives ethane-1,2-diol.

The purple colour of the permanganate disappears, and a brown precipitate of manganese(IV) oxide appears. This is Baeyer's test for a double bond. It also shows for an alkyne, and for some other compounds.

2. Hot, concentrated or acidified permanganate. Now the double bond is cut. Each carbon of it takes oxygen atoms until it can go no further.

• A carbon with two hydrogen atoms ends as carbon dioxide and water.
• A carbon with one hydrogen atom ends as a carboxylic acid.
• A carbon with no hydrogen atom ends as a ketone.

Propene gives ethanoic acid and carbon dioxide. But-2-ene gives two molecules of ethanoic acid. 2-Methylpropene gives propanone and carbon dioxide.

3. Ozonolysis. Ozone adds to the double bond at low temperature and gives an unstable compound called an ozonide. Zinc and water then split it into two carbonyl compounds. The zinc stops the aldehydes from being oxidised further.

• $\text{CH}_{2}\text{=}$ becomes methanal, $\text{HCHO}$.
• $\text{RCH}\text{=}$ becomes an aldehyde, $\text{RCHO}$.
• $\text{R}_{2}\text{C}\text{=}$ becomes a ketone, $\text{R}_{2}\text{C}\text{=}\text{O}$.

Working back to the alkene

Ozonolysis is how chemists once found the position of a double bond. Remove the oxygen atoms of the two carbonyl compounds and join the two carbon atoms with a double bond. Ethanal and propanone give $\text{CH}_{3}\text{CH}\text{=}\text{C}(\text{CH}_{3})_{2}$, which is 2-methylbut-2-ene.

If only one carbonyl compound forms, the alkene was symmetrical. A ring gives one chain with a carbonyl group at both ends: cyclohexene gives hexanedial, $\text{OHC}(\text{CH}_{2})_{4}\text{CHO}$.

Practise with new numbers

Alkynes: the acidic hydrogen

The hydrogen on a triple-bonded carbon is slightly acidic. The carbon is $\text{sp}$ hybridised, with 50 per cent $s$ character, so its electrons sit nearer the nucleus than in an $\text{sp}^2$ or $\text{sp}^3$ carbon. When the hydrogen leaves as a proton, the lone pair left on carbon is held more tightly and the anion is more stable.

The order of acidity is ethyne $>$ ethene $>$ ethane. The pKa values are about 25, 44 and 50. Ethyne is a much weaker acid than water, though, so it needs a strong base.

• Sodium reacts with ethyne to give sodium ethynide and hydrogen: $2\text{HC}\equiv \text{CH}\ +\ 2\text{Na}\ \longrightarrow \ 2\text{HC}\equiv \text{CNa}\ +\ \text{H}_{2}$.
• Sodium amide, $\text{NaNH}_{2}$, in liquid ammonia also removes the proton.
• Ammoniacal silver nitrate (Tollens' reagent) gives a white precipitate of silver ethynide from ethyne. Other terminal alkynes give the silver salt of the alkyne.
• Ammoniacal copper(I) chloride gives a red precipitate of copper(I) ethynide.

Only a terminal alkyne has a hydrogen on a triple-bonded carbon. So these two precipitates are the tests that tell propyne from but-2-yne, and an alkyne from an alkene. Dry silver and copper ethynides can explode. A teacher destroys them with acid.

Other reactions of alkynes

• Hydrogenation. Nickel, platinum or palladium add two molecules of hydrogen and give the alkane. Lindlar's catalyst stops at the cis alkene. Sodium in liquid ammonia stops at the trans alkene.
• Halogens. Two molecules of bromine add. Ethyne gives 1,2-dibromoethene and then 1,1,2,2-tetrabromoethane. Bromine in tetrachloromethane loses its colour.
• Hydrogen halides. Two molecules add, each by Markovnikov's rule. Propyne with hydrogen bromide gives 2-bromopropene and then 2,2-dibromopropane.
• Hydration. Ethyne, passed into dilute sulphuric acid with mercury(II) sulphate at about $333\ \text{K}$, gives ethanal. The first product is an enol, a compound with OH on a double-bonded carbon. It at once turns into the keto form, with a C=O group. Terminal alkynes other than ethyne give methyl ketones: propyne gives propanone.

Polymerisation

• Ethene makes polythene, by joining thousands of molecules: $n\ \text{CH}_{2}\text{=}\text{CH}_{2} \to (\text{CH}_{2}\text{CH}_{2})\text{n}$. The older method uses high pressure, high temperature and a trace of oxygen or a peroxide. A Ziegler-Natta catalyst, a titanium compound with an aluminium alkyl, gives a denser, stronger polythene at low pressure.
• Propene makes polypropene, and tetrafluoroethene makes Teflon. Chloroethene makes PVC.
• Ethyne passed through a red-hot iron tube at about $873\ \text{K}$ makes benzene by joining three molecules: $3\ \text{C}_{2}\text{H}_{2} \to \text{C}_{6}\text{H}_{6}$. This is the link to the next course.

Try it in Python

The program shows what ozonolysis and hot permanganate make from five alkenes.

Common mistakes

• Using hot permanganate to make a diol. It breaks the double bond. Use cold, dilute, alkaline permanganate for the diol.
• Forgetting that a carbon with no hydrogen gives a ketone. It cannot become an acid.
• Expecting an aldehyde from every alkyne hydration. Only ethyne gives an aldehyde.
• Saying but-2-yne gives a precipitate with Tollens' reagent. It has no terminal hydrogen.
• Stopping addition to an alkyne after one molecule. With excess reagent, two molecules add.

Remember these

• Cold dilute alkaline $\text{KMnO}_{4}$: diol. Hot acidified $\text{KMnO}_{4}$: the bond is cut to acids and ketones.
• Ozonolysis, with zinc and water: $\text{CH}_{2}\text{=}$ gives methanal, $\text{RCH}\text{=}$ an aldehyde, $\text{R}_{2}\text{C}\text{=}$ a ketone.
• The $\text{sp}$ C-H bond is acidic: sodium, sodium amide, ammoniacal silver and copper(I). Only terminal alkynes.
• Alkynes add hydrogen, halogens and hydrogen halides twice. Hydration gives a ketone, or ethanal from ethyne.
• Ethene gives polythene. Three ethyne molecules give benzene.

Check yourself

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

1. What does ethene give with cold, dilute, alkaline permanganate? What do you see?

2. Name the products of the ozonolysis of 3-methylbut-1-ene, followed by zinc and water.

3. An alkene gives only propanone on ozonolysis. Which alkene is it?

4. How can you tell propyne from but-2-yne with one test tube reagent?

5. What does propyne give with water, in the presence of dilute sulphuric acid and mercury(II) sulphate?

References

• OpenStax Organic Chemistry (McMurry), chapter 8, "Epoxidation and Hydroxylation" and "Cleavage to Carbonyl Compounds"; chapter 9, "Hydration of Alkynes", "Reduction of Alkynes" and "Alkyne Acidity".
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons" (oxidation, ozonolysis, alkynes).