Substitution, dehydration, esterification and oxidation of alcohols

Three places where an alcohol can react

An alcohol has three sites. The O-H bond can break, which gives the acid reactions and the esters. The C-O bond can break, which gives substitution and dehydration. The hydrogen on the OH carbon can be removed, which gives oxidation. Lesson 1 covered the acid reactions. This lesson covers the rest.

Replacing OH by a halogen

With hydrogen halides. An alcohol and a hydrogen halide give a haloalkane and water.

The reactivity order of the alcohols is tertiary, then secondary, then primary. The order of the halides is HI, then HBr, then HCl. The reaction goes through a protonated alcohol. Water leaves, and the halide ion attacks. The more stable the carbocation, the faster the reaction.

Hydrogen chloride alone is slow with primary and secondary alcohols. Anhydrous zinc chloride speeds it up. The mixture of concentrated hydrochloric acid and anhydrous zinc chloride is the Lucas reagent.

The Lucas test. Shake a small amount of the alcohol with the Lucas reagent at room temperature. The alkyl chloride that forms is not soluble in the acid layer, so the mixture turns cloudy.

• Tertiary: cloudy at once.
• Secondary: cloudy in about five minutes.
• Primary: clear at room temperature. It turns cloudy only on warming.

Allylic and benzylic alcohols also react fast, because their carbocations are stable. The test works for alcohols of up to about six carbons that dissolve in the reagent.

With phosphorus halides and thionyl chloride. Phosphorus tribromide gives bromoalkanes: $3 \text{ROH} + \text{PBr}_{3} \to 3 \text{RBr} + \text{H}_{3}\text{PO}_{3}$. Phosphorus pentachloride and thionyl chloride give chloroalkanes. Thionyl chloride is preferred, because the other two products, $\text{SO}_{2}$ and $\text{HCl}$, are gases and leave the mixture: $\text{ROH} + \text{SOCl}_{2} \to \text{RCl} + \text{SO}_{2} + \text{HCl}$.

Dehydration

Heating an alcohol with a strong acid takes out water. Concentrated sulphuric acid or phosphoric acid is used.

The ease of dehydration is the reverse of the order for acidity: tertiary, then secondary, then primary. The conditions in NCERT are about $443\ \text{K}$ for a primary alcohol, $440\ \text{K}$ for a secondary alcohol and $358\ \text{K}$ for a tertiary alcohol. The steps are the same as for the reaction with HX: the alcohol is protonated, water leaves to give a carbocation, and a proton is lost from a neighbouring carbon.

If two alkenes can form, the more substituted one is the main product. This is Saytzeff's rule (also spelled Zaitsev). Butan-2-ol gives mainly but-2-ene and a little but-1-ene.

Temperature decides between two products from ethanol. At $443\ \text{K}$ with excess acid the product is ethene. At $413\ \text{K}$ with excess alcohol the product is ethoxyethane, an ether. Course 3 comes back to this ether.

Esterification

An alcohol and a carboxylic acid, heated with a little concentrated sulphuric acid, give an ester and water.

The ester here is ethyl ethanoate, which smells of fruit. The reaction is reversible, with $K_c$ about 4 for these two. Use an excess of one reagent, or remove the water, to push it towards the ester. An acid chloride or an acid anhydride gives an ester without any such limit, because the reaction is not reversible: $\text{ROH} + (\text{CH}_{3}\text{CO})_{2}\text{O} \to \text{CH}_{3}\text{COOR} + \text{CH}_{3}\text{COOH}$.

With one mole of each, two thirds of the acid becomes ester. With ten moles of alcohol the yield passes $97\ \%$.

Oxidation

Oxidation of an alcohol removes the hydrogen from the OH group and one from the OH carbon. A carbon with no hydrogen on it cannot be oxidised that way.

• Primary alcohol. It gives an aldehyde, and then a carboxylic acid. Pyridinium chlorochromate (PCC) in dichloromethane stops at the aldehyde. Acidified potassium dichromate or potassium permanganate, heated, goes on to the acid.
• Secondary alcohol. It gives a ketone, with any of these agents.
• Tertiary alcohol. It is not oxidised under mild conditions. Under severe conditions the carbon chain breaks, and a mixture of acids with fewer carbons forms.

A hot copper catalyst at $573\ \text{K}$ takes hydrogen from the alcohol: a primary alcohol gives an aldehyde, a secondary alcohol gives a ketone, and a tertiary alcohol gives an alkene.

Orange dichromate turns green when it oxidises a primary or a secondary alcohol, and stays orange with a tertiary one. This is a colour test. Another test uses iodine and sodium hydroxide. Ethanol, and any alcohol with the unit $\text{CH}_{3}\text{CH}(\text{OH})-$ such as propan-2-ol, is first oxidised to a methyl carbonyl compound and gives yellow iodoform, $\text{CHI}_{3}$.

Practise with new numbers

Try it in Python

The program gives the product of PCC and of a strong oxidising agent for one alcohol of each class.

Common mistakes

• Expecting a tertiary alcohol to oxidise to a ketone. It has no hydrogen on the OH carbon.
• Stopping at an aldehyde with hot acidified dichromate. It goes on to the acid, unless the aldehyde is distilled out as it forms.
• Mixing up the Lucas test order. The tertiary alcohol reacts at once, the primary one not at room temperature.
• Forgetting that esterification is reversible. Use excess reagent or remove water.
• Forgetting Saytzeff's rule. The more substituted alkene is the main product of dehydration.

Remember these

• Reactivity with HX and ease of dehydration: tertiary, then secondary, then primary.
• Lucas test: cloudy at once for tertiary, in about five minutes for secondary, and not at room temperature for primary.
• Ethanol with concentrated $\text{H}_{2}\text{SO}_{4}$: ethene at 443 K, ethoxyethane at 413 K.
• Primary alcohol: aldehyde, then acid. Secondary alcohol: ketone. Tertiary alcohol: none.

Check yourself

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

1. Which alcohol gives a cloudy mixture first with the Lucas reagent: butan-1-ol, butan-2-ol or 2-methylpropan-2-ol?

2. What is the main product when butan-2-ol is dehydrated by concentrated sulphuric acid?

3. How can ethanal be obtained from ethanol without going on to ethanoic acid?

4. Why does a larger amount of ethanol improve the yield of ethyl ethanoate?

References

• NCERT Chemistry Part II (Class 12), Chapter 7, the sections on the reactions of alcohols.
• Morrison and Boyd, Organic Chemistry, the chapters on the reactions of alcohols.
• OpenStax Chemistry 2e, Chapter 20, section 20.2 on alcohols and ethers.