Loading the lesson
Loading the lesson
Search AllRounder.ai
Search the courses, subjects, tracks, games and features, or jump straight to a page.
Search the courses, subjects, tracks, games and features, or jump straight to a page.
Learn it your way
A short class with Sarah and her students. The other ways are under More.
Sign up free to listen. Transcripts are free to read.
Drop a small piece of sodium into ethanol. What do you see?
Bubbles of hydrogen, and sodium ethoxide is left in solution.
So ethanol is acting as an acid. Which is more acidic, a primary or a tertiary alcohol?
Primary. Alkyl groups push electrons towards oxygen and make the alkoxide ion less stable.
Right. And compared with water?
Alcohols are weaker acids than water, because an alkoxide ion takes a proton back from water.
In the Lucas test we add concentrated HCl and zinc chloride. What makes the solution cloudy?
The alkyl chloride that forms. It does not dissolve, so we see turbidity.
2-Methylpropan-2-ol turns cloudy at once. What would you expect from propan-2-ol?
Turbidity in about five minutes, because it is secondary.
And from ethanol?
No turbidity at room temperature. Here the order is the reverse of acidity: 3° > 2° > 1°.
Warm propan-2-ol with acidified potassium dichromate. What happens?
The orange colour turns green and propanone forms.
And with 2-methylpropan-2-ol?
Nothing under these conditions. A tertiary alcohol has no hydrogen on the carbon carrying the –OH group.
Good. Over hot copper, what does the tertiary alcohol do instead?
It loses water and forms 2-methylpropene.
alcohols react with sodium to give hydrogen and alkoxides, and with acids to give esters; acidity is 1° > 2° > 3°.
reactions with HX, PCl₃, PCl₅ and SOCl₂ give haloalkanes; with HX the reactivity is 3° > 2° > 1°.
ethanol with concentrated H₂SO₄ gives ethene at 443 K and ethoxyethane at 413 K.
1° alcohols give aldehydes and then acids, 2° alcohols give ketones, 3° alcohols resist mild oxidation.
1° gives an aldehyde, 2° a ketone and 3° an alkene.
given by ethanol and CH₃CH(OH)– alcohols, not by methanol.
2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂: hydrogen gas shows the acidic nature of ethanol.
With Lucas reagent, 2-methylpropan-2-ol turns cloudy at once, propan-2-ol in about five minutes and propan-1-ol not at all at room temperature.
Propan-2-ol warmed with acidified K₂Cr₂O₇ gives propanone, and the orange solution turns green.
Acid strength falls from one to three; Lucas speed climbs from one to three.
Three alcohol brothers met hot copper at 573 K. The primary one became an aldehyde, the secondary one a ketone, and the tertiary one, having no hydrogen to give up from its carbon, lost water instead and became an alkene.
P-A-C, S-K, T-No - Primary gives Aldehyde then Carboxylic acid, Secondary gives Ketone, Tertiary says No to mild oxidation.
E.E. at 4-4-3 and 4-1-3 - Ethene at 443 K, Ether at 413 K.
Term
What are the products of ethanol and sodium?
Click to flip
Term
What does ethanol give with concentrated sulphuric acid at 443 K?
Click to flip
Term
Which class of alcohol gives turbidity at once with Lucas reagent?
Click to flip
Term
What does a secondary alcohol give on oxidation with acidified dichromate?
Click to flip
Term
Which test distinguishes ethanol from methanol?
Click to flip
The anion RO⁻ formed when an alcohol loses the hydrogen of its –OH group, as in sodium ethoxide.
The reaction of an alcohol with a carboxylic acid to form an ester and water.
Concentrated hydrochloric acid with anhydrous zinc chloride, used to distinguish 1°, 2° and 3° alcohols.
Removal of water from a molecule, as when ethanol forms ethene.
In elimination reactions, the more substituted alkene is the main product.
A mild oxidising agent that converts primary alcohols to aldehydes without further oxidation.
A yellow precipitate of CHI₃ formed with iodine and sodium hydroxide, given by ethanol and alcohols with a CH₃CH(OH)– group.
Removal of hydrogen from a molecule, as when ethanol forms ethanal over hot copper.
Alcohols react either by breaking the O–H bond, acting as weak acids and forming esters, or by breaking the C–O bond, forming haloalkanes, alkenes and ethers; they can also be oxidised or dehydrogenated. The reactions differ for primary, secondary and tertiary alcohols, which is how chemists tell the three classes apart.
Breaking the O–H bond, alcohols react with sodium to give hydrogen and alkoxides, and with carboxylic acids to give esters; their acidity falls in the order 1° > 2° > 3°, and they are weaker acids than water. Breaking the C–O bond, they react with hydrogen halides (the basis of the Lucas test), with PCl₃, PCl₅ and SOCl₂, and with concentrated sulphuric acid, which dehydrates ethanol to ethene at 443 K or to ethoxyethane at 413 K; here reactivity runs 3° > 2° > 1°. Oxidation turns primary alcohols into aldehydes and then acids and secondary alcohols into ketones, while tertiary alcohols resist it. Heated copper at 573 K dehydrogenates 1° and 2° alcohols but dehydrates 3° alcohols. The iodoform test picks out ethanol and alcohols with a CH₃CH(OH)– group.
An alcohol, R–OH, can react in two main ways:
Oxidation, dehydrogenation and the iodoform test make up the rest of the chemistry.
1. Acidic nature: reaction with sodium. Alcohols react with active metals such as sodium, giving hydrogen gas and an alkoxide: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂ (sodium ethoxide)
2. Esterification. With a carboxylic acid and a little concentrated sulphuric acid, an alcohol forms an ester in a reversible reaction: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O (ethyl ethanoate) With an acid chloride the reaction is faster and goes to completion; a base such as pyridine removes the HCl formed: CH₃CH₂OH + CH₃COCl → CH₃COOCH₂CH₃ + HCl
3. Reaction with hydrogen halides. R–OH + HX → R–X + H₂O. Reactivity of the acids is HI > HBr > HCl, and of the alcohols 3° > 2° > 1°.
The Lucas test uses Lucas reagent, concentrated HCl with anhydrous ZnCl₂. The alkyl chloride formed is insoluble and makes the solution turbid:
4. Reaction with phosphorus halides and thionyl chloride.
5. Dehydration. Heated with concentrated sulphuric acid, ethanol loses water:
The ease of dehydration is 3° > 2° > 1°, so tertiary alcohols lose water under much milder conditions. When more than one alkene can form, the more substituted alkene is the main product (Saytzeff rule): butan-2-ol gives mainly but-2-ene, with some but-1-ene.
6. Oxidation. Acidified potassium dichromate or potassium permanganate oxidises alcohols according to their class:
To stop a primary alcohol at the aldehyde stage, a milder reagent such as pyridinium chlorochromate (PCC) is used, or the aldehyde is distilled off as it forms. With dichromate, the orange colour of Cr₂O₇²⁻ turns green as Cr³⁺ forms, a change once used in breath tests for alcohol.
7. Dehydrogenation over copper at 573 K.
Ethanol, and secondary alcohols with the group CH₃CH(OH)–, such as propan-2-ol, give a yellow precipitate of iodoform (CHI₃) when warmed with iodine and sodium hydroxide: CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃ + HCOONa + 5NaI + 5H₂O Methanol does not give the test, so it distinguishes ethanol from methanol.