Physical & Chemical Properties of Alcohols

A molecule of two parts

An alcohol consists of two pieces — an alkyl group and a hydroxyl (–OH) group — and the chemistry is governed chiefly by the hydroxyl group, with the alkyl group only modulating it. The oxygen of the –OH is joined to carbon by a sigma bond formed from the overlap of an sp³ orbital of carbon with an sp³ orbital of oxygen; the bond angle is slightly less than the tetrahedral 109°28′ because the two lone pairs on oxygen repel the bond pairs.

That oxygen sits between two electronegative-difference bonds. The O–H bond is polar — oxygen carries a partial negative charge and hydrogen a partial positive charge — and this single feature explains both the physical properties (high boiling points, water solubility) and the chemical reactivity (acidity, the O–H and C–O cleavages). Throughout this note we sort the chemical reactions by which bond breaks: alcohols act as nucleophiles when the O–H bond breaks, and as electrophiles (after protonation) when the C–O bond breaks.

Boiling points and hydrogen bonding

Alcohols boil far higher than other classes of comparable molecular mass — hydrocarbons, ethers, haloalkanes. The reason is intermolecular hydrogen bonding: the partially negative oxygen of one molecule attracts the partially positive hydroxyl hydrogen of a neighbour, so the molecules are associated. These hydrogen bonds must be broken before any molecule can escape into the vapour, and that demands extra energy.

Ethanol and propane have comparable molecular masses, yet ethanol (b.p. 351 K) boils well above propane (231 K). Methoxymethane, an ether of the same formula as ethanol, lies in between — it is polar but has no O–H hydrogen to donate, so it cannot hydrogen-bond to itself. Within the alcohol family, boiling point rises with chain length (greater van der Waals forces) but falls with branching, because a more compact branched molecule has a smaller surface area.

The take-away NCERT states plainly: the high boiling points of alcohols are due mainly to intermolecular hydrogen bonding, which is lacking in ethers and hydrocarbons.

Solubility in water

Lower alcohols are miscible with water in all proportions because the –OH group forms hydrogen bonds with water molecules — the alcohol's oxygen accepts an H from water and its hydroxyl hydrogen donates to water's oxygen. As the alkyl chain lengthens, the hydrophobic hydrocarbon portion grows and increasingly resists this hydration, so solubility decreases with increasing chain length. Methanol, ethanol and propanol are infinitely miscible; butanol and higher members are only partly soluble.

Acidic nature of alcohols

Alcohols react with active metals — sodium, potassium, aluminium — to liberate hydrogen and form the corresponding alkoxide. This proves they are Brønsted acids, able to donate the hydroxyl proton.

$$\ce{2R-OH + 2Na -> 2R-O^{-}Na^{+} + H2 ^}$$

The acidity arises from the polarity of the O–H bond. An electron-releasing alkyl group (–CH₃, –C₂H₅) pushes electron density onto oxygen, which lowers the O–H polarity and also destabilises the resulting alkoxide ion (the alkoxide already bears a full negative charge that the alkyl groups make worse). The more alkyl groups attached, the weaker the acid. Hence the acid strength runs:

$$\underset{\text{1° (strongest acid)}}{\ce{R-CH2-OH}} \;>\; \underset{\text{2°}}{\ce{R2CH-OH}} \;>\; \underset{\text{3° (weakest acid)}}{\ce{R3C-OH}}$$

Alcohols are nevertheless weaker acids than water. When an alkoxide meets water, water donates a proton back, showing it is the stronger acid; equivalently, an alkoxide ion is a stronger base than hydroxide. NCERT lists ethanol at pK_a ≈ 15.9 against water's 15.74, and phenol at 10.0 — so phenol is roughly a million times more acidic than ethanol.

Reactions with O–H cleavage: esterification

When alcohols behave as nucleophiles, the O–H bond breaks. Beyond reaction with metals, the key O–H reaction is esterification: alcohols react with carboxylic acids, acid chlorides and acid anhydrides to give esters.

$$\ce{R-OH + R'COOH <=>[\text{conc. } H2SO4] R'COOR + H2O}$$

With a carboxylic acid the reaction is reversible and carried out with a little concentrated sulphuric acid, removing water to drive it forward. With an acid chloride the reaction is run in the presence of a base such as pyridine, which neutralises the HCl produced and shifts the equilibrium to the right:

$$\ce{R-OH + R'COCl ->[\text{pyridine}] R'COOR + HCl}$$

Introducing an acetyl (CH₃CO–) group this way is called acetylation; acetylation of salicylic acid gives aspirin. NIOS notes that phenols behave the same way, the acetyl group replacing the phenolic –OH hydrogen.

Reactions with C–O cleavage: HX, Lucas test, PCl3 / PCl5 / SOCl2

When a protonated alcohol acts as an electrophile, the C–O bond breaks and the –OH is replaced. These C–O cleavages occur only in alcohols (phenols essentially do not, because the aryl C–O is strengthened by partial double-bond character).

Reaction with hydrogen halides and the Lucas test

Alcohols react with hydrogen halides to give alkyl halides:

$$\ce{R-OH + HX -> R-X + H2O}$$

The difference in reactivity of the three classes underlies the Lucas test. Lucas reagent is concentrated HCl with anhydrous ZnCl₂; alcohols dissolve in it but their alkyl chlorides are immiscible and show up as turbidity (cloudiness). The reactivity order toward C–O cleavage is:

$$\underset{\text{immediate turbidity}}{\text{tertiary}} \;>\; \underset{\text{within } \sim5\text{ min}}{\text{secondary}} \;>\; \underset{\text{no turbidity at RT}}{\text{primary}}$$

A tertiary alcohol forms its halide instantly because it goes through a stable tertiary carbocation; a primary alcohol gives no turbidity at room temperature. This ordering — and the trap of confusing it with the acidity order — is examined repeatedly in NEET.

Reaction with PCl₃, PCl₅ and SOCl₂

Alcohols are also converted to alkyl halides by phosphorus halides and thionyl chloride. These reagents differ chiefly in their by-products, which is exactly what NEET 2024 tested.

SOCl₂ (the Darzens method) is the cleanest because both its by-products are gases that escape, leaving pure alkyl chloride. PBr₃ is used analogously to make alkyl bromides.

Dehydration to alkenes

On heating with a protic acid (concentrated H₂SO₄ or H₃PO₄) or a catalyst such as alumina, alcohols lose a molecule of water to form alkenes. Ethanol dehydrates to ethene with concentrated H₂SO₄ at 443 K.

$$\ce{CH3CH2OH ->[\text{conc. }H2SO4][443\,K] CH2=CH2 + H2O}$$

Secondary and tertiary alcohols dehydrate under milder conditions, giving the order:

$$\text{tertiary} \;>\; \text{secondary} \;>\; \text{primary}$$

The reason is the mechanism. Acid-catalysed dehydration follows three steps and passes through a carbocation in the slow, rate-determining step. A tertiary carbocation is the most stable (three electron-releasing alkyl groups) and forms most easily, so the tertiary alcohol is the easiest to dehydrate.

When more than one alkene is possible, the more highly substituted alkene predominates — Saytzeff's rule. NEET 2023 used exactly this idea, asking which alcohol dehydrates most readily under acidic conditions (the answer being the one that gives the most stable carbocation). A related warning: a carbocation can rearrange to a more stable one by a hydride shift, changing the product — the basis of another 2023 question on 3-methylbutan-2-ol with HBr.

Oxidation and dehydrogenation

Oxidation of an alcohol forms a carbon–oxygen double bond and involves cleavage of both an O–H and a C–H bond on the carbinol carbon. Because dihydrogen is effectively removed, these are also called dehydrogenation reactions. The outcome depends on the class of alcohol and on how many C–H bonds the carbinol carbon has.

A primary alcohol is oxidised first to an aldehyde and then, with a strong oxidant such as acidified KMnO₄, on to a carboxylic acid. To stop cleanly at the aldehyde, a mild reagent — pyridinium chlorochromate (PCC), or CrO₃ in anhydrous medium — is used.

$$\ce{R-CH2-OH ->[PCC] R-CHO ->[KMnO4/H+] R-COOH}$$

A secondary alcohol is oxidised to a ketone by chromic anhydride (CrO₃); the ketone resists further oxidation under normal conditions.

$$\ce{R2CH-OH ->[CrO3] R2C=O}$$

A tertiary alcohol has no hydrogen on the carbinol carbon, so it cannot be oxidised in the ordinary way. Only under forcing conditions (strong KMnO₄, high temperature) do C–C bonds break, giving a mixture of carboxylic acids with fewer carbon atoms.

Passing the vapour of a primary or secondary alcohol over heated copper at 573 K achieves dehydrogenation — a primary alcohol gives an aldehyde, a secondary gives a ketone — whereas a tertiary alcohol simply dehydrates to an alkene over copper. (This catalytic-copper distinction is the basis of the optional Victor Meyer–style class identification.)