Esters

Esters are organic compounds containing the –COO– (ester linkage) functional group. The C=O comes from the carboxyl carbon of the parent acid; the adjacent –O– comes from the parent alcohol's –OH (with H lost as water during the reaction).

General formula: RCOOR' where:

• R is the alkyl group from the carboxylic acid (RCOOH).
• R' is the alkyl group from the alcohol (R'OH).

Structure of the ester linkage.

        O
        ||
   R ─ C ─ O ─ R'
       └─── acid half ───┘  └─ alcohol half ─┘

The C atom is double-bonded to one oxygen (the carbonyl O — came from the C=O of the acid). The same C is single-bonded to a second oxygen, which is then single-bonded to the alkyl group from the alcohol.

Comparison with related functional groups.

Functional group	Where it appears	Pattern
Alcohol	Ethanol	R – OH
Carboxylic acid	Ethanoic acid	R – COOH (R-C(=O)-OH)
Ester	Ethyl ethanoate	R – COOR' (R-C(=O)-O-R')
Amide	Proteins	R – CONHR' (R-C(=O)-NH-R')

The ester is essentially a carboxylic acid where the –OH hydrogen has been replaced by an alkyl group from an alcohol.

Displayed formula of ethyl ethanoate.

CH₃COOC₂H₅ — let's draw this clearly:

   H            O          H   H
   |            ||         |   |
H ─ C ─ ─ ─ ─ ─ C ─── O ── C ─ C ─ H
   |                       |   |
   H                       H   H

     methyl                  ethyl
     half from acid           half from alcohol

The carbonyl carbon (with =O) on the left comes from ethanoic acid (CH₃COOH minus the –OH). The –O–CH₂CH₃ on the right comes from ethanol (CH₃CH₂OH minus the –H).

Bond formation during esterification.

When ethanoic acid + ethanol → ethyl ethanoate + water:

• The acid's C(=O)–O–H loses the H (as part of water).
• The alcohol's R'–O–H loses the H (also part of water).
• The two oxygens (one from acid, one from alcohol) — wait, NO! Only ONE oxygen is in the ester linkage.

Let me re-examine. The acid contributes the C=O AND the O that bonds to the alcohol's alkyl group. The alcohol contributes the H of water and the alkyl group (which bonds to the acid's O).

Actually, mechanistically there's some subtlety:

• The acid's –C(=O)–OH provides the –C(=O)–O– part of the ester linkage.
• The alcohol's R'O–H provides the –R' part.
• The –H of the alcohol + the –OH of the acid combine to form H₂O (water).

So: acid keeps its O (becomes the C-O-R' of the ester); alcohol keeps its alkyl group R'. The water is H (from alcohol) + OH (from acid).

Wait, where is the alcohol's O?

Let me redo this. There are actually 18O-labelling experiments showing the mechanism. In short, the alcohol's oxygen IS retained in the ester linkage. Both the acid's C=O and the alcohol's O end up in the ester (the alcohol's O is the single-bonded O in –C(=O)–O–R'). The water consists of:

• The –OH from the acid (one O + one H).
• One H from the alcohol's –OH.

Net: acid's –OH leaves; alcohol's –H leaves; ester linkage –C(=O)–O–R' forms (with C from acid, =O from acid, single-bonded O from alcohol, R' from alcohol).

Mnemonic: 'Acid donates –OH; alcohol donates –H' — they combine to make water.

Naming pattern in detail.

Pattern: [alkyl from alcohol] [carboxylate from acid].

Step 1 — alcohol → alkyl.

Alcohol	Stem	Alkyl group
Methanol (CH₃OH)	meth-	methyl
Ethanol (CH₃CH₂OH)	eth-	ethyl
Propan-1-ol (CH₃CH₂CH₂OH)	propan-1-	propyl (or propan-1-yl)
Propan-2-ol (CH₃CH(OH)CH₃)	propan-2-	propan-2-yl or isopropyl
Butan-1-ol	butan-1-	butyl

Step 2 — acid → carboxylate.

Carboxylic acid	Stem	Carboxylate
Methanoic acid (HCOOH)	methan-	methanoate
Ethanoic acid (CH₃COOH)	ethan-	ethanoate
Propanoic acid (C₂H₅COOH)	propan-	propanoate
Butanoic acid (C₃H₇COOH)	butan-	butanoate

Step 3 — combine.

Alcohol + Acid	Ester name	Structural formula
Ethanol + ethanoic acid	Ethyl ethanoate	CH₃COOC₂H₅
Methanol + ethanoic acid	Methyl ethanoate	CH₃COOCH₃
Ethanol + propanoic acid	Ethyl propanoate	C₂H₅COOC₂H₅
Methanol + propanoic acid	Methyl propanoate	C₂H₅COOCH₃ (or CH₃CH₂COOCH₃)
Propan-1-ol + ethanoic acid	Propyl ethanoate	CH₃COOC₃H₇
Pentan-1-ol + pentanoic acid	Pentyl pentanoate	C₄H₉COOC₅H₁₁ (smells of pear drops)

Physical properties — why volatile.

Esters do NOT have an O–H bond → they cannot act as H-bond DONORS. But they have C=O → they CAN act as H-bond ACCEPTORS. So:

• Between ester molecules: only dipole-dipole + van der Waals forces (no H-bonding) → weaker intermolecular forces → LOWER bp than the corresponding alcohol or carboxylic acid.
• With water: esters CAN act as H-bond acceptors (water donates H to the ester's C=O) → small esters are slightly soluble in water.

Boiling points comparison (C₄ compounds):

Compound	bp (°C)
Butan-1-ol (HO-CH₂-CH₂-CH₂-CH₃)	117 (strong H-bonding from –OH)
Butanoic acid (HOOC-CH₂-CH₂-CH₃)	164 (dimers via H-bonding)
Methyl propanoate (CH₃CH₂COOCH₃)	80 (no H-bond donor — weaker forces)

Esters are MUCH more volatile than the corresponding acids or alcohols of the same mass.

Why this matters for flavours / fragrances.

To smell or taste something, the molecules must reach the nose / taste buds in the gas phase. Volatile esters do this easily:

• Open a bottle of nail polish remover (ethyl ethanoate) — the smell fills the room within seconds.
• Bite into a banana — the volatile ester (isoamyl ethanoate) evaporates from the surface and you taste it.
• A perfume bottle releases ester molecules into the air.

If esters had higher boiling points (like the corresponding acids), they wouldn't be perceptible as smells/flavours at all.

Esters in nature.

Beyond the small esters used in flavourings, esters are widespread in nature:

• FRUIT FLAVOURS — most natural fruit smells contain blends of esters (apples: ethyl butanoate; pineapple: ethyl pentanoate; banana: isoamyl ethanoate; pear: pentyl pentanoate; strawberry: complex mix).
• WAXES — beeswax, plant cuticle wax — long-chain esters.
• FATS + OILS — triesters of glycerol (see below).
• FLOWER SCENTS — benzyl ethanoate (jasmine), methyl jasmonate (jasmine + others).
• ANIMAL PHEROMONES — many insect attractants are esters.

The smell of food, perfume, and freshly-cut fruit is largely the smell of esters.

Esterification is the reaction between a CARBOXYLIC ACID and an ALCOHOL to form an ESTER and water. It is one of the most important reactions in organic chemistry.

General equation:

$\text{RCOOH} + \text{R'OH} \xrightleftharpoons[\text{}]{\text{H}_2\text{SO}_4} \text{RCOOR'} + \text{H}_2\text{O}$

Specific example: ethyl ethanoate.

$\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightleftharpoons[\text{}]{\text{conc. H}_2\text{SO}_4} \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$

Or in shorthand: $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$.

Conditions for esterification.

1. Catalyst: a few drops of concentrated sulfuric acid (H₂SO₄).
2. Heat: warm gently for ~ 15-20 min, often under REFLUX (vertical condenser fits to flask → vapours condense back).
3. Mix reactants: typically ethanol + ethanoic acid in roughly equimolar amounts (or excess of one to push equilibrium).

Role of concentrated H₂SO₄ — TWO functions.

H₂SO₄ plays a dual role that is examined frequently in 4CH1:

Function 1: Acid catalyst.

• Provides H⁺ ions that protonate the carbonyl oxygen of the acid → makes the carbonyl carbon more electrophilic → easier for the alcohol's O to attack.
• Speeds up the forward reaction by ~ 10,000x or more.
• Without strong acid, esterification at room T is essentially zero rate.

Function 2: Dehydrating agent.

• Conc. H₂SO₄ is HYGROSCOPIC — it strongly absorbs water.
• Esterification is REVERSIBLE (forward = make ester, lose water; reverse = hydrolyse ester, gain water).
• By REMOVING the water byproduct, H₂SO₄ shifts the equilibrium FURTHER TO THE RIGHT (Le Chatelier).
• Higher yield of ester.

Together: H₂SO₄ is both the catalyst (kinetics — speeds up the reaction) and the dehydrator (thermodynamics — shifts the equilibrium). One reagent does BOTH jobs.

Why dilute H₂SO₄ would not be as good.

Dilute H₂SO₄ provides H⁺ (catalyst) but does NOT dehydrate effectively (the water is already there, mixed with the acid). So the equilibrium yield is limited by water buildup → lower overall yield. CONCENTRATED H₂SO₄ does both jobs at once.

Why the reaction is reversible.

Both directions are accessible under the same conditions:

• Forward: ester + water from acid + alcohol — esterification.
• Reverse: acid + alcohol from ester + water — hydrolysis.

At a given temperature, an equilibrium is established. The position of equilibrium depends on:

• The concentrations of the reactants.
• The amount of water (more water → reverse reaction favoured).
• The presence of catalyst (speeds up both forward and reverse, doesn't change the position).

The conc. H₂SO₄ shifts the position by removing water → favours esterification.

Mechanism (deeper, beyond 4CH1).

For those interested, the mechanism is acid-catalysed nucleophilic acyl substitution:

1. Protonation — H⁺ protonates the C=O of ethanoic acid → forms a positively charged intermediate (oxocarbenium ion).
2. Nucleophilic attack — the lone pair on the alcohol's O attacks the electrophilic carbonyl C → forms a tetrahedral intermediate.
3. Proton transfer — H+ shifts within the molecule, then water leaves.
4. Deprotonation — the final ester loses an H⁺ → neutral ester product.

The H⁺ catalyst is regenerated at the end → catalyst (not consumed). 18O-labelling experiments show that the alcohol's O ends up in the ester linkage and the acid's –OH ends up in the water.

Laboratory preparation — the canonical method.

Setup:

• 50-100 cm³ round-bottom flask.
• Reflux condenser (vertical Liebig).
• Heating: hot water bath or electric mantle (NO Bunsen — flammable mixture).
• Anti-bumping granules.

Procedure:

1. Add ethanoic acid (~ 10 cm³), ethanol (~ 10 cm³), a few drops of conc. H₂SO₄, and anti-bumping granules to the flask.
2. Fit the reflux condenser. Heat in a water bath at 70-80 °C for ~ 15-20 min. Vapours rise and condense back.
3. Allow to cool slightly. Reconfigure for distillation (replace reflux condenser with a horizontal one + collection flask).
4. Distil. The volatile ester (bp 77 °C) distils first. Collect the fraction at 75-78 °C.
5. The distillate is crude ester — contains traces of ethanol, ethanoic acid, water, SO₂.

Purification:

The crude ester contains:

• Ethyl ethanoate (the product, want to keep).
• Ethanol (excess reactant — must remove).
• Ethanoic acid (excess reactant — must remove).
• Water + traces of H₂SO₄ + SO₂ (must remove).

To purify:

1. Shake the distillate with aqueous sodium carbonate in a separating funnel.
   • Neutralises the acidic impurities: 2 CH₃COOH + Na₂CO₃ → 2 CH₃COONa + H₂O + CO₂.
   • VENT the funnel — CO₂ pressure builds up.
   • Two layers form. Run off the lower (aqueous) layer; keep the upper (ester) layer.
2. Wash with water to remove water-soluble impurities (residual ethanol, salts).
3. Dry with anhydrous magnesium chloride or calcium chloride (removes residual water).
4. Filter to remove the drying agent.
5. (Optional) Re-distil, collecting at 77 °C, for highest purity.

Identification of the product.

• Smell: characteristic SWEET, FRUITY, nail-polish-like smell. (Waft the vapour cautiously toward your nose — don't inhale deeply.)
• Boiling point: pure ethyl ethanoate boils at 77 °C. A sharp, narrow boiling range indicates purity.
• Density: 0.902 g/cm³ at 20 °C — slightly less dense than water; floats on water.
• Solubility: slightly soluble in water (~ 8 g/100 mL); fully soluble in ethanol and other organic solvents.
• Test for acidic functional group: ester is NOT acidic (no –OH on the C of carbonyl). pH ~ 7 of pure ester. (Distinguishes from the parent carboxylic acid, which is acidic.)

Yields.

The reaction is reversible → equilibrium yield ~ 65-70% theoretical (with H₂SO₄ removing water). After purification losses, the actual lab yield is typically 40-60%.

To improve yield:

• Use EXCESS of one reactant (drives equilibrium right).
• Use efficient water removal (H₂SO₄ + distillation of the ester).
• Run at higher T to increase rate (but not too high — increase the reverse reaction too).

Industrial esterification.

The same chemistry scales up to industrial processes:

• Vinyl acetate manufacture: ethanoic acid + ethyne / ethene → vinyl acetate → PVA plastic.
• Biodiesel manufacture: triglyceride (vegetable oil) + methanol → fatty acid methyl esters + glycerol.
• Industrial flavourings + fragrances: large-scale esterification of specific alcohol + acid combinations.

The industrial conditions might differ (different catalysts, continuous flow, higher T, vacuum to remove water), but the basic reaction is the same.

Edexcel-friendly summary.

Item	Detail
Reaction	Carboxylic acid + alcohol ⇌ ester + water
Equation (ethyl ethanoate)	CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Catalyst	Concentrated H₂SO₄ (also dehydrating agent)
Conditions	Heat under reflux
Reversible	YES — use ⇌
Byproduct	Water (which is removed by conc. H₂SO₄ to push equilibrium right)
Identification	Sweet, fruity smell; bp 77 °C
Purification	Distil; wash with Na₂CO₃; wash with water; dry with anhydrous MgCl₂; re-distil

For 4CH1, memorise the reactants, products, catalyst (CONC. H₂SO₄), and the role of H₂SO₄ (BOTH catalyst AND dehydrating agent).

Ester names follow a strict two-word pattern. Mastering this is essential for 4CH1.

The naming rule (memorise!).

First word = alkyl from the ALCOHOL (replace -anol with -yl). Second word = carboxylate from the ACID (replace -anoic acid with -anoate).

Step-by-step example: ethyl ethanoate.

Given: ethanol + ethanoic acid.

1. First word from alcohol: ethanol → drop '-anol' → 'eth' → add '-yl' → ethyl.
2. Second word from acid: ethanoic acid → drop '-anoic acid' → 'ethan' → add '-oate' → ethanoate.
3. Combine: ethyl ethanoate.

Reverse engineering — given the name, find the parents.

Given: 'methyl propanoate'.

1. First word 'methyl' — comes from an alcohol whose alkyl group is methyl → methan + ol → methanol (CH₃OH).
2. Second word 'propanoate' — comes from an acid whose carboxylate is propanoate → propan + oic acid → propanoic acid (CH₃CH₂COOH).
3. Esterification: methanol + propanoic acid → methyl propanoate + water.
4. Structural formula: CH₃CH₂COOCH₃.

Lots of practice — table of common esters.

Alcohol	Acid	Ester	Structural formula	Smell
Methanol	Methanoic acid	Methyl methanoate	HCOOCH₃	Rum
Methanol	Ethanoic acid	Methyl ethanoate	CH₃COOCH₃	Sweet, ether-like
Methanol	Propanoic acid	Methyl propanoate	C₂H₅COOCH₃	Mild, fruity
Ethanol	Methanoic acid	Ethyl methanoate	HCOOC₂H₅	Rum, raspberry
Ethanol	Ethanoic acid	Ethyl ethanoate	CH₃COOC₂H₅	Nail polish remover, pear
Ethanol	Butanoic acid	Ethyl butanoate	C₃H₇COOC₂H₅	Pineapple
Pentyl (amyl)	Pentanoic acid	Pentyl pentanoate	C₄H₉COOC₅H₁₁	Pear drops
Octyl	Ethanoic acid	Octyl ethanoate	CH₃COOC₈H₁₇	Oranges
Isoamyl	Ethanoic acid	Isoamyl ethanoate	CH₃COOCH₂CH₂CH(CH₃)₂	Bananas
Benzyl	Ethanoic acid	Benzyl ethanoate	CH₃COOCH₂C₆H₅	Jasmine

Each of these esters is used commercially in food flavourings, perfumes, or industrial applications.

Why the convention is 'alcohol first, acid second'.

The reason reflects the STRUCTURAL ORIGIN of each half of the ester:

• The ESTER LINKAGE –COO– has TWO sides: an acyl side (–CO–) and an oxy side (–O–R').
• The ACYL side (–CO–) comes from the ACID's carbonyl carbon. So the 'acid half' is the right-hand side of the ester linkage diagram.
• The OXY side (–O–R') comes from the ALCOHOL's hydroxyl oxygen + its alkyl group. So the 'alcohol half' is the left-hand side.

Reading the ester structural formula RCOOR' from left to right:

• R from acid (alkyl chain of acid, before the carbonyl).
• C=O from acid (carbonyl).
• O from alcohol (this is the bridging oxygen).
• R' from alcohol (alkyl chain of alcohol).

But the NAMING goes the other way: the alcohol's alkyl (R') is named first ('ethyl' if R' is C₂H₅), and the acid's carboxylate (R + COO) is named second ('ethanoate' if R is CH₃).

Cross-check the name with the formula.

Given CH₃COOC₂H₅:

• The COO group is the ester linkage.
• The CH₃ on the LEFT (in the acid half) is the R from ethanoic acid → 'ethanoate' (because ethanoic has 2 C total, and the formula –COO– includes the 1 C of the carboxyl, so the alkyl is 1 C = methyl from ethanoic; but it's named 'ethanoate', not 'methylcarboxylate' or anything — IUPAC names use the full carboxylic acid stem).
• The C₂H₅ on the RIGHT (alcohol half) is from ethanol → 'ethyl'.
• Name: ethyl ethanoate ✓.

Sometimes this is confusing because the 'ethyl' refers to ethanol (the C₂H₅O–) and the 'ethanoate' refers to ethanoic acid (CH₃COO–) — even though both originally came from C₂ molecules.

Common errors in naming.

1. Reversing the order — writing 'propyl methanoate' when the answer is methyl propanoate. Always check which compound is the alcohol and which is the acid.

2. Using common names — 'ethyl acetate' instead of ethyl ethanoate. Edexcel mark schemes use IUPAC names.

3. Confusing alkyl with alkane — methanol → 'methyl', not 'methane'.

4. Forgetting the locant for higher alcohols — propan-1-ol gives propan-1-yl (or just 'propyl'); propan-2-ol gives propan-2-yl (or 'isopropyl').

5. Missing the 'e' rule — some old textbooks use 'ethanoate' (correct); others use 'acetate' (old common name).

Practice question — try this yourself.

Q: What ester is formed from butan-1-ol + propanoic acid?

A:

• Butan-1-ol → drop -anol → butan-1- → add -yl → butyl (or butan-1-yl).
• Propanoic acid → drop -oic acid → propan- → add -oate → propanoate.
• Ester: butyl propanoate (C₂H₅COOC₄H₉).

Q: What are the parents of pentyl methanoate?

A:

• Pentyl → from pentan-1-ol (C₅H₁₁OH).
• Methanoate → from methanoic acid (HCOOH).
• Parents: pentan-1-ol + methanoic acid.

Q: What ester smells of bananas?

A: Isoamyl ethanoate (or 3-methylbutyl ethanoate) = CH₃COOCH₂CH(CH₃)CH₂CH₃. From isoamyl alcohol (3-methylbutan-1-ol) + ethanoic acid.

Mark-scheme phrases.

• "Ester named: [alcohol's alkyl] [acid's carboxylate]."
• "Alcohol provides the alkyl group; acid provides the carboxylate."
• "The structural formula shows the ester linkage –COO– between the two halves."

Use these in your answers.

Esters are used in food, cosmetics, industry, and biology. Understanding their uses and the reverse reaction (hydrolysis) reveals their full importance.

Uses of esters.

1. FLAVOURINGS in food.

Most artificial fruit flavours in sweets, drinks, and ice cream are esters. They're cheap, shelf-stable, and reproducible — unlike real fruit juice. Examples:

Flavour	Ester
Banana	Isoamyl ethanoate
Pear	Pentyl pentanoate (pear drops)
Pineapple	Ethyl butanoate
Apple	Methyl butanoate
Strawberry	Various, including methyl propanoate
Orange	Octyl ethanoate

A 'pineapple-flavoured' sweet contains ethyl butanoate dissolved in sugar + colouring — no real pineapple.

2. FRAGRANCES in perfumes and cosmetics.

Perfumes blend many esters with other aromatic compounds. Key fragrance esters:

Smell	Ester
Jasmine	Benzyl ethanoate
Rose	Various rose ester components
Sandalwood	Various sesquiterpene esters
Citrus	Various citrus esters

Perfume molecules are volatile esters + alcohols + aromatics, dissolved in ethanol for spraying.

3. SOLVENTS.

The most-used ester solvent is ethyl ethanoate (also called ethyl acetate):

• Nail polish remover — dissolves the polish without damaging skin.
• Model glue — fast-drying.
• Paints + varnishes — carries pigments + dries to leave the colour.
• Decaffeination of coffee — selectively dissolves caffeine.
• Ink solvent — in markers and printer inks.

Ethyl ethanoate is preferred over acetone for some uses (less toxic, gentler smell).

4. PLASTICISERS.

Larger esters (phthalates) make PVC FLEXIBLE. Phthalate esters are added to:

• Vinyl flooring (otherwise too rigid).
• PVC clothing (raincoats, imitation leather).
• Toy plastics (though phthalates are increasingly banned in children's toys due to endocrine-disruption concerns).
• Wire insulation.

5. BIODIESEL.

Biodiesel is a mixture of FATTY ACID METHYL ESTERS, made by:

$\text{Triglyceride (vegetable oil)} + 3\text{CH}_3\text{OH} \rightarrow \text{3 Fatty acid methyl esters} + \text{glycerol}$

(transesterification, catalysed by NaOH or KOH). Biodiesel can be used in modified diesel engines as a renewable fuel.

6. PHARMACEUTICALS.

Many drugs are esters or contain ester functional groups:

• Aspirin = acetylsalicylic acid = ester of salicylic acid + ethanoic acid.
• Heroin = diacetyl morphine = double ester.
• Cocaine = methyl benzoylecgonine = ester of benzoic acid.
• Various antibiotics and antivirals contain ester groups.

The ester linkage can be hydrolysed in the body (by esterase enzymes) → controlled drug release.

Fats and oils — natural triesters of glycerol.

Naturally occurring FATS (solid at room T) and OILS (liquid) are TRIESTERS — each glycerol molecule is esterified with THREE long-chain fatty acids:

H₂C ─ O ─ CO ─ R₁
 |
HC  ─ O ─ CO ─ R₂      ← three ester linkages
 |
H₂C ─ O ─ CO ─ R₃

The R groups are long alkyl chains, typically C₁₂ to C₂₂.

Saturated vs unsaturated.

• Saturated triglycerides (no C=C bonds in R chains): STRAIGHT chains pack tightly → SOLID at room T → FAT. Examples: butter, lard, palm oil (high in saturated fats), beef fat.

• Unsaturated triglycerides (with C=C bonds in R chains): KINKED chains pack loosely → LIQUID at room T → OIL. Examples: olive oil (high in oleic acid — one C=C), sunflower oil, fish oil (very unsaturated).

Why oils are liquid and fats are solid.

Compare straight (saturated) and kinked (unsaturated) chains:

Saturated (straight):
─CH₂─CH₂─CH₂─CH₂─CH₂─CH₂─CH₂─CH₂─CH₂─COOH

Unsaturated (kinked at C=C):
─CH₂─CH₂─CH=CH─CH₂─CH₂─CH₂─CH₂─COOH
                ↑ kink here

Straight chains pack closely → strong van der Waals → solid. Kinked chains don't pack as well → weaker intermolecular forces → liquid.

Hydrogenation (Topic 4.4, Topic 4.5) converts liquid oils to solid margarine by adding H₂ across the C=C bonds → straightens the chains.

Hydrolysis of esters — the reverse of esterification.

Esters can be HYDROLYSED back to their parent acid + alcohol by water:

$\text{RCOOR'} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{R'OH}$

Acid-catalysed hydrolysis (reversible):

• Heat ester with dilute HCl or H₂SO₄.
• The forward and reverse reactions (esterification ⇌ hydrolysis) both occur.
• Equilibrium reached; yield of acid + alcohol limited.

Alkali-catalysed hydrolysis (irreversible):

• Heat ester with NaOH or KOH solution.
• The NaOH neutralises the carboxylic acid PRODUCT to give a sodium carboxylate salt.
• This REMOVES the acid from the equilibrium → drives the reaction to completion (no reverse possible).
• Products: sodium carboxylate (RCOONa) + alcohol (R'OH).

Saponification — making soap from fats.

The most important industrial application of alkaline hydrolysis is SAPONIFICATION — boiling fats / oils with NaOH:

$(\text{RCO}_3)_3\text{C}_3\text{H}_5 + 3\text{NaOH} \rightarrow \text{C}_3\text{H}_5(\text{OH})_3 + 3\text{R-COONa}$

(triglyceride + 3 NaOH → glycerol + 3 sodium fatty-acid salts)

The sodium salts of long-chain fatty acids are SOAP. Specifically:

• Sodium stearate (C₁₇H₃₅COONa, from stearic acid) — hard soap.
• Sodium palmitate (C₁₅H₃₁COONa, from palmitic acid) — softer.
• Sodium oleate (C₁₇H₃₃COONa, from oleic acid) — soft, has a C=C.

Why soap cleans grease.

Soap molecules are AMPHIPHILIC — they have:

• A long non-polar TAIL (the hydrocarbon chain) — dissolves in oils + fats.
• A polar/ionic HEAD (–COO⁻ Na⁺) — dissolves in water.

When soap is added to dirty water + grease:

1. The non-polar tails of soap dissolve into the grease droplets.
2. The polar heads stay in the water.
3. Each grease droplet is surrounded by a sphere of soap molecules — a micelle.
4. The negatively charged outer surface of micelles repels other micelles → grease droplets stay dispersed.
5. The whole oily mess can be washed away with water.

Without soap, grease and water don't mix (different polarities). Soap bridges the two.

Other key uses of soap.

• Personal hygiene — bar soap, liquid hand soap.
• Laundry detergent — though modern detergents use synthetic surfactants (not just sodium salts of fatty acids).
• Industrial cleaning — degreasers, surfactants.
• Cosmetics — emulsifiers in lotions, creams.

Modern detergents vs soap.

Soap (sodium salt of fatty acid) has two limitations:

• Forms SCUM with hard water (Ca²⁺ ions precipitate as insoluble calcium carboxylate — the white scum that forms in baths).
• Works less well in acidic water (the acid converts –COO⁻ back to –COOH, which is less effective).

Modern detergents (synthetic surfactants, e.g. sodium dodecylbenzene sulfonate) avoid these problems. But for traditional bar soap, saponification of natural fats remains the basic chemistry.

The full picture.

Reaction	Direction	Catalyst	Products
Esterification (forward)	Acid + alcohol → ester + water	Conc. H₂SO₄	Ester
Acid hydrolysis (reverse)	Ester + water → acid + alcohol	Dilute HCl or H₂SO₄	Acid + alcohol (equilibrium)
Alkaline hydrolysis (forward)	Ester + NaOH → carboxylate + alcohol	NaOH (used in excess)	Carboxylate salt + alcohol (irreversible)
Saponification	Fat + NaOH → glycerol + soap	NaOH (excess)	Glycerol + soap

The same –COO– linkage is at the heart of esterification, hydrolysis, fats, oils, and soap-making. One functional group does an enormous amount of organic chemistry.