# Reactions of Organic Acids and Bases

> Carboxylic acids, amines and amides, their boiling points and solubility, esterification by reflux, making and cleaning an ester, and how soaps, detergents and surfactants clean.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/organic-acids-and-bases/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Bananas without bananas
Taste a lolly that says "banana flavour". There is probably no banana in it. Instead, it contains a few drops of a liquid called **pentyl ethanoate** (often sold as "banana oil"), made by joining an alcohol to an acid.

Chemists call these sweet-smelling compounds **esters**. Pear drops, pineapple and rum flavours are all esters too. The same chapter of chemistry also explains soap: a molecule with one end that loves water and one end that loves grease.

**In short:** Carboxylic acids (−COOH) are weak acids, and amines (−NH₂) are weak bases. A carboxylic acid reacts with an alcohol, with concentrated sulfuric acid as a catalyst, to make an ester and water, a reaction called esterification.

## Carboxylic acids

**Carboxylic acids** have the **carboxyl group**, −COOH. They are **weak acids**:

$$\mathrm{CH_3COOH(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{CH_3COO^-(aq)} + \mathrm{H_3O^+(aq)}$$

They react like other acids:

- with metals: $\mathrm{2CH_3COOH} + \mathrm{Mg} \rightarrow \mathrm{(CH_3COO)_2Mg} + \mathrm{H_2}$
- with carbonates (fizzing): $\mathrm{CH_3COOH} + \mathrm{NaHCO_3} \rightarrow \mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}$
- with bases: $\mathrm{CH_3COOH} + \mathrm{NaOH} \rightarrow \mathrm{CH_3COONa} + \mathrm{H_2O}$

**Test**: adding sodium carbonate or sodium hydrogen carbonate gives bubbles of $\mathrm{CO_2}$. Alcohols, esters and aldehydes do not.

## Amines and amides

**Amines** have an **−NH₂** group (or N with carbon groups). Like ammonia, they are **weak bases**, because the lone pair on N accepts a proton:

$$\mathrm{CH_3NH_2(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{CH_3NH_3^+(aq)} + \mathrm{OH^-(aq)}$$

Amines react with acids to form salts: $\mathrm{CH_3NH_2} + \mathrm{HCl} \rightarrow \mathrm{CH_3NH_3Cl}$.

**Amides** have the **−CONH₂** group. They are made from a carboxylic acid and an amine (or ammonia). Amides are **neutral**: the C=O pulls the lone pair on N, so it cannot accept a proton.

## Boiling points and solubility

For molecules of similar size:

| Series | Strongest force | Boiling point |
|---|---|---|
| Alkane | Dispersion | Lowest |
| Ester, aldehyde, ketone | Dipole-dipole (no O−H, so no hydrogen bonds between molecules) | Low |
| Amine | Hydrogen bonds (N−H, weaker than O−H) | Medium |
| Alcohol | Hydrogen bonds (O−H) | High |
| Carboxylic acid | Two hydrogen bonds per pair of molecules (C=O and O−H) | Higher |
| Amide | Several hydrogen bonds (N−H and C=O) | Highest |

All the small ones (up to about 4 carbons) dissolve in water because they can hydrogen bond with water. Esters can **accept** hydrogen bonds from water through their oxygens, so small esters are slightly soluble. As the carbon chain grows, solubility falls.

## Esterification

An **alcohol** and a **carboxylic acid** react to form an **ester** and **water**. This is a **condensation** reaction (a small molecule, water, is lost).

$$\mathrm{CH_3COOH} + \mathrm{CH_3CH_2OH} \rightleftharpoons \mathrm{CH_3COOCH_2CH_3} + \mathrm{H_2O}$$

ethanoic acid + ethanol ⇌ ethyl ethanoate + water

- **Catalyst**: concentrated sulfuric acid. It also absorbs water, pushing the equilibrium right.
- The reaction is **slow** and **reversible**, so the yield is limited.
- To raise the yield: use an **excess** of the cheaper reactant, or remove water.

## Refluxing

The reactants are heated under **reflux**: a condenser stands upright on the flask.

- Volatile reactants and products **boil, condense and drip back**, so nothing escapes. The mixture can be heated for a long time.
- Heat with a **heating mantle** or water bath, not a Bunsen flame: alcohols and esters are flammable.
- Add **boiling chips** for smooth boiling.
- Water flows **in at the bottom** of the condenser and out at the top.

## Purifying the ester

1. Pour the cooled mixture into a **separating funnel** with water.
2. Add **sodium carbonate solution** to neutralise leftover acid ($\mathrm{CO_2}$ bubbles; release the pressure often).
3. The ester is not very soluble, so it forms a **separate layer** (on top). Drain off the lower aqueous layer.
4. Dry the ester with **anhydrous** magnesium sulfate.
5. **Distil**, collecting the liquid at the ester's boiling point.

## Hydrolysis of esters

Heating an ester with dilute acid or base breaks it back into an acid and an alcohol. With NaOH this is **saponification**, which is how soap is made from fats (esters of glycerol and long-chain fatty acids).

## Soaps and detergents

A **surfactant** molecule has two ends:

- a long non-polar **hydrocarbon tail** that is **hydrophobic** (dissolves in grease),
- a charged **head** that is **hydrophilic** (dissolves in water).

**Soap** is the sodium salt of a long-chain fatty acid, such as sodium stearate, $\mathrm{CH_3(CH_2)_{16}COO^-Na^+}$.

**How cleaning works:**

1. The tails dissolve in a grease droplet; the heads stay in the water.
2. Many molecules surround the droplet, forming a **micelle** with negative heads on the outside.
3. The negative micelles repel each other, so the grease breaks into small droplets (an **emulsion**) that wash away.
4. Surfactants also lower the **surface tension** of water, so it wets surfaces better.

**Soap in hard water**: $\mathrm{Ca^{2+}}$ and $\mathrm{Mg^{2+}}$ ions form insoluble **scum** with soap. **Detergents** do not have this problem.

| Detergent | Head group | Example use |
|---|---|---|
| **Anionic** | Negative, such as sulfonate $\mathrm{-SO_3^-}$ | Laundry powder |
| **Cationic** | Positive, such as $\mathrm{-N(CH_3)_3^+}$ | Hair conditioner, fabric softener |
| **Non-ionic** | No charge, polar chain with many O atoms | Dishwashing liquid, low foam |

**Mind map**

```text
Organic acids and bases
  Carboxylic acids
    −COOH, weak acids
    Fizz with carbonates
  Amines
    −NH2, weak bases
  Amides
    −CONH2, neutral
  Boiling points
    Acids and amides highest
    Esters lowest of these
  Esterification
    Acid + alcohol
    Conc. H2SO4
    Reflux
    Purify: Na2CO3, separate, distil
  Surfactants
    Hydrophobic tail
    Hydrophilic head
    Micelles
    Anionic, cationic, non-ionic
```

### Year 11: Calculating yield
**Percentage yield** $= \dfrac{\text{actual mass}}{\text{theoretical mass}} \times 100$

**Example.** 4.61 g of ethanol (0.100 mol) reacts with excess ethanoic acid. In theory, 0.100 mol of ethyl ethanoate (88.11 g/mol) forms: 8.81 g. The student collects 5.29 g.

Yield $= 5.29 \div 8.81 \times 100 = 60\%$.

**Multi-step yields multiply.** Two steps each with 70% yield give $0.70 \times 0.70 = 0.49$, so only 49% overall.

### Year 12: Naming esters from their parts, and why reflux
**From ester to parts.** In $\mathrm{CH_3CH_2COOCH_2CH_2CH_3}$, the part with the C=O, $\mathrm{CH_3CH_2CO}$, came from **propanoic acid**. The part on the other O, $\mathrm{CH_2CH_2CH_3}$, came from **propan-1-ol**. Name: **propyl propanoate**.

**Making banana oil.** Pentyl ethanoate needs pentan-1-ol and ethanoic acid. From pent-1-ene you could hydrate it (but this gives mostly pentan-2-ol by Markovnikov's rule). From ethanol you could oxidise it to ethanoic acid under reflux.

**Why reflux and not an open beaker?** Ethanol (78 °C) and ethyl ethanoate (77 °C) boil at low temperatures. Heating in an open beaker for the long time needed would boil them away, lowering the yield and creating a fire risk.

**K for esterification.** Starting with 1.00 mol each of acid and alcohol, 0.67 mol of ester and water form, leaving 0.33 mol of each reactant. The volume cancels, so $K = \dfrac{0.67 \times 0.67}{0.33 \times 0.33} = 4.1$.

**Telling $\mathrm{C_3H_6O_2}$ isomers apart.** Propanoic acid fizzes with sodium hydrogen carbonate. Methyl ethanoate (an ester) does not fizz and has no C=C or easily oxidised group, so it leaves permanganate purple too.

**Saponification products.** Boiling an ester with NaOH gives the **sodium salt** of the acid (not the acid itself) and the alcohol: $\mathrm{CH_3COOCH_2CH_3} + \mathrm{NaOH} \rightarrow \mathrm{CH_3COONa} + \mathrm{CH_3CH_2OH}$.

**Detergents in hard water.** The calcium salts of sulfonate detergents dissolve, so no scum forms. The calcium salts of soap (carboxylates) do not.

**Amides and water.** Pentanamide's $\mathrm{-CONH_2}$ group can hydrogen bond with water through its N−H and C=O. Hexane cannot, so it does not dissolve.

## Quick summary

- Carboxylic acids (−COOH) are weak acids; they fizz with carbonates.
- Amines (−NH₂) are weak bases. Amides (−CONH₂) are neutral.
- Boiling points: ester < amine < alcohol < carboxylic acid < amide (similar size).
- Esterification: acid + alcohol ⇌ ester + water, conc. $\mathrm{H_2SO_4}$, under reflux.
- Purify with sodium carbonate, a separating funnel and distillation.
- Surfactants have a hydrophobic tail and hydrophilic head and form micelles around grease.

## Quiz

Try these questions from the chapter quiz.

1. The functional group of a carboxylic acid is:
   - A. −OH
   - B. −COOH
   - C. −CHO
   - D. −NH₂
   - Answer: B. −COOH. The carboxyl group.

2. Carboxylic acids are:
   - A. Strong acids
   - B. Weak acids
   - C. Bases
   - D. Neutral
   - Answer: B. Weak acids. They only partly ionise.

3. Which test shows a carboxylic acid is present?
   - A. Bromine water decolourises
   - B. Sodium carbonate fizzes
   - C. Dichromate turns green
   - D. Litmus turns blue
   - Answer: B. Sodium carbonate fizzes. The acid releases $\mathrm{CO_2}$ from carbonate.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/organic-acids-and-bases/#quiz