Grade by Grade
B. Organic Chemistry · Organic Chemistry

Carboxylic Acids

B2.2 B3.1 B3.2 B3.3 A1.13About 70 minutes
Learning goals
  • I can name and draw carboxylic acids and their salts.
  • I can explain the high boiling points and the solubility of carboxylic acids using hydrogen bonding.
  • I can explain why carboxylic acids are weak acids and write equations for their reactions with bases and carbonates.
  • I can describe how carboxylic acids are made by oxidation, and find the concentration of acetic acid in vinegar from titration data.

Words to know

carboxylic acid
An organic compound containing the carboxyl group, –COOH.
carboxyl group
The –COOH group: a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon.
weak acid
An acid that only partly ionizes in water, so most of its molecules stay whole.
carboxylate ion
The ion R–COO⁻ formed when a carboxylic acid loses the H⁺ from its –COOH group.
fatty acid
A carboxylic acid with a long hydrocarbon chain (often 12 to 20 carbons), found in fats and oils.
dimer
Two identical molecules joined together, for example two acid molecules held by hydrogen bonds.

Sour by design

The sour taste of vinegar, lemon juice, yogurt and rhubarb comes from carboxylic acids. Their functional group, the carboxyl group, –COOH, combines a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. Together they behave very differently from either one alone: the H of a –COOH group can leave as H⁺, so these compounds are acids.

Remember from Grade 11: in Solutions and Solubility you met Arrhenius acids, strong and weak acids, and you titrated vinegar with sodium hydroxide. Vinegar's acid is ethanoic acid (acetic acid), CH₃COOH.

CH₃COOHcarbonylhydroxylethanoic acid, CH₃COOHcarbonyl + hydroxyl= carboxyl group, –COOHThe C of –COOH is always C1.Written –COOH or –CO₂H.
The carboxyl group is a carbonyl and a hydroxyl on the same carbon atom. Its carbon is part of the parent chain and is always numbered C1.

Naming carboxylic acids

  1. The parent chain is the longest chain that includes the –COOH carbon.
  2. Replace the final -e of the alkane with -oic acid: methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid, butanoic acid.
  3. The –COOH carbon is C1, so it never needs a number. Number branches from it: (CH₃)₂CHCOOH is 2-methylpropanoic acid.
  4. Two –COOH groups: -dioic acid, keeping the -e: HOOC–COOH is ethanedioic acid (oxalic acid, found in rhubarb leaves, which is one reason the leaves are not eaten). HOOC(CH₂)₄COOH is hexanedioic acid, used to make nylon.
  5. A –COOH on a benzene ring: benzoic acid.

Common names: formic acid (methanoic), acetic acid (ethanoic), butyric acid (butanoic). Many natural acids also have common names: citric acid (citrus fruit, three –COOH groups), lactic acid (sour milk and working muscles) and ASA, acetylsalicylic acid (aspirin).

Salts: when the H⁺ is replaced by a metal ion, name the metal and change -oic acid to -oate: CH₃COONa is sodium ethanoate (sodium acetate); C₆H₅COONa is sodium benzoate, a food preservative.

Worked example

Example 1: Naming acids

Name (a) CH₃CH₂CH₂COOH, (b) CH₃CH(CH₃)CH₂COOH, (c) HCOOH, (d) CH₃CH₂COOK.

  1. (a) 4 C including the COOH carbon: butanoic acid.
  2. (b) Number from COOH: C1 = COOH, C2 = CH₂, C3 = CH(CH₃), C4 = CH₃. 3-methylbutanoic acid.
  3. (c) 1 C: methanoic acid (formic acid).
  4. (d) The potassium salt of propanoic acid: potassium propanoate.

(a) butanoic acid, (b) 3-methylbutanoic acid, (c) methanoic acid, (d) potassium propanoate.

Physical properties

  • The –COOH group can both donate (O–H) and accept (C=O and O lone pairs) hydrogen bonds. Two acid molecules often pair up into a dimer, held by two hydrogen bonds.
  • So carboxylic acids have the highest boiling points of the families in this unit, for molecules of similar mass: ethanoic acid (60 g/mol) boils at 118 °C, while propan-1-ol (60 g/mol) boils at 97 °C and methoxyethane (60 g/mol) at 7 °C.
  • Methanoic to butanoic acid mix completely with water. Solubility falls as the non-polar chain grows; long-chain fatty acids such as stearic acid (C₁₈) are insoluble waxy solids.
  • Small acids have sharp or unpleasant odours: ethanoic acid smells of vinegar, butanoic acid of rancid butter and hexanoic acid of goats (its old name, caproic acid, comes from capra, Latin for goat).
CH₃COOHCH₃COOHH-bondH-bondtwo ethanoic acid molecules held together by two hydrogen bonds (a dimer)
In pure ethanoic acid, pairs of molecules are held together by two hydrogen bonds. Extra energy is needed to pull them apart, which helps explain the high boiling point.

Carboxylic acids are weak acids

In water, a carboxylic acid gives up its –COOH hydrogen as H⁺ (forming H₃O⁺), leaving a carboxylate ion:

CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)

The double arrow shows that the reaction goes both ways and only a small fraction of the molecules ionize: about 1% in a 0.10 mol/L solution. That is why carboxylic acids are weak acids. Vinegar has a pH of about 2.5 to 3, while a hydrochloric acid solution of the same concentration would have a pH near 0. You will calculate these values with equilibrium constants in the unit Acid-Base and Solubility Equilibria.

The H atoms on carbon are not acidic. Only the H of –COOH leaves. Alcohols are not acidic either: their O–H is far less willing to lose H⁺.

CH₃COOH (whole molecule)ionized: CH₃COO⁻ + H₃O⁺100 molecules of ethanoic acid in 0.10 mol/L solution
In a 0.10 mol/L solution of ethanoic acid, only about 1 molecule in 100 is ionized at any moment (1.3%); the rest stay whole. This is what "weak acid" means.

Reactions of carboxylic acids

Neutralization with bases gives a salt and water:

CH₃COOH + NaOH → CH₃COONa + H₂O

With carbonates and hydrogen carbonates they give a salt, water and carbon dioxide (the fizz when vinegar meets baking soda):

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Making them: oxidize a primary alcohol or an aldehyde (Lesson 9): CH₃CH₂OH → CH₃CHO → CH₃COOH. This is also what happens when wine left open turns to vinegar: bacteria oxidize the ethanol.

Carboxylic acids also react with alcohols to make esters (Lesson 11) and with amines to make amides (Lesson 12).

Worked example

Example 2: Neutralization equations

Write balanced equations, naming the organic product: (a) propanoic acid + potassium hydroxide; (b) methanoic acid + sodium carbonate.

  1. (a) One acid H reacts with one OH⁻: CH₃CH₂COOH + KOH → CH₃CH₂COOK + H₂O.
  2. (b) Carbonate (CO₃²⁻) can react with two acid H atoms: 2 HCOOH + Na₂CO₃ → 2 HCOONa + H₂O + CO₂.

(a) potassium propanoate + water; (b) sodium methanoate + water + carbon dioxide.

Worked example

Example 3: How strong is your vinegar?

A 10.00 mL sample of white vinegar is titrated with 0.200 mol/L NaOH. The endpoint (phenolphthalein turns pink) is reached after 41.60 mL. Find the concentration of ethanoic acid in mol/L and in g per 100 mL.

  1. n(NaOH) = c × V = 0.200 mol/L × 0.04160 L = 0.008320 mol.
  2. CH₃COOH + NaOH → CH₃COONa + H₂O is 1 : 1, so n(CH₃COOH) = 0.008320 mol.
  3. c = n ÷ V = 0.008320 mol ÷ 0.01000 L = 0.8320 mol/L.
  4. Mass per litre = 0.8320 mol/L × 60.06 g/mol = 49.97 g/L. Divide by 10 for the mass in 100 mL: 4.997 g per 100 mL.

0.832 mol/L, or about 5.00 g per 100 mL (5% acetic acid, as most Canadian white vinegar labels say).

Hands-on activity

Red cabbage acid test

You need: 2 or 3 red cabbage leaves, a kettle, a heat-proof bowl, a strainer, 5 clear glasses, white vinegar, lemon juice, baking soda, water, a teaspoon, and a grown-up. Grown-up note: an adult should pour the boiling water and do any chopping. Let the liquid cool before using it.

  1. Tear the leaves into small pieces, cover them with about 500 mL of boiling water, and let them steep for 10 minutes. Strain. The purple liquid is a natural acid-base indicator.
  2. Put 50 mL of indicator into each of 5 glasses.
  3. Add: (1) nothing (control), (2) 1 teaspoon of vinegar, (3) 1 teaspoon of lemon juice, (4) 1 teaspoon of baking soda, (5) 1 teaspoon of vinegar, then slowly baking soda until the fizzing stops.
  4. Record the colour of each glass and the fizzing in glass 5.
  5. Write the equation for the reaction in glass 5 and name the salt formed.

What to look for: vinegar (ethanoic acid) and lemon juice (citric acid) turn the indicator pink-red; baking soda turns it blue-green. In glass 5 the fizzing is CO₂: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂, forming sodium ethanoate. As the acid is used up, the colour moves back toward purple.

Just for fun

Methanoic acid's common name, formic acid, comes from formica, the Latin word for ant. In the 1600s a naturalist first collected it by distilling a large number of ants, which is a commitment to science few of us would make.

Your turn

1

Name CH₃CH₂COOH.

2

Draw (condensed) 2-methylbutanoic acid.

3

Name the salt CH₃COOK.

4

Why does ethanoic acid (60 g/mol) boil higher than propan-1-ol (60 g/mol)?

5

Write the equation for butanoic acid with NaOH.

6

Which alcohol would you oxidize to make butanoic acid?

7

Is a 0.10 mol/L ethanoic acid solution mostly CH₃COOH molecules or mostly ions? Explain.

More practice: warm-up

1

What is the functional group of a carboxylic acid?

2

What ending does a carboxylic acid name have?

3

Give the common name of ethanoic acid.

4

What gas forms when an acid reacts with baking soda?

5

What ending is used for the salt of a carboxylic acid?

More practice: on your own

1

Name HOOCCH₂CH₂COOH.

2

Rank by boiling point: propanoic acid, butan-1-ol, pentane (all about 72 to 74 g/mol). Explain.

3

Write a balanced equation for ethanoic acid reacting with calcium carbonate (as when vinegar dissolves kettle scale).

4

A 25.00 mL sample of 0.150 mol/L propanoic acid is titrated with 0.120 mol/L NaOH. What volume of NaOH reaches the equivalence point?

5

Why does sodium benzoate dissolve in water much better than benzoic acid?

Challenge

1

Stearic acid, C₁₇H₃₅COOH, and oleic acid, C₁₇H₃₃COOH, are both C₁₈ fatty acids. Stearic acid melts at about 69 °C and oleic acid at about 13 °C. Use structure to explain the difference.

2

A student titrates 10.00 mL of vinegar with 0.200 mol/L NaOH and gets 39.85 mL, 41.60 mL and 41.55 mL in three trials. Which trials should be averaged, what is the concentration (with correct significant figures), and what might explain the odd trial?

3

Explain why the H in –COOH is acidic but the H in the –OH of ethanol is not, in terms of the ion left behind.

Check your understanding

Quiz

1.

What is the IUPAC name of CH₃CH₂CH₂CH₂COOH?

2.

Which compound has the highest boiling point?

3.

What are the products of CH₃COOH + NaHCO₃?

4.

Why is ethanoic acid called a weak acid?

5.

What is the name of CH₃CH₂COONa?

6.

Which compound is oxidized to propanoic acid?

For parents and teachers

Big ideas (B2.2, B3.2, B3.3): the carboxyl group –COOH makes these compounds weak acids with high boiling points (strong hydrogen bonding, dimers). They are named with -oic acid, their salts with -oate, and they are made by oxidizing primary alcohols or aldehydes.

How to help: Cook with your teen and point out the acids: acetic acid in vinegar, citric acid in lemons, lactic acid in yogurt. Ask, "Where is the –COOH?"

Common difficulties:

  • Numbering the –COOH carbon (it is always C1 and is not numbered in the name).
  • Confusing weak (partly ionized) with dilute (low concentration).
  • Thinking all the H atoms in CH₃COOH are acidic.
  • Unit conversions in titration problems (mL to L).

Safety: the red cabbage activity uses boiling water; an adult should handle it.