10.2 Structures of Alcohols, Carboxylic Acids and Esters

A free GCSE Chemistry lesson for AQA (8462) from OBA School: functional groups, homologous series, and the names and structures of the first four alcohols and carboxylic acids and of ethyl ethanoate. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Recognise the functional groups of alcohols (–OH), carboxylic acids (–COOH) and esters (–COO–)
• Name and draw the first four alcohols and the first four carboxylic acids
• Draw the structure of ethyl ethanoate and explain where each part comes from
• Write molecular, structural and displayed formulae and use them to calculate Mr

Key words

• Homologous series: A family of compounds with the same functional group and general formula, and similar chemical properties.
• Functional group: The atom or group of atoms that decides how a compound reacts.
• Alcohol: A compound containing the –OH functional group. Names end in “-ol”.
• Carboxylic acid: A compound containing the –COOH functional group. Names end in “-oic acid”.
• Ester: A compound containing the –COO– functional group, made from an alcohol and a carboxylic acid.
• Displayed formula: A drawing showing every atom and every bond.

Homologous series and functional groups

Organic compounds are grouped into families called homologous series. Members of a series have the same functional group, so they react in similar ways. Each member differs from the next by CH₂.

• Alkanes; Functional group C–C (single bonds only); Name ending -ane; Example ethane, C₂H₆
• Alkenes; Functional group C=C; Name ending -ene; Example ethene, C₂H₄
• Alcohols; Functional group –OH; Name ending -ol; Example ethanol, C₂H₅OH
• Carboxylic acids; Functional group –COOH; Name ending -oic acid; Example ethanoic acid, CH₃COOH
• Esters; Functional group –COO–; Name ending -yl …-oate; Example ethyl ethanoate, CH₃COOC₂H₅

Key fact: The stem of the name still counts the carbon atoms: meth- 1, eth- 2, prop- 3, but- 4. The ending tells you the functional group.

The alcohols

Alcohols contain the –OH group bonded to a carbon atom. Their general formula is CnH₂n₊₁OH. To name one, take the alkane with the same number of carbon atoms and change the ending “-ane” to “-anol”: ethane → ethanol. Notice that the O–H bond is drawn in full in a displayed formula.

Worked example: relative formula mass of ethanol (C = 12, H = 1, O = 16)

Ethanol is C₂H₅OH, which contains 2 C, 6 H and 1 O.
Mr = (2 × 12) + (6 × 1) + 16 = 46

Answer: Mr = 46

Exam tip: Count all the hydrogen atoms, including the one in –OH. Ethanol has six H atoms, not five.

The carboxylic acids

Carboxylic acids contain the –COOH group. In this group the carbon atom has a double bond to one oxygen atom and a single bond to an –OH group. The carbon in –COOH counts in the name: ethanoic acid, CH₃COOH, has two carbon atoms. Ethanoic acid is the acid in vinegar.

Esters: ethyl ethanoate

An ester is made when an alcohol reacts with a carboxylic acid. Ethyl ethanoate is made from ethanol and ethanoic acid. The ester link, –COO–, joins the two parts.

Worked example: percentage by mass (C = 12, H = 1, O = 16)

Calculate the percentage by mass of oxygen in ethyl ethanoate, C₄H₈O₂.
Mr = (4 × 12) + (8 × 1) + (2 × 16) = 48 + 8 + 32 = 88
% oxygen = 32 ÷ 88 × 100 = 36.36…

Answer: 36.4%

Different ways to write a formula

• Molecular; Shows the number of each type of atom; Ethanol C₂H₆O; Ethanoic acid C₂H₄O₂
• Structural (condensed); Shows what is bonded to each carbon atom, with the functional group clear; Ethanol CH₃CH₂OH or C₂H₅OH; Ethanoic acid CH₃COOH
• Displayed; Shows every atom and every bond; Ethanol see diagrams; Ethanoic acid see diagrams

Molecular formulae can hide the functional group. Ethyl ethanoate and butanoic acid both have the molecular formula C₄H₈O₂, but they belong to different families. Structural formulae such as CH₃COOC₂H₅ and C₃H₇COOH show the difference.

Exam tip: Structural formulae can be written in full or shortened: CH₃CH₂CH₂COOH is the same as C₃H₇COOH (butanoic acid), and CH₃COOCH₂CH₃ is the same as CH₃COOC₂H₅. Read along the chain and count every carbon.

Common mistakes

• Calling ethanoic acid “ethanol acid”, or mixing up ethanol (alcohol) and ethanoic acid. The ending tells you the family.
• Forgetting to count the C in –COOH. Propanoic acid has three carbon atoms: C₂H₅COOH.
• Calling alcohols alkalis or bases because of the “OH”. The –OH in an alcohol is covalently bonded; it is not a hydroxide ion.
• Naming esters the wrong way round. The alcohol part (“ethyl”) comes first and the acid part (“ethanoate”) second.

Revision questions

Try these, then check your answers below each one.

Question 1. Name the alcohol with the formula C₃H₇OH.

Answer: propanol

Working: Propanol (3 carbon atoms, –OH group).

Question 2. What is the formula of ethanoic acid? (Choose from: C₂H₅OH, CH₃COOH, HCOOH, C₂H₅COOH.)

Answer: CH₃COOH

Working: CH₃COOH has two carbons. C₂H₅OH is ethanol, HCOOH is methanoic acid and C₂H₅COOH is propanoic acid.

Question 3. Calculate the relative formula mass of butanol, C₄H₉OH. (C = 12, H = 1, O = 16)

Answer: 74

Working: (4 × 12) + (10 × 1) + 16 = 48 + 10 + 16 = 74.

Related lessons

• Previous lesson: 10.1 Reactions of the Alkenes
• Next lesson: 10.3 Reactions and Uses of Alcohols
• In this chapter: 10.4 Carboxylic Acids and Esters

Continue in the full course

This lesson is section 10.2 (AGCCH1002) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.