Alcohols and Carboxylic Acids

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alcohols: Contain the –OH functional group bonded to a saturated carbon. General formula: $C_nH_{2n+1}OH$ or $ROH$.

Classification of Alcohols:

• Primary (1°): –OH on carbon attached to ONE other carbon (e.g., ethanol: $CH_3CH_2OH$)
• Secondary (2°): –OH on carbon attached to TWO other carbons (e.g., propan-2-ol: $CH_3CH(OH)CH_3$)
• Tertiary (3°): –OH on carbon attached to THREE other carbons (e.g., 2-methylpropan-2-ol)

⚡ WAEC Exam Tip: WAEC Paper 2 frequently asks you to classify an alcohol given its structure. Count how many carbon atoms are bonded to the carbon bearing the –OH group. This determines whether it is 1°, 2°, or 3°.

Carboxylic Acids: Contain the –COOH functional group. General formula: $RCOOH$.

Key Members:

• Methanoic acid (formic acid): $HCOOH$ — ant venom, stinging nettles
• Ethanoic acid (acetic acid): $CH_3COOH$ — vinegar (5–8% solution)
• Propanoic acid: $C_2H_5COOH$ — bread preservative

Distinguishing Test: Sodium hydrogencarbonate ($NaHCO_3$) test — carboxylic acids effervesce (produce $\ CO_2$) with $NaHCO_3$; alcohols do not. $$RCOOH + NaHCO_3 \rightarrow RCOONa + H_2O + CO_2 \uparrow$$

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Reactions of Alcohols:

Oxidation: Alcohols are oxidised by acidified potassium dichromate(VI), $K_2Cr_2O_7/H^+$. The colour changes from orange to green.

• Primary alcohol $\rightarrow$ aldehyde $\rightarrow$ carboxylic acid $$CH_3CH_2OH \xrightarrow[H^+]{K_2Cr_2O_7} CH_3COOH$$
• Secondary alcohol $\rightarrow$ ketone (no further oxidation) $$CH_3CH(OH)CH_3 \xrightarrow[H^+]{K_2Cr_2O_7} CH_3COCH_3$$
• Tertiary alcohol: NO oxidation (no hydrogen on the –OH carbon)

Dehydration: Concentrated sulphuric acid removes water from alcohols at 170°C to give alkenes (elimination). At 140°C, it gives an ether instead. $$CH_3CH_2OH \xrightarrow{conc\ H_2SO_4, 170°C} CH_2=CH_2 + H_2O$$

Esterification: Alcohol + carboxylic acid $\xrightarrow{H_2SO_4}$ ester + water. $$CH_3COOH + C_2H_5OH \xrightarrow{H_2SO_4} CH_3COOC_2H_5 + H_2O$$ (Ethyl ethanoate — banana smell)

⚡ WAEC Exam Tip: The esterification reaction is reversible. WAEC questions ask you to identify the role of concentrated sulphuric acid as a catalyst and dehydrating agent (it removes water, shifting equilibrium to the right). Do not write “oxidising agent.”

Reactions of Carboxylic Acids:

1. Reaction with carbonates/hydrogencarbonates: $2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2$
2. Reaction with alkalis: $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$
3. Esterification (with alcohols): Same as above — acid acts as proton donor
4. Reduction: Lithium aluminium hydride ($LiAlH_4$) reduces carboxylic acids to primary alcohols (not in WAEC scope for reagent names, but the product is important)

Preparation:

• Ethanol: Fermentation of glucose (yeast, 30–37°C anaerobic): $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$
• Ethanoic acid: Oxidation of ethanol with $K_2Cr_2O_7/H^+$ or by bacterial oxidation of ethanol in wine to vinegar

Physical Properties:

• Lower alcohols (methanol, ethanol) are miscible with water due to hydrogen bonding
• Boiling points decrease as the hydrocarbon chain length increases (hydrogen bonding decreases)
• Carboxylic acids have higher boiling points than alcohols of similar molar mass due to dimerisation via two hydrogen bonds

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanism of Esterification: The reaction between a carboxylic acid and an alcohol proceeds via nucleophilic acyl substitution. The key steps:

1. Protonation of the carbonyl oxygen of the carboxylic acid (makes the carbonyl carbon more electrophilic)
2. Nucleophilic attack by the alcohol oxygen lone pair
3. Proton transfers within the tetrahedral intermediate
4. Loss of water to form the ester

The dehydrating agent (concentrated $H_2SO_4$) removes the water produced, driving the equilibrium forward according to Le Chatelier’s principle. This is the core explanation WAEC examiners want.

Oxidation Products — Systematic Summary:

Alcohol Type	Reagent	Product Class	Further Oxidation?
Primary	$K_2Cr_2O_7/H^+$	Aldehyde (distil off)	Yes → acid
Secondary	$K_2Cr_2O_7/H^+$	Ketone	No
Tertiary	$K_2Cr_2O_7/H^+$	No reaction	N/A

Acid Strength in Carboxylic Acids: Carboxylic acids are weak acids. The acidity arises from resonance stabilisation of the carboxylate anion ($RCOO^-$) formed after proton donation. Electron-donating groups (e.g., –CH₃) decrease acidity; electron-withdrawing groups (e.g., –Cl, –NO₂) increase acidity. Methanoic acid is slightly stronger than ethanoic acid because the methyl group in ethanoic acid is weakly electron-donating.

Comparative Study — Alcohols vs Carboxylic Acids:

Property	Alcohols	Carboxylic Acids
Functional group	–OH	–COOH
Action with $NaHCO_3$	No reaction	Effervescence of $\ CO_2$
Oxidation product (1°)	Aldehyde → acid	Cannot oxidise further
Ester formation	With acids	With alcohols
pH of aqueous solution	~6–7 (neutral)	~2–4 (weakly acidic)
$K_2Cr_2O_7/H^+$ test	Colour changes (orange→green)	No colour change

WAEC Past Question Patterns: WASSCE Paper 2 frequently asks:

• Classifying an alcohol as primary, secondary, or tertiary from its structural formula
• Writing balanced equations for esterification
• Identifying the dehydrating agent and its role in esterification
• Distinguishing between alcohols and carboxylic acids using $NaHCO_3$ or $K_2Cr_2O_7/H^+$
• Calculating the molecular formula of an ester given the reacting alcohol and acid (e.g., ethyl propanoate = $C_2H_5COOCH_3$)

⚡ WAEC Exam Tip: When drawing the structural formula of a carboxylic acid, ensure the –COOH group is written distinctly from –OH. Students often lose marks by writing –CHO (aldehyde) or –COO– (ester linkage) incorrectly. The –COOH has a carbonyl (C=O) AND a hydroxyl (–OH) on the same carbon.