Comprehensive Study Guide to Alkanoic Acids and Ethanoic Acid Chemistry

Introduction and Molecular Structure of Alkanoic Acids

Essential Curriculum Questions:
- State the natural sources and molecular structures of alkanoic acids.
- State the physical properties of alkanoic acids.
- Describe the chemical properties of alkanoic acids with balanced chemical equations.
- Describe a standard laboratory test for identifying alkanoic acids.
- Highlight the industrial and domestic uses of named alkanoic acids.
General Molecular Formula:
- The homologous series of alkanoic acids conforms to the general molecular formula: $C_nH_{2n+1}COOH$ where $n \ge 0$ .
- For methanoic acid ( $HCOOH$ ), which is the simplest and the first member of the family, $n = 0$ : $CH_{2 \times 0 + 1}COOH = HCOOH$
- For ethanoic acid ( $CH_3COOH$ ), which is the second member of the family, $n = 1$ : $C_1H_{2 \times 1 + 1}COOH = CH_3COOH$
Functional Group Structure:
- Alkanoic acids possess the carboxylic functional group ( $-COOH$ ).
- This group is composed of two distinct parts bonded to the same carbon atom: a carbonyl group ( $-C=O$ ) and a hydroxyl group ( $-OH$ ).

Physical Properties and Homologous Series Trends

Boiling Point Progression:
- Physical properties vary progressively as the molecular sizes, molar masses, or the number of alkyl groups in the molecules increase.
- Boiling points increase with carbon chain length in the order: $\text{Ethanoic acid} < \text{Propanoic acid} < \text{Butanoic acid}$
Intermolecular Hydrogen Bonding:
- Due to the simultaneous presence of the carbonyl ( $-C=O$ ) and hydroxyl ( $-OH$ ) functional groups, alkanoic acids form two hydrogen bonds per pair of molecules (forming stable dimers).
- Consequently, alkanoic acids possess significantly higher boiling points than corresponding alkanols (which form only one hydrogen bond per molecule) and corresponding alkanes (which have no hydrogen bonding, possessing only weak van der Waals forces).
- Compounds containing hydrogen bonding exhibit very high boiling points relative to non-polar organic molecules of comparable molar mass.
Aqueous Solubility:
- Solubility in water decreases progressively as the molecular sizes and molar masses of the members increase.
- The lower members of the series are completely miscible and very soluble in water.
- High water solubility of the lower members is due to the polar $-C=O$ and $-OH$ functional groups, which readily form intermolecular hydrogen bonds with water molecules.
Litmus Paper Reaction:
- Aqueous solutions of alkanoic acids turn blue litmus paper red.

Acidic Characteristics and Natural Sources

Acidic Behavior in Solution:
- Alkanoic acids contain the $-COOH$ functional group, in which the hydrogen atom of the $-OH$ group ionizes in aqueous solution to yield hydronium/hydrogen ions ( $H^+$ ).
- Because of this ionization, alkanoic acids exhibit all the typical acidic properties of inorganic mineral acids:
  - Characteristic action on litmus paper (turning blue litmus red).
  - Readily soluble in aqueous media.
  - Effervescent displacement reactions with electropositive metals.
  - Effervescent gas evolution when reacted with metal trioxocarbonates (IV) and hydrogen trioxocarbonates (IV).
  - Neutralization reactions with alkalis to form salts and water.
Natural Sources of Key Alkanoic Acids:
- Ethanoic acid: Naturally present in vinegar.
- Latic acid (2-hydroxypropanoic acid): Naturally present in sour milk.

Structural formula of lactic acid (2-hydroxypropanoic acid)

*   Butanoic acid: Naturally present in rancid butter.
*   Citric acid (2-hydroxypropan-1,2,3-trioic acid): Naturally present in citrus fruits such as oranges.
*   Ethanedioic acid: Naturally present in cabbage, tomatoes, sweet potatoes, and spinach.
*   Fatty acids: Naturally present in biological fats and oils.

Classification of Alkanoic Acids

Basis of Classification:
- Alkanoic acids are categorized into three distinct classes based entirely on the number of carboxylic functional groups ( $-COOH$ groups) contained in each molecule:
  - Monocarboxylic acids
  - Dicarboxylic acids
  - Tricarboxylic acids
Monocarboxylic Acids:
- Definition: Alkanoic acids that contain exactly one carboxylic functional group per molecule of the acid.
- Prevalence: They represent the most common class of alkanoic acids.
- Representative lower members include:
  - Methanoic acid ( $HCOOH$ )

Structural formula of methanoic acid

    *   Ethanoic acid ( $CH_3COOH$ )
    *   Propanoic acid ( $C_2H_5COOH$  or  $CH_3CH_2COOH$ )

Structural formula of propanoic acid

    *   Butanoic acid ( $C_3H_7COOH$  or  $CH_3(CH_2)_2COOH$ )

Structural formula of butanoic acid

    *   Pentanoic acid ( $CH_3(CH_2)_3COOH$ ), and higher homologs.

Survey of Straight-Chain Saturated Monocarboxylic Acids (Carbons 1 to 20):

Table of straight-chain saturated carboxylic acids from 1 to 20 carbons

*    $1$  Carbon: Common Name: Formic acid; IUPAC Name: Methanoic acid; Chemical Formula:  $HCOOH$ ; Common location or use: Insect stings.
*    $2$  Carbons: Common Name: Acetic acid; IUPAC Name: Ethanoic acid; Chemical Formula:  $CH_3COOH$ ; Common location or use: Vinegar.
*    $3$  Carbons: Common Name: Propionic acid; IUPAC Name: Propanoic acid; Chemical Formula:  $CH_3CH_2COOH$ ; Common location or use: Preservative for stored grains, body odour, milk, butter, cheese.
*    $4$  Carbons: Common Name: Butyric acid; IUPAC Name: Butanoic acid; Chemical Formula:  $CH_3(CH_2)_2COOH$ ; Common location or use: Butter.
*    $5$  Carbons: Common Name: Valeric acid; IUPAC Name: Pentanoic acid; Chemical Formula:  $CH_3(CH_2)_3COOH$ ; Common location or use: Valerian plant.
*    $6$  Carbons: Common Name: Caproic acid; IUPAC Name: Hexanoic acid; Chemical Formula:  $CH_3(CH_2)_4COOH$ ; Common location or use: Goat fat.
*    $7$  Carbons: Common Name: Enanthic acid; IUPAC Name: Heptanoic acid; Chemical Formula:  $CH_3(CH_2)_5COOH$ ; Common location or use: Fragrance.
*    $8$  Carbons: Common Name: Caprylic acid; IUPAC Name: Octanoic acid; Chemical Formula:  $CH_3(CH_2)_6COOH$ ; Common location or use: Coconuts.
*    $9$  Carbons: Common Name: Pelargonic acid; IUPAC Name: Nonanoic acid; Chemical Formula:  $CH_3(CH_2)_7COOH$ ; Common location or use: Pelargonium plant.
*    $10$  Carbons: Common Name: Capric acid; IUPAC Name: Decanoic acid; Chemical Formula:  $CH_3(CH_2)_8COOH$ ; Common location or use: Coconut and Palm kernel oil.
*    $11$  Carbons: Common Name: Undecylic acid; IUPAC Name: Undecanoic acid; Chemical Formula:  $CH_3(CH_2)_9COOH$ ; Common location or use: Anti-fungal agent.
*    $12$  Carbons: Common Name: Lauric acid; IUPAC Name: Dodecanoic acid; Chemical Formula:  $CH_3(CH_2)_{10}COOH$ ; Common location or use: Coconut oil and hand wash soaps.
*    $13$  Carbons: Common Name: Tridecylic acid; IUPAC Name: Tridecanoic acid; Chemical Formula:  $CH_3(CH_2)_{11}COOH$ ; Common location or use: Plant metabolite.
*    $14$  Carbons: Common Name: Myristic acid; IUPAC Name: Tetradecanoic acid; Chemical Formula:  $CH_3(CH_2)_{12}COOH$ ; Common location or use: Nutmeg.
*    $15$  Carbons: Common Name: Pentadecylic acid; IUPAC Name: Pentadecanoic acid; Chemical Formula:  $CH_3(CH_2)_{13}COOH$ ; Common location or use: Milk fat.
*    $16$  Carbons: Common Name: Palmitic acid; IUPAC Name: Hexadecanoic acid; Chemical Formula:  $CH_3(CH_2)_{14}COOH$ ; Common location or use: Palm oil.
*    $17$  Carbons: Common Name: Margaric acid; IUPAC Name: Heptadecanoic acid; Chemical Formula:  $CH_3(CH_2)_{15}COOH$ ; Common location or use: Pheromone in various animals.
*    $18$  Carbons: Common Name: Stearic acid; IUPAC Name: Octadecanoic acid; Chemical Formula:  $CH_3(CH_2)_{16}COOH$ ; Common location or use: Chocolate, waxes, soaps, and oils.
*    $19$  Carbons: Common Name: Nonadecylic acid; IUPAC Name: Nonadecanoic acid; Chemical Formula:  $CH_3(CH_2)_{17}COOH$ ; Common location or use: Fats, vegetable oils, pheromone.
*    $20$  Carbons: Common Name: Arachidic acid; IUPAC Name: Icosanoic acid; Chemical Formula:  $CH_3(CH_2)_{18}COOH$ ; Common location or use: Peanut oil.

Dicarboxylic Acids:
- Definition: Alkanoic acids containing two carboxylic acid functional groups per molecule of the acid.
- Primary Representative: Ethane-1,2-dioic acid (IUPAC name), known by the common name oxalic acid.

Structural formula of ethane-1,2-dioic acid (oxalic acid)

*   Applications and Natural Distribution:
    *   Used in the manufacture of terylene, a polyester polymer widely used as a synthetic fibre in the textile industry.
    *   Used as a primary standard in analytical laboratory acid-base titrations.
    *   Found in natural vegetable matter such as cabbage, tomatoes, spinach, and sweet potatoes.

Tricarboxylic Acids:
- Definition: Alkanoic acids containing three carboxylic functional groups per molecule of the acid.
- Primary Representative: 2-hydroxypropan-1,2,3-trioic acid (IUPAC name), formerly and commonly called citric acid.

Structural formula of 2-hydroxypropan-1,2,3-trioic acid (citric acid)

*   Occurrence: Concentrated naturally in citrus oranges and related citrus fruits.

General Chemical Reactions of Alkanoic Acids

Reaction with Metal Carbonates and Hydrogen Carbonates:
- Alkanoic acids produce vigorous effervescence with the evolution of colourless bubbles of carbon dioxide gas ( $CO_2$ ) when treated with saturated solutions of sodium trioxocarbonate (IV) ( $Na_2CO_3$ ) or sodium hydrogen trioxocarbonate (IV) ( $NaHCO_3$ ).
- Reaction of Propanoic Acid with Sodium Trioxocarbonate (IV): $2C_2H_5COOH_{(l)} + Na_2CO_3 \rightarrow 2C_2H_5COONa + CO_2 + H_2O$
- Reaction of Butanoic Acid with Sodium Hydrogen Trioxocarbonate (IV): $C_3H_7COOH_{(l)} + NaHCO_3 \rightarrow C_3H_7COONa + CO_2 + H_2O$
Esterification Reactions:
- Alkanoic acids react reversibly with alkanols in the presence of an inorganic acid catalyst ( $H^+$ ) under reflux/heat to produce pleasant-smelling esters (alkanoates) and water.
- Propanoic acid reacting with ethanol: C_2H_5COOH_{(l)} + C_2H_5OH_{(aq)} \xrightarrow{H^+ / \text{Heat}} C_2H_5COOC_2H_5_{(l)} + H_2O_{(l)} yielding ethyl propanoate.
- Butanoic acid reacting with ethanol: C_3H_7COOH_{(l)} + C_2H_5OH_{(aq)} \xrightarrow{H^+ / \text{Heat}} C_3H_7COOC_2H_5_{(l)} + H_2O_{(l)} yielding ethyl butanoate.
Neutralization Reactions with Alkalis:
- Alkanoic acids react with sodium hydroxide ( $NaOH$ ) solutions to yield alkanoate salts and water.
- Propanoic acid neutralization: $C_2H_5COOH_{(l)} + NaOH_{(aq)} \rightarrow C_2H_5COONa + H_2O$ yielding sodium propanoate.
- Butanoic acid neutralization: $C_3H_7COOH_{(l)} + NaOH_{(aq)} \rightarrow C_3H_7COONa + H_2O$ yielding sodium butanoate.
Reduction to Alkanols:
- Alkanoic acids undergo reduction in the presence of nascent hydrogen ( $[H]$ ), lithium aluminumhydride ( $LiAlH_4$ ), or sodium bromohydride ( $NaBH_4$ ) to yield corresponding primary alkanols: $C_2H_5COOH + 4[H] \rightarrow C_3H_7OH + H_2O$
Halogen Substitution Reactions:
- Alkanoic acids undergo substitution reactions with chlorine gas ( $Cl_2$ ) in the presence of ultraviolet sunlight or boiling heat to form chloro-substituted alkanoic acids and hydrogen chloride gas.
- Propanoic acid chlorination: $C_2H_5COOH_{(l)} + Cl_2 \rightarrow C_2H_4ClCOOH + HCl_{(g)}$ producing 2-chloropropanoic acid.
- Butanoic acid chlorination: $C_3H_7COOH_{(l)} + Cl_2 \rightarrow C_3H_6ClCOOH + HCl_{(g)}$ producing 2-chlorobutanoic acid.
Substitution Reactions with Electropositive Metals:
- Alkanoic acids react directly with strongly electropositive metals (such as sodium, $Na$ ) to liberate colourless bubbles of hydrogen gas ( $H_2$ ) with effervescence.
- Reaction of propanoic acid with sodium: $2C_2H_5COOH + 2Na \rightarrow 2C_2H_5COONa + H_2$ yielding sodiumpropanoate.
- Reaction of butanoic acid with sodium: $2C_3H_7COOH + 2Na \rightarrow 2C_3H_7COONa + H_2$ yielding sodiumbutanoate.
Substitution Reactions with Phosphorus Halides:
- Alkanoic acids react with phosphorus trichloride ( $PCl_3$ ) and phosphorus pentachloride ( $PCl_5$ ) to produce acyl chlorides (ethonyl chloride).

Properties and Synthesis of Ethanoic Acid

Molecular Identity and Characteristics:
- Ethanoic acid is the second and most commercially common member of the alkanoic acid family.
- Represented by the general formula where $n = 1$ : $C_1H_{2(1)+1}COOH \rightarrow CH_3COOH$
- Molecular Formula: $CH_3COOH$ or $C_2H_4O_2$
- Empirical Formula: $CH_2O$
- Functional Group: Carboxylic group ( $-COOH$ )

Structural formula of ethanoic acid

Natural Sources and Industrial Applications:
- Source: Constituent of vinegar.
- Solvent use: Employed extensively as an organic solvent.
- Polymer and chemical manufacturing: Used in the synthesis of cellulose acetate, an essential intermediate utilized in commercial paints and varnishes.
Laboratory Preparation by Ester Hydrolysis:
- Prepared via the hydrolysis of ethyl ethanoate in the presence of concentrated sulphuric acid catalyst ( $\text{Conc. } H_2SO_4$ ), with the reaction mixture heated to a boil: CH_3COOC_2H_5_{(l)} + H_2O \xrightarrow{H^+ / \text{Heat}} CH_3COOH_{(l)} + C_2H_5OH_{(aq)}
- Apparatus Requirement: The distillation must be carried out in an all-glass apparatus (e.g., a glass retort flask) because hot ethanoic acid vapor vigorously corrodes and destroys rubber and cork fittings.
- Catalyst Restrictions: Dry concentrated hydrochloric acid ( $\text{Conc. } HCl$ ) cannot be used as an acid catalyst because it is highly volatile and will co-distil with the ethanoic acid vapour, contaminating the distillate.
Oxidation Route Apparatus:
- Ethanoic acid can also be prepared through the reflux oxidation of ethanol using acidified potassium dichromate (VI) ( $K_2Cr_2O_7$ ) and concentrated sulphuric acid in a water bath fitted with a Liebig condenser.

Reflux apparatus for the oxidation of ethanol using a Liebig condenser and water bath

Chemical Reactions and Diagnostic Tests of Ethanoic Acid

Diagnostic Test for the Carboxylic Acid Group ( $-COOH$ ):
- Ethanoic acid reacts with a saturated solution of sodium trioxocarbonate (IV) ( $Na_2CO_3$ ) or sodium hydrogen trioxocarbonate (IV) ( $NaHCO_3$ ) to produce brisk effervescence of colourless carbon dioxide gas ( $CO_2$ ) and aqueous sodium ethanoate: $2CH_3COOH_{(aq)} + Na_2CO_3{(aq)} \rightarrow 2CH_3COONa_{(aq)} + CO_2{(g)} + H_2O_{(l)}$
- Reaction with Solid Calcium Carbonate: When solid $CaCO_3$ is added, the solid dissolves completely with effervescence of colourless $CO_2$ gas to form a clear calcium ethanoate solution.
- Analytical Utility: The evolution of carbon dioxide gas from aqueous $Na_2CO_3$ or $NaHCO_3$ serves as the definitive qualitative laboratory test confirming the presence of the $-COOH$ functional group in an unknown organic sample.
Esterification Reaction:
- Reacts with ethanol when heated to boil in the presence of concentrated sulphuric acid ( $\text{Conc. } H_2SO_4$ ), forming ethyl ethanoate and water: $CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$
Neutralization Reaction:
- Neutralizes sodium hydroxide solution to form sodium ethanoate and water: $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$
- This organic neutralization mirrors inorganic acid-alkali reactions: neutral salt solutions are generated, and the process is strictly exothermic.
Substitution Reaction with Electropositive Metals:
- Sodium metal dissolves vigorously in ethanoic acid with effervescence, liberating colourless, odourless hydrogen gas ( $H_2$ ) and leaving an aqueous solution of sodium ethanoate: $2CH_3COOH_{(aq)} + 2Na_{(s)} \rightarrow 2CH_3COONa_{(aq)} + H_2{(g)}$
Substitution Reaction with Chlorine Gas:
- Reacts with chlorine gas in the presence of sunlight or when heated to boil to yield 2-chloroethanoic acid, substituting a hydrogen atom on the methyl carbon: $CH_3COOH_{(aq)} + Cl_{2(g)} \rightarrow CH_2ClCOOH_{(l)} + HCl_{(g)}$
Substitution with Phosphorus Pentachloride ( $PCl_5$ ):
- Reacts with liquid phosphorus pentachloride to yield ethanoyl chloride as the principal product, alongside hydrogen chloride gas and phosphorus oxychloride: $CH_3COOH_{(aq)} + PCl_{5(l)} \rightarrow CH_3COCl_{(l)} + HCl_{(g)} + POCl_3$
Substitution with Phosphorus Trichloride ( $PCl_3$ ):
- Reacts with phosphorus trichloride to yield ethonyl chloride and phosphorous acid: $3CH_3COOH_{(aq)} + PCl_3 \rightarrow 3CH_3COCl_{(l)} + H_3PO_4{(aq)}$
Reduction to Ethanol:
- Ethanoic acid is reduced to ethanol by reducing agents such as nascent hydrogen ( $Zn/NaOH$ ), aqueous sodium bromohydride ( $NaBH_4$ ), or lithium aluminumhydride ( $LiAlH_4$ ) in dry ether: $CH_3COOH \xrightarrow{[H]} C_2H_5OH + H_2O$

Review Questions and Academic Exercises

Preparative Laboratory Review Questions:
- Question: State why hydrochloric acid ( $HCl$ ) cannot be used as the catalyst in the preparation of ethanoic acid.
  - Response: Dry concentrated hydrochloric acid is volatile; upon heating, it readily vaporizes and distils over simultaneously with the ethanoic acid vapour, contaminating the final product.
- Question: Explain why an all-glass apparatus is mandatory in the preparation of ethanoic acid.
  - Response: An all-glass apparatus (such as a glass retort flask) is required because hot ethanoic acid vapour vigorously corrodes and attacks laboratory rubber tubing and cork stoppers.
Prescribed Homework Assignments:
- Reference Text: Essential Chemistry
  - Exercise 1: Answer questions $2$ and $3$ on page $592$ and page $593$ .
  - Exercise 2: Answer question $20$ on page $595$ .