Chemistry Form 4 Notes : ORGANIC CHEMISTRY 2 – ALKANOLS (Alcohols)

ORGANIC CHEMISTRY 2

ALKANOLS (Alcohols)

(A) INTRODUCTION

Alkanols belong to a homologous series of organic compounds with the general formula C_nH_2n+1OH, featuring the -OH functional group. The first ten alkanols include:

Alkanols, like hydrocarbons (alkanes, alkenes, alkynes), form a homologous series characterized by the following:

• The general name is derived from the alkane name ending with “-ol“.
• The members have the –OH functional group.
• They share the general formula represented by R–OH, where R is an alkyl group.
• Each member differs from the next or previous by a –CH₂– group.
• They exhibit similar and gradual changes in physical properties such as boiling and melting points.
• They also show similar and gradual changes in chemical properties.

B. ISOMERS OF ALKANOLS

Alkanols exhibit both structural and position isomerism. The isomers are named using the following guidelines:

• Identify the longest carbon chain as the parent name.
• Identify the position of the -OH functional group, assigning it the lowest possible number.
• Identify the type and position of any side branches.

Practice examples of isomers of alkanols:

(i) Isomers of propanol C₃H₇OH

CH₃CH₂CH₂OH – Propan-1-ol

CH₃CH(OH)CH₃ – Propan-2-ol

Propan-2-ol and Propan-1-ol are position isomers because only the position of the –OH functional group changes.

(ii) Isomers of Butanol C₄H₉OH

CH₃CH₂CH₂CH₂OH – Butan-1-ol

CH₃CH₂CH(OH)CH₃ – Butan-2-ol

2-methylpropan-2-ol

Butan-2-ol and Butan-1-ol are position isomers because only the position of the -OH functional group changes.

2-methylpropan-2-ol is both a structural and position isomer because both the position of the functional group and the arrangement of atoms in the molecule change.

(iii) Isomers of Pentanol C₅H₁₁OH

Pentan-1-ol, Pentan-2-ol, Pentan-3-ol (position isomers)

2-methylbutan-2-ol, 2,2-dimethylbutan-1-ol, 2,3-dimethylbutan-1-ol (position/structural isomers)

(iv) 1,2-dichloropropan-2-ol

CClH₂CClCH₃OH

(v) 1,2-dichloropropan-1-ol

CClH₂CHClCH₂OH

(vi) Ethan-1,2-diol

HOCH₂CH₂OH

(vii) Propan-1,2,3-triol

HOCH₂CHOHCH₂OH

C. LABORATORY PREPARATION OF ALKANOLS

Fermentation is the process where sugar is converted to alcohol (alkanol) using biological catalysts (enzymes) in yeast. It involves three main steps:

1. Conversion of starch to maltose using the enzyme diastase:
   
   (C₆H₁₀O₅)_n (s) + H₂O (l) –diastase enzyme→ C₁₂H₂₂O₁₁ (aq)
2. Hydrolysis of maltose to glucose using the enzyme maltase:
   
   C₁₂H₂₂O₁₁ (aq) + H₂O (l) –maltase enzyme→ 2 C₆H₁₂O₆ (aq)
3. Conversion of glucose to ethanol and carbon(IV) oxide gas using the enzyme zymase:
   
   C₆H₁₂O₆ (aq) –zymase enzyme→ 2 C₂H₅OH (aq) + 2 CO₂ (g)

At ethanol concentrations greater than 15% by volume, the ethanol produced kills the yeast enzyme, stopping the reaction. To increase the concentration, fractional distillation is performed to produce spirits (e.g., Brandy contains about 40% ethanol).

School laboratory preparation of ethanol from fermentation of glucose:

• Measure 100 cm³ of pure water into a conical flask.
• Add about five spatula ends full of glucose and stir to dissolve.
• Add about one spatula end full of yeast.
• Set up the apparatus as shown below and preserve the mixture for about three days.

D. PHYSICAL AND CHEMICAL PROPERTIES OF ALKANOLS

Use the prepared sample above for the following experiments that demonstrate the characteristic properties of alkanols:

1. Role of yeast

Yeast is a single-cell fungus containing the enzymes maltase and zymase that catalyze the fermentation process.

2. Observations in lime water

A white precipitate forms initially and later dissolves to a colourless solution. Lime water (calcium hydroxide) reacts with carbon(IV) oxide produced during fermentation to form insoluble calcium carbonate and water:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)

H₂O(l) + CO₂(g) + CaCO₃(s) → Ca(HCO₃)₂(aq)

3. Effects on litmus paper

Test the prepared sample with both blue and red litmus papers. Repeat with pure ethanol and methylated spirit.

Explanation: Alkanols are neutral compounds/solutions with a characteristic sweet smell and taste. They have no effect on either blue or red litmus papers.

4. Solubility in water

Mix equal volumes of the prepared sample and distilled water. Repeat with pure ethanol and methylated spirit.

Observation: No layers form between the two liquids.

Explanation: Ethanol is miscible in water because both ethanol and water are polar compounds. The solubility of alkanols decreases as the alkyl chain length or molecular mass increases. The alkyl group is insoluble in water, while the –OH functional group is soluble. As the molecular chain becomes longer, the effect of the alkyl group increases and the effect of the functional group decreases.

5. Melting/boiling point

Determine the boiling point of pure ethanol.

Observation: Pure ethanol has a boiling point of 78°C at sea level (one atmosphere pressure).

Explanation: The melting and boiling points of alkanols increase with increasing molecular chain length or mass due to stronger intermolecular (van der Waals) forces of attraction between molecules. More heat energy is required to overcome these forces during melting and boiling.

6. Density

Density of alkanols increases with stronger intermolecular forces of attraction, causing molecules to pack more closely. This reduces the volume occupied and increases mass per unit volume (density).

Summary table showing the trend in physical properties of alkanols:

7. Burning

Place the prepared sample in a watch glass and ignite. Repeat with pure ethanol and methylated spirit.

Observation/Explanation: Fermentation produces ethanol with a high water content (about a 1:3 ratio), which prevents the alcohol from igniting. Pure ethanol and methylated spirit ignite easily and burn with an almost colourless, non-sooty blue flame, producing carbon(IV) oxide (in excess air/oxygen) or carbon(II) oxide (in limited air) and water.

Chemical equations:

C₂H₅OH(l) + 3O₂(g) → 3H₂O(l) + 2CO₂(g) (excess air)

C₂H₅OH(l) + 2O₂(g) → 3H₂O(l) + 2CO(g) (limited air)

2CH₃OH(l) + 3O₂(g) → 4H₂O(l) + 2CO₂(g) (excess air)

2CH₃OH(l) + 2O₂(g) → 4H₂O(l) + 2CO(g) (limited air)

2C₃H₇OH(l) + 9O₂(g) → 8H₂O(l) + 6CO₂(g) (excess air)

C₃H₇OH(l) + 3O₂(g) → 4H₂O(l) + 3CO(g) (limited air)

2C₄H₉OH(l) + 13O₂(g) → 20H₂O(l) + 8CO₂(g) (excess air)

C₄H₉OH(l) + 3O₂(g) → 4H₂O(l) + 3CO(g) (limited air)

Due to its flammability, ethanol is used:

• As a fuel in spirit lamps
• As gasohol when blended with gasoline

8. Formation of alkoxides

Cut a very small piece of sodium and place it in a beaker containing about 20 cm³ of the prepared sample. Test the products with litmus papers. Repeat with absolute ethanol and methylated spirit.

Explanation: Sodium or potassium reacts slowly with alkanols to form basic solutions called alkoxides and produce hydrogen gas. If the alkanol contains some water, the metals react faster with the water to form soluble hydroxides/alkalis.

Examples:

1. Sodium metal reacts with ethanol to form sodium ethoxide:

2CH₃CH₂OH(l) + 2Na(s) → 2CH₃CH₂ONa(aq) + H₂(g)

2H₂O(l) + 2Na(s) → 2NaOH(aq) + H₂(g)

2. Potassium metal reacts with ethanol to form potassium ethoxide:

2CH₃CH₂OH(l) + 2K(s) → 2CH₃CH₂OK(aq) + H₂(g)

2H₂O(l) + 2K(s) → 2KOH(aq) + H₂(g)

3. Sodium metal reacts with propanol to form sodium propoxide:

2CH₃CH₂CH₂OH(l) + 2Na(s) → 2CH₃CH₂CH₂ONa(aq) + H₂(g)

2H₂O(l) + 2Na(s) → 2NaOH(aq) + H₂(g)

4. Potassium metal reacts with propanol to form potassium propoxide:

2CH₃CH₂CH₂OH(l) + 2K(s) → 2CH₃CH₂CH₂OK(aq) + H₂(g)

2H₂O(l) + 2K(s) → 2KOH(aq) + H₂(g)

5. Sodium metal reacts with butanol to form sodium butoxide:

2CH₃CH₂CH₂CH₂OH(l) + 2Na(s) → 2CH₃CH₂CH₂CH₂ONa(aq) + H₂(g)

2H₂O(l) + 2Na(s) → 2NaOH(aq) + H₂(g)

6. Sodium metal reacts with pentanol to form sodium pentoxide:

2CH₃CH₂CH₂CH₂CH₂OH(l) + 2Na(s) → 2CH₃CH₂CH₂CH₂CH₂ONa(aq) + H₂(g)

2H₂O(l) + 2Na(s) → 2NaOH(aq) + H₂(g)

9. Formation of Esters / Esterification

Place 2 cm³ of ethanol in a boiling tube. Add an equal amount of ethanoic acid. Add carefully 2 drops of concentrated sulphuric(VI) acid. Warm gently. Pour the mixture into a beaker containing about 50 cm³ of cold water. Smell the products. Repeat with methanol.

Explanation: Alkanols react with alkanoic acids to form a homologous series of sweet-smelling compounds called esters and water. This reaction is catalyzed by concentrated sulphuric(VI) acid in the laboratory. Naturally, esterification is catalyzed by sunlight. Each ester has a characteristic smell derived from the many possible combinations of alkanols and alkanoic acids.

General reaction:

Alkanol + Alkanoic acid –Conc. H₂SO₄→ Ester + Water

Esters derive their names from the alkanol first, then the alkanoic acid. The alkanol “becomes” an alkyl group and the alkanoic acid “becomes” alkanoate, hence alkylalkanoate. For example:

• Ethanol + Ethanoic acid → Ethyl ethanoate + Water
• Ethanol + Propanoic acid → Ethyl propanoate + Water
• Ethanol + Methanoic acid → Ethyl methanoate + Water
• Ethanol + Butanoic acid → Ethyl butanoate + Water
• Propanol + Ethanoic acid → Propyl ethanoate + Water
• Methanol + Ethanoic acid → Methyl ethanoate + Water
• Methanol + Decanoic acid → Methyl decanoate + Water
• Decanol + Methanoic acid → Decyl methanoate + Water

During the formation of the ester, the oxygen atom joining the alkanol and alkanoic acid comes from the alkanol:

R₁-COOH + R₂-OH → R₁-COO-R₂ + H₂O

Example:

Ethanol reacts with ethanoic acid to form ethyl ethanoate and water:

C₂H₅OH (l) + CH₃COOH (l) –Conc. H₂SO₄→ CH₃COOC₂H₅ (aq) + H₂O (l)

Other examples follow similarly.

10. Oxidation

Place 5 cm³ of absolute ethanol in a test tube. Add three drops of acidified potassium manganate(VII). Shake thoroughly for one minute or warm. Test the solution mixture using pH paper. Repeat by adding acidified potassium dichromate(VI).

Explanation: Both acidified KMnO₄ and K₂Cr₂O₇ are oxidizing agents. They oxidize alkanols to alkanals and then further to alkanoic acids. The oxidizing agents are themselves reduced, changing their colour:

• Purple KMnO₄ is reduced to colourless Mn²⁺
• Orange K₂Cr₂O₇ is reduced to green Cr³⁺

The pH of alkanoic acids shows they have few H⁺ ions because they are weak acids:

Alkanol + [O] → Alkanal + [O] → Alkanoic acid

NB: The [O] comes from the oxidizing agents acidified KMnO₄ or K₂Cr₂O₇.

Examples of oxidation follow similarly.

(K) Hydrolysis / Hydration and Dehydration

I. Hydrolysis/Hydration is the reaction of a compound with water. Alkenes react with water vapour/steam at high temperatures and high pressures in the presence of phosphoric acid catalyst to form alkanols:

Alkene + Water –H₃PO₄ catalyst→ Alkanol

Examples:

• Ethene + water –60 atm/300°C/H₃PO₄→ Ethanol
• Propene + water –60 atm/300°C/H₃PO₄→ Propanol
• Butene + water –60 atm/300°C/H₃PO₄→ Butanol

II. Dehydration is the process where concentrated sulphuric(VI) acid (dehydrating agent) removes water from a compound.

Concentrated sulphuric(VI) acid dehydrates alkanols to the corresponding alkenes at about 180°C:

Alkanol –Conc. H₂SO₄/180°C→ Alkene + Water

Examples:

• Ethanol –180°C/H₂SO₄→ Ethene + Water
• Propanol –180°C/H₂SO₄→ Propene + Water
• Butanol –180°C/H₂SO₄→ Butene + Water
• Pentanol –180°C/H₂SO₄→ Pentene + Water

Similarities of alkanols with hydrocarbons

I. Similarity with alkanes:

Both alkanols and alkanes burn with a blue non-sooty flame to form carbon(IV) oxide (in excess air) or carbon(II) oxide (in limited air) and water.

II. Similarity with alkenes/alkynes:

Both alkanols (R–OH) and alkenes/alkynes (with C=C double or C≡C triple bonds) decolorize acidified KMnO₄ and turn orange acidified K₂Cr₂O₇ to green.

Alkanols are oxidized to alkanals and then to alkanoic acids. Alkenes are oxidized to alkanols with two functional groups.

III. Differences with alkenes/alkynes:

Alkanols do not decolorize bromine and chlorine water, whereas alkenes do, forming halogenoalkanols.

IV. Differences in melting and boiling points with hydrocarbons:

Alkanols have higher melting points than corresponding hydrocarbons because most alkanols exist as dimers joined by hydrogen bonding, requiring more energy to break these bonds.

E. USES OF SOME ALKANOLS

• Methanol is used as industrial alcohol and in making methylated spirit.
• Ethanol is used:
  • As alcohol in alcoholic drinks (beer, wines, spirits).
  • As antiseptic to wash wounds.
  • In manufacture of varnishes, ink, glue, and paint because it is volatile and evaporates easily.
  • As a fuel when blended with petrol to make gasohol.