Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Alkanols
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Alkanols
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 7
Theme: Chemistry Of Life
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Relate the structure of alkanes to that of alkanols Identify -OH as the functional group in alkanols Explain the in crease, in boiling point of alkanolscompared with hydrocarbons in terms of hydrogen bonding Explain the polar natureof alcohols and its effect on the solubility substances. Determine solubility of common materials in waterand alcohols. Identify di, tri and polyhydroxy compounds by the ir structures and namethem appropriately
2. 1. Introduction to Alkanols and their Structure Alkanols (also commonly known as alcohols) are a homologous series of organic compounds that contain a hydroxyl (-OH) functional group attached to a saturated carbon atom. They can be considered derivatives of alkanes where one or more hydrogen atoms have been replaced by a hydroxyl group.
Relationship to Alkanes: Alkanes are saturated hydrocarbons with the general formula CnH2n+
2. When one hydrogen atom from an alkane is removed and replaced by a hydroxyl group, an alkanol is formed.
Example: Methane (CH4) → Methanol (CH3OH)
Example: Ethane (C2H6) → Ethanol (C2H5OH)
General Formula: The general formula for monohydric alkanols (those with one -OH group) is CnH2n+1O
H. Functional Group: The hydroxyl group (-OH) is the characteristic functional group of alkanols. It is responsible for most of the chemical and physical properties of this class of compounds. 2.
2. Nomenclature of Alkanols (IUPAC and Common Names) The naming of alkanols follows the IUPAC (International Union of Pure and Applied Chemistry) rules:
1. Identify the longest continuous carbon chain containing the -OH group. This forms the parent alkane name.
2. Replace the '-e' ending of the alkane name with '-ol'.
3. Number the carbon chain starting from the end closest to the -OH group, giving the carbon atom bonded to -OH the lowest possible number.
4. Indicate the position of the -OH group by placing its number before the '-ol' suffix (or before the parent name, though less common now).
5. Identify and name any alkyl substituents (branches), indicating their positions.
Examples: Methanol (CH3OH): Derived from methane.
IUPAC: Methanol.
Common: Methyl alcohol.
Ethanol (C2H5OH): Derived from ethane.
IUPAC: Ethanol.
Common: Ethyl alcohol.
Propan-1-ol (CH3CH2CH2OH): The -OH group is on the first carbon.
IUPAC: Propan-1-ol.
Common: n-Propyl alcohol.
Propan-2-ol (CH3CH(OH)CH3): The -OH group is on the second carbon.
IUPAC: Propan-2-ol.
Common: Isopropyl alcohol.
Butan-2-ol (CH3CH2CH(OH)CH3):
1. Longest chain is butane. 2. -OH is on carbon 2 (numbering from right).
3. IUPAC: Butan-2-ol. 2.
3. Physical Properties of Alkanols 2.3.
1. Boiling Point Comparison (Alkanols vs. Hydrocarbons) Alkanols generally have significantly higher boiling points compared to hydrocarbons (alkanes) of comparable molar mass. This difference is primarily due to the presence of hydrogen bonding in alkanols.
Intermolecular Forces in Alkanes: Alkanes are non-polar molecules and exhibit only weak Van der Waals forces (specifically London dispersion forces) between molecules. These forces are easily overcome, resulting in low boiling points.
Intermolecular Forces in Alkanols: The hydroxyl (-OH) group in alkanols is highly polar. The oxygen atom is highly electronegative and pulls electron density away from the hydrogen atom, creating a partial negative charge (δ−) on oxygen and a partial positive charge (δ+) on hydrogen. This polarity allows for the formation of hydrogen bonds between the partially positive hydrogen of one alkanol molecule and the partially negative oxygen of an adjacent alkanol molecule. Hydrogen bonds are much stronger intermolecular forces than Van der Waals forces.
Explanation: More energy is required to break these strong hydrogen bonds to allow the alkanol molecules to escape into the gaseous phase. Consequently, alkanols have higher boiling points than alkanes of similar molar mass.
Example: Ethane (C2H6, Molar Mass ≈ 30 g/mol) has a boiling point of -89°C. Methanol (CH3OH, Molar Mass ≈ 32 g/mol) has a boiling point of 65°C. Ethanol (C2H5OH, Molar Mass ≈ 46 g/mol) has a boiling point of 78°C. The significant increase in boiling point despite similar molar masses highlights the impact of hydrogen bonding. 2.3.
2. Polarity and Solubility of Alkanols The polar nature of the -OH group also dictates the solubility characteristics of alkanols.
Polarity: The O-H bond in the hydroxyl group is highly polar due to the electronegativity difference between oxygen and hydrogen. This makes the -OH group the hydrophilic (water-loving) part of the molecule.
Solubility in Water: The "like dissolves like" principle applies: polar compounds tend to dissolve in polar solvents (like water), and non-polar compounds dissolve in non-polar solvents. Short-chain alkanols (e.g., methanol, ethanol, propan-1-ol) are completely miscible with water. This is because they can form hydrogen bonds with water molecules, effectively overcoming dictates the solubility characteristics of alkanols.
Polarity: The O-H bond in the hydroxyl group is highly polar due to the electronegativity difference between oxygen and hydrogen. This makes the -OH group the hydrophilic (water-loving) part of the molecule.
Solubility in Water: The "like dissolves like" principle applies: polar compounds tend to dissolve in polar solvents (like water), and non-polar compounds dissolve in non-polar solvents. Short-chain alkanols (e.g., methanol, ethanol, propan-1-ol) are completely miscible with water. This is because they can form hydrogen bonds with water molecules, effectively overcoming the hydrogen bonds between water molecules themselves and between alkanol molecules. As the length of the non-polar hydrocarbon chain (CnH2n+1-) increases, the non-polar character of the molecule dominates the polar character of the -OH group. This makes the alkanol less able to form effective hydrogen bonds with water molecules. Consequently, solubility in water decreases significantly for longer-chain alkanols (e.g., butanol is moderately soluble, pentanol is sparingly soluble, hexanol and above are practically insoluble).
Solubility as Solvents: Alkanols, especially ethanol, are excellent solvents for a wide range of organic compounds (which are often non-polar or moderately polar) due to their hydrocarbon chain. Because they possess both a polar -OH group and a non-polar hydrocarbon chain, they can dissolve both polar and some non-polar substances. They act as good "intermediate" solvents.
Example: Tincture of iodine is a solution of iodine (non-polar) in ethanol (an alkanol). Ethanol is also used to dissolve many oils, fats, resins, and some inorganic salts. This makes them useful in industries like pharmaceuticals, cosmetics, and paints in Nigeria. 2.
4. Di-, Tri-, and Polyhydroxy Compounds These are alkanols containing more than one hydroxyl (-OH) group.
Dihydroxy Alkanols (Diols): Contain two -OH groups.
Structure and Naming: The suffix is '-diol'. Positions of both -OH groups must be indicated. Ethan-1,2-diol (Ethylene Glycol): HO-CH2-CH2-OH Used as an antifreeze component in car radiators (though less relevant in tropical Nigeria, it's a known industrial chemical) and in the production of polyester fibers. Propan-1,2-diol: CH3-CH(OH)-CH2-OH Used in cosmetics, food, and pharmaceuticals.
Trihydroxy Alkanols (Triols): Contain three -OH groups.
Structure and Naming: The suffix is '-triol'. Positions of all -OH groups must be indicated. Propan-1,2,3-triol (Glycerol or Glycerine): HO-CH2-CH(OH)-CH2-OH A viscous, sweet-tasting liquid. Highly soluble in water due to multiple hydrogen bonding sites. Widely used in the pharmaceutical industry (syrups, suppositories), cosmetics (lotions, soaps, moisturizers), and food industry (sweetener, humectant). It's a byproduct of soap making.
Polyhydroxy Alkanols (Polyols): A general term for compounds with many -OH groups. Often refers to sugar alcohols like sorbitol or mannitol. * These compounds generally exhibit very high solubility in water and high boiling points due to extensive hydrogen bonding. --- 3.
1. Teacher Activities Introduction (10 minutes): Begin by asking students to name some common substances containing "alcohol" that they encounter in everyday life (e.g., hand sanitizers, drinks, medicine). Introduce alkanols as a class of organic compounds, emphasizing their derivation from alkanes. Display or draw the structures of simple alkanes (methane, ethane) and their corresponding alkanols (methanol, ethanol). Clearly identify the hydroxyl (-OH) group as the functional group of alkanols.
Boiling Point Explanation (15 minutes): Guide a discussion on intermolecular forces in alkanes (Van der Waals forces). Introduce the concept of hydrogen bonding, explaining its formation in alkanols due to the polar O-H bond. Use a simple diagram on the board to illustrate hydrogen bonding between alkanol molecules. Compare the strength of hydrogen bonds to Van der Waals forces. Lead students to conclude why alkanols have higher boiling points than alkanes of similar molar mass. Provide comparative data for methanol/ethane and ethanol/propane. Solubility Explanation and Demonstration (15 minutes): Explain the polar nature of the -OH group and the non-polar nature of the alkyl chain. Discuss the "like dissolves like" principle. Practical Demonstration (if facilities permit): Have three test tubes.
Test tube 1: Water + a few drops of vegetable oil (to show immiscibility of polar and non-polar).
Test tube 2: Water + ethanol (to show miscibility of short-chain alkanol in water).
Test tube 3: Ethanol + a small amount of vegetable oil (to show ethanol's ability to dissolve some non-polar substances).
Teacher note: Be cautious with ethanol, ensure good ventilation.* Discuss observations and relate them to the polarity of the molecules. Explain how increasing chain length affects water solubility. Di-, Tri-, and Polyhydroxy Compounds (10 minutes): Introduce the terms diol, triol, and polyol. Draw the structures of Ethan-1,2-diol (ethylene glycol) and Propan-1,2,3-triol (glycerol). Explain their IUPAC naming and mention their common uses in Nigeria (e.g., glycerol in soap, cosmetics, food). Emphasize how multiple -OH groups enhance water solubility and increase boiling points even further.
Activity and Consolidation (10 minutes): Provide structural formulas for various alkanols (including branched ones and a diol/triol) and ask students to name them, or vice-versa. Facilitate a class discussion to review key concepts. 3.
2. Student Activities Brainstorming and Observation: Students will identify everyday uses of "alcohol" and observe the teacher's solubility demonstration, noting changes. Structure and Functional Group Identification: Students will draw simple alkane and alkanol structures and circle/identify the -OH functional group.
Comparative Analysis: Students will compare boiling point data for alkanes and alkanols, attempting to explain the differences based on the teacher's explanation.
Discussion and Reasoning: Students will participate in discussions about polarity and solubility, relating observations from the demonstration to chemical principles.
Nomenclature Practice: Students will practice naming given structures of monohydric, dihydric, and trihydric alkanols, and draw structures from given names.
Question and Answer: Students will ask questions and answer teacher-posed questions to clarify understanding. --- Question 1: Propane has the formula C3H8. a) Draw the structural formula of propane. b) Draw the structural formula of a corresponding alkanol where one hydrogen atom is replaced by a hydroxyl group. Name this alkanol using IUPAC nomenclature.
Solution 1: a)
Propane: ``` H H H | | | H - C - C - C - H | | | H H H ``` b)
Propan-1-ol (or Propan-2-ol): (Choosing Propan-1-ol for simplicity) ``` H H H | | | H - C - C - C - OH | | | H H H ``` IUPAC Name: Propan-1-ol
Commentary: This question helps students visualize the structural relationship between alkanes and alkanols and practice basic naming.
Question 2: Identify the functional group present in all alkanols and state its chemical formula. Explain why this functional group is crucial to the characteristic properties of alkanols.
Solution 2: Functional Group: Hydroxyl group Chemical Formula: -OH Explanation: The hydroxyl (-OH) group is crucial because it is highly polar due to the electronegative oxygen atom. This polarity enables alkanols to form hydrogen bonds with other alkanol molecules and with water molecules. Hydrogen bonding is responsible for the relatively high boiling points of alkanols (compared to alkanes) and the solubility of short-chain alkanols in water. Without this group, alkanols would behave similarly to non-polar alkanes.
Commentary: This reinforces the identification of the functional group and its direct link to physical properties.
Question 3: Ethanol (C2H5OH) has a boiling point of 78°C, while ethane (C2H6) has a boiling point of -89°C, despite their similar molar masses. Explain this significant difference in boiling points.
Solution 3: The significant difference in boiling points is due to the types of intermolecular forces present: Ethanol: Contains a polar hydroxyl (-OH) group, which allows its molecules to form strong hydrogen bonds with each other. These hydrogen bonds require a substantial amount of energy to overcome, leading to a much higher boiling point.
Ethane: Is a non-polar hydrocarbon and only exhibits weak Van der Waals forces (London dispersion forces) between its molecules. These forces are easily broken, resulting in a very low boiling point.
Therefore, the presence of hydrogen bonding in ethanol necessitates more energy for boiling compared to ethane.
Commentary: This directly addresses Performance Objective 3, requiring students to explain hydrogen bonding and its effect on boiling point.
Question 4: Explain why methanol (CH3OH) is completely soluble in water, but hexan-1-ol (CH3(CH2)4CH2OH) is practically insoluble in water.
Solution 4: Methanol's Solubility: Methanol is a short-chain alkanol. Its hydroxyl (-OH) group is highly polar and can form strong hydrogen bonds with water molecules. The small, non-polar methyl (CH3-) group does not significantly hinder this interaction, allowing methanol to mix completely with water.
Hexan-1-ol's Insolubility: Hexan-1-ol has a long, non-polar hydrocarbon chain (six carbon atoms). While it still has a polar -OH group that can form hydrogen bonds, the large non-polar chain is dominant. This long hydrocarbon chain disrupts the hydrogen bonding network of water more than the single -OH group can compensate for. Consequently, the non-polar character outweighs the polar character, making hexan-1-ol practically insoluble in water.
Commentary: This question assesses understanding of polarity, the "like dissolves like" principle, and the effect of alkyl chain length on solubility.
Question 5: Identify and name the following compounds: a) HO-CH2-CH2-OH b)
HO-CH2-CH(OH)-CH2-OH Solution 5: a)
HO-CH2-CH2-OH Identification: This compound has two hydroxyl groups attached to a two-carbon chain. It is a diol.
Name: Ethan-1,2-diol (Common name: Ethylene glycol) b)
HO-CH2-CH(OH)-CH2-OH Identification: This compound has three hydroxyl groups attached to a three-carbon chain. It is a triol.
Name: Propan-1,2,3-triol (Common name: Glycerol or Glycerine)
Commentary: This directly targets Performance Objective 6, requiring identification and naming of di- and trihydroxy compounds. ---
Hand Sanitizers and Antiseptics (Health & Hygiene): In Nigeria, especially after the COVID-19 pandemic, alcohol-based hand sanitizers are ubiquitous. Students learn that the active ingredient is typically ethanol or propan-2-ol. The lesson connects this to the antiseptic properties of alkanols, which denature proteins in microorganisms, explaining why they are effective disinfectants in hospitals, homes, and public spaces like markets and schools. Solvents in Industries (Economy & Manufacturing): Ethanol is a versatile solvent.
Pharmaceuticals: Used to extract active ingredients from plants for traditional medicine preparations (e.g., tinctures) or modern drug formulations. Many cough syrups and some topical medications (like iodine tincture) use ethanol as a solvent.
Cosmetics: Perfumes, lotions, and hair sprays often contain alkanols as solvents or preservatives.
Paints and Varnishes: Some paints and varnishes, locally produced in industrial clusters in places like Lagos or Aba, use alcohols as solvents to dissolve resins and pigments. Beverage Production and Biofuel Potential (Culture & Energy): Local Beverages: The fermentation of palm sap or cassava for "ogogoro" (local gin) or palm wine production in many Nigerian communities demonstrates the natural production of ethanol. While teaching the chemistry, the teacher can also emphasize responsible consumption and the dangers of unregulated production.
Biofuel: Ethanol can be produced from biomass (e.g., sugarcane, cassava – crops abundant in Nigeria). This links to the national conversation about renewable energy sources and reducing reliance on fossil fuels, potentially creating an alternative energy sector and job opportunities for Nigerian youth. ---