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Week 12 Content
Overview of Week 12 Inquiry Question – What are the properties of organic acids and bases?
Learning Objective #1 – Investigate the structural formulae, properties and functional group including:
- primary, secondary and tertiary alcohols
- aldehydes and ketones
- amines and amides
- carboxylic acids
Learning Objective #2 – Explain the properties within and between the homologous series of carboxylic acids amines and amides with reference to the intermolecular and intramolecular bonding present
Learning Objective #3 – Investigate the production, in a school laboratory, of simple esters
Learning Objective #4 – Investigate the differences between an organic acid and organic base
Learning Objective #5 – Investigate the structure and action of soaps and detergents
Learning Objective #6 – Draft and construct flow charts to show reaction pathways for chemical synthesis, including those that involve more than one step
NEW HSC Chemistry Syllabus Video – Reactions of Organic Acids and Bases
Week 12 Homework Questions
Week 12 Curveball Questions
Week 12 Extension Questions
Solutions to Week 12 Questions
Overview of Week 12 Inquiry Question
Welcome to Week 12 of your HSC Chemistry Syllabus Notes!
This week’s learning objectives have, for the majority, been covered in previous weeks’ notes. You know what that means? An easy week for us!
Learning Objective #1 and #2 of this week’s inquiry question have already been covered in previous weeks’ notes.
By order, we will first introduce and cover a new class of organic molecule known as ester which we have no talked about previously. We will explain how it is produced, structural formula as well as the nomenclature in naming ester molecules.
Next, we will explain the difference between organic acids and organic bases.
After that, we will have a look at soaps and detergents as organic molecules used in our daily lives and how they work.
Lastly, we will have an organic synthesis roadmap that summarises how we can convert one organic molecule to another. For example, how to convert an alcohol into an ester.
Learning Objective #1 - Investigate the structural formulae, properties and functional groups including:
-Primary, secondary and tertiary alcohols
- Aldehydes and Ketones
- Amines and Amides
- Carboxylic acids
The structural formulae and functional groups have been covered in Week 8 Notes.
The properties of organic molecules containing the above functional groups was covered in Week 9 Notes.
Feel free to revisit Week 9’s Notes to recap or revise.
Learning Objective #2 - Explain the properties within and between homologous series of carboxylic acid, amines, amides with reference to the intermolecular and intramolecular bonding present
This necessary information required to address this learning objective have been covered in Week 9 Notes.
Learning Objective #3 - Investigate the production, in a school laboratory, of simple esters
Esters are substances formed via a condensation reaction called esterification using an alcohol (alkanol) and an carboxylic acid (alkanoic acid).
Nomenclature of Esters
Nomenclature of Ester
Step 1: Identify the relevant carboxylic acid and alcohol molecules that make up the ester.
Step 2: Name the alcohol first according to the name of the corresponding alkyl group by counting the number of carbon atoms that makes up the alcohol.
Step 3: Name the carboxylic acid by replacing the ‘ic’ with ‘ate’.
Step 4: Name the ester with a space between the alcohol and carboxylic acid component.
PRACTICAL PROBLEM: What is the name of the ester made from the following alkanoic acid and alkanol?
Name of alkanoic acid (carboxylic acid): Ethanoic Acid.
Name of alkanol (alcohol): Butan-1-ol.
Well, we just follow Step 1-4 as we have explored in the above section.
We don’t need to do Step 1 as the relevant carboxylic acid and alcohol is already provided to us here.
For step 2, we need to name butan-1-ol according to the name of the corresponding alkyl group. Well, butan-1-ol is made up four carbon atoms due to the prefix ‘but‘ as we have learn in organic chemistry nomenclature in Week 8’s Notes.
So, this gives us the corresponding alkyl (single-bonded) group, 1-butyl.
In step 3, we need to replace the ‘ic’ with ‘ate’. Hence, ethanoic acid becomes ethanoate.
In step 4, we just name the ester with a space between the two components. So, the name of the ester in this example is 1-butyl ethanoate.
Now, say you were given the an ester molecule.
Can you determine the name of carboxylic acid and alcohol from which the ester is made from?
Let’s see an example!
Remember we mention that condensation reaction involves the elimination of a water molecule?
Well, have a look at the following diagram.
NOTE: At HSC Chemistry level, you should just memorise that the OH group comes from the carboxylic acid and NOT from the alcohol. The alcohol only contributes a hydrogen atom.
- The reasoning for this has to do with substitution reactions that are taught at university.
NOTE: In reality, this process involves many steps, meaning that the reaction pathway is not as straight forward as depicted in the above diagram.
If you decide to take Higher Chemistry 1A at university (e.g. as an free elective subject), you will learn about the various reaction stage leading to the final product of a condensation reaction.
For HSC Chemistry purposes, the above diagram is all you need to know how a condensation reaction works.
Example of a procedure used to produce esters in a school laboratory
Step 1: Measure 15mL of ethanol and 10mL of ethanoic acid using separate 20mL measuring cylinder and transfer the solutions into a 50mL round-bottomed flask.
Step 2: Add ten drops of concentrated sulfuric acid into the round-bottomed flask using a dropper alongside three to five boiling chips.
Step 3: Attach the condenser to the neck of the round-bottomed flask (upright) and hold it in place using a clamps and retort stand, attaching hoses to the condenser and turning on the tap to allow water to flow through.
Step 4: Set up a tripod with gauze mat on top and a water bath using a large beaker containing 300mL of water.
Step 5: Lower the round-bottom flask into the water bath using the retort stand such that the mixture is under the water level.
Step 6: Heat the water bath using a bunsen burner so that it is gently boiling under reflux for approximately 30 minutes.
Step 7: Turn off bunsen burner and allow equipment and solution to cool while leaving water running through the condenser before dissembling the apparatus.
Step 8: Pour the mixture into a separating funnel and add 50mL of distilled water, followed by swirling to allow dissolution of any remaining soluble alcohol, alkanoic acid and sulfuric acid.
Step 9: Drain off bottom aqueous layer containing the alcohol, alkanoic acid, sulfuric acid and water using the stopcock of the separating funnel.
Step 10: Add sodium hydrogen carbonate to neutralise any remaining traces of sulfuric acid that is present in the remaining mixture containing the ester.
Step 11: Pour the remaining ester solution into a clean, empty beaker for drying and distillation (or fractional distillation) to increase the purity of the ester.
[Write equation] ; justify use of excess ethanol to shift equilibrium position to the right ; use of sulfuric acid as a dehydrating agent to shift equilibrium to the right.
The Scent of Esters~
The following esters derived from ethanoic acids (aka. acetic Acid as IUPAC prefers) have the sweet scent of:
Butyl ethanoate – Banana
Pentyl ethanoate – Pear
Octyl ethanoate – Orange
EXTRA: Ethyl ethanoate is used as a solvent in nail polishers.
Feel free to explore the scent of esters derived from other alkanoic acids on the worldwide web 🙂
Learning Objective #4 - Investigate the differences between an organic acid and organic base
We can essentially divide organic acids into two categories being neutral or positively charged.
Comparatively, we can separate organic bases into the categories of negatively charged or being neutral.
Neutral organic bases
This category involving neutrally charged bases are those that contain one or more lone pairs of electrons as the lone pair of electrons have the ability to attract and bond with a hydrogen ion by which is positively charged by donating a lone pair of electrons.
The basic nature of organic bases deals with the nature of organic bases acting as a lewis base, that is, the ability to donating a lone pair of electrons. The number lone pairs of electron an organic base does not necessarily correspond with the compound being a stronger base.
At HSC level, the strength of an organic base (acting as a lewis base) will depend on the stability of the lone pair of electrons.
The more stable the lone pair of electrons in the base molecule, the less capable the molecule is able to donate its lone pair of electrons which reduces its ability act as a lewis base. Thus, weaker base.
- The most common neutral organic base are those containing the amine functional group (R-NH2) which has a nitrogen atom with one lone pair of electrons.
- Another example are those organic compounds that has ether group (R-O-R) where the oxygen in the ether functional group has two lone pairs of electrons.
In terms of the Bronsted-Lowry theory, both organic bases and inorganic bases undergo the same reaction with acids bonding to a hydrogen ion.
That is, when you react (or dissolve) organic base in water, it is able to accept a hydrogen ion from water to produce a basic solution, similar to an inorganic base like sodium hydroxide.
This also implies that inorganic bases have the same properties as organic bases. However, generally speaking most organic bases are weak bases compared to the plentiful of strong inorganic bases that we know of.
Neutral organic acids
For HSC Chemistry purposes, there are two types of neutral organic acids. The first type are organic compounds with carboxylic acid functional group and the second type are those with the hydroxyl functional group. That is, alkanoic acids and alcohol molecules respectively.
- There are other types of neutral organic acids (outside scope of syllabus) such as those with the phenol functional group which essentially is a phenyl group (C6H5) but with one hydrogen atom is substituted with a hydroxyl group, i.e (C6H4OH).
The important trend here is that all of these neutral organic acids have the hydroxyl group in common. Compared to alcohols, alkanoic acids (i.e. carboxylic acids) is more acidic although it is a weak acid.
The reason why alkanoic acids (R-COOH) are more acidic than alcohol (R-OH) for instance can be explained using the logic we have talked about for neutral organic bases previously. We said that the more stable that a lone pair electrons is in a base, the weaker the base will be.
So, if you recall from Module 6, we said that the stronger the acid, the weaker its conjugate base. Hence, the more stable the conjugate base, the more acidic or the stronger the acid. This is because the formation of a stable conjugate base occurs more readily than forming an unstable conjugate base. Therefore, the more stable the conjugate base, the stronger the acid which the conjugate base is derived from.
From this, the reason why alkanoic acids (-COOH) is more acidic than alcohol (-OH) is that its conjugate base is more stable than the conjugate base of formed by alcohol (when the alcohol acts as an acid).
This is because there is resonance in the carboxylate ion that is formed after the removal (deprotonation) of the hydrogen in the COOH group. This resonance structure allows the delocalisation of electrons across multiple atoms rather than being confined on just one oxygen atom in alcohols.
This distribution of negative charge across multiple atoms therefore allows the conjugate base of alkanoic acids to be more stable than alcohols and thus more acidic.
Resonance of the COO- (carboxylate ion)
In terms of Bronsted-Lowry theory, the reaction in which organic acids undertakes is same as inorganic acids. For instance, if you react an organic acid with water, an acidic solution will be produced as the water accepts a hydrogen ion released by the organic acid.
This reaction is similar to how inorganic acids, such as hydrochloric acid, reacts with water to produce an acidic solution.
In general, most organic acids are weak acids compared to the large variety of strong inorganic acids that we know of.
Example of organic acid (weak acid): CH3COOH(aq) + H2O(l) <-> H3O+(aq) + CH3COO–(aq)
EXTRA: Negatively charged organic bases and Positively charged organic acids
These are basically bases with a negatively charged central carbon atom, attached to three other groups or atoms, with a lone pair of electron.
Likewise, positively charged organic acids are those compounds with a positively charged central atom bonded to three other groups with one missing pair of electron.
Learning Objective #5 - Investigate the structure and action of soaps and detergents
- By definition, soaps are non-synthetic detergents.
- However, note that from this point onwards in our set of notes, the term ‘detergent’ will be referring to ‘synthetic detergents’.
- In HSC exams, you should be specific by writing ‘synthetic (soap) or non-synthetic’ detergents.
The structure of soaps consists of a long hydrocarbon chain (‘tail’) that is attached to deprotonated carboxylic acid anion group (‘head’), i.e. carboxylate ion, with an ionic bond to a cation such as Na+ or K+.
- Therefore, soap is an non-synthetic anionic detergent.
Comparatively, (synthetic) detergents do not have the COO- group that is attached to the long hydrocarbon chain (or tail). Rather, instead of having the COO- group, detergents have a SO3- group (negatively charge) instead which has an ionic bond with a metal ion such as Na+. These are called anionic detergents.
The second type of detergent is called cationic detergent. These have a long hydrocarbon tail attached to a nitrogen atom which has bonded to three alkyl groups. This gives the nitrogen atom a positive charge which is able to ionic bond with a chloride or bromide anion.
The third and last kind of detergent is called non-ionic detergent whereby there is no ionic bond in its composition or structure as it is uncharged. One example of this is the product formed from the reaction between stearic acid and polyethyeneglycol.
- We will be exploring anionic, cationic and non-ionic detergents in more detail shortly.
Action of soaps
Explanation of above diagram for a grease-soap micelle.
- The red lines are the non-polar organic molecules such as dirt, grease or oil.
- The green molecules with non-polar tails and negatively charged heads are the soap molecules.
- Lastly, the blue molecules are water that performs hydrogen bonding with the negatively charged heads of soap molecules.
- Collectively, the above diagram is a zoomed-in picture of a grease-soap micelle.
- The collective mixture of micelle (consisting of soap molecules, grease and water molecule) is called an emulsion.
- The process of forming emulsion (emulsification) can be assisted using hot water to facilitate the water to be attracted to the soap molecule, thus surround and stabilise the soap-grease structure.
As soap molecules is dissolved in water, the ionic head (negatively charged) of the soap allows hydrogen bonds to be formed with water molecules by breaking the hydrogen bonding between water molecules.
The non-polar hydrocarbon tail is allowed to trap non-polar substances such as oil and dirt through dispersion forces, forming oil droplets in the water. These are droplets which is comprise of water molecules, soap molecules, oil, dirt, etc are known as micelles.
A surfactant is a substance that is able to modify the physical properties of another substance’s surface.
Due to the formation of hydrogen bonds between soap molecules and water, the hydrogen bonds between water molecules are reduced and so the water surface tension decreases. As a result of this, water molecules are able to spread over a wider area to allow greater surface area of contact between the surfactant (soap or detergent) and the oil, dirt or stain. Therefore, more oil, dirt or stain can be removed saving the amount of water and soap required as well as time used in the washing.
As the oil droplets are essentially coated with water molecules that has the soap molecules trapping non-polar substances inside, the oil droplets can be washed/rinsed away using a fresh batch of water by turning on the tap once again.
The problem with soaps is that, if the water used contains high concentrations of calcium and magnesium
ions, precipitates known as scum will form in water where the calcium and/or magnesium ion can bind to the oxygen atom in the COO- group.
Ca2+ (aq) + 2CH3(CH2)16COO–(aq) -> Ca(CH3(CH2)16COO)2(s)
- The precipitate (scum) here is calcium stearate.
Water that contains high concentrations of calcium and/or magnesium is called hard water.
After washing, these scum can stick to clothing and even hair if soap is used as an ingredient in shampoo, resulting in dull hair.
NOTE: In acidic water or solutions, the carboxylate anion in soap molecule will be protonated (accepts H+ ion) to form insoluble fatty acids. As a result, this eliminates the soap’s ability to lather and cannot form bond with water to form micelles. This therefore means that the grease cannot be washed away.
Action of detergents
Detergents’ use or action is similar to soaps although it does not form precipitates with calcium or magnesium ions in the water, i.e. no scum is formed, as there is still foaming/lathering even in hard water.
This is because there is no COO- group in detergents to form ionic bonds with aqueous magnesium or calcium ions to create scum precipitate.
Anionic detergents: This class of detergent is able to readily lather upon contact with water and so can readily form micelles. Since glass is negatively charged and anionic detergents’ heads are negatively charged too, this class of detergent can be used as washing liquids for washing glass plates or cups. This class of detergent can also be dried to form a chemically stable washing powder that can be used in washing machines to remove oil, dirt and stains.
Cationic detergents: As glass surfaces are negatively charged, this class of detergent is not used to clean grease off such surfaces as it will adhere to them, resulting in a greasy texture. Rather, this class of detergent can be used to clean plastic materials and used as a flocculant in water purification to improve the clarity of water (by removing very light and small suspended particles known as colloids).
Non-ionic detergents: This type of detergent has hydrophilic heads, e.g. containing the polar OH group, as shown in the detergents diagram that we seen in the above section. For such reasons, it can hydrogen bond with water. Non-ionic detergents can used as an ingredient added to paint and cosmetic products (e.g. skin skin exfoliation scrubs) to allow the paint or cosmetic product to mix well with water. This class of detergent also does not for scum in hard water.
Learning Objective #6 - Draft and construct flow charts to show reaction pathways of chemical synthesis, including those that involve more than one step
NEW HSC Chemistry Syllabus Video - Reactions of Organic Acids and Bases
Week 12 Homework Questions
Week 12 Curveball Questions
Week 12 Extension Questions
Solutions to Week 12 Questions