Week 5 Polymers - Workbook PDF

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Week 5: Polymers - Workbook

Site: Monash University Unit: CHM1022 - Chemistry II - S2 2018 Book: Week 5: Polymers - Workbook Printed by: Kellie Vanderkruk Date: Monday, 27 August 2018, 6:26 PM

Table of contents Week 5: Pre-lectorial Work Carbonyl containing compounds Carboxylic Acids Esters and Amides Polymers – Applications, synthesis and measurement Common Polymers Answer for Hydrolysis Reaction Activity Are you ready? Pre-Test Checklist Week 5: Pre-lectorial Test Week 5: Lectorial/Session 1 Activity 1: Lecture on Carboxylic Acids and Mechanism of the Fischer Esterification Activity 2: Post-Lecture Q&A Activity 3: Structure and Properties of Carboxylic Acids, Esters and Amides Activity 3: Answers Activity 4: Demonstration of Acid with Alcohol Activity 4: Answers Week 5: Lectorial/Session 2 Activity 1: Lecture on Hydrolysis Activity 2: Case Study of Hydrolysis Reactions Activity 2: Answers Activity 3: Demonstration of Polymer Reactions Activity 3: Answers Activity 4: Lecture on Polymers Activity 5: Demonstration of Polyurethane Activity 6: Polymer Classification Activity 6: Answers Summary Page

Week 5: Pre-lectorial Work

Introduction This section of the workbook will take you through the required pre-lectorial readings and activities. It is important that you take the time to carefully review the material provided and complete any activities. We recommend that you allocate at least 65 minutes to work through the topics and activities in this section of the workbook, with additional time to complete the prompted readings as necessary (this will vary depending on your reading speed and any prior knowledge you may have). Activity type

Time to complete (approx)

Reading 50 mins Activity

5 mins

Pre-quiz 10 mins Total

65 mins

Learning Outcomes Understand and be able to predict relationship between structure and properties of: carboxylic acids, esters, amides, acid chloride Compare possible transformations within the family of acids, esters and amides Predict the results of combining an acid with an alcohol Identify the difference in reactivities / stability ester vs amide Describe polymerisation Identify molecular weights of polymers Discuss categories of polymer materials Describe polyesters and polyamides

What do I need to do? The following activities must be completed prior to attending your first lectorial for this week. Read through the eBook slides and the associated chapters of the textbook. Work through some problems. Finally, complete an activity that will test your understanding of the material covered in this eBook.

Carbonyl containing compounds

Examples of carboxylic acid-derived compounds The carbonyl group consists of carbon atom double bonded to an oxygen atom and is a key feature in carboxylic acids. For derivatives of carboxylic acids such as esters and amides, the carbonyl is the common feature. There are numerous examples of carboxylic acid, ester and amide containing groups in nature. In order to expose you to the breadth of compounds which contain these functional groups we will explore a few examples. Salicylic acid is an organic carboxylic acid which is derived from the metabolism of salicin which is produced in willow bark. Although salicylic acid displays its own pain relieving properties, its acidity is too high and can cause severe stomach pain when ingested. Esterification of salicylic acid with acetic anhydride yields acetyl salicylic acid (Aspirin) (Figure 1).

Figure 1: Esterification of salicylic acid with acetic anhydride

Source: picture alliance / dpa / dpa central image / Peter Endig

Morphine, heroin and codeine are all examples of drugs from the opiate family (Figure 2). In a series of transformations, morphine is esterified with acetic acid to yield heroin. Codeine is another opiate in which the ester is replaced by an ether. It is obvious that all three compounds have different uses and effects on a human body. Yet, the chemical change is small, highlighting the importance of these variations in medicinial chemistry.

Figure 2: Structures of morphine, heroin and codeine

Poly(ethylene terephtalate) (PET) is the most common type of thermoplastic polymer which belongs to the polyester family. PET is well known and used in many applications such as clothing fibers, food and liquid containers and solar panels to name a few. PET consists of polymerised units of the

ethylene terephthalate monomer unit (Figure 3). These monomer units are connected via an ester bond, hence the name polyester.

Figure 3: Structure of poly(ethylene terephtalate) (PET)

Protein and peptides are fundamental components of cells which carry out daily important biological functions. Structurally, proteins and peptides are similar. They are both made up of chains of amino acids which are connected by amide bonds (also referred to a peptide bond) (Figure 4). The distinguishing factor between the two is the size; peptides are smaller than proteins.

Figure 4: Structure of protein and peptides Source: CNX OpenStax

Fats and oils are composed of molecules known as triglycerides. These species are esters composed of three fatty acid units which are bonded to glycerol (Figure 5). If a triglyceride is solid at 25°C, it is a fat and if the triglyceride is liquid at 25°C it is an oil.

Figure 5: Structure of fats and oils

Carboxylic Acids

The Carbonyl Group The previous examples of chemical structures you just saw all have one thing in common; they contain a carbonyl group. The carbonyl group is common to several classes of organic compounds and are distinguished according to what is bonded to the carbonyl (Figure 7). An aldehyde consists of a carbonyl center bonded to a hydrogen and an R group. A ketone consists of a carbonyl center bonded to two hydrocarbon groups. We will be focussing on the following three classes; carboxylic acids, esters and amides. A carboxylic acid has a hydrocarbon group and an alcohol about the carbonyl center (-OH). An ester is similar to this, however, the hydrogen of the acid is replaced by another alkyl group (-OR’). An amide consists of a carbonyl center bonded to a hydrocarbon group and an amine (-NH 2, -NHR’ or -NR 2).

Figure 7: Classes of carbonyl containing compounds

Carboxylic acids, esters and amides are similar in structure. These structural similarities account for their similar properties. They all have higher boiling points in comparison to alkanes of similar size. Carboxylic acids and their derivatives commonly undergo carbonyl-group substitution reactions, in which a group we represent as —Z replaces (substitutes for) the group bonded to the carbonyl carbon atom (Figure 8).

Figure 8: Carbonyl-group substitution reactions

The portion of the carboxylic acid that does not change during a carbonyl-group substitution reaction is known as an acyl group (Figure 9). In biochemistry, carbonyl-group substitution reactions are quite often called acyl transfer reactions. They play an important role in the metabolism of biomolecules.

Figure 9: The acyl group

Carboxylic acids Many carboxylic acids are colourless liquids with unpleasant odours. Carboxylic acids with 5 to 10 carbons have a characteristic “goaty” odor. These acids are also produced by the action of skin bacteria on human oils. Acids with an excess of 10 carbon atoms are wax like solids. Odor is diminished with increasing size due to the decrease in volatility. Carboxylic acids as their name may suggest, are acidic. The acidity of carboxylic acids is higher than that of alcohols or even phenols. Carboxylic acids readily liberate the hydrogen of the carboxylic acid leading to the formation of the carboxylate anion. The carboxylate anion is stabilised by resonance leading to a markedly increased acidity. Carboxylic acids with 1 to 4 carbon atoms are miscible with water as the carbonyl is readily able to engage in hydrogen bonding with water. Solubility decreases with increasing carbon chain length because dipole forces become less predominant while dispersion forces become more predominant. Carboxylic acids exhibit strong hydrogen bonding between molecules and therefore exhibit high boiling points in comparison to compounds of comparable molar mass. Carboxylic acids are named according to the IUPAC system where the suffix (-e) is replaced with -oic acid (Figure 10).

Figure 10: Naming of carboxylic acids

Common names of carboxylic acids are commonly used over the IUPAC naming system. In this system the carbon atoms attached to the —COOH group are identified by Greek letters α, β, ϒ and so on, rather than numbers (Figure 11).

Figure 11: Greek letters in naming of carboxylic acids

Figure 12: Physical properties of various carboxylic acids

Carboxylic acids are typically weak acids, meaning they partially dissociate to form H+ ions and RCOO- anions. Carboxylic acids establish equilibria in aqueous solution with carboxylate anions (RCOO-). Carboxylate anions are named by replacing the -ic ending in the carboxylic acid name with -ate (Figure 13).

Figure 13: Carboxylate anions

Something to do: Ranking by Property The following activity must be completed prior to attending your first lectorial for this week. Rank the following molecules, from highest to lowest boiling points.

If you have printed this workbook, write your answer here and bring this to class.

Esters and Amides

Esters Esters are colourless and volatile liquids with pleasant odors. They have distinctive fruit-like odors and many occur naturally in essential oils of plants. Esters are derived from carboxylic acids. The -OH group of the carboxylic acids is converted to an -OR in an ester. This loss of the hydrogen in the molecule also leads to a loss in possibility for molecules to form hydrogen bonds in esters. Therefore, boiling points are not as high as boiling points of carboxylic acids. Small esters are fairly soluble in water but the solubility decreases with increasing chain length. Their slight solubility is attributed to the fact that although esters can’t form hydrogen bonds with themselves, they can form hydrogen bonds with water molecules. Esters consist of two words. The first is the name of the alkyl group R' of the ester group and it ends with -yl. The second is the name of the parent acid, with the family-name ending -ic acid replaced by ate (Figure 14).

Figure 14: Naming an ester

Amides Compounds with a nitrogen directly attached to the carbonyl carbon atom are amides. The nitrogen of an amide may be unsubstituted (-NH 2), monosubstituted (-NRH) or disubstituted (-NR 2). With the exception of formamide (HCONH 2), which is a liquid, amides are solids (Figure 15). Importantly, amides are not basic like amines.

Figure 15: Hydrogen bonding of amides

Unsubstituted amides (-NH2) can form multiple hydrogen bonds to other amide molecules (Figure X) resulting in generally high boiling points. Due to this hydrogen bonding, unsubstituted amides are also soluble in water. Monosubstituted amides are able to form hydrogen bonds with each other but

disubstituted amides cannot leading to lower boiling points (Figure 15). Amides are not basic like amines are. The lack of basicity is due to the fact the lone pair of electrons on the nitrogen of the amide are delocalised between the nitrogen and the oxygen through resonance.

Figure 16: Comparison of boiling points of unsubstituted, monosubstituted and disubstituted amides

Amides with an unsubstituted —NH2 group are named by replacing the -ic acid or -oic acid of the corresponding carboxylic acid name with -amide. If the nitrogen atom has alkyl substituents on it, the compound is named by specifying the alkyl group and then identifying the amide name. The alkyl substituents are preceded by the letter N to identify them as being attached directly to nitrogen (Figure 17).

Figure 17: Naming of amides

Reactions of Carboxylic Acids The conversion of carboxylic acids to esters using acid and alcohol is a type of reaction named Fischer Esterification (Figure 18). Esterification reactions are equilibrium reactions and can be reversed. Esters can be split into carboxylic acids and alcohols by acid catalysis (H+) or the action of a base (OH-).

Figure 18: Ester formation

Figure 19: Amide bond formation The amide bond is synthesised when the carboxyl group reacts with an amine (Figure 19). The simplest amides are derived with ammonia (NH 3), leading to RC(O)-NH 2 species. Amines can, however, also be mono- or disubstituted to yield substituted amides, see following example:

Figure 20: Formation of substituted amides Tertiary amines do not react with carboxylic acids to amides due to the lack of a N-H bond. Tertiary amines hence only react in acid-base reactions with organic acids (see Figure 21).

Figure 21: Acid-base reaction with a tertiary amine

Hydrolysis of esters and amides Esters and amides can be hydrolized to yield back the parent carboxylic acid and corresponding alcohol or amine. The most well known hydrolysis reaction is saponofication, in which triglycerides (fats) are reacted with soldium hydroxide to yield fatty acids and glycerol. Soap is produced in this way. Amides are considerable more stable than esters. While esters can be hydrolyzed in acid or base catalyzed reactions, amides only undergo hydrolysis in acid catalyzed reactions. This difference in + reactivity stems from the fact that -NH 2 cannot be split of from the amide easily, while -NH 3 can leave the molecule and be replaced by an OH group. The reactivity of carboxylic acids and their derivatives depends on the basic strength of the acylbound substituent. -NH2 groups are the most basic, followed by -OR and OH groups. Halogens are the weakest bases and are thus replaced most easily (see Figure 22):

Figure 22: Reactivity series for acyl substituents

As can be seen from Figure 22, amides are the most stable compounds. Amides can also be obtained from reacting amines with esters in presence of a acid or base catalyst. Acyl chlorides on the other hand are so reactive that they will spontaneously react with alcohols and amines to esters and amides, respectively, even in absence of a catalyst. These reactions are typically very exothermic and fast. A reaction of an acid chloride is given below (Figure 23).

Figure 23: Reaction of an acid chloride with an alcohol to yield an ester Instead of acid chlorides, also acid anhydrides can be used. Acid anhydrides are often somewhat better to handle in the laboratory. The reaction product is the same, only that one equivalent of acid is produced alongside the desired product (Figure 24).

Figure 24: Reaction of an acuid anhydride with a n alcohol to yield an ester Something to do: Hydrolysis Reaction The following activity must be completed prior to attending the second lectorial for this week. In the session you will be completing similar problems to this one. Attempt to answer the question and review the notes to understand the correct answer. Fill in the blanks to create a correct equation.

The answer will be shown at the end of the pre-class reading.

Polymers – Applications, synthesis and measurement

Polymers - Introduction One of the many uses of carboxylic acids and amides is in the formation of polymers (we’ll come back to this!). Polymers are incredibly common in your everyday life and some examples are shown below:

Figure 1 – Polymers in everyday life

A polymer is a large molecule made by linking together repeating units of small molecules called monomers.

Figure 2 – A pictorial representation of polymers being formed from monomers

Other common examples include polystyrene (often referred to as Styrofoam and used in packaging) or poly(ethylene terephthlate) (often to referred to as PET or polyester and used in clothing and many other applications).

Figure 3a – The synthesis of the polymer polystyrene from the monomer styrene

Figure 3b – The synthesis of the polymer poly(ethylene terephthalate) from the monomers dimethyl terephthlate and 1,2-ethanediol

Up to this point, we’ve drawn polymers as though they are two-dimensional structures that lie flat on the page. As is often the case in organic chemistry, this is not a true representation of the many possibilities that can actually occur. Polymers can exist in all three dimensions:

Figure 4 – Depiction of polymers in one (1D), two (2D) or three (3D) dimensions

Or they can form into many different complex straight chain formations:

Figure 5 – The incredible many types of polymers that can form!

Lastly, stereochemistry can play a role, leading to vastly different polymers.

Figure 6 – Different polymers caused by the presence of various stereogenic centres

You’d be forgiven if you were overwhelmed by all of this new information! The main take home message is that polymers: are large molecules formed by the combination of one (or several) monomers that form a repeating unit. can exist in one-two or three dimensions. can be highly complex and form extremely complicated structures. can be affected by the sterochemistry of the monomers/repeating units. are used everywhere in many different modern environments.

Polymers – from structure to size After the structure and the composition of the polymer is defined, what is the size of a polymer? i.e. How big is an individual unit of a given polymer? Typically, the answer is that that when a polymer is first made, it is generally polydisperse. This means that there are many different polymer units of varying lengths and sizes in the final product. Very few polymers are monodisperse wherein all of the individual chains are exactly the same length and size! Hence, we tend to deal with polymer distributions where the molecular weight is used to represent the length of the chains being considered. The following graph shows the absolute molecular weight measured in a given polymer (x-axis) and a measure of how often that molecular weight was found 5 (y-axis). You can see that this polymer has an average molecular weight around 1.3 x 10 g/mole – which is massive when compared to normal individual molecules!

Figure 7 – The distribution of a given polymer as per the average molecular weight

Take particular note of the difference between an M N value and an M W value:

Figure 8 – Two measurements of the same polymer - MN and MW

The important piece of information to take here is that the two values are two different measurements of the same polymer. The M N value is a measure of the concentration of the chain lengths of the polymer whereas the MW value is a measure of its physical properties. The relationship between them is highly important and is known as polydispersity:

If the MN value and the MW value are the same (PDI=1), the polymer is considered uniform with all chain lengths approximately the same length and size. If there is a large amount of irregularity in the polymer, then the MW value will become larger than the M N value and result in a PDI > 1. The images below represent two systems, the left being monodisperse (PDI=1) and the right being significantly polydisperse (PDI>1).

Figure 9 – Images of monodisperse (left) and polydisperse (right) materials

Calculations and further explanation – This is not examinable in CHM1022 or CHM1052 The distribution of the molecular weights is described in statistics as:

where k is the statistical moment of the distribution and x P= concentration of chains with length P. The exact mean...