Condensation Polymers (2) Polyamides

Relevant Specification Extracts:

OCR

6.2.3 Polyesters and Polyamides

Condensation polymers

(a) condensation polymerisation to form: 

1. polyesters (ii) polyamides 

Formation from carboxylic acids/dicarboxylic acids (or respective di acyl chlorides) and from alcohols/diols or amines/diamines.

Learners will not be expected to recall the structures of synthetic polyesters and polyamides or their monomers. 

Edexcel 18B

14. know that the formation of a polyamide is a condensation polymerisation reaction 

15. be able to draw the structural formulae of the repeat units of condensation polymers formed by reactions between: 

i  dicarboxylic acids and diols 

ii  dicarboxylic acids and diamines 

iii amino acids 

AQA

3.3.12.1 Condensation polymers (A-level only) 

Condensation polymers are formed by reactions between: 

dicarboxylic acids and diols
dicarboxylic acids and diamines
amino acids.
The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units.
Typical uses of these polymers.
Students should be able to:
draw the repeating unit from monomer structure(s)
draw the repeating unit from a section of the polymer chain
draw the structure(s) of the monomer(s) from a section of the polymer
explain the nature of the intermolecular forces between molecules of condensation polymers.

Polyamides

In this second post on condensation polymers, I’m looking at the formation of polyamides, defining the repeat units of polyamidess (e.g Kevlar or Nylon 6:6), and the nature of the intermolecular forces that exist between these polymer molecules.

I’m going to discuss how dicarboxylic acids (or acyl chlorides ) combine with diamines and how to spot the repeat unit and the linkage between repeat units of polyamide.

Formation of polyamides

As their name suggests polyamides are part of the amide family of molecules and contain the –CONH– functional group.

Like aliphatic and aromatic amides, polyamides are formed from either a carboxylic acid and an amine or a carbonyl chloride and an amine.

However, there is a distinguishing feature of these carboxylic acids and carbonyl chlorides: they are di–carboxylic acids and di–chlorides i.e. they have two acid groups or two acid chloride groups.  Similarly the amines used are diamines.

A typical dicarboxylic acid is hexan–1,4–dioic acid and a typical amine might be 1,6–diaminohexane. See below for their structures: 

hexan–1,4–dioic acid

1,6–diaminohexane

When these molecules combine a small molecules is eliminated in the case above that molecule is water H₂O.

You can see that described in the reaction scheme below:  

You can see how the structures allow for a chain of units of acid and alcohol linked by amide groups to develop.

You should be able to draw the repeat unit of a typical polyamide such as Kevlar or Nylon 6:6.  These are shown below: 

Nylon 6:6

Kevlar

You should also be able to highlight the amide functional group in the polyamide structure as below.

If the acid used is hexan–1,4–dioic acid and the diamine is 1,6–diaminohexane then the resultant polymer was originally called Nylon 6:6.

And here is a picture of the actual material Nylon6:6 before it is spun and. stitched into a textile:

Such polyamide molecular chains stick together through the use of polar bonds (between CO groups and NH groups) and temporary dipole forces (between aromatic groups).  

Condensation Polymers (1) Polyesters

Relevant Specification Extracts:

OCR

6.2.3 Polyesters and Polyamides

Condensation polymers

(a) condensation polymerisation to form: 

1. polyesters (ii) polyamides 

Formation from carboxylic acids/dicarboxylic acids(or respective di acyl chlorides) and from alcohols/diols or amines/diamines.

Learners will not be expected to recall the structures of synthetic polyesters and polyamides or their monomers. 

Edexcel 18B

14. know that the formation of a polyamide is a condensation polymerisation reaction 

15. be able to draw the structural formulae of the repeat units of condensation polymers formed by reactions between: 

i  dicarboxylic acids and diols 

ii  dicarboxylic acids and diamines 

iii amino acids 

AQA

3.3.12.1 Condensation polymers (A-level only) 

Condensation polymers are formed by reactions between: 

dicarboxylic acids and diols
dicarboxylic acids and diamines
amino acids.
The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units.
Typical uses of these polymers.
Students should be able to:
draw the repeating unit from monomer structure(s)
draw the repeating unit from a section of the polymer chain
draw the structure(s) of the monomer(s) from a section of the polymer
explain the nature of the intermolecular forces between molecules of condensation polymers.

Polyesters

In this first post on condensation polymers, I’m looking at the formation of polyesters, defining the repeat units of polyesters (e.g Terylene), and the nature of the intermolecular forces that exist between these polymer molecules.

I’m going to discuss how dicarboxylic acids (or acyl chlorides ) combine with diols and how to spot the repeat unit and the linkage between repeat units of polyester.

Formation of polyesters

As their name suggests polyesters are part of the ester family of molecules and contain the –COO– functional group.

Like aliphatic and aromatic esters polyesters are formed from either a  carboxylic acid and an alcohol or a carbonyl chloride and an alcohol.

However there is a distinguishing feature of these carboxylic acids and carbonyl chlorides: they are di carboxylic acids and chloride I.e. they have two acid groups or two acid chloride groups.  Similarly the alcohols used are diols.

A typical dicarboxylic acid is 1,4-benzene dicarboxylic acid and a typical alcohol might be ethan–1,2–diol. See below for their structures: 

When these molecules combine a small molecules is eliminated in the case above that molecule is water H₂O.

You can that described in the reaction scheme below:  

You can see how the structures allow for a chain to develop of units of acid and alcohol linked by ester groups.

You should be able to draw the repeat unit of a typical polyester such as Terylene.

You should also be able to highlight the ester functional group in the structure as below.

And you can also see the repeat units outline in two different ways.

If the acid used is 1,4- benzene dicarboxylic acid and the alcohol is ethan–1,2–diol then the resultant polymer was originally called Terylene at ICI back in the day (thats the old Imperial Chemical Industries).

Here is a list of some of the properties both useful and not so helpful of typical polyesters:

Advantages and Disadvantages of Polyesters

Advantages:

Tough and rigid

Recycled into useful products as the basis for resins in such applications as shower units and floor tiles etc..

PET flakes from PET bottles are in great demand for fibrefill for pillows and sleeping bags, carpet fibre and sheet mouldings etc..

Easy to print or mass colour or dye

Disadvantages:

Subject to attack from acids and bases

Low thermal resistance

Poor solvent resistance

And here is a picture of the actual material Terylene before it is spun and stitched into a textile:

Such polyester molecular chains stick together through the use of polar bonds (between CO groups) and temporary dipole forces (between aromatic groups).  

Aromatic Chemistry (5) Phenol: electrophilic substitution reactions

Specification extracts:

OCR

Phenols

6.1.1.  

• (i)  the electrophilic substitution reactions of phenol: 
• (i)  with bromine to form 2,4,6-tribromophenol
  
• (ii)  with dilute nitric acid to form a mixture of 2-nitrophenol and 4-nitrophenol 
• Note that nitration with phenol does not require concentrated HNO3 or the presence of a concentrated H2SO4 catalyst.
• (j)  the relative ease of electrophilic substitution of phenol compared with benzene, in terms of electron pair donation to the π-system from an oxygen p-orbital in phenol (see also 4.1.3 a)
  Explanation is only in terms of susceptibility of ring to 'attack' and not in terms of stability of intermediate. 
• (k)  the 2- and 4-directing effect of electron- donating groups (OH, NH2) and the 3-directing effect of electron-withdrawing groups (NO2) in electrophilic substitution of aromatic compounds
  Learners will not be expected to know further electron-donating or electron-withdrawing groups; relevant additional data will be supplied in examinations. 
• (l)  the prediction of substitution products of aromatic compounds by directing effects and the importance to organic synthesis

Edexcel

6. understand the reaction of phenol with bromine water 

7. understand reasons for the relative ease of bromination of phenol, compared to benzene 

Aromatic Chemistry (5) Phenol: electrophilic substitution reactions

In this fifth post on aromatic chemistry, I’m looking at the electrophilic substitution reactions of phenol with bromine water and nitric acid.

I’m going to discuss the fact that phenol is more easily substituted than benzene and explain this difference illustrating it by looking at the reaction with bromine water and with nitric acid solution.

Phenol with bromine water

Phenol is an aromatic alcohol but it behaves very differently from benzene in electrophilic substitution reactions.

The structure of phenol (C₆H₅OH) means that the OH group attached to the benzene ring can add extra electrons to the ring π system and as result enhance the electron density at certain of the carbon atoms in the ring.  (see below).

The additional electron pair comes from a p orbital in the oxygen of the OH group.  The p orbital overlaps with the ring π system extending the system to include the extra electron pair. 

This addition of electrons to the ring π system is called ring activation.  The carbon atoms at positions 2, 4 and 6 are given enhanced electron density and become activated towards electrophiles such as the bromonium ion Br+ or the nitronium ion NO₂+.

The resultant effect is that much milder conditions are required to brominate phenol i.e. no halogen carrier and simply the addition of bromine water.  The resultant white precipitate is the 2,4,6-tribromophenol. (see the equation below).

The -OH group then is said to be 2,4,6 directing.  

Phenol with nitric acid

In much the same as with bromine water, phenol also reacts under very mild conditions with nitric acid.  Unlike in the case of benzene where concentrated acid was required in the presence of concentrated sulphuric acid, the nitration of phenol takes place in the presence of just dilute nitric acid.  

Two products usually result at room temperature 2-nitrophenol and 4-nitrophenol.  

They can be separated since the molecules have different boiling points.  

Again we see the 2,4,6 directing effect of the OH group in phenol since these positions on the ring are given enhanced electron density and make the ring more susceptible to electrophilic substitution. 

The effect of other groups on the benzene ring is to do the same as phenol so for example the amine group NH₂ is also 2,4,6 directing towards electrophiles as it too adds an electron lone pair from a nitrogen p orbital to the ring π system and makes the ring more susceptible to electrophilic substitution.

Bromination and nitration of phenylamine happen under the same kind of mild conditions as they do in phenol.  (see equation below)

However if the substituent on the benzene ring is not electron pushing but electron withdrawing ( like a nitro group NO2) then the opposite effect on electrophiles is observed.

So for example the nitration of nitrobenzene (at temperature greater than 55^oC ) produces 1,3–dinitrobenzene because the nitro group is 3,5 directing as it deactivates the ring towards electrophilic substitution.

Aromatic Chemistry (4) Phenol

Aromatic Chemistry (4) Phenol

Specification extracts:

OCR

Phenols

6.1.1.(h)  

the weak acidity of phenols shown by the neutralisation reaction with NaOH but absence of reaction with carbonates 

Aromatic Chemistry (4) Phenol as a weak acid

In this fourth post on aromatic chemistry, I’m looking at the acidic properties of phenol.

I’m going to discuss the fact that phenol is a weak acid and illustrate this looking at its reaction with sodium hydroxide solution and the reaction of carboxylic acids with carbonates.

Phenol as a weak acid

Phenol is an aromatic alcohol but it behaves very differently from aliphatic alcohols like ethanol.

The structure of phenol (C₆H₅OH) means that the OH group attached to the benzene ring can add extra electrons to the ring π system and as result lose the proton from the OH group.  (see below)

The resultant anion (C₆H₅O^–) is stabilised as the negative charge on the oxygen can be distributed around the ring π system. 

The image below shows how the phenoxide anion is stabilised.

As a result phenol is a weak acid.  It has a K_a of 1.6 × 10^—10. and a pK_a of 9.8

As a weak acid it displays some of the conventional properties of acids.  So it will neutralise sodium hydroxide solution forming sodium phenoxide. (see below)

However, if we compare the pKa of phenol (9.8) with those of carboxylic acids such as ethanoic acid (4.76) and with carbonic acid (dissolved CO₂ solution) (pK_a2 10.32) we note that the CO₂ solution has a final second pK_a greater than that of phenol in aqueous solution so addition of phenol to a carbonate does NOT result in the effervescence of CO₂ gas.

However, we can also see that the pK_a of ethanoic acid is less than that of phenol AND dissolved carbon dioxide solution.  Thus addition of carbonates to ethanoic acid and other carboxylic acids will result in the effervescence of CO₂ gas.

Thus we have a simple test to distinguish a carboxyl group from a phenolic group.  Add a few drops of sodium carbonate solution and the carboxyl group will cause the solution to fizz!!.

This is a useful test since many carboxylic acids have a pKa less than that of the carbonate ion as this table of selected acids shows:

Aromatic Chemistry (3) Comparing bromination of benzene and ethene

Aromatic Chemistry (3) Bromination of Benzene and Alkenes compared

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(f) and (g) 

—the explanation of the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the π-system in benzene compared with the localised electron density of the π-bond in alkenes.
—the interpretation of unfamiliar electrophilic substitution reactions of aromatic compounds, including prediction of mechanisms.

Aromatic Chemistry (3) Bromination of Benzene and Alkenes compared

In this third post on aromatic chemistry, I’m comparing the reaction mechanism of the electrophilic substitution of benzene with that of the electrophilic addition of bromine to an alkene such as propene (C₃H₆).  

I’m also going to discuss the general mechanism for the electrophilic substitution of benzene and other aromatic compounds such as phenol.

Comparing the mechanisms for the bromination of benzene (C₆H₆) and propene (C₃H₆).

1. The benzene problem

Benzene is a relatively unreactive molecule.  The reason for its unreactivity lies in its distinctive electronic structure.

The 6 carbon ring is bonded in two ways.  There is the end-on overlap of s orbitals to form a series of six sigma bonds between the carbon atoms.  However that does not account for all the electrons in the molecule.  6 electrons remain unbonded in sigma bonds.  There is one delocalised electron per carbon atom.  These in each carbon atom occupy a half filled p orbital.  The genius of the molecule is that these six half filled p orbitals combine in a side-on overlap move to form a ring shaped molecular orbital that resides above and below the plane of the sigma bond ring.

You can see how that forms in the illustration below:

So for a species to access the benzene molecule and add or substitute itself in the molecule it has to at least disrupt this ring shaped molecular orbital and that requires considerable energy around 150kJmol^-1 to be precise.  Hence there is considerable resistance to reaction.

The energy profile for the bromination of benzene reaction is found below:

In an alkene of course there is only one double bond in which the π bond is exposed above and below the plane of the single sigma bond.  

The energy required to break this bond is much less than that required to disrupt benzene’s π system.  Alkenes tend to undergo addition reactions fairly quickly and easily with reagents like bromine and hydrogen bromide.  

The reaction energy profile for the action of hydrogen bromide on an alkene is shown below.

Bromination of benzene on the other hand requires a raised temperature and the use of a halogen carrier to generate the electrophile the bromonium ion Br+.

If the benzene ring is already substituted with an electron rich group such as —OH or —NH₂ then these extra electrons enable the ring to be more susceptible to electrophilic substitution since they become incorporated in the π system.  The result being that the conditions for nitration of phenol are much less severe than they are for benzene as we can see illustrated in the two reactions below.