A2 Chemistry topic 6: Carboxylic acid derivatives, nitrogen-containing compounds and polymers

• produced by the oxidation of alkylbenzenes

• the alkyl side-chain in alkylbenzenes, e.g. methylbenzene, can be oxidised to a carboxylic acid

• alkylbenzene is heated under reflux with a solution of hot alkaline KMnO4 (this is the oxidising agent)

  • purple colour disappears and forms a brown ppt (from the formation of MnO2) as Mn7+ reduces to Mn4+

• the mixture is then acidified with dilute acid (e.g. HCl) to protonate the organic product form and produce benzoic acid

how is benzoic acid produced

• liquid PCl3 and heat

• solid PCl5

• liquid SOCl2

what are the three ways in which an acyl chloride can be formed from a carboxylic acid

3CH3CO2H (l) + PCl3 (l) → 3CH3COCl (l) + H3PO3 (l)

describe the reaction for the formation of an acyl chloride from a carboxylic acid using PCl3 and heat, using ethanoic acid as an example

CH3CO2H (l) + PCl5 (s) → CH3COCl (l) + POCl3 (l) + HCl (g)

HCl gas produces steamy white fumes

describe the reaction for the formation of an acyl chloride from a carboxylic acid using PCl5, using ethanoic acid as an example

CH3CO2H (l) + SOCl2 (l) → CH3COCl (l) + SO2 (g) + HCl (g)

describe the reaction for the formation of an acyl chloride from a carboxylic acid using SOCl2, using ethanoic acid as an example

• methanoic acid is a strong reducing agent and gets further oxidised to form CO2

• this occurs by warming methanoic acid with mild oxidising agent such as Fehling’s or Tollens’ reagent

  • Fehling’s: Cu2+ reduced to Cu+, forms red ppt - Cu2O

  • Tollens’: Ag+ reduced to Ag,

• stronger oxidising agents such as acidified KMnO4 or K2Cr2O7 can also be used

  • purple KMnO4 solution turns colourless, Mn7+ ions reduced to Mn2+

  • orange K2Cr2O7 solution turns green, Cr6+ ions reduced to Cr3+

describe how carboxylic acids, such as methanoic acid, can be further oxidised

use strong oxidising agent such as warm acidified KMnO4, forms CO2

describe how carboxylic acids, such as ethanedioic acid, can be further oxidised

• electrons in the O-H bond are drawn towards the C-O bond

• the electrons in the C-O bond are drawn towards the C=O bond

• overall, the O-H bond is weakened due to the carbonyl group removing electron density from it and drawing it towards itself

• carboxylic acids can therefore more easily lose a proton compared to the phenols and alcohols which lack this electron-withdrawing group, therefore carboxylic acids are stronger acids than alcohols and phenols

describe how the strength of the O-H bond explains the acidity of a carboxylic acid relative to alcohols and phenols

• conjugate base of carboxylic acids is the carboxylate ion

• the charge density on the oxygen atom is spread out over the carboxylate ion

• this is because charge is delocalised on the electronegative carbonyl oxygen atom

• as a result, the electrons on the oxygen atom are less available for bond formation with an H+ ion to reform the undissociated acid molecule with -COOH group

• the position of the dissociation equilibrium lies more to the right compared to alcohols and phenols

describe how the stability of the carboxylate ion explains the acidity of a carboxylic acid relative to alcohols and phenols

• conjugate base of alcohols is the alkoxide ion

• the alkyl group in the ion is an electron-donating group that donates electron density to the oxygen atom

• as a result, the electron density on the oxygen atom is more readily available for bond formation with an H+ ion

• alkoxide ions also lack the ability to delocalise the charge density on the entire ion

• therefore alkoxides are less stable than the alcohol itself so is more likely to reform the alcohol

• alcohols are therefore weaker acids than carboxylic acids and phenols

• dissociation equilibrium position lies to the left

describe how the stability of the alkoxide ion explains the acidity of a carboxylic acid relative to alcohols and phenols

• charge density on the oxygen atom is spread out across the entire ion

  • this delocalisation stabilises the phenoxide ion

• as a result, the electrons on the oxygen atom are less available for bond formation with a proton (H+ ion)

• conjugate base of phenols are therefore more stable than the phenol

• however, since the delocalisation of charge density is on the carbon atoms and not on electronegative oxygen atoms like in the carboxylate ion, phenoxide ions are less stable than carboxylate ions

• therefore phenols are weaker acids than carboxylic acids

• position of dissociation equilibrium lies more to the left than carboxylic acids and more to the right than alcohols

describe how the stability of phenoxide ions explains the acidity of a carboxylic acid relative to alcohols and phenols

• chlorine atoms act as electron-withdrawing groups that make the carboxylic acid stronger

• the O-H bond is further weakened as the chlorine atom draws even more electron density away from this bond

• this further extends the delocalisation of the negative charge on the -COO- group of the carboxylate ion henceforth making it more stable and less likely to bond with the H+ ion

• the more electron-withdrawing groups there are on the carbon attached to the -COOH group, the stronger the acid

• on the contrary, carboxylic acids have an electron-donating methyl group which strengthens the O-H bond, the methyl group donates negative charge towards the -COO- group which becomes more likely to accept the H+ ion

describe and explain the relative acidities of chlorine-substituted carboxylic acids

• acyl chloride + alcohol → ester

• ethanoyl chloride + ethanol → ethyl ethanoate + HCl

• benzoyl chloride + phenol → phenyl benzoate + HCl

describe the reaction of alcohols and acyl chlorides to form esters, using ethyl ethanoate and phenol benzoate as examples

• results in the formation of a carboxylic acid and an HCl molecule

• this is an addition-elimination reaction

• water molecule is added across the C=O bond

• HCl molecule is eliminated

describe the hydrolysis of acyl chlorides

• acyl chlorides reacts with alcohols and phenols to form esters

• esters can also be formed from carboxylic acids reacting with alcohols but this reaction is slower as carboxylic acids are less reactive than acyl chlorides and therefore the reaction does not go to completion (so less product is formed)

• esterification of acyl chlorides is an addition-elimination reaction

  • alcohol/phenol adds across the C=O bond

  • HCl molecule is eliminated

describe the process in which ester is formed from an acyl chloride

• acyl chlorides can form amides from their condensation reactions with amines and ammonia

• nitrogen atom in ammonia and amines has a lone pair of electrons which attacks the carbonyl carbon atom in the acyl chloride

• product is a non-substituted amide (when reacted with ammonia) or substituted amide (when reacted with a primary and secondary amine)

• this is an example of an addition-elimination reaction

  • the amine or ammonia molecule adds across the C=O bond

  • HCl molecule is eliminated

describe the formation of amides from acyl chlorides

1. addition of a nucleophile across the C=O bond

2. elimination of a small molecule such as HCl or H2O

describe the general mechanism of addition-elimination reactions

• water molecule acts as nucleophile

  1. lone pair of electrons on oxygen atom initiate attack on the carbonyl carbon atom

  2. this is followed by the elimination of an HCl molecule

describe the mechanism of hydrolysis of acyl chlorides

• alcohols or phenols act as a nucleophile

  1. the lone pair of electrons on the oxygen atoms initiate an attack on the carbonyl atom

  2. this is followed by the elimination of an HCl molecule

• with phenols, reaction requires heat and presence of a base for the reaction to proceed

• the base deprotonates the phenol to form a phenoxide ion which is a better nucleophile than a phenol molecule

  1. the phenoxide ion initiates attack on the carbonyl carbon atom

  2. small molecule of NaCl is eliminated

describe the reaction mechanism for the formation of an ester from an acyl chloride

• nitrogen in ammonia and primary/secondary amines act as a nucleophile

  1. lone pair on the nitrogen atom initiate an attack on the carbonyl carbon atom

  2. HCl molecule is eliminated

• both reactions of acyl chlorides with ammonia and amines are vigorous but have differences

  • reaction with ammonia produces a non-substituted amide and white fumes of HCl are formed

  • reaction with amines produces a substituted amide and the HCl formed reacts with the unreacted amine to form a white organic ammonium salt

describe the reaction mechanism for the formation of amides from acyl chlorides

acyl chloride>alkyl chloride>aryl chloride

what is the relative ease of hydrolysis of acyl chlorides, alkyl chlorides, and halogenoarenes

• acyl chlorides are most readily hydrolysed at room temp.

  • this is because the carbon bonded to the chlorine atom is also bonded to an oxygen atom

  • There are two strong electronegative atoms pulling electrons away from the carbonyl carbon, leaving it very δ⁺

  • C-Cl bond is therefore weakened and nucleophilic attack of the carbonyl carbon is much more rapid

• the carbonyl carbon in alkyl chlorides is only attached to one electronegative atom which pulls electrons from it

  • this carbon atom is therefore not very δ⁺ and the C-Cl bond is stronger than the C-Cl bond in acyl chlorides

  • hydrolysis of alkyl chlorides therefore requires a strong alkali (such as OH-) to be refluxed with it

  • an OH- ion will hydrolyse the alkyl chloride as it is a stronger nucleophile than H2O

• in aryl chlorides, the carbon atom bonded to the chlorine atom is part of the delocalised π bonding system of the benzene ring

  • one of the lone pairs of electrons of the Cl atom overlaps with this delocalised system

  • C-Cl bond therefore has some double-bond character causing it to become stronger

  • as a result, the C-Cl bond is difficult to break and hydrolysis will not occur

explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides, and halogenoarenes

• nucleophilic substitution reaction occurs where the lone pair on the nitrogen acts as a nucleophile and replaces the halogen in the halogenoalkane

• when a halogenoalkane is reacted with excess, hot ethanolic ammonia under pressure, a primary amine is formed

describe how amines can be produced from the reaction of halogenoalkanes with ammonia

• nucleophilic substitution

• nitrogen on primary amine acts as nucleophile and replaces the halogen on the halogenoalkane

• when a halogenoalkane is reacted with a primary amine in ethanol and heated in a sealed tube, under pressure a secondary amine

describe how secondary amines can be produced the reaction of halogenoalkanes with primary amines

• amines can be formed from the reduction of amides by LiAlH4 in dry ether

• whether a primary or secondary amine is formed depends on the nature of the amide

describe how amines are produced from the reduction of amides

• nitriles contain a -CN functional group that can be reduced to an -NH2 group

• the nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH4 in dry ether can be used to form a primary amine

describe how amines are produced from the reduction of nitriles

• the chlorine atom in acyl chlorides are electronegative and draw electron density away from the carbonyl carbon

• this means the carbonyl carbon becomes electron deficient and therefore susceptible to attack by nucleophiles

• the nitrogen atom in ammonia and amines have a lone pair of electrons that can act as a nucleophile and attack the carbonyl carbon

• as a result the C-Cl bond is broken and an amide is formed

• reaction with primary and secondary amines will give a substituted amide

• reaction with ammonia will produce a non-substituted amide

describe how amides are produced from the condensation reaction between an acyl chloride and ammonia or an amine

• the strength of basicitiy of an amine depends on the availability of the lone pair of electrons on the nitrogen atom to form a dative covalent bond with a proton

• the more readily the lone pair of electrons is available, the stronger the base is

• factors that affect the basicity of an amine are:

  • positive inductive effect - some groups such as alkyl groups donate electron density to the nitrogen atom causing the lone pair of electrons to become more available therefore increasing the amines basicitiy

  • delocalisation - presence of an aromatic ring such as the benzene ring causes the lone pair of electrons on the nitrogen to be delocalised into the benzene ring

    • the lone pair becomes less available to form a dative bond with ammonia and hence decreases the amines basicitiy

describe and explain the basicitiy of aqueous solutions of amines

• produced in a three-step synthesis reaction followed by the separation of phenylamine from the reaction mixture

  1. benzene undergoes nitration with concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at 25 degrees to 60 degrees to form nitrobenzene

  2. nitrobenzene is reduced with hot tin (Sn) and concentrated hydrochloric acid (HCl) under reflux to form an acidic reaction mixture that contains the organic product C₆H₅N⁺H₃

  3. sodium hydroxide (NaOH) is added to the acidic reaction mixture to form phenylamine

  4. the phenylamine is separated from the reaction mixture by steam distillation

describe how phenylamines are prepared

• electrophilic substitution reaction

• the lone pair of electrons on the nitrogen atom of the phenylamine donates electron density into the benzene ring

• the delocalisation of electrons causes an increased electron density in the benzene ring

• the benzene ring therefore becomes activated and becomes more readily attacked by electrophiles

• the incoming electrophiles are directed to the 2,4, and 6 positions

• phenylamine therefore reacts under mild conditions with aqueous bromine at room temperature to form 2,4,6-tribromophenylamine

describe the bromination of phenylamine

• diazonium compounds are very reactive compounds containing an -N2+ group

• the amine (-NH2) group of phenylamines will react with nitrous acid (HNO2) at a temperature below 10 degrees to form diazonium salts

  • since nitrous acid is unstable, it has to be made in a test tube by reacting sodium nitrite (NaNO2) and dilute acid (such as HCl)

• the diazonium salts are so unstable that upon further warming with water they will form phenol

describe the reaction of the formation of diazonium salt from phenylamine

• ethylamine>ammonia>phenylamine

• ethylamine

  • electron-donating alkyl groups has a positive inductive effect by donating electron density to the nitrogen atom causing its lone pair of electrons to become more available to form a dative covalent bond with a proton

• ammonia

  • no electron-donating group hence it is a weaker base than ethylamine

  • more basic than phenylamine as there are no aromatic rings to cause the delocalisation of nitrogen’s lone pair of electrons

• phenylamine

  • lone pair of electrons overlap with the conjugated system on the benzene ring and becomes delocalised; as a result, the lone pair of electrons become less readily available to form a covalent dative bond with a proton

describe and explain the relative basicity of aqueous ammonia, ethylamine, and phenylamine

• organic compounds that have an R₁-N=N-R₂

• they are often used as dyes and are formed in a coupling reaction between the diazonium ion and an alkaline solution of phenol

what are azo compounds

1. formation of nitrous acid

   • nitrous acid is very unstable so has to made in a test tube using sodium nitrate (NaNO2) and hydrochloric acid (HCl)

2. diazotization

   • the reaction between nitrous acid and phenylamine to form the diazonium ion is called diazotization

   • reaction mixture must be kept under 10 degrees using ice otherwise the diazonium ion will thermally decompose to benzene and nitrogen (N2)

   • requires dilute acid (e.g. HCl)

3. coupling reaction

   • the diazonium ion acts as an electrophile and substitutes into the benzene ring of the phenol at the 4th position

   • alkaline conditions are required to deprotonate the organic product and form the azo compound

describe the coupling of benzenediazonium chloride with phenol in NaOH(aq) to form an azo compound

• the delocalised electrons in the π bonding systems of the two benzene rings are extended through the -N=N- which acts as a bridge between the two rings

• as a result of the delocalisation of electrons throughout the compound, azo compounds are very stable

why are azo compounds very stable

• the amide link can be broken down by hydrolysis by refluxing with aqueous alkali or acid

• the products of a non-substituted amide:

  • carboxylic acid

  • ammonia

• the products of a substituted amide:

  • carboxylic acid

  • primary amine

• ammonia will react in excess acid to form an ammonium salt

• carboxylic acid will get deprotonated in excess base to form a carboxylate ion

describe the hydrolysis of amides with aqueous alkali or acid

• the C=O group in amides can be reduced by the strong reducing agent LiAlH4

• the product of a non-substituted amide are:

  • primary amine

  • water

• the products of a substituted amide are:

  • secondary amine

  • water

describe the reduction of amides with LiAlH4 to form an amine

• due to the presence of the electron-withdrawing oxygen atom in the amide group, electron density is removed from the nitrogen atom

• the lone pair on the nitrogen atom, therefore, becomes less readily available and is not available to donate to an electron-deficient species

• since this electron-withdrawing oxygen is characteristic of amides and not amines, amides are much weaker bases than amines

why are amides much weaker bases than amines

• organic compounds that contain the two functional groups:

  • a basic amino (-NH2) group

  • an acidic carboxylic acid (-COOH) group

what are amino acids

• due to the presence of both a basic and acidic group, they are said to be amphoteric meaning they can act as both acids and bases

why are amino acids said to be amphoteric

an ion with both a positive and negative charge

what is a zwitterion

• amino acids interact intramolecularly within themselves to form a zwitterion

• because of these charges in a zwitterion, there are strong intermolecular forces of attraction between amino acids

  • amino acids are therefore soluble crystalline solids

describe how amino acids interact within themselves to form a zwitterion and how it affects the properties of an amino acid

• a solution of amino acids in water will exist as zwitterions with both acidic and basic properties

• they act as a buffer solution as they resist change in pH when small amounts of acid or alkali are added

• if an acid is added (and pH is lowered)

  • the -COO- part of the zwitterion will accept an H+ ion to reform the -COOH group

  • this causes the zwitterion to become a positively charged ion

• if base is added (and pH is raised)

  • the -NH3+ part of the zwitterion will donate an H+ ion to reform the -NH2 group

  • this causes the zwitterion to become negatively charged

• the pH can be slightly adjusted to reach a point where neither the negatively charged or positively charged ions dominate and the amino acid exists as a neutral zwitterion

  • this is called the isoelectric point of the amino acid

describe what the isoelectric point is

• the -NH2 group of one amino acid can react with the -COOH group of another amino acid in a condensation reaction to form a dipeptide

  • the new bond form between the two amino acids is called a peptide link or peptide bond

• since this is a condensation reaction, a small molecule (H2O) is removed

• the dipeptide still contains an -NH2 and -COOH group at each end of the molecule so can therefore undergo another condensation reaction to form a tripeptide

describe the formation of peptide bonds

analytical technique used in biochemical analysis to identify and purify proteins by separating ions by placing them in an electric field

what is electropheresis

• sample of amino acids is placed between two oppositely charged electrodes

  • the positively charged ions will move towards the negatively charged electrode

  • the negatively charged ions will move towards the positively charged electrode

• the rate (how fast) at which ions move towards electrodes depends on:

  • the size of the ions: larger ions move more slowly

  • the charge of the ions: highly charged ions move more quickly

• An electropherogram is the series of bands which are observed on the paper or gel after electrophoresis has occurred

  • Each band in the electropherogram corresponds to a particular species

describe how electropheresis works

• occurs via the condensation reaction between a diol and a dicarboxylic acid

  • a diol contains 2 -OH groups

  • a dicarboxylic acid contains 2 -COOH groups

• forms an ester bond

• when the polyester is formed, one of the -OH groups on the diol and the hydrogen atom of the -COOH are expelled as a water molecule (H2O)

• the resulting polymer is a polyester

describe the formation of polyesters via condensation between a diol and a dicarboxylic acid

• hydroxycarboxylic acids contain an alcohol group at one end of the molecule and a carboxylic acid at the other

• molecules undergo a condensation reaction to form a polyester, expelling water as a small molecule

• forms ester bond

describe the formation of a polyester via condensation reaction between two molecules of hydroxycarboxylic acids

• a diamine and dicarboxylic acid is required to make a polyamide

  • a diamine contains 2 -NH2 groups

  • a dicarboxylic acid contains 2 -COOH groups

• dioyl chlorides can also be used to react with the diamine instead of the acid

  • a dioyl chloride contains 2 -COCl groups

• condensation reaction occurs and forms an amide link

describe the formation of polyamides from monomers

• using aminocarboxylic acids, you can carry out a condensation reaction/polymerisation using only one monomer which provides both of the function groups necessary for an amide/peptide link

• condensation reaction occurs and forms an amide link/bond with the release of a water molecule

describe the formation of polyamides from aminocarboxylic acids

• When a section of polymer is presented, the monomers can be identified by considering the small molecules expelled from the monomers

• If a water molecule is expelled, the -OH must have been from an acid group

• The hydrogen atom may be from an amine group of a monomer.

• If the molecule was hydrochloric acid (HCl), a dioyl chloride monomer may have been used

how can the monomers that make up a condensation polymer be identified

• polyalkene chains are non-polar and saturated

• this makes them chemically inert, and therefore, non-biodegradable

• (Poly)alkenes can be melted and recycled for new uses

  • However, even in the new applications, the (poly)alkenes are not biodegradable

• Recycling plants can burn used plastic materials

  • The energy released from burning can be used to generate electricity

  • Burning plastics in oxygen releases carbon dioxide and water (complete combustion) which can contribute to global warming

describe the biodegradability of poly(alkenes)

• Polyesters and polyamides are biodegradable polymers for a number of reasons

  • One such reason is their ability to breakdown with the use of light

• Carbonyl groups (C=O) along polymer chains are able to absorb energy from the Electromagnetic Spectrum, in particular ultraviolet (UV) light

  • Absorbing UV light weakens the carbonyl areas of polymers and breaks them down into smaller molecules

describe the photodegradation of polymers

• Despite this ability being a great advantage of polyesters and polyamides, it may pose problems when the polymers are repurposed

• When applied to a new use, the biodegradability could give a weaker polymer

• Breaking down polymers also poses another challenge

  • Once used, polymeric materials are taken to landfill sites where many other materials are piled on top of each other

  • This could mean that photodegradable polyesters or polyamides do not have access to UV light in order to break down naturally

what are the disadvantages of photo degradability

• Hydrolysis is the breakdown of molecules using water

• In acidic hydrolysis, an acid (such as hydrochloric acid) acts as the catalyst

  • Polyamides such as Kevlar are heated with dilute acid

  • This reaction breaks the polyamide into a dicarboxylic acid and ammonium ions

• Alkaline hydrolysis

  • The polyamide is heated with a species containing hydroxide ions (eg. sodium hydroxide)

  • This breaks the polymer into the sodium salts of its monomers (dicarboxylic acid salt and diamines

describe the hydrolysis of polyamides to biodegrade them

• Ester linkages can also be degraded through hydrolysis reactions

• Acid hydrolysis forms the diol and dicarboxylic acid that were used to form the polyesters

• Alkaline hydrolysis forms the diol and dicarboxylic acid salt 

describe the hydrolysis of polyesters to biodegrade them