Chemistry (6.1): Aromatic compounds, carbonyls and acids

Benzene

a naturally occurring aromatic compound, which is a very stable planar ring structure with delocalized electrons

Models of benzene (2)

1. Kekule’s model

2. delocalised model

Kekule’s model

a six-membered ring with alternating single and double bonds between the carbon atoms

Problems with the Kekule model (3)

1. unlike alkenes, benzene is resistant to addition reactions

2. enthalpy of hydration of benzene shows that benzene is more stable than predicted

3. all 6 carbon bonds are the same length

Delocalised structure of benzene

each of the six carbon atoms donates one electron from its p-orbital which combine to form a ring of delocalised electrons above and below the plane of the molecule

substitution reaction

where a group or atom is exchanged for another group or atom in a chemical reaction

benzene derivative

a benzene ring that has undergone a substitution reaction

What groups can substitute a hydrogen atom on a benzene ring?

chlorine, bromine, NO₂ and alkyl chains

Electrophilic substitution

a substitution reaction where an electrophile is attracted to an electron-rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair

Reaction mechanism

a model with steps to explain and predict a chemical reaction

Reactions of benzene (2)

1. nitration

2. halogenation

Reagent and catalyst for nitration of benzene

nitric acid and sulfuric acid catalyst

Equation showing how sulfuric acid generates the NO₂⁺ electrophile in the nitration of benzene

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄^- + H₂O

Equation showing the H₂SO₄ catalyst is regenerated in the nitration of benzene

H⁺ + HSO₄^- → H₂SO₄

Why doesn’t benzene directly react with halogens?

the aromatic ring is too stable

What is used to make benzene react with halogens?

a halogen carrier

What does a halogen carrier do?

generates a positive halogen ion

Examples of halogen carriers

iron halides or aluminium halides

Friedel-Crafts reaction

a substitution reaction where hydrogen is exchanged for an alkyl or acyl chain

Why doesn't benzene react with bromine water?

benzene has a lower electron density, so there isn’t enough electron density to induce a dipole and start the reaction

What can react with benzene in a Friedel-Crafts reaction? (2)

1. haloalkane

2. acyl chloride

acyl chloride functional group

RCOCl

Conditions for acylation

60 degrees, AlCl₃ catalyst, reflux

Reaction between benzene and an acyl chloride

acylation

Phenols

a class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring

What type of acid is phenol?

weak acid

What does phenol react with?

strong bases to form a salt and water

What doesn’t react with phenol?

carbonates

Is phenol or benzene more reactive?

phenol is more reactive than benzene

Why is phenol more reactive than benzene?

aromatic ring is more electron-dense so can induce a dipole in the non-polar bromine molecule

Why is the pi-system in phenol more nucleophilic?

the lone pair of electrons on the oxygen atom in the phenol group become part of the delocalised ring, increasing the electron density in the ring

directing effect

how a functional group attached directly attached to an aromatic ring affects which carbon atoms are more likely to undergo substitution

What are the two groups in further substitution of benzene?

electron releasing and electron withdrawing

Examples of electron releasing groups

OH, NH₂, CH₃

Electron density of electron releasing groups

increases

Ease of substitution for electron releasing groups

easier

Position of substitution of electron releasing groups

2,4 and 6

Example of an electron withdrawing group

NO₂

Electron density of electron withdrawing groups

decreases

Ease of substitution of electron withdrawing groups

harder

Position of substitution for electron withdrawing groups

3 and 5

Carbonyl functional group

C=O

What reactions to carbonyls undergo? (2)

1. oxidation

2. nucleophilic addition

What do carbonyls react with in nucleophilic addition reactions?

1. sodium tetrahydridoborate(II) (NaBH₄)

2. hydrogen cyanide

What do aldehydes form after they undergo oxidation?

carboxylic acids

Reagents in the oxidation of aldehydes

K₂Cr₂O₇ and H₂SO₄

What technique is used to oxidise aldeydes?

reflux

What would you observe when an aldehyde is oxidised?

colour change from orange to green

Why is there a colour change when aldehydes are oxidised?

oxidation sta

What makes carbonyl compounds susceptible to nucleophilic attack?

the C=O functional group has a dipole

sodium tetrahydridoborate(III) formula

NaBH₄

What can an oxidising agent be summarised as?

[O]

What can a reducing agent be summarised as?

[H]

What acts as the nucleophile in the reaction between NaBH₄ and carbonyls?

hydride ion H^-

Where does the hydride ion attack in nucleophilic addition of carbonyls?

the delta positive carbon atom

What is produced in the reaction between carbonyls and NaBH₄?

alcohol

What are aldehydes reduced to?

primary alcohols

What are ketones reduced to?

secondary alcohols

Sources of cyanide

hydrogen cyanide and sodium cyanide

What charge does a cyanide nucleophile have?

-1

Are aldehydes or ketones easier to oxidise?

aldehydes

Reagents and conditions in the reaction between carbonyls and cyanide

product in the nucleophilic addition reaction between cyanide and carbonyls

hydroxynitrile

What kind of isomerism do hydroxynitriles show?

optical isomerism

Why do hydroxynitriles show optical isomerism?

cyanide can attack from above or below the carbon atom

How do you test for a carbonyl functional group?

1. Brady’s reagent (2,4-DNP)

2. Tollen’s reagent

Positive test for a carbonyl group when adding 2,4-DNP

yellow/orange ppt formed

How can you identify a specific aldehyde or ketone after a positive Brady’s reagent test?

compare melting point of product with values in a database

What is Tollen’s reagent?

ammoniacal silver nitrate

Why can Tollen’s reagent only react with aldehydes?

it is a weak oxidising agent

What is observed when Tollen’s reagent is added to an aldehyde?

silver mirror

Functional group of carboxylic acids

-COOH

Physical properties of carboxylic acids

soluble in water

Why are carboxylic acids soluble in water?

H-bonds formed between carboxylic acid functional group and water

As the hydrocarbon chain of a carboxylic acid increases in size solubility …

decreases

Why does solubility decrease as the hydrocarbon chain of a carboxylic acid increases in size?

more of the molecule becomes non-polar

What do carboxylic acids react with? (4)

1. metals

2. metal oxides

3. metal hydroxides

4. metal carbonates

carboxylic acid + metal →

metal salt + hydrogen

How does the suffix change when a salt is generated from a carboxylic acid

carboxylic acid + metal oxide →

metal salt + water

carboxylic acid + metal hydroxide →

metal salt + water

carboxylic acid + metal carbonate →

metal salt + water + carbon dioxide

Ester functional group

R-COO-R

Uses of synthetic esters

solvents, artificial fruit flavourings and perfumes

How do you name esters?

first part from alcohol, second part from carboxylic acid

What is the chemical reaction used to make an ester known as?

esterification

conditions of making an ester from an alcohol and carboxylic acid

reflux with strong acid catalyst (e.g. H₂SO₄)

How is an ester separated from the reaction mixture?

distillation (it has the lowest boiling point)

Ways to make esters (2)

1. carboxylic + alcohol

2. acid anhydride + alcohol

How is an acid anhydride made?

removal of a molecule of water from two carboxylic acid molecules

Characteristics of the formation of an ester from an acid anhydride and alcohol (4)

1. not reversible so higher yield

2. slow rate

3. safer (less exothermic)

4. less toxic

hydrolysis

a chemical reaction where water causes the breaking of bonds in a decomposition reaction

What are the conditions for the hydrolysis of esters?

acidic or alkaline

Conditions for hydrolysis of esters in acidic conditions

refluxed with catalyst of hot aqueous acids (e.g. sulfuric or hydrochloric acid)

Conditions for hydrolysis of esters in alkaline conditions

refluxed with hot aqueous alkali (e.g. potassium or sodium hydroxide)

Products of hydrolysis of esters in acidic conditions

carboxylic acid and alcohols

Products of hydrolysis of esters in alkaline conditions

carboxylate salts and alcohols

Acyl chloride functional group

R-COCl

Acyl chlorides

carboxylic acid derivatives where the -OH group has been replaced with Cl

Characteristics of acyl chlorides

very reactive, fuming colourless liquids