Benzene
a naturally occurring aromatic compound, which is a very stable planar ring structure with delocalized electrons
Models of benzene (2)
Kekule’s model
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)
unlike alkenes, benzene is resistant to addition reactions
enthalpy of hydration of benzene shows that benzene is more stable than predicted
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)
nitration
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)
haloalkane
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)
oxidation
nucleophilic addition
What do carbonyls react with in nucleophilic addition reactions?
sodium tetrahydridoborate(II) (NaBH4)
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?
Brady’s reagent (2,4-DNP)
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)
metals
metal oxides
metal hydroxides
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)
carboxylic + alcohol
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)
not reversible so higher yield
slow rate
safer (less exothermic)
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