6.1.1 Aromatic Compounds

What are arenes?

aromatic hydrocarbons containing one or more benzene rings

What is the relationship between aromatic and aliphatic compounds?

they are mutually exclusive

What is the molecular formula of a benzene ring?

C6H6

What is the skeletal formula of a benzene ring?

What is the displayed formula of a benzene ring?

What is the empirical formula of benzene

CH

How are benzene rings with a halogen group named?

halo-benzene

How are benzene rings with an alkyl group named?

alkyl-benzene

How are alcohols with a benzene ring named?

phenyl-alcohol

How are benzene with a ketone group named?

phenyl-ketone

What is a nitro group?

-NO2

How are arenes with a nitro group named?

nitrobenzene

How are arenes with a halogen and alkyl group named?

halo-alkyl-benzene

How is an arene with a carboxylic acid named?

benzoic acid

What is an amine group?

-NH2

How are arenes with an amine group named?

phenylamine

How are are arenes with an OH group named?

phenol

When is the phenyl-prefix used when naming arenes?

when the benzene ring is attached to a alkyl group with 7 or more carbons or that contains a functional group

When is the benzene suffix used when naming arenes?

attached to an alkyl group woth less than 7 carbons or directly attached to a group

What are the exceptions when naming arenes?

• phenol

• benzoic acid

• benzaldehyde

• phenylamine

What is the name of C6H4OHNO2?

2-nitro-phenol

Who suggested that the structure of benzene is the same as triene?

Kekule von Stradonitz

What is the Kekule structure of benzene?

triene

Give 3 pieces of evidence that suggest the Kekule structure of benzene is incorrect?

• resistance to reacting with bromine

• bond lengths

• enthalpy change of hydrogenation

Does benzene decolourise bromine at room temperature?

no

Why doesn’t benzene react with bromine?

it doesnt contain C=C bonds

How can the bond lengths of benzene be found?

xray diffraction analysis

Describe the bond lengths in benzene?

• all C-C bond lengths are the same

• the C-C bond length is between that of C-C and C=C length

What do the bond lengths of benzene suggest?

benzene does not consist of alternating, isolated C-C and C=C bonds

Compare the enthalpy change for the hydrogenation of benzene and the hydrogenation of triene?

the enthalpy change for hydrogenation of benzene is less exothermic than predicted

What does the enthalpy change for the hydrogenation of benzene suggest?

benzene does not contain 3 C=C bonds and is more stable than expected

Compare the stability of benzene and triene

benzene is more stable

What is the theoretical enthalpy change for hydrogenation of triene?

-360

What is the enthalpy change for benzene?

-208

What is the correct structure of benzene?

the delocalised structure

Describe the delocalised structure of benzene

• cyclic molecule

• shape around each C molecule is trigonal planar (120)

• sigma bonds between each carbon atom

• a pi system above and below the plane of carbon rings that consists of 6 delocalised electrons

What is the shape and around each C atom of benzene?

trigonal planar, 120

What bonds exist between the C atoms and CH atoms?

sigma bond

How are sigma bonds formed?

single overlap of orbitals directly between the nuclei of C atoms

How is rge pi system of a benzene ring formed?

formed by the sideways double overlap of p orbitals on each c atom on the carbon ring

How many delocalised electrons does the benzene pi system consist of?

6

Describe the delocalised electrons in the benzene system?

they are mobile across all 6 carbon atoms on the benzene ring

What are the similarities between the pi bond in cyclohexene and the pi system in benzene

• pi bond formed in the same way

• pi bond above and below the plane

What are the differences between the pi bond in cyclohexene and the pi system in benzene

• pi system made up of 6 delocalised electrons while pi bond made up of 2

• pi system - electrons are delocalised, pi bond- electrons are localised

What are elecrophiles?

electron pair acceptor

Why does benzene attract elecrtophiles?

it has an electron-rich pi system

Does the pi system in benzene make it more or less stable?

more stable

What does benzene maintain when it takes part in chemical reactions?

its delocalised pi system

What type of reaction does benzene undergo?

electrophilic substitution

Why is cyclohexene more reacrive toward electrophiles than benzene?

• the pi bond in an alkene is more electron dense than the pi system in benzene

• the 2pi electrons in an alkene are localised bwteen 2 C atoms while the 6pi electrons in a benzene are delocalised between 6 C atoms

• the pi bond in alkene is sufficiently electron dense to induce a dipole in Br2 and so it attracts electrophiles more densely

Define electrophilic substitution in benzene?

• a H atom on the benzene ring is replaced by an electrophile

• the electrophile is attracted to the electron-rich pi system where it accepts a pair of electrons

• forms a new covalent bond with a C atom if the ring

What does the electrophile do to benzene?

accepts a pair of electrons

What is formed when the electrophile accepts a pair of electrons from the benzene pi system?

a new covalent bond with a C atom of the ring

Draw the general mechanism of electrophilic substitution in benzene

How many electrons are involved in the pi bonding in the intermediate of electrophilic substitution?

4

How many electrons are involved in the pi system of the organic product of electrophilic substitution?

6

Why is the intermediate of electrophilic substitution unstable?

the 4 electrons are delocalised about 5C atoms

What are the reagents for the nitrification of benzene?

concentrated nitric acid

What are the conditions for the nitrification of benzene?

concentrated sulfuric acid, heat under reflux at 50*C

If the temperature for nitrification of benzene was increased to 100*C what would happen?

further substitution

What is the overall equation for the nitrification of benzene?

How many steps are involved in the nitrification of benzene?

4

What is the first step of the nitrifcation of benzene?

generation of the electrophile

What is the equation for the generation of NO2+

HNO3 + H2SO4 → NO2+ + H2O + HSO4-

Draw the reaction mechanism between benzene and NO2+

What is the final step in the nitrification of benzene?

regeneration of H2SO4

Write the equation for the regeneration of H2SO4

H+ + HSO4- → H2SO4

What are the reagents for the chlorination of benzene (and bromine)?

chlorine/bromine

What are the conditions for the chlorination of benzene (and bromine)?

FeCl3 / FeBr3

Write the overall equation for the chlorination or bromination of benzene

Write the generation of the Cl+/Br+ electrophile

Cl2 + FeCl3 → FeCl4- + Cl+

Draw the mechanism for the electrophilic substitution of Cl+/Br+ and benzene

Write the equation for the regeneration of the FeCl3/FeBr3 catalyst

FeCl4- + H+ → FeCl3 + HCl

What are the reagents for the alkylation of benzene?

haloalkane

What are the conditions for the alkylation of benzene?

aluminium chloride

What is the equation for the alkylatio of benzne

What is the equation for the acylation of benzene?

What are the reagents for the acylation of benzene?

acyl chloride

What are the conditions for the acylation of benzene?

aluminium chloride

What is the directing effect of an -OH group on benzene?

activates- 2,4 directing

What is the directing effect of a -NH2 group?

activates, 2,4 directing

What is the directing effect of an -NHR group?

activating 2,4 directing

What is the effect of an -NO2 group?

deactivating, 3 directing

What is the effect of an electron donating group?

they make the arene more reactive

Why do electron donating groups make arenes more reactive?

they make the pi system more electron dense, making the ring more susceptible to attack by electrophiles

What is the effect of electron withdrawing groups?

less reactive

Why do electron withdrawing groups make arenes less reactive?

they make the pi system less electron dense making the ring less susceptible to attack by electrophiles

What is the molecular formula of phenol?

C6H6O

What is the displayed formula of phenol?

What is the skeletal formula of phenol?

What are common uses of phenol?

• antiseptic

• disinfectants

• plastics

• resins in paints

What is the use of trichlorophenol?

antiseptic

What is the use of thymol?

antiseptic (thyme)

what is the use of bisphenol A?

plastics and resins

What state is phenol at room temperature?

solid

Why is phenol solid at room temperature?

it forms hydrogen bonds with itself which require lots of heat energy to overcome

Why is phenol soluble in water?

it forms hydrogen bonds with water

What type of reaction does phenol undergo?

electrophilic substitution

Compare the reactivity of benzene and phenol?

phenol is more reactive

What are the reagents for the nitrification of phenol?

dilute nitric acid