Unit 3) SYNTHESIS ) aromatic hydrocarbons

Question 1

what are aromatic hydrocarbons

Answer 1

a subset of the larger set of hydrocarbons and the simplest of these is benzene

Question 2

benzene properties

• formula (and what does this suggest)
• colour or colourless

Answer 2

• C6H6- shows that benzene is deficient in hydrogens so suggesting it is an unsaturated hydrocarbon
• colourless

Question 3

explain the real structure of benzene

Answer 3

• each carbon atom in benzene is sp2 hybridised , the 3 sp2 hybrid orbitals on each carbon, form sigma bonds with 1 hydrogen and 2 neighbouring carbon atoms
• leaves one unhybridized p orbital on each carbon. these p orbitals overlap side on with an adjacent carbon atom, forming a pi cloud above and below the benzene ring

Question 4

so benzene contains ___ sigma bonds and one pi ____ _____

Answer 4

12 sigma bonds

1 pi electron cloud

Question 5

the 6 p electrons spread over the whole ___
they are _____ and give regions of electron density above and below the ring
they are shared by all ___ c atoms

Answer 5

ring
delocalised
6

Question 6

why is the benzene ring stable

- why does benzene not take part in addition reactions

Answer 6

due to the delocalisation of electrons in the conjugated system. the delocalised electrons being present explains why the benzene ring does not take part in addition reactions - addition reactions would disrupt the electron delocalisation and so reduce the stability of the ring

Question 7

what is a phenyl group

- formula of phenyl group

Answer 7

a benzene ring in which one hydrogen atom has been substituted by another group
-c6h5

Question 8

when a hydrogen in benzene is replaced by a hydroxyl group , what is formed

Answer 8

phenol - a weak acid

Question 9

phenol can lose a hydrogen ion as the phenoxide ion formed is _____ because the negative charge on the oxygen atom is delocalised around the ring

Answer 9

stabilised

Question 10

benzene can undergo electrophilic substitution reactions in which the pi cloud is disturbed by then __ ____ in the final product

Answer 10

re forms

Question 11

electrophilic substitution reactions that benzene can undergo

Answer 11

• nitration
• sulfonation
• alkylation
• halogenation

Question 12

nitration

• what is it
• mechanism
• how is it done ( what reagents are used)
• what is the electrophile

Answer 12

• nitration of benzene is where a hydrogen is replaced with a nitro group (NO2)
• notes for mechanism
• to nitrate benzene, a mixture of concentrated sulfuric acid and concentrated nitric acid is heated under reflux at 50degrees.
• this generates the nucleophile NO2+

Question 13

sulfonation

• mechanism
• how is it done - what reagents are used
• what is the electrophile

Answer 13

• notes for mechanism
• when benzene is heated under reflux for hours with concentrated sulfuric acid , benzenesulfonic acid is formed
• electrophile is formed from the sulfuric acid HOSO2+
• if oleum is used the electrophile is SO3+

Question 14

alkylation

• mechanism
• how is it done ( what catalyst is used)
• what is the electrophile

Answer 14

• notes for mechanism
• when benzene is alkylated , it reacts with a haloalkane in the presence of aluminium chloride as a catalyst
• electrophile from using chloromethane + AlCl3 is CH3+

Question 15

explain alkylation

Question 16

halogenation

• mechanism
• how is it done - what catalysts are used
• example electrophile

Answer 16

• notes for mechanism
• benzene can undergo chlorination or bromination when it reacts with chlorine or bromine in the presence of a catalyst usually aluminium chloride or iron iii chloride
• in chlorination the electrophile would be cl+