Chapter 6.1- Aromatic compounds, carbonyls and acids

Question 1

What is benzene?

Answer 1

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

Question 2

What is the Kekule model of benzene?

Answer 2

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

Question 3

What was Kekule’s evidence for his model?

Answer 3

• the number of isotopes of benzene:
• when you add one group to benzene there is 1 isomer
• when you add two groups to benzene there are 3 isomers

Question 4

What are the 3 problems with Kekule’s model?

Answer 4

• unlike alkenes, benzene is resistant to addition reactions
• enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted
• all six carbon atoms in benzene are the same length

Question 5

Why does the enthalpy change of hydrogenation of benzene disprove the Kekule model?

Answer 5

• the enthalpy change of hydrogenation of cyclohexane is -120KJmol^-1, and this contains one double bond. If you multiply this by 3 then the enthalpy change of hydrogenation of benzene should be -360KJmol^-1.
• instead experimentally it was found that it was -208KJmol^-1 and therefore was 152KJmol^-1 more energetically stable than predicted

Question 6

Why does the bond lengths in benzene disprove Kekule’s model?

Answer 6

• X-ray diffraction techniques have shown that all six carbon bonds in benzene are 0.140nm, which is between a C-C bond at 0.147nm and a double bond at 0.135nm.
• Kekule’s model suggests that there should be three shorter C=C double bonds and three longer C-C single bonds.

Question 7

What is the delocalised structure of benzene?

Answer 7

• each of the 6 carbon atoms donates one electron from its p-orbital. These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule.
• the electrons are said to be delocalised as they are able to move freely within the ring and do not belong to a single atom.

Question 8

How does the delocalised structure of benzene explain the 3 problems with his model?

Answer 8

• lots of energy is needed to disrupt the shared electrons so benzene will not readily undergo addition reactions
• the delocalisation of electrons makes the molecule 152KJmol^-1 more stable than three separate double bonds
• the electrons are shared evenly so all the bonds are the same length

Question 9

What is a substitution reaction?

Answer 9

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

Question 10

What is a benzene derivative?

Answer 10

benzene ring that has undergone a substitution reaction

Question 11

How would you name a single-substituted benzene derivative?

Answer 11

• prefixbenzene

- for example when a ethyl group replaces a hydrogen atom in benzene the name becomes ethylbenzene

Question 12

How would you name a double-substituted benzene derivative?

Answer 12

• if two different groups replace the hydrogen atoms on benzene, then the name is written as a prefix in alphabetical order. prefixbenzene
• one group will be added first and be given carbon number 1, the second group will then be given a number to state which carbon atom it is on with respect to the first substituted group.
• the smallest possible numbers should be used

Question 13

How would you name a aromatic compound where benzene is not the main functional group?

Answer 13

-benzene is no longer part of the name
-the prefix changes and the name takes the form:
PhenylMainFunctionalGroup
-example: phenylethene, when ethene replaces a hydrogen atom on a aromatic ring

Question 14

When you substitute one group into the benzene ring, how many isomers do you get?

Answer 14

1

Question 15

When you substitute two groups into the benzene ring, how many isomers do you get?

Answer 15

3

Question 16

What is electrophilic substitution?

Answer 16

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

Question 17

What is a electrophile?

Answer 17

species that accepts an electron pair

Question 18

What are the steps in the electrophilic substitution of benzene?

Answer 18

1. Electrons above and below the plane of atoms in the benzene ring attract an electrophile
2. The electrophile accepts a pair of pie electrons from the delocalised ring and makes a covalent bond. This is the slowest step and known as the rate determining step.
3. A reactive intermediate is formed where the delocalised electrons have been disrupted
4. The unstable intermediate releases an H+ ion and the stable product is formed. This is a very fast step

Question 19

What is the molecular and empirical formula of benzene?

Answer 19

molecular: C6H6
empirical: CH

Question 20

What is nitration of benzene?

Answer 20

electrophilic substitution reaction where a hydrogen atom is exchanged for a nitro group (-NO2)

Question 21

What are the reagent and what is the catalyst in the nitration of benzene?

Answer 21

• reagent :concentrated nitric acid

- catalyst: concentrated sulfuric acid

Question 22

How is the N02+ electrophile generated and what is the equation?

Answer 22

HN03 + H2SO4——-NO2^+ +HS04^- + H20

Question 23

How is sulfuric acid the catalyst in the nitration of benzene?

Answer 23

• the HSO4^- combines with the H+ ion that is released from the unstable intermediate and this forms sulfuric acid (H2S04)
• H^+ + HSO4^- =====H2SO4

Question 24

What is halogenation?

Answer 24

this is an electrophilic substitution reaction where a halogen ion is the electrophile and so replaces a hydrogen atom in benzene

Question 25

What required for benzene to undergo halogenation?

Answer 25

- halogen carrier | - benzene will not react with halogens because the electron ring is too stable

Question 26

What are the different halogen carriers which can be used in chlorination (Cl+ halogen ion is substituted for a H atom)?

Answer 26

- AlCl3 - FeCl3 - Fe

Question 27

What are the different halogen carriers which can be used in bromination (Br^+ halogen ion is substituted for a H atom)?

Answer 27

- AlBr3 - FeBr3 - Fe

Question 28

How is a Br+ halogen electrophile formed from the reaction with iron (III) bromide and bromine?

Answer 28

Br2 + FeBr3====Br^+ + FeBr4^-

Question 29

How are halogen carriers catalysts, using the example of the bromination of benzene with the FeBr3 halogen carrier?

Answer 29

FeBr4^- + H^+======HBr + FeBr3

Question 30

What is a Friedel-Crafts reaction?

Answer 30

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

Question 31

What happens when bromine water is shaken with alkenes?

Answer 31

- this will cause decolorisation as the coloured bromine is used to form the colourless 1,2-dibromoethane. - This is because alkenes have a pie bond between the two carbon atoms. This is a region of high electron density and will therefor readily undergo addition reactions to become saturated

Question 32

What happens when benzene is mixed with bromine water?

Answer 32

- no reaction occurs - this is because benzene is not able to induce a dipole in the non-polar bromine molecule and therefore electrophilic substitution does not occur

Question 33

How are haloalkanes used to attach alkyl groups to benzene rings?

Answer 33

- mix the haloalkane (such as chloromethane) with a halogen carrier (such as iron(III) chloride) which generates the alkyl electrophile (CH3+) - the carbocation undergoes electrophilic substitution with the benzene ring

Question 34

In Friedel-Crafts reactions how is the halogen carrier acting as a catalyst and give a example using FeCl3

Answer 34

- the FeCl4^- reacts with the H atom on the unstable intermediate - this forms HCL and FeCl3 (catalyst regenerated)

Question 35

What is the functional group of acyl chloride?

Answer 35

RCOCL

Question 36

What do acyl chlorides do in Friedel-Crafts reactions?

Answer 36

attach carbonyl groups (ketone) to the benzene ring

Question 37

How are carbonyl groups added to benzene rings through the use of acyl chlorides, using CH3COCL and FeCl3?

Answer 37

1. mix the acyl chloride with a halogen carrier which generates the electrophile 2. CH3COCL + FeCl3====CH3O^+ + FeCl4^- 3. the electrophile undergoes electrophilic substitution with the benzene ring and a benzene ring attached to a carbonyl is made

Question 38

What are the conditions required in acylation?

Answer 38

- 60 degrees for 30 minutes under reflux | - anhydrous conditions

Question 39

Why does only one substitution occur in acylation but multiple substitutions can occur in alkylation?

Answer 39

- the carbonyl group removes electron density from the benzene ring and makes it less susceptible to attack from electrophiles so only one substitution occurs - in alkylation, multiple substitutions occur because the alkyl chain donates electrons to the aromatic ring making it more negative and more susceptible to attack by electrophiles

Question 40

What are phenols?

Answer 40

class of aromatic compounds where a hydroxyl group (-OH) is directly attached to the aromatic ring

Question 41

Is phenol a weak or strong acid?

Answer 41

-weak acid -partially dissociates in water C6H5OH + H20===H30^- + C6H50^-

Question 42

What happens when phenol reacts with NaOH?

Answer 42

- neutralisation reaction which produces a salt and water | - C6H5OH +NaOH=====C6H5O^-Na^+. +H20

Question 43

What happens when phenol reacts with carbonates?

Answer 43

-doesn't react with carbonates such as sodium carbonate as they are weak bases

Question 44

Why is phenol more reactive than benzene?

Answer 44

- due to the p-orbital electrons from the oxygen of the hydroxyl group adding to the delocalised electrons of the aromatic ring - the pie-system becomes more nucleophilic as there is an increase in electron density which allows the aromatic ring in phenol to be more susceptible to electrophilic attack as it can induce a dipole in a non-polar molecule

Question 45

What is the bromination of phenol?

Answer 45

- electrophilic substitution reaction - tripple substitution reaction - C6H5OH + 3Br2===C6H2Br3OH + 3HBr - white precipitate forms which smells of antiseptic

Question 46

What happens when phenol undergoes nitration with dilute nitric acid?

Answer 46

- single substitution reaction with dilute nitric acid (HNO3) - reaction forms a mixture of either 2-nitrophenol and 4-nitrophenol - C6H5OH + HNO3==C6H4(NO2)OH + H20

Question 47

What happens when phenol undergoes nitration with concentrated nitric acid?

Answer 47

- triple electrophilic substitution reaction occurs | - 2,4,6-trinitrophenol is formed

Question 48

What is the directing effect?

Answer 48

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