Chapter 6.1- Aromatic compounds, carbonyls and acids Flashcards
What is benzene?
naturally occurring aromatic compound, which is very stable planar ring structure with delocalised electrons
What is the Kekule model of benzene?
-six -membered carbon ring with alternating single and double bonds between the carbon atoms
What was Kekule’s evidence for his model?
- 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
What are the 3 problems with Kekule’s model?
- 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
Why does the enthalpy change of hydrogenation of benzene disprove the Kekule model?
- 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
Why does the bond lengths in benzene disprove Kekule’s model?
- 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.
What is the delocalised structure of benzene?
- 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.
How does the delocalised structure of benzene explain the 3 problems with his model?
- 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
What is a substitution reaction?
where a group or atom is exchanges for another group or atom in a chemical reaction
What is a benzene derivative?
benzene ring that has undergone a substitution reaction
How would you name a single-substituted benzene derivative?
- prefixbenzene
- for example when a ethyl group replaces a hydrogen atom in benzene the name becomes ethylbenzene
How would you name a double-substituted benzene derivative?
- 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
How would you name a aromatic compound where benzene is not the main functional group?
-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
When you substitute one group into the benzene ring, how many isomers do you get?
1
When you substitute two groups into the benzene ring, how many isomers do you get?
3
What is electrophilic substitution?
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
What is a electrophile?
species that accepts an electron pair
What are the steps in the electrophilic substitution of benzene?
- Electrons above and below the plane of atoms in the benzene ring attract an electrophile
- 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.
- A reactive intermediate is formed where the delocalised electrons have been disrupted
- The unstable intermediate releases an H+ ion and the stable product is formed. This is a very fast step
What is the molecular and empirical formula of benzene?
molecular: C6H6
empirical: CH
What is nitration of benzene?
electrophilic substitution reaction where a hydrogen atom is exchanged for a nitro group (-NO2)
What are the reagent and what is the catalyst in the nitration of benzene?
- reagent :concentrated nitric acid
- catalyst: concentrated sulfuric acid
How is the N02+ electrophile generated and what is the equation?
HN03 + H2SO4——-NO2^+ +HS04^- + H20
How is sulfuric acid the catalyst in the nitration of benzene?
- the HSO4^- combines with the H+ ion that is released from the unstable intermediate and this forms sulfuric acid (H2S04)
- H^+ + HSO4^- =====H2SO4
What is halogenation?
this is an electrophilic substitution reaction where a halogen ion is the electrophile and so replaces a hydrogen atom in benzene
What required for benzene to undergo halogenation?
- halogen carrier
- benzene will not react with halogens because the electron ring is too stable
What are the different halogen carriers which can be used in chlorination (Cl+ halogen ion is substituted for a H atom)?
- AlCl3
- FeCl3
- Fe
What are the different halogen carriers which can be used in bromination (Br^+ halogen ion is substituted for a H atom)?
- AlBr3
- FeBr3
- Fe
How is a Br+ halogen electrophile formed from the reaction with iron (III) bromide and bromine?
Br2 + FeBr3====Br^+ + FeBr4^-
How are halogen carriers catalysts, using the example of the bromination of benzene with the FeBr3 halogen carrier?
FeBr4^- + H^+======HBr + FeBr3
What is a Friedel-Crafts reaction?
a substitution where hydrogen is exchanged for an alkyl, or acyl, chain
What happens when bromine water is shaken with alkenes?
- 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
What happens when benzene is mixed with bromine water?
- 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
How are haloalkanes used to attach alkyl groups to benzene rings?
- 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
In Friedel-Crafts reactions how is the halogen carrier acting as a catalyst and give a example using FeCl3
- the FeCl4^- reacts with the H atom on the unstable intermediate
- this forms HCL and FeCl3 (catalyst regenerated)
What is the functional group of acyl chloride?
RCOCL
What do acyl chlorides do in Friedel-Crafts reactions?
attach carbonyl groups (ketone) to the benzene ring
How are carbonyl groups added to benzene rings through the use of acyl chlorides, using CH3COCL and FeCl3?
- mix the acyl chloride with a halogen carrier which generates the electrophile
- CH3COCL + FeCl3====CH3O^+ + FeCl4^-
- the electrophile undergoes electrophilic substitution with the benzene ring and a benzene ring attached to a carbonyl is made
What are the conditions required in acylation?
- 60 degrees for 30 minutes under reflux
- anhydrous conditions
Why does only one substitution occur in acylation but multiple substitutions can occur in alkylation?
- 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
What are phenols?
class of aromatic compounds where a hydroxyl group (-OH) is directly attached to the aromatic ring
Is phenol a weak or strong acid?
-weak acid
-partially dissociates in water
C6H5OH + H20===H30^- + C6H50^-
What happens when phenol reacts with NaOH?
- neutralisation reaction which produces a salt and water
- C6H5OH +NaOH=====C6H5O^-Na^+. +H20
What happens when phenol reacts with carbonates?
-doesn’t react with carbonates such as sodium carbonate as they are weak bases
Why is phenol more reactive than benzene?
- 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
What is the bromination of phenol?
- electrophilic substitution reaction
- tripple substitution reaction
- C6H5OH + 3Br2===C6H2Br3OH + 3HBr
- white precipitate forms which smells of antiseptic
What happens when phenol undergoes nitration with dilute nitric acid?
- 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
What happens when phenol undergoes nitration with concentrated nitric acid?
- triple electrophilic substitution reaction occurs
- 2,4,6-trinitrophenol is formed
What is the directing effect?
how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution
