Arenes

  • Created by: Hindleyc
  • Created on: 26-02-19 18:20
What are the two major classes of organic chemicals
Aliphatic- straight and branched chain organic substances. Aromatic/arene- Includes one or more ring of 6 carbon atoms with delocalised bonding eg Benzene
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Benzenes formula (simplest arene)
C6H6
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What is its basic structure
6 C atoms in a hexagonal ring with one H atom bonded to each C atom
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What is each C atom bonded to? how?
2 other C atoms and one H atom by single covalent sigma bonds
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What does this leave?
1 unused electron on each C atom in a P orbital perpendicular to the plane of the ring
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What does this mean?
the 6 p electrons are delocalised in a ring structure above and below the plane of carbon atoms
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What type of molecule is benzene
Planar
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What does the evidence suggest
That all the c-c bonds are the same and have a length and bond energy b/w a c-c single and c=c double bond
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In formulae what do we draw to show this delocalised system
A circle
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What is the H-C-C bond angle in benzene?
120
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What are the 6 electrons in the pi bond
Delocalised - not attached to a particular atom and spread out over the whole ring
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Theoretically what would be expected because there are 3 double bonds in theoretical cyclohexane-1,3,5-triene
amount of energy to be 3x as much as cyclohexene
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However in actual benzene what is the amount of energy ?
less
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why
6 Pi electrons are delocalised and not arranged in 3 double bonds
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What is this inc in stability connected to delocalisation called?
delocalisation energy
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What is the delocalised benzene?
More thermodynamically stable than theoretical structure
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In cyclohexa-1,3-diene what would there be some of
delocalisation and extra stability as the pi electrons close together and so overlap.
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What would hydrogenation value be
Less -ve than -240 kJ/mol (more stable- endothermic = more stable)
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In cyclohexa-1,4-diene what wouldn't there be
delocalisation because pi electrons too far apart so don't overlap and hydrogenation value =-240 kj/mol
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How to name simple molecules
Have benzene at the root of their name eg methylbenzene
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If 2 or more substituents present on benzene ring?
their positions must be indicated by the use of numbers - give lowest possible numbers to substituents
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When 2 or more substituents present what do you do?
list in alphabetical order and di, tribute prefixes should be used eg 1,3-dimethylbenzene
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What about in other molecules?
Benzene ring can be regarded as a substituent side group on another molecule (like alkyl groups are) .
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What is the c6h5 group known as
The phenyl group eg phenylethanoate
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What doesn't benzene generally undergo
Addition reactions because would involve breaking up delocalised system
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What does most of its reactions involve
Substituting H for another atom or group of atoms
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What does benzene have? what does this mean?
High electron density so attracts electrophiles- reactions usually ELECTROPHILIC SUBSTITUTION
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What is benzene
Carcinogen
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What is methylbenzene
less toxic and also reacts more readily than benzene as methyl side group releases electrons into delocalised system making it more attractive to electrophiles
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What is thef nitration of benzene and other arenes an important step in
Synthesising useful compounds eg explosive manufacture like TNT and formation of amines from which dye stuffs are manufactured
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Change in FG for nitration of benzene
Benzene- nitrobenzene
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Reagents
Conc nitric acids in presence of sulphuric acid (catalyst)
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Mechanism
Electrophilic substitution
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Electrophile
NO2 ^+
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eon for the formation of the electrophile
HNO3+2H2SO4=NO2^+ +2HSO4^- +H3O^+
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What is this
Acid base reaction HNO3 acts as a base
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Overall eqn
Benzene+ NO2^+ = Benzene-NO2 +H^+
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Mechanism
Ring to NO2+, H from branch to horse shoe plus then benzene-no2 + H+
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What does the H+ ion rejoin with
The HSO4^- to reform H2SO4 catalyst
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What temp is run done at
60 *c
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What on using a higher temperature
A second nitrogroup can be substituted
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Change in FG for dreidel craft acylation?
Benzene- phenyl ketone
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Reagents
Acyl chloride in presence of anhydrous aluminium chloride catalyst
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Conditions
Heat under reflux (50 c)
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Mechanism
Electrophilic substitution
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What can be used
Any acyl chloride RCOCl where R is any alkyl group eg -ch3, -c2h5
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What is the electrophile
RCO^+
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EQN for the formation of electrophile
AlCl3+ CH3COCl = CH3CO^+ + AlCl4^-
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What is the AlCl3
Halogen carrier
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Why is it an important reaction in organic synthesis
Introduce a reactive functional group on the benzene ring
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At the end, what does the H+ and AlCl4- react to form
To reform AlCl3 catalyst and HCl
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Mechanism steps
Arrow from ring to plus, Arrow from H bond to plus ring
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What can the nitro group on an arene be reduced to
An amine group
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How? Reagent
Sn or Fe AND HCl
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Conditions
Heating
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Mechanism
Reduction
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(How else can it be done )
Rxn can be done with catalytic hydrogenation (H2 using Ni catalyst)
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EQN
Nitrobenzene + 6[H]= Phenylamine + 2H2O
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As the reaction is carried out in HCl, what will be formed
Ionic salt- C6H5NH3Cl-
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What is this?
Soluble in water
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What will reacting this salt with NaOH give
Insoluble Phenyl amine
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What if an -OH group, Cl atom or an NH2 group is directly attached to a benzene ring/
The delocalisation in the benzene ring will extend to include the lone pair on the N,O and Cl.
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What does this change
Properties and reactions of the side group
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What happens in Chlorobenzene
The C-Cl bond is made stronger, typical halogenoalkane subs and elimination rxns don't occur
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What will the electron rich benzene ring repel?
Nucleophiles
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What happens in Phenol
Delocalisation makes the C-O bond stronger and O-H bond weaker
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What doesn't phenol act like why
An alcohol - is more acidic and doesn't oxidise
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What does happens in phenyl amine?
Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton
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Other cards in this set

Card 2

Front

Benzenes formula (simplest arene)

Back

C6H6

Card 3

Front

What is its basic structure

Back

Preview of the front of card 3

Card 4

Front

What is each C atom bonded to? how?

Back

Preview of the front of card 4

Card 5

Front

What does this leave?

Back

Preview of the front of card 5
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