organics - aromatics Flashcards
what does aromatic mean in chemistry
molecules that contain more than one benzene ring
what is a benzene ring
a ring with conjugated π systems
-Conjugated π systems arise from alternating double and single bonds in which the electrons are delocalised
what is the structure of benzne
- structure consists of 6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds
- each carbon atom forms 3 sigma bonds using the sp2 orbital
- the remaning p orbitals overlap laterally with p orbtials of neighbouring carbon atoms to for a π system
- forms electron cloud above and below the ring
how can the electrons in benzene be delocalised
-the extensive overlap of p orbitals result in the electrons being delocalised as they can spread over the entire ring - causing a π system
what is a π system
-two ring shaped clouds of electron density - one above the plane and one below
what is the shape of benzene and its bond angles
trigonal planar - 120
what does the delocalisation of electrons cause the bonds to be
intermediate between double and single bonds
identical
-single and double bond character
-bond are all the same length
-140 pm
why is kekule example not correct
- C-C bond length - inbetween double/single -triene would expect there to be alternating double + single
- cannot undergo addition reactions - if was triene we would expect it would as has double bond
- triene react with 3H2 to form cyclohexane releasing 360 ( 3x120)- but only releases 208 so stucture is more stable than triene due to delocalisation of electrons- so new strucutre is more stable as is at a lower resting energy
What reaction can benzene undergo
Electrophilic substitutions
Cannot undergo addition as they would loose their delocalisation
What is the nitration of benzene reaction
Hydrogen is replaced by nitro (-NO2) group
Using conc HNO3 and H2SO4
This is to generate the nitro group
HNO3 + 2H2SO4 = NO2+ + H3O+ + HSO4-
Draw arrow from ring to carbon that is being nitrated and the pit horseshoe in direction of the nitro group addition and arrow from H
regenerate the catalyst
HSO4- + H+ = H2SO4
What is the Friedel-Crafts Reactions
In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring
An acyl group is an alkyl group containing a carbonyl, C=O group
The benzene ring is reacted with an acyl chloride in the presence of an AlCl3 catalyst or acid anhydride
What are the steps of acylation of friedel-crafts
H is replaced by a R-C=O group in anhydrous conditions
Produces aromatic ketones which are useful as u can add carbon onto the ring
Generate electrophile - react acylchloride with AlCl3 and produce RCO+ and AlCl4-
Mechanism which is the same as the nitro sub regeneration of catalyst AlCL4- + H+ = AlCl3 + HCl
WITH ACID ANHYDRIDE
Generate Cl3Al-O-C=O-R
And same mechanism
How does carbocations fit into benzene
Methyl are e- releasing so increase electron density in the ring therefore it is easier to nitrate than addition reactions
The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene.
Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.
Value within range –239 to –121
Double bonds separated by one single bond / alternating
Allows some delocalisation/overlap of p orbitals
what does the HNO3 act as in nitration of benzene
a base
