6.1.1 Aromatic Compounds Flashcards
Aliphatic
Straight or branched chain organic substancex
Aromatic/arene
Includes one of more rings of six carbons with delocalised bonding
Benzene structure
C6H6
Carbon to 2 carbons and one hydrogen by single covalent sigma bonds
One unused electron in each C atom in a p orbital, perpendicular to plane of ring
6 p electrons are delocalised into a ring above and below carbon atoms
Kekule model
Alternate single and double covalent bonds
What type of molecule is benzene what does this suggest
Planar
All C-C bonds are the same and have a length and bond energy between a CC single and CC double
HCC bond angle in benzene
120 degrees
What is delocalisation energy
Increase in stability connected to delocalisation
Enthalpies of hydrogenation
Would expect when 3 double bonds for the hydrogenation to be 3 times greater
Real amount is less as 6 pi electrons are delocalised
Delocalised is more thermodynamically stable
Evidence for why benzene has a delocalised structure
Bond length intermediate between short C=C and long C-C
🔼H hydrogenation less exothermic than expected when compared to that of Kekule structure
Only reacts with Br2 at high temp or presence of halogen carrier
Doesn’t undergo addition reactions
Reactions of benzene
Doesn’t undergo addition reactions as they’d have to break delocalised system
Electrophillic substitution
Toxicity of benzene
Benzene is a carcinogen
Methylbenzene less toxic and reacts more readily as methyl side group releases electrons into delocalised ring, more attractive to electrophiles
Nitration of benzene
Benzene to nitrobenzene Conc nitric scid Conc sulfuric acid catalyst Electrophillic substitution Uses NO2+ as electrophile 50 degrees temp Reacts slowly
Bonding in delocalised
Draw hexagon with p orbitals above and below
Show overlap
Then draw ring
Enthalpy of hydrogenation
Enthalpy change when one mole of unsaturated compound reacts with an excess of hydrogen to become fully saturated
Nitration full stages
1. Create electrophile: HNO3 + H2SO4 -> NO2+ +H2SO4- +H2O 2. Dative covalent bond (mechanism) 3. Catalyst regenerated: H+ + HSO4- -> H2SO4
Halogenation
Benzene to bromobenzene Bromine Iron 3 bromide catalyst (FeBr3) Electrophillic substitution Can be done with chlorine with (AlCl3 or FeCl3)
naming aromatic compounds
if group is: alkyl, halogen, nitro then added as a prefix ___ benzene
if group is: amine,ester, alcohol or ketone then phenyl___
which groups get ___benzene
alkyl
halogen
nitro
which groups get phenyl ___
amine
ester
alcohol
ketone
Halogenation full stages
1. Create electrophile: AlBr3 + Br2 -> AlBr4- + Br+ 2. Dative covalent bond forms unstable intermediate 3. Catalyst regenerated: H+ + AlBr4- -> AlBr3 + HBr
which category of reactions are alkylation and acylation?
Friedel Crafts
Alkylation
benzene to alkyl benzene
chloroalkane in the presence of anhydrous aluminium chloride catalyst
heat under reflux
electrophillic substitution
Alkylation full steps
1. Create electrophile: AlCl3 + CH3Cl -> AlCl4- + CH3+ 2. Accepts lone pair, dative covalent bond is formed 3. Catalyst is regenerated AlCl4- + H+ -> AlCl3 + HCl
Acylation
benzene to phenyl ketone
acyl chloride in the presence of anhydrous aluminium chloride catalyst
heat under reflux at 50 degrees
electrophilic substitution
Acylation full steps
1. Create electrophile AlCl3 + CH3COCl -> AlCl4- + CH3CO+ 2. Forms a dative covalent bond 3. Catalyst regeneration: AlCl4- + H+ -> AlCl3 + HCl
naming acyl chlorides
look like a carboxylic acid but have Cl in replacement of OH
Name carbon chain first (ethanoyl) then place “chloride” on the end
phenol definition
aromatic compound where a hydroxyl (OH) group is directly bonded to the benzene ring
phenols as acids
very weakly acidic
weaker than carboxylic acids
reactions: phenols vs carboxylic acids
both: react with sodium metal and sodium hydroxide (metals and strong bases)
carboxylic acids: react with sodium carbonate (weak base)
phenols: bromine
phenols in water, proof of weak acid
less soluble in water than alcohols due to non-polar benzene ring
dissolved in water partially dissociates to form phenoxide ion and a proton
as it only partially dissociates phenol is a weak acid
phenols reaction with alkalis
forms a salt (sodium phenoxide) and water in neutralisation reactions
acid + alkali -> salt + water
C6H5OH + NaOH -> C6H5 O- Na+ + H2O
if more than one OH then excess alkali, put alkali on each
make sure ions aren’t bonded but just next to each other
phenols reactions with metals
need to balance this equation
acid + metal -> salt + hydrogen
phenol + sodium -> sodium phenoxide + hydrogen
bromination with phenol
Br2
room temperature
decolourised water forms a white ppt
bromination with phenol full steps
max 3 bromine added
decolarised water forms a white ppt
why is phenol more reactive with bromine than benzene
one of the lone pairs of electrons on p orbital of oxygen atom in OH is partially delocalised into the ring
increases the electron density
electrophile is more polarised
so can react more readily
nitration with phenol
reacts with dilute nitric acid
at room temperature
adds to carbon 2,4 or 6
nitration of phenol full steps
react with dilute nitric acid
forms 2-nitrophenol or 4-nitrophenol
2 nitro is more common as 6 would be the same as 2 is the second carbon was free
also forms water
effect of electron donating
increases electron density
effect of electron withdrawing
decreases electron density
electron donating
side chains increase the density
allow electrophiles to react faster, activating groups
in OH and NH2 the O and the N lone pairs overlap the ring
substitute onto position 2,4,6
2,4 directing
electron withdrawing
side chains decrease density
slow electrophile reactions, de-activating groups
NO2, the O is more electronegative so electrons are drawn away from the ring
groups direct substitution to 3 and 5
3-directing
2,4-directing
OH
NH2
F, Cl, Br
R (carbon chain)
3-directing
NO2
COOH
CHO
COOR (ester)
