Aromatic Chemistry Flashcards
What are aromatic hydrcarbons?
Where some or all of the carbons are found in a benze ring
What is the benzene ring classed as if there is an alkyl, halogen or nitro group attached?
Classed as the parent group so benzene = prefix
eg. Chlororbenzene
What groups have the top priority?
Alcohols and amines
What is benzene’s prefix when an alcohol or amine is present?
Phenyl
What is a benzene ring with an OH group present called?
Phenol
Where must the line go when attaching a group?
Straight to first element
eg. CH3 —-> C
What was Kekules propsed structure?
Planar
Cyclic with alternating double and single bonds
120 degrees bond angles
How was it proved there was no double bonds in the structure? PROBLEM 1
Chemist tried reacgting C6H6 with Br2, HBr and H2SO4 but didn’t work
Told us no double bonds as molecule cound not undergo electrophilic addition reactions
What was problem 2?
Expected to be irregular hexagon
But used x-ray diffraction to measure bond length
Showed regular shape = c-c bonds same length
+ each bond is an intermediate between a single and a double
What was problem 3?
Cyclohexene’s enthalpy change = -120kjmol-1
Kekules model expected benzene to have -360kjmol-1
But in reality had -208kjmol-1
So showed bonds more stable than alkenes as less exothermic than expected
Delocalised model of benzene
LEARN IT
Each carbon has 3 sigma bonds = leaves a spare electron in 2p orbital
2p orbitals overlap above + below ring
Form pi bond
Electrons in pi bond = delocalised
What is the general equation for halogenation?
Benzene + Br2 —-> bromobenzene + HBr
What catalyst is used in halogenation make the halogen carrier?
AlBr3
What must the halogen carrier catalyst contain?
The same halogen as the halogen carrier
What is the equation for creating the electrophile (step 1) in halognation?
Br2 + AlBr3 —-> AlBr4- + HBr
What is the equation for regenerating the catalyst (step 3) in halogenation?
AlBr4- + H+ —-> AlBr3 + HBr
What are the products of the mechanisms for halogenation?
Bromobenzene + H+
What is the general equation for nitration?
Benzene + Nitric acid —-> Nitrobenzene + H2O
What is the equation for the first step of nitration?
Nitric acid + sulphuric acid —-> NO2 + + HSO4 -
What are the products of the mechanism for nitration?
Nitrobenzene + H+
What is the equation for step 3 of nitration?
HSO4- + H+ —-> H2SO4
What are the conditions needed for nitration?
H2SO4 catalyst
50 degrees
What is the order you draw out electrophilic substitution for aromatic chemistry?
General equation
Creating the electrophile
Mechanism
Regenerating the catalyst
What type of reaction is alkylation ans acylation?
Friedel Crafts reaction
What is different about alkylation?
The alkyl group needs to be attached to a halogen
What is the general equation for alkylation?
Benzene + Ch3Cl —-> methylbenzene + HCl
What is step 1 of alkylation?
CH3Cl + AlCl3 —-> CH3+ + AlCl4-
What is the products of alkylation mechanism?
Methylbenzene + H+
What is the third step of alkylation?
AlCl4- + H+ —-> AlCl3 + HCl
What is the catalyst of alkylation?
AlCl3
But must have the same halogen on as the alkyl group
What is the general equation for acylation?
Benzene + CH3COCl —-> phenylethanone + HCl
What is the equation for step 1 of alcylation?
CH3COCl +AlCl3 —-> AlCl4- + CH3CO+
What are the products formed in the mechanism for acylation?
Pheylethanone (ketone) + H+
What is step 3 of acylation?
AlCl4- + H+ —-> AlCl3 + HCl
What must an electrophile have in electrophilic substitution?
A strong positive charge so tha it can attack the benezene ring
Most compounds not polar enough
What does a halogen carrier do in electrophilic substitution?
Accepts a lone pair of electrons from the halogen carrier
As the lone pair is pulled away polaristaion increases, forming a strong electrophile
What is a phenol?
Type of organic chemical containing a hydroxyl fuctional group bonded directly to an aromatic ring
What type of acid is phenol?
Weak acid
Is phenol more soluble in water then benzene?
YES
Is phenol more soluble in water than alcohols?
NO
Why is phenol less soluble in water than alcohols?
Due to non-polar benzene ring on phenol
What happens when phenol is dissolved in water?
It partially dissociates to form a phenoxide ion and a proton
What state is phenol at R.T?
Solid
What doesn’t ethanol react with?
Strong bases (NaOH) or weak bases (Na2CO3)
What is the difference between ethanol and phenols and carboxylic acids?
Phenols + carboxylic acids will react with solutions of strong bases
What is the difference between carboxylic acids and pehnols?
Carboxylic acids are the only ones strong enough to react with a weak base
What would you observe if a carboxylic acid reacted with a weak base like Na2CO3?
Gas/bubbles
What is the reaction of phenol with an alkali?
Phenol + NaOH —-> sodium phenoxide + H2O
What is the reaction of phenol with a metal?
Phenol + Na —-> sodium phenoxide + H2
In bromination of phenols what does phenol react with?
Aqueous solution of bromine (bromine water)
At what conditons an bromination of phenols be carried out at?
R.T
How do you know when bromination of phenols are complete?
Br2 water decolourises
OR white precipitate forms
What is the reaction of bromination of phenol?
Phenol + 3Br2 —-> 2,4,6 tribromophenol + 3HBr
What the products of bromination of phenol in the mechanism?
Bromophenol + H+
Why does phenol react more readily with bromie than benzene?
One of the lone pairs on the p-orbital of O atom in OH is partially delocalised into the ring
Increases elcetro density
Electrophile becomes polarised
What is different about bromination phenols?
Doesn’t require a halogen carrier
What does phenols react with in nitration?
Nitric acid
What is different about nitration of phenols?
Doesn’t require a halogen carrier
Can produce different products depending where the NO2 group joins
What is the equation for nitration of phenols?
Phenol + nitric acid —-> nitrophenol + H2O
Why can the nitration of phenols produce either product?
Because the -OH group has a 2,4 directing effect
What are the products of the mechanism of nitration of phenols?
Nitrophenol + H+
What is it called when aromatic compounds undergo second substitution?
Di-substitution
What is the directing effect?
The presence of substituted groups on a benzene ring will have an effect on the substitution of further groups
How can substituted groups be classified?
By the effect they have on electron density of the ring
What does electron donating do?
Increase electron density
What does withdrawing electrons do?
Decrease electron density
What do side chains that increase electron density do?
Allow electrophiles to react faster than with benzene, so are “activating groups”
Why are -OH and -NH2 groups activating groups?
O + N lone pair orbitals overlap with delocalised ring
So increase electron density at positions 2,4 + 6
So electrophiles likely to react at these positions
What is 2,4 directing groups?
Electron donating
Why do second substituted groups join at positoon 3 if the 1st substituted group is 2,4 directing?
Weaker position at 3
What do side chains that decrease electron density do?
Slow electrophiles reactions compared to benzene so are “deactivating groups”
Why is NO2 group a deactivating group?
O is more electronegative, drawing electrons away from delocalised ring
Withdraws electro density at positions 2,4 + 6
So electrophiles unlikely to react at these positions
What is 3-directing groups?
Electron withdrawing
Why do second substituted groups join at position 2,4/6 if the 1st substituted group is 3-directing?
Waeker positoon at 2,4 + 6
What are the 2,4 directing groups?
NH2 OH OR F, Cl, Br, I R
What are the 3-directing groups?
NO2
COOH
CHO
COOR
Explain the relative resistance to bromination of benzene compared with alkenes
Benzene pi bond = delocalised
Alkene pi bond = localised
So alkenes have higher electron density compared to benzene
So benzene polarised Br2 LESS
