Aromatic chemistry Flashcards
notes from spec
The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double.
Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.
electrophilic subsitution
Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.
Nitration is an important step in synthesis, including the manufacture of explosives and formation of amines.
Friedel–Crafts acylation reactions are also important steps in synthesis.
benzene
-Benzene doesn’t undergo addition reactions like expected
-Benzene – planar, bond angles of 120 degrees
C6H6
overlap of orbtials
-carbon can form 2 bonds as it promotes electrons from 2s orbital to 2p
-overlap of orbitals = delocalised electrons
=in beneze each carbon forms 3 sigma bonds
-no delocalised electrons = no alternating double and single bonds (carbon-carbon) (p orbtials involving in forming pi bond can overlap)
aromatic compound
-when compounds contain 6 carbon ring and overlapping p orbital (benzene) then they are known as an aromatic compound
-when a compound has delocalised electrons it is more stable
-more stable = electron density decreases = electrons spread out
-benzene has a lower electron density than alkenes
explain the bonding in benzene
You should say that each carbon atom forms three covalent bonds.
You should say that the remaining p orbitals overlap above and below the ring to form a pi system.
You should say that the electrons in these p orbitals are delocalise
explain the shape of benzene
You should say that benzene is planar.
You should say that the bond angles in benzene are 120°.
You should say that the C-C bonds are equal in length.
naming aromatic compounds
-other atoms bonded to benzene ring = called phenyl group
-alkene has higher priortiy than benzene
-suffix = benzene when highest priority
-prefix = phenyl when not high priority
nitro group
Nitro group:
NO2 = 1,2,3trinitrobenzene
benzene and bromine electrophilic subsitution
-Benzene and bromine don’t undergo addition reactions because benzene has a lower electron density than alkenes
-this means it doesn’t induce a large dipole in bromine
-aluminium bromide (halogen carrier) + benzene –> bromobenzne + H+ in electrophillic subsitution reaction
Br+ is stronger electrophille than Br2
reagents for the nitration of benzene
-concentrated HNO3
-concentrated H2SO4
needs to be concentrated bc otherwise would react with water instead of benzene as electrophiles are very reactice
classes of organic molecules
There are two major classes of organic chemicals
aliphatic : straight or branched chain organic substances
aromatic or arene: includes one or more ring of six carbon
atoms with delocalised bonding.
single and double bonds
Benzene is a planar molecule. The
evidence suggests all the C-C bonds
are the same and have a length and
bond energy between a C-C single
and C=C double bond.
bond angle in benzene
The H-C-C bond angle is 120o
in
benzene
thermodynamically stable
This when represented on an energy level
diagram shows that the delocalised benzene
is more thermodynamically stable than the
theoretical structure.
Theoretically because there are 3
double bonds in the theoretical
cyclohexa-1,3,5-triene one might
expect the amount of energy to be
3 times as much as cyclohexene.
However, in actual benzene the
amount of energy is less. The 6 pi
electrons are delocalised and not
arranged in 3 double bonds
toxicity of benzene
Benzene is a carcinogen (cancers causing
molecule) and is banned for use in
schools.
Methylbenzene is less toxic and also
reacts more readily than benzene as the
methyl side group releases electrons into
the delocalised system making it more
attractive to electrophiles.
nitration of benzene
Change in functional group: benzene nitrobenzene
Reagents: concentrated nitric acid in the presence of
concentrated sulfuric acid (catalyst)
Mechanism: Electrophilic substitution
Electrophile: +NO2
overall equation for formation of electrophile
Overall equation for formation of the electrophile:
HNO3 + 2H2SO4 –> +NO2 + 2HSO4- + H3O+
Fridel crafts acylation
Change in functional group: benzene phenyl ketone
Reagents: acyl chloride in the presence of anhydrous
aluminium chloride catalyst
Conditions: heat under reflux (50OC)
Mechanism: Electrophilic substitution
Formation of electrophile:
AlCl3 + CH3COCl [CH3CO]+ + [AlCl4]-
benzene = attract electrophiles
-electrophile = electron pair acceptor
-benzene = high electron density = delocalised ring of electrons
benzene undergoes electrophilic subsitution rather than addition as it would disrupt the stability of the molecule as the delocalised ring of electrons would have to break
AlCl3 = halogen carrier equation
RCOCl + AlCl3 –> RC+O + AlCl4-
AlCl3 accepts pair of electrons from acyl group
carbocation is formed due to increased polarisation
stronger electrophile is produced which can now react with benzene
nitration of benzene = x2 equations
HNO3 + H2SO4 –> H2NO3+ + HSO4-
H2NO3+ –> NO2+ + H2O
use these data to show that benzene is 152Kjmol-1 more stable than the hypothetical compound cyclohexa-1,3,5-tirene
-120 x 3 = -360
-360 – 208 = 152
state in terms of bonding why benzene is more stable than cyclohexa-1,3,5-triene
-each carbon has 3 covalent bonds
-electrons in p orbital form pi system (delocalised electrons)
write the letters in increaseing bond length
X Y W
explain your answer the bonding of these 2 dienes
proximity for 1,3 C=C bonds are close together
delocalisation = overlap of electrons in p orbital = extra stability
closer bonds =
partial delocalsation
C=C bonds are closer = repel so less stable
they become stabilised by delocalisation
pentanedioyl chloride
O=C-Cl – (CH2)3 - Cl-C=O
electrophile in electrophilic subsitution
NO2+
methylbenzene + HNO3 + H2SO4
2-nitromethylbenzene
methylbenzene + ethanoic anhydride + AlCl3 to make
2-methylphenylethanone
write an equation for the reaction of concentrated HNO3 and concentrated H2SO4
HNO3 + 2H2SO4 –> NO2+ + H3O+ + 2HSO4-
name and outline the mechanism for the reaction of this species with nitrobenzene to form 1,3-dinitrobenzene
electrophilic subsitution
equation for ethanoic anhydride and propan-2-ol
(CH3)2CHOH + (CH3CO)2O –> (CH3)CH3COOH
ester = methylethylethanonate
give the formula of the biodisel molecule with the highest mr that can be produced from the reaction of this ester with methanol
C17H33COOCH3
biodiesel formula
RCOOCH3
arenes
benzene = high melting points due to stability of delocalised ring
but low boiling points as they are non-polar
benzene
-C6H6 = alternating double and single bonds
-Kekule’s structure = cyclohex-1,3,5-triene
-Benzene has a delocalised electron within the pi system meaning it is more stable than kekule’s model
bonding in benzene
Bonding in benzene –> planar structure and bond length intermediates between double and single bonds
HNO3 + H2SO4
HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
aromatic vs aliphatic
Aromatic:
-carbon and hydrogen arranged in ring structures with delocalised pi electrons
Aliphatic:
-organic compounds arranged in straight chains = branched and non-aromatic
kekule structure
-C=C bond length in cyclohexane is shorter than C-C bond length in benzene
-C=C bond in cyclohexane is larger than benzne
Ethalphy of hydrogenation for kekule’s model is more exothermic than cyclohexene suggesting benzene is more stable and the bonding is stronger
-benzene = less reactive than kekule’s model
delocalisation of benzene
-pi delocalised system in benzene formed by overlap of p orbitals spreads electron density throughout the structure
-benzene = planar structure = each carbon has 3 covalent bonds
bonding / shape/ stability of benzene
Bonding –> each carbon has 3 covalent bonds, spare electrons in p orbital overlap, delocalisation
Shape –> Planar (120 degree), 6 carbon ring, C-C bonds equal in length
Stability = benzene = more stable
electrophilic ring
-electrophilic ring = area of high electron density so is attacked by electrophiles
Benzene doesn undergo addition reactions in the same way as ethene. It would take a lot of energy to break the delocalisation
nitrated arene
Nitrated arene are useful for making explosives such as TNT and industrial dyes
The NO2+ ion is produced by a 2 step reaction. Both acids are concentrated
Sulfuric acid is stronger acid than nitric acid
H2SO4 + HNO3 –> H2NO3+ + HSO4-
HNO3 + 2H2SO4 –> NO2+ + 2HSO4- + H3O+
phenyl
Benzene which has lost 1 H is called phenyl
general mechanism for electrophilic subsitution of benzene
-two of the electrons in the delocalised system attracted towards the X+ and form a bond with it
-a lone pair of electrons on Y- forms a bond with the hydrogen atom at the topic of the ring
2 types of electrophilic subsitution
1) nitration = concentrated nitric acid and concentrated sulfuric acid
2) Friedel craft acylation of benzene
electrophiles
X = NO2+
Y = HSO4- (has lone pair of electrons)
equation for nitration of benzene
-benzene + HNO3 –> nitrobenzene + H2O
-generation of electrophile = HNO3 + 2H2SO4 –> NO2+ + 2HSO4- + H3O+
friedel crfats acylation
-reagent = acyl chloride or acid anhydride
-conditions = anhydrous = H atom on ring is replaced by acyl group
-products = aromatic ketones
acyl chloride equation
benzene + acyl chloride –> aromatic ketone + HCL
acid anhydride equation
benzene + acid anhydride –> atomatic ketone + carboxylic acid
Equation for regeneration of catalyst in acyl chloride:
-AlCl4- + H+ –> AlCl3 + HCl
-electrophile = RCO+ (acylium ion)
friedel crafts acylation conditons and reagents
Type of reaction: (Friedel-Crafts) Acylation
Reagent(s): +CH3COCl
Conditions: AlCl3
reduction nitrobenzene to aromatic amine
Sn and Conc HCL
followed by NaOH
reduction of nitrile to primary aliphatic amine
Ni/H2
