Unit 3: Section 6 - aromatic compounds and amines MDY * Flashcards
aromatic compounds amines and amides
what is benzene’s formula?
C6H6
what is benzene’s structure?
planar cyclic
- 6 carbons carbon atoms joined together in a flat ring
bond angle = 120*
what is the bonding like in benzene?
each carbon forms a single covalent bond to the carbons on either side of it and to 1 hydrogen atom
what happens to the fourth electron on the carbon atoms in benzene?
it’s located in a p orbital that sticks out above and below the plane of the ring,
each p orbital overlaps with the neighbouring p orbitals to form a pi bond
neighbouring pi bonds overlap
they form a ring of delocalised electrons above and below the plane - a pi system
how long are the bonds in benzene?
they are all the same length and are between the lengths of a single C-C bond and a double C=C bond
why is benzene so stable?
the enthalpy of hydrogenation of cyclohexene (with 1 double bond) is -120 kJ/mol
you would then expect the energy to be -360kJ/mol for 3 double bonds
but in benzene it is -208kJ/mol
meaning more energy was put into breaking the bonds in benzene than making them, so it is more stable than it should be
due to the delocalised ring of electrons (delocalisation stability)
what are aromatic compounds?
a.k.a. arenes
compounds containing a benzene ring
how are arenes named?
chlorobenzene (has Cl) nitrobenzene (NO2) methylbenzene (methyl group) phenol (OH) phenylamine (NH2)
why do arenes undergo electrophilic substitution?
the benzene ring has a high electron density, so it attracts electrophiles
the benzene ring is stable so it doesn’t undergo addition
it swaps a hydrogen for another group or atom to keep the delocalised ring of electrons intact
what are friedel-crafts acylation reactions?
they are used to add an acyl group to the benzene ring
how can benzene be made into useful chemicals?
using friedel-crafts acylation
once an acyl group has been added, the side chains can be modified using further reactions to make useful products
how are electrophiles made to attack benzene?
catalysts called halogen carriers can be used to make stronger electrophiles that have a positive enough charge o attack the stable benzene ring
what halogen carrier is used in Friedel-crafts acylation?
AlCl3
how does AlCl3 make an acyl chloride electrophile stronger?
AlCl3 accepts a lone pair of electrons from the acyl chloride, increasing it’s polarisation and forming a carbocation
it is now a strong enough electrophile
what is the mechanism for friedel-crafts acylation?
electrons in benzene ring attracted to carbocation
2 electrons from benzene bond with carbocation
delocalised ring partially breaks and has positive charge
AlCl4 - ion attracted to positively charged ring
1 chloride breaks away and bonds with hydrogen ion
this removes hydrogen from the ring forming HCl and allows catalyst to reform
how is the electrophile made for the nitration of benzene?
HNO3 + H2SO4 –> NO2 + + HSO4 - + H2O
what conditions are needed to only add one NO2 group?
below 55*C
why are nitration reactions useful?
nitro compounds can be reduced to form aromatic amines (used to manufacture dyes and pharmaceuticals
2,4,6-trinitromethylbenzene (trinitrotoluene - TNT)
what are amines?
ammonia (NH3) with one or more H’s replaced by an organic group
what are the different types of amines?
primary amine (RNH2) secondary amine (R2NH) tertiary amine (NR3) quaternary ammonium ion (NR4 +)
what is phenylamine?
benzene-NH2
a primary amine
what is a quaternary ammonium salt?
quaternary ammonium ions are positively charged so they hang around with negative ions, forming salts
what are cationic surfactants?
quaternary ammonium salts with at least 1 long hydrocarbon chain
what do cationic surfactants do?
long hydrocarbon tail binds to nonpolar substances like grease
cationic head dissolves in water
what are cationic surfactants used for?
fabric cleaners and hair products
fabric conditioners
why can cationic surfactants be used as fabric conditioners?
the positively charged part will bind to negatively charged substances like hair and fibre to get rid of static
how can amines act as weak bases?
they accept protons
the lone pair of electrons on the nitrogen can form a dative covalent bond with an H+ ion
what determines the strength of amine bases?
the more available the lone pair is, the more likely the amine is to accept a proton, the stronger the base
a lone pair is more available if it’s electron density is higher
which amines are the strongest bases?
tertiary amine - strongest secondary amine primary amine ammonia phenylamine - weakest
why are tertiary amines the strongest bases?
alkyl groups push electrons onto attached groups, so electron density on nitrogen increases.
more alkyl groups = lone pair on nitrogen more available
why are aromatic amines the weakest bases?
the benzene ring draws electrons towards itself and the nitrogen lone pair gets partially delocalised onto the ring.
electron density on the nitrogen decreases, making lone pair less available
how can aliphatic amines be produced?
from halogenoalkanes
by reducing nitriles
how can amines be made from halogenoalkanes?
by heating halogenoalkanes with excess ammonia
the ammonia acts as a nucleophile, replacing the halogen
what if there is an excess of halogenoalkanes when making amines?
the primary amine has a lone pair so can act as a nucleophile to join to another halogenoalkane and become a secondary amine
this can keep happening until a quaternary ammonium ion is formed
excess amines can also become ammonium salts with the halogen
how are nitriles reduced to form amines?
using LiAlH4 in dry ether with dilute acid
catalytic hydrogenation
what is catalytic hydrogenation?
reducing a nitrile using hydrogen gas with a metal catalyst e.g. platinum at high temperature and pressure
what is the equation for the reduction of a nitrile?
R-CN + 4[H] –> R-CH2-NH2
how are aromatic amines produced?
by reducing a nitro compound like nitrobenzene
how to reduce nitrobenzene to phenylamine?
heat a mixture of a nitro compound, tin metal and concentrated hydrochloric acid under reflux
add sodium hydroxide
nitrobenzene + 6[H] – tin, HCl, NaOH –> phenylamine + 2H2O
how are aromatic amines useful?
used as starting molecules for dyes and pharmaceuticals
what are amides?
they have functional group CONH2
carbonyl group pulls electrons away from NH2, so amides behave differently to amines
what is kekule’s structure of benzene?
a triene, every other bond is double
if this were true benzene would behave like a alkene
what are the properties if the pi system in benzene?
electrons are delocalised, they don’t belong to any 1 carbon
delocalised electrons repel each other and spread out, making benzene more stable
what does aliphatic mean?
a substance made up of carbon chains
what is a pi bond?
a bond caused by the overlap of p orbitals, found in double C=C bonds and aromatic compounds
what is a pi system?
an area where close pi bonds overlap to create extra stability in a molecule
why does benzene react more slowly than alkenes?
pi bond is delocalised in benzene but it is localised in alkenes
benzene has lower electron density than C=C
so it polarises other molecules less (to create dipoles), so doesn’t react with electrophiles as strongly
how to name an aromatic if it has a carboxylic acid and a hydroxyl group?
carboxylic acids always take priority so it is -hydroxybenzenecarboxylic acid
the carboxylic acid is on carbon 1
