3.6 aromatic compounds Flashcards
what do aromatic compounds all contain?
a benzene ring
what is the structure of benzene?
C6H6
planar,cyclic structure
each c forms single covalent bonds to the carbons on either side and to 1 hydrogen
unpaired electron on each c is located in a p orbital that sticks above and below the plane of the ring.
p-orbitals on each c form a ring of delocalised electrons
bond angles- 120
(6 electrons in a pi bond above and below the plane of the atoms)
what is the length of the bonds in a benzene ring?
all the same length (140pm)
between the length of a single C-C and double C=C
why is benzene more stable than cyclohexa-1,3,5-triene would be?
because of the delocalised electron ring system
in a delocalised ring, the electron density is shared over more atoms so energy of the molecule is lowered and it becomes more stable
evidence for benzene being more stable than cyclohexa-1,3,5-triene
cyclohexatriene would be expected to have a ∆H of 3x -120KJmol-1 (because it would have 3 double bonds)
benzene is more stable because the actual hydration enthalpy is -208 KJmol-1 so 152 JKmol-1 less exothermic than expected
so less energy is given out than the expected so must be used elsewhere (to break bonds) this energy is used to break the more stable electron ring system
what are the problems with kekules structure? (cyclohexa-1,3,5-triene)
- C-C bonds are all the same length (halfway between c-c and c=c)
- benzene doesnt undergo electrophilic addition readily
- you dont get isomers like those in my book from 5/9/24
- ∆H ∆H is 152KJmol-1 less than expected
how to name aromatic compounds
practice
suffix benzene or prefix phenyl
why does benzene attract electrophiles?
because it is a region of high electron density
what mechanism does benzene go through?
why?
electrophilic substitution
because it is so stable so doesnt undergo electrophilic addition as this would destroy the electron ring
describe the electrophilic substitution mechanism of benzene
electron dense region at centre of ring attracts a nucleophile
electrophile steals pair of e- from centre of ring and gives molecule a + charge
to regain stability of benzene ring, c which is now bound to electrophile loses a h
conditions of nitration?
overall equation?
when you warm benzene and conc sulfuric acid, you get nitrobenzene.
overall eq is:
HNO3 + benzene -> nitrobenzene + H2O
what is the sulfuric acid catalyst for in nitration?
equation for making NO2+
helps make nitronium ion (NO2+) which is the electrophile
formation of NO2+ is first step of reaction mechanism
eq is
HNO3 + H2SO4 -> HSO4- + NO2+ + H2O
(once formed it can react w benzene through electrophilic sub)
what are the uses of nitration reactions?
nitro compounds can be reduced to form aromatic amines which are used to manufacture dye and pharmaceuticals
nitro compounds decompose violently when heated so they are heated so are used as explosive such as TNT
why are friedel crafts acylation reactions important?
they add an acyl group to a benzene ring which can be modified using further reactions to make useful products
this is good because benzene is stable so is fairly unreactive
what is Friedel-Crafts acylation?
conditions?
Adding an acyl group to a benzene ring
reactants need to be heated under reflux in a non-aqueous solvent
what is the equation for friedel crafts acylation?
acyl chloride + benzene -> phenylketone + HCl
what is a halogen carrier?
what is the halogen carrier and electrophile in friedel crafts acylation?
electrophiles need to be very + to attack stable benzene and most arent polar enough so are made stronger by halogen carrier catalyst
acyl chloride provides electrophile, AlCl3 is halogen carrier
how does AlCl3 make electrophile stronger?
accepts lp from acyl chloride which increases the polarisation of the acyl chloride forming a carbocation
what is the first step of friedel crafts acylation (electrophilic addition) mechanism
forming the catalyst
CH3COCl + AlCl3 -> Ch3CO+ + AlCl4
what are amines?
when one or more hydrogens in ammonia (NH3) is replaced with an organic group eg alkyl
1 hydrogen replaced- primary amine
2- secondary amine
3- tertiary amine
4- quaternary ammonium ion
how are quaternary ammonium salts fromed?
quaternary ammonium ion is + charged so attracted to any - ions near and form quaternary ammonium salts eg tetramethyl ammonium chloride
naming amines
amine is suffix or amine ion for quaternary
prefix is what is attached
add di tri tetra is multiple of same group attached
if more than 1 type of group, alphabetical order
what is a surfactant?
what are cationic surfactants?
compound which is partly soluble and partly insoluble and is a quaternary ammonium salts with at least one long hydrocarbon chain
why are cationic surfactants useful in detergents?
non polar end (- tail which is insoluble in water) binds to grease and polar end (+ head which is soluble in water) dissolves in water allowing grease to mix w water and be washed away
why can cationic surfactants be used in fabric conditioners and hair products?
hair/ fabric gets wet and picks up - charges on surface
+ part of surfactant is attracted to this and forms coating over surface
this prevents build up of static electricity
why are amines weak bases?
they can accept protons
lp on nitrogen forms coordinate bond with H+
strength of this depends on how available the N lp is (high electron density= more available so more likely to accept H+ so stronger)
are primary aromatic amines strong or weak bases? why?
weak
benzene ring draw electrons closer to itself and N lp gets partly delocalised onto the ring so electron density on N decreases making lp less available
are primary aliphatic amines strong or weak bases? why?
strong
alkyl group has a positive inductive effects as it pushes electrons onto N so N’s electron density increases and nitrogen’s lp becomes more available to accept a proton
how strong is ammonia as a base?
between aromatic amine and aliphatic amine as it doesnt have aromatic group to pull lp away or alkyl group to push lp forwards
what are amides?
derivatives of carboxylic acids
CONH2
carbonyl group pulls electrons away from NH2 group so amides behave differently from amines
what is an N-substituted amide?
amides where one of the H attached to N has been substituted with alkyl group
name starts with N-(alkyl group)
difference between amide and amine?
amiDes have nitrogen bonded directly to carbon with C=O Double bond
amines have no double bond
how are amides produced?
produced from acyl chlorides or acid anhydrides
what are the 2 ways of producing aliphatic (straight chain) amines?
reaction of NH3 with halogenoalkanes
reduction of nitrile compounds
how do you produce aliphatic amines using halogenoalkanes and NH3?
conditions?
nucleophillic substitution
heat halogenoalkane with excess ethanolic ammonia
what is the problem of reacting NH3 with halogenoalkanes to produce primary amines?
further substitution reactions occur so you get a mix of primary, secondary, tertiary, quaternary ammonium salts
excess ammonia condition leads to…
(nucleophillic sub to produce amines)
mostly primary amines
excess halogenoalkanes leads to….
(nucleophillic sub to produce amines
mostly quaternary ammonium salts
why does the further substitutions of amine stop at quaternary ammonium salt?
it doesnt have a lone pair of electrons so cant react further
equations for producing aliphatic amines by reducing nitrile compounds?
R-X + KCN -> R-C≡N + KX (reflux, aq ethanol)
then
R-C≡N + 4[H] -> R-CH2-NH2 (dry ether, LiAlH4)
or
R-C≡N + 2H2 -> R-CH2-NH2
(Ni)
what do you get when acyl chlorides and ammonia react?
what do you get when acyl chlorides react with primary amines?
what is the mechanism for these?
primary amides
N-substituted amides
nucleophilic addition-elimination- see pg 215
how to form phenylamine from nitrobenzene?
nitrobenzene + 6[H] -> phenylamine + 2H2O (Sn and HCl)
or
nitrobenzene + 3H2 -> phenylamine + 2H2O (Ni)
uses of aromatic amines?
dyes
making pharmaceutical chemicals
why wont ammonia react with benzene?
delocalised electrons in benzene repel lp in ammonia- therefore cant act as electrophile
delocalised electrons reduce polarity of C-halogen bond
is phenylamine a strong or weak base? why?
nitrogens lp becomes partially delocalised onto benzene ring so lp is less available to accept a proton (so lower base strength and less basic)
order of base strength of amines
tertiary > secondary > primary > phenylamine
