Aromatic chemistry Flashcards
BENZENE
what is aromatic chemistry?
the study of organic compounds containing benzene rings
Why is the name ‘aromatic’ used?
because a lot of these compounds have a distinct smell (an aroma)
what are aromatics important in the synthesis of?
aromatics are important in the synthesis of substances such as dyes, drugs, explosives and some plastics
what is the molecular formula for benzene?
C6H6
What is benzene said to be the ‘parent’ of?
benzene is said to be the ‘parent’ of a group of cyclic unsaturated compounds called the ARENES
what is the empirical formula for benzene?
CH
what is the molecular mass for benzene?
78
is benzene or alkenes more reactive?
benzene does not react like toher unstaurated compounds.
Benzene is fairly unreactive compared to the alkenes
How is benzene hydrogenated?
In the presence of Hydrogen gas + Nickel catalyst
Give the symbol equation to show the hydrogenation of benzene?
C6H6 +3H2 —–> C6H12
when benzene is hydrogenated, (h2 gas/Ni catalyst), 3 moles of H2 react with each mole of the benzene to form cyclohexane
what is formed when benzene is hydrogenated?
cyclohexane
What did Kekulè propose about the structure of benzene?
Kekulè proposed that benzene had a cyclic structure with alternating single and double carbon-carbon bonds
Draw the two proposed structures of benzene
what was benzene considered to be, but does not actually exist?
benzene is often considered to be a resonance hybrid of the 2 Kekulè structures, neither of which actually exists
what are the 4 main problems with kekulè structures?
- if benzene was unsaturated it should undergo electrophilic addition readily but it does not
- the reaction with benzene with chlorine, does in fact not produce 2 products as would be expected, instead only one product is formed (1,2-dichlorobenzene)
- X-ray diffraction studies have shown that the benzene molecule is planar and a regular hexagon. all single C-C bond lengths are equal (all 0.139nm). However, in the kekulè structure, the bond lengths would be different because single C-C bonds are longer than double C=C bonds. So c-c bond lengths would be (0.154nm) and c=c bonds to be (0.134nm)
- the enthalpy of hydrogenation value for benzene provides further evidence that the kekulè structures are incorrect
explain bond lengths in a real benzene molecule?
- the c-c bonds are all of equal length and equal strength (intermediate between single and double)
- Sigle c-c bonds are long and weak
- benzene c-c bonds are intermediate length and strength
- double c=c bonds are short and strong
enthalpies of hydrogenation
what evidence shows that benzene is more stable than kekulè proposed structure?
thermochemical evidence - enthalpies of hydrogenation
write out the symbol equation for the hydrogenation of cyclohexene
How much energy is released when cyclohexene is hydrogenated?
120KJ/mol-1
so when we hydrogenate kekulè structure (cyclohexa-1,3,5triene) we expect how much energy to be released?
3x more energy to be released than cyclohexene (has an extra 2 double bonds)
draw ou the symbol equation of kekulè structure being hydrogenated
what is the actual observed value for the hydrogenation of benzene?
208KJ/Mol-1
what do these results tell us?
- this is 152KJ/Mol-1 less than expected
- these results suggest that more energy is needed to break the ‘double bonds’ in benzene than to break 3 double bonds in an alkene
- benzene is therefore more stable than expected and less energy is released when benzene molecules are hydrogenated
Delocalisation stability
Why is benzene more stable?
the explanation for the extra stability is that π electrons in the carbon- carbon double bonds are delocalised around the benzene ring. the delocalisation of the electrons bring more stability to the benzene molecule
draw the skeletal and structural formula of benzene
what does te circle in benzene represent?
the circle represents the 6 delocalised electrons and all the carbon atoms bonded identically
describe the structure and bonding in benzene?
- the 6 carbon atoms in benzene are planar and arranged as a regular hexagon (all bond angles 120°)
- all bonds in benzene are identical
- each carbon atom is covalently bonded to 2 other carbon atoms and 1 hydrogen atom
- therefore, each carbon atom has 1 ‘unused’ p-electron
- delocalisation of the 6 ‘unused’ p-electrons produces electron density above and below the plane of the ring (pi electrons)
because the delocalisation satbilises the benzene molecule, this means that benzene is able to undergo?
overall addition reactions with difficulty
what is the delocalised energy/ resonance energy?
the increase in stability associated with electron delocalisation
Benzene - Electrophilic Substitution
Why is the benzene molecule attacked by electrophiles?
- benzene has a region of high density above and below the plane of the molecule. so the molecule is attacked by positively charged species (electrophiles)
what reaction can benzene undergo but with difficulty?
Benzene can undergo addition (e.g hydrogenation) but only with difficulty (delocalisation energy is lost)
Electrophilic substitution of benzene mechanism. What is step 1?
The formation of the electrophile (E+) - a positively charged species
Which two main electrophiles are regenerated?
e.g +NO2 nitronium (for nitration), R-C(+)=O acylium cation (for Friedel-crafts reaction)
describe what happens in step 2 of the reaction mechanism?
- Addition of the electrophile forms a..
- highly unstable cation intermediate which readily loses (eliminates) a proton (H+) to reform the stable delocalised ring
- left with substituted product
why does benzene undergo substitution reactions and not really any other?
because these reactions retain the extra stability given by the delocalised electrons
What is Nitration?
what is the electrophile for this reaction?
a nitro group is joined onto the benzene ring
+NO2 ( Nitronium ion)
What is acylation?
what is the electrophile for this reaction?
a acyl group is joined onto a benzene ring (a Friedel-Crafts reaction)
acylium carbon R-C(+)=O
what is the overall equation for Nitration?
Benzene + HNO3 —-conc H2SO4 + 50—–> Nitrobenzene + H2O
what are the reagents for nitration?
concentrated HNO3 and concentrated H2SO4
which reagent acts as a catalyst?
concentrated H2SO4 (homogenous)
Draw and explain the electrophilic substitution of benzene via Nitration. Include both step 1 the summary of step 1 and full mechanism of step 2
Step 1: making the electrophile (+NO2)
HNO3 + 2H2SO4 —> +NO2 + H3O+ + 2HSO4-
summary:
HNO3 +H2SO4 —> [H2NO3]+ + HSO4-
[H2NO3]+ –> H20 + +NO2
H2O + H2SO4 –> H3O+ + HSO4-
Step 2: nucleophilic substitution of Nitronium ion
Electrophilic substitution of the benzene ring by the positive electrophile (E+)
highly unstable cation intermediate readily loses (eliminates) a proton (H+) to reform the stable delocalised ring
left with substituted product AND
H+ reformed
what are nitrated benzene derivatives used as?
Explosives (e.g TNT)
what does TNT stand for?
TriNitroTolulene or methyl-2,4,6-trinitrobenzene
how is TNT formed?
when methylbenzene is nitrated
what makes methylbenzene attractive to electrophiles? this means there are more..?
the methyl group is electron-releasing and so makes the benzene ring even more attractive to electrophiles. therefore more nitro groups present in the molecules
the nitro group can be reduced to make..?
aromatic amines
Which two processes can nitro groups be reduced by?
- catalytic hydrogenation (H2/Ni)
- using metal/acid combinations (Sn/HCl)
the nitro benzene ring is then converted into..?
primary amines
uses of primary aromatic amines?
very useful intermediates in organic synthesis
what are primary aromatic amines used to make?
Synthetic dyes - (Azo dyes)
Friedel-Crafts Acylation
why is Friedel-crafts important in synthesis?
leads to the formation of the C-C bond
what is the electrophile?
acylium cation
what does benzene react with during electrophilic substitution of benzene via Friedel-Crafts mechanism?
reacts with acyl chlorides or acid anhydrides in the presence of an aluminium chloride catalyst
what is formed?
A aromatic ketone
what is the overall equation?
benzene + ethanoyl chloride –> phenylethanone + HCL
what are the reagents for this reaction?
ethanoyl chloride and aluminium chloride catalyst
Draw and explain the electrophilic substitution mechanism vias Friedel-crafts acylation.
Step 1; making the electrophile
CH3COCL + AlCl3 —> CH3-C(+)=O + AlCl4-
Step 2: electrophilic substitution
draw
regeneration of the catalyst:
H+ + AlCl4- –> HCL + AlCl3
what does the aluminum chloride catalyst act as?
a Lewis acid
what is a Lewis acid?
Lewis acids act as electron pair acceptors
how does the aluminium chloride catalyst help to form an electrophile?
the aluminium atom has a vacant orbital and can accept (attract0 a pair of electrons to activate a molecule to make an electrophile
show how this reaction occurs in a symbol equation?
What bond does AlCl4- contain?
contains a dative or coordinate bond (1 chlorine donates both electrons to make the covalent bond)
when an acid anhydride is used instead of an acyl chloride, what does the regeneration of the catalyst look like?
(RCO)2O + AlCl3 –> R-C(+)=O + RCOOAlCl3-
explain why multiple substitutions do not occur with acylation?
the acyl group is electron withdrawing and makes the ring less attractive to electrophiles (deactivates the ring)
Naming Aromatics
when are the four instances where the root of the name ‘benzene’ is used?
- an alkyl group (methyl, ethyl)
- a halogen (Br, Cl, F)
- a nitro group (NO2)
- a carboxylic acid group (COOH)
how to name aromatics with one substituent?
include the substituent first then add benzene
how to name aromatics with more than 1 substituent?
lowest possible numbers for ring positions and listed alphabetically (together with appropriate number)
Which group takes priority over another group?
nitro group takes priority over methyl group when naming
(also carboxylic acid group takes priority over alcohol group)
how to name aromatics when an alkyl group is attached to a benzene ring?
the carbon atom attached to the alkyl group linked to the ring is always allocated the prefix 1- e.g (1-methylpropyl) benzene
what else can benzene ring be considered as?
a constituent
when the benzene ring is regarded as a constituent, what is the benzene group known as?
phenyl group
draw 2-phenylbutane. why is this not a straight chain with a butyl group?
the phenyl group is a substituent on carbon number 2
what is hydroxybenzene also commonly known as?
Phenol - this can be used in exam questions - get used to this
Draw out the structure of Phenylethanote and methylbenzenecarboxylate. why are they named like this?
phenylethanoate - benzene is on the –O- section of the ester functional group, so comes first as phenyl group
Methylbenzenecarboxylate - benzene is on the –C=O section of the ester func. group. the methyl group comes first, and then the ester is formed from benzenecarboxylic acid so has second name ‘benzenecarboxylate.
Making Polystyrene
what are the uses of Polystyrene/ (poly(phenylethene))?
is an extremely useful polymer used in packaging, insulation and cups
draw the two structures of polystyrene
how is a polymer of polystyrene made?
by the addition polymerisation of the monomer phenylethene
draw the structure of phenylethene
what is a possible synthetic route to making polystyrene from benzene?
Draw each route
step 1- Friedel-Crafts acylation (CH3COCl +AlCl3)
Step 2 - Reduction (NaBH4)
Step 3 - Dehydration using Conc. H3PO4
Step 4 - addition polymerisation
FINISH
