Chapter 25 - Benzene Flashcards
(40 cards)
What is the molecular formula of Benzene and what are its key properties
C6H6
- colourless
- sweet smelling
- highly flammable
- carcinogen
what was Kekule’s Model for Benzene
cyclohex 1,3,5 tri-ene
effectively cyclohexane with alternating single bond, double bond
what are the three key pieces of evidence to disprove Kekule’s model
1) lack of reactivity
2) length of C-C bonds in Benzene
3) energies of hydrogenation of cycloalkenes
explain the evidence for the lack of reactivity of Benzene
- if benzene did have a C=C double bond it would be very reactive e.g. decolourise bromine
However:
- it doesn’t undergo electrophilic addition and therefore doesn’t decolourise bromine water
This suggests it doesn’t have a C=C double bond
explain the evidence for the bond lengths in benzene
with X-ray diffraction you can measure bond lengths
- If Kekule’s model was correct you would expect alternating short C=C bond and long C-C bond lengths
- It was actually a constant intermediate bond length
explain the evidence for the hydrogenation of cycloalkenes
1) cyclohexene + H2 –> cyclohexane,
enthalpy change = -120kjmol^-1
2) cyclohex 1,3, diene + 2H2 –> cyclohexane
enthalpy change = -240kjmol^-1
so therefore you would expect kekule’s model to have an enthalpy of -360kjmol^-1 but when Benzene was reacted with it, the enthalpy change was only -208kjmol^-1
- this is lower than expected so the molecule is more stable than expected
what are the 6 main points of the delocalised model
- Benzene is a planar, hexagonal hydrocarbon, C6H6
- Each carbon has 3 shared pairs of electrons/3 bonds
- Each carbon has an electron in a P orbital perpendicular to the plane of the molecule
- each adjacent P-orbital overlaps sideways to form rings of electron density above and below the plane of the molecule
- this creates a system of Pi bonds, all 6 electrons in it are delocalised and can move freely over all the carbons
- the bond angle is 120 degrees
points on naming aromatic compounds
what are the exceptions
- where there are groups attached to the benzene you name it as such e.g. chlorobenzene, nitrobenzene etc.
- where benzene is attached to a carbon chain either of 7 or more carbons or with another functional group, Benzene becomes the substituent e.g. phenyl
the exceptions are
Benzoic acid
Phenylamine
Benzaldehyde
what is the type of reaction that benzene typically undergoes, give a generalised equation
Electrophilic substitution
C6H6 + E+ —> C6H5E + H+
benzene doesn’t undergo addition
what occurs in the nitration of Benzene, give an overall equation and name the conditions
- benzene will react slowly with concentrated nitric acid to form nitrobenzene
- a catalyst of concentrated H2SO4 is used at 50 celcius
C6H6 + HNO3 —(H2SO4)(50 degrees)–> C6H5NO2 +H2O
what is the first step in the nitration of Benzene, name the electrophile
1) Formation of the electrophile from the catalyst
H2SO4 + HNO3 —> HSO4(-) + NO2(+) + H2O
the electrophile is NO2+
what is the second step in the nitration of benzene, draw out the mechanism, explain what happens
2) attack from the electrophile
- the nitronium ion accepts a pair of electrons from the Pi system of the benzene ring forming a daitive covalent bond
- the organic intermediate formed is positively charged and is unstable, the hydrogen donates an electron pair to the Pi ring to form a hydrogen ion and the product
what is the third step in the nitration of benzene, give an equation
3) reforming of catalyst
HSO4- + H+ —> H2SO4
- catalyst reforms using the H+ ion from step 2
what occurs in the Halogenation of Benzene, give an overall equation
- benzene reacts with a halogen e.g. Cl2
- they react at RTP with a catalyst halogen carrier of AlCl3
C6H6 + Cl2 —(RTP)(AlCl3)–> C6H5Cl + HCl
what occurs in the first step of the halogenation of Benzene, give an equation
1) formation of electrophile
Cl2 + AlCl3 —> Cl(+) + AlCl4(-)
this can occur with any halogen
what occurs in the second step of the halogenation of benzene, draw the mechanism, explain what occurs
2) electrophilic attack
- the positive halogen ion accepts an electron pair from the pi system forming a daitive covalent bond
- the positive intermediate is unstable and breaks down to form a hydrogen ion and a halogenobenzene as the carbon- hydrogen bond breaks by heterolyic fission
what occurs in the third step of the Halogenation of Benzene, give and equation
3) reforming of catalyst
AlCl4(-) + H(+) —> AlCl3 + HCl
what occurs in the Alkylation of Benzene, give the conditions and the equation, state the type of reaction
- an Alkyl group substitutes for a hydrogen on a benzene ring through electrophilic substitution
- react Benzene and a haloalkane with a halogen carrier catalyst of AlCl3 at RTP
- don’t need to know mechanism but likely to work similarly to before with AlCl4(-) being formed
overall equation
C6H6 + C2H5Cl —(AlCl3)–> C6H5(C2H5) + HCl
what occurs in the Acylation of Benzene, give the conditions, type of reaction and equation
- An acyl group substitutes for one of the ring’s hydrogens
- electrophilic substitution
- AlCl3 acts as a catalyst, RTP
equation
Benzene + acyl Chloride —> Phenyl ketone + HCl
C6H6 + CH3(CH2)nCOCl —-> C6H5(CO(CH2)nCH3) + HCl
explain why Alkenes are more reactive than arenes
Alkenes will undergo electrophilic addition because:
- Pi-bond contains localised electrons above or below the C=C bond
- this gives a higher electron density so they can induce greater dipoles on molecules, and attract more electrophiles
- the induced dipoles allow molecules to act as electrophiles
Arenes won’t undergo electrophilic addition because:
- the electron density in the delocalised ring is too low
- so it doesn’t attract or induce big enough dipoles on electrophiles for addition to take place
Define a Phenol
“Phenols are a type of organic compound containing an -OH hydroxyl group directly attached to a Benzene ring”
what is the solubility of Phenol
- it is more soluble than Benzene due to the presence of the -OH group to hydrogen bond
- but it is less soluble than Alcohols
write the equation for what happens when Phenol dissolves and state the effect of this
IN DYNAMIC EQUILIBRIUM
Phenol —> Phenoxide ion + H+
C6H5OH —> C6H5O- + H+
this makes phenol a weak acid
explain why phenol is a stronger acid than an alcohol
- there is a positive inductive (electron pushing) effect of an Alkyl group in an alcohol, increasing the dipole on the OH
- but in phenol, a lone pair on the oxygen is partially donated to the pi ring reducing the dipole on the OH
- this means there is a greater dipole on the O-H bond in an alcohol
- so the H+ ion is lost less easily in an alcohol or more easily in Phenol
