Chapter 25 - Benzene Flashcards
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
state the order of acidity of Alcohols, Caboxylic acids, and Phenols and state the reactions that show it
in increasing acidity
Alcohol - doesn’t react with NaOH or Na2CO3
Phenol - only reacts with NaOH
Carboxylic acid - reacts with both
state the equation for the reaction of Phenol with NaOH
Phenol + Sodium Hydroxide —> Sodium Phenoxide + water
C6H5OH + NaOH —-> C6H5O-Na+ + H2O
generally how do phenols react and how is it different to arenes
- they generally undergo electrophilic substitution reactions
- but they occur at milder conditions than those for Benzene, e.g. at RTP and without a Halogen Carrier
state the equation and conditions for the reaction of Phenol with an excess of Bromine and state what you would see
Phenol + Aqueous Bromine —> 2,4,6 Tribromophenol + hydrogen Bromide
C6H5OH + 3Br2 —> C6H2Br3OH + 3HBr
- it decolourises the bromine water then forms a white precipitate of the 2,4,6 tribromophenol
- occurs at RTP with no catalyst
state the equation of the nitration of Phenol (major and minor products) and the conditions
Phenol + Nitric acid —> 2 or 4 Nitrophenol + water
- occurs at RTP with no catalyst
C6H5OH + HNO3 —> C6H4(NO2)OH + H2O
explain the differences in reactivity of Benzene and Phenol
- Phenol reacts more readily than Benzene
- This is because the lone pair of electrons on the oxygen P-orbital are PARTIALLY donated to the Pi-System
- this gives Phenol a greater electron density than benzene
- so it is more able to POLARISE molecules and attract electrophiles, making it more susceptible to attack
what occurs when a Benzene ring is activated
- some groups (on Benzene) activate the ring
- this is where a lone pair on the group is donated to the ring
- this increases its electron density and reactivity with electrophiles
- these reactions usually occur at lower temperatures and milder conditions
where are groups directed to when the ring is activated
2 and 4
what occurs when a ring is deactivated
- some groups deactivate the ring
- this is where they attract or ‘withdraw’ electrons from the ring
- so its reactivity to electrophiles is decreased
- this means higher temperatures and a halogen carrier catalyst are usually required
where are groups directed when the ring is deactivated
position 3
which groups are 2 and 4 directing
NH2 or NHR OH OR (Not or) R or C6H5 F, Cl, Br, I
which groups are 3 directing
RCOR COOR SO3H CHO COOH CN NO2 NR3+
what 3 things should you always mention when talking about the bonding in benzene/comparing to others
- DELOCALISED electrons, which can move to ALL 6 CARBONS
- ABOVE AND BELOW the plane of the molecule
- C-C bond length is an intermediate length
what to ALWAYS mention when writing about the conditions for the nitration of benzene (or comparing to phenol/cyclohexene etc.)
CONCENTRATED HNO3
CONCENTRATED H2SO4
when Phenol forms a phenoxide ion, what can we say about the thing it reacts with
- it will form a phenoxide ion when in solution or reacting with a base such as NaOH
- the phenol loses a proton
- therefore whatever it reacts with acts as a base
- this means water/OH can act as a base (despite the mechanism looking like its a nucleophile)
how can we use the isomers of a di-substituted benzene compound to provide evidence for the delocalised model
- for 1,2 substituted molecule, there is only 1 isomer possible with the delocalised model
- however there are two possible with the Kekulé model
- there has only been the one for the delocalised model found
- thus providing evidence for the delocalised model
