Chapter 25 - Benzene

C6H6 • colourless • sweet smelling • highly flammable • carcinogen

cyclohex 1,3,5 tri-ene effectively cyclohexane with alternating single bond, double bond

1) lack of reactivity 2) length of C-C bonds in Benzene 3) energies of hydrogenation of cycloalkenes

• 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

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

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

• 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

• 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

Electrophilic substitution C6H6 + E+ —> C6H5E + H+ benzene doesn’t undergo addition

• 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

1) Formation of the electrophile from the catalyst H2SO4 + HNO3 —> HSO4(-) + NO2(+) + H2O the electrophile is NO2+

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

3) reforming of catalyst HSO4- + H+ —> H2SO4 - catalyst reforms using the H+ ion from step 2

• 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

1) formation of electrophile Cl2 + AlCl3 —> Cl(+) + AlCl4(-) this can occur with any halogen

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

3) reforming of catalyst AlCl4(-) + H(+) —> AlCl3 + HCl

• 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

• 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

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

“Phenols are a type of organic compound containing an -OH hydroxyl group directly attached to a Benzene ring”

• it is more soluble than Benzene due to the presence of the -OH group to hydrogen bond • but it is less soluble than Alcohols

IN DYNAMIC EQUILIBRIUM Phenol —> Phenoxide ion + H+ C6H5OH —> C6H5O- + H+ this makes phenol a weak acid

• 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