Benzene & Phenols

What is the general structure of benzene

• 12 sigma bonds

• 3 pi bonds

• trigonal planar around C atom

• each C atom provides 3 of its 4 outer shell electrons to bond to 2 other C and 1 H atom, leaving 1 electron in a p orbital

What is Kekule’s structure of benzene

• p orbitals overlap sideways above and below the plane

• pi bond electrons are localised between 2 C atoms

What is evidence 1 against Kekule’s model of benzene

• benzene has low reactivity as it doesn’t take part in electrophillic addition reactions with bromine water (remains orange)

• halogen carrier required for this reaction

How did he refine his benzene structure

• pi bonds alternate

What is evidence 2 against Kekule’s model of benzene

• the carbon-carbon bonds in benzene are of equal length

What is evidence 3 against Kekule’s model of benzene

• enthalpy change of hydrogenation of benzene is less exothermic than expected, as it is more stable

• actual: -208kJ mol-1, estimated: -357kJ mol-1 (3x more than cyclohexene)

What is the delocalised model of benzene (planar structure)

• all 6 p orbitals overlap sideways above and below the plane

• all 6 pi electrons are delocalised and spread across all 6 C atoms, increasing stability

Explain why benzene is less reactive and brominates less than alkenes

• in alkenes the π bond electrons are localised across 2 carbon atoms

• whereas in benzene the π bond electrons are delocalised across all 6C atoms (more stable)

• benzene has a lowER electron density

• benzene polarises Br2 molecules less

How does benzene react with an electrophile through electrophillic substitution

• a pair of electrons leave the delocalised system to form a bond to the electrophile, disrupting the system

• an unstable intermediate is formed

• the pair of electrons in the C-H moves back into the ring it restore stability

• there is a substitution of hydrogen

What is halogenation and what are the reagants and conditions required

• benzene + halogen → halobenzene + HHalide

• halogen, halogen carrier (catalyst)

• reflux in presence of halogen carrier (AlCl3, FeCl3)

How is a catalyst used in halogenation

• Cl2 + AlCl3 → Cl+ + AlCl4-

• AlCl4- + H+ → AlCl3 + HCl

Why does halogenation require a catalyst

• Cl2 is non polar and benzene has a low pi density

• All 6 pi electrons are delocalised across 6 C atoms

• As Cl2 approaches benzene, aromatic ring is unable to polarise the Cl2 molecule

How does does the structure and properties of benzene compare to alkenes

• 6 pi electrons delocalised around 6C atoms in benzene whereas pi electrons are localised between 2C atoms alkenes

• benzene has a lower pi electron density

• benzene is more stable

• benzene has a lower reactivity

• benzene has less polarisation of electrophiles

• benzene attracts electrophiles less, alkenes more

What is nitration and what are the reagents and conditions required

• benzene + nitric acid → nitrobenzene + water

• conc HNO3, conc H2SO4 (catalyst)

• reflux at 55°C

What is the use of nitrobenzene

• formation of dyes, explosives

How does H2SO4 act as a catalyst in nitration

• H2SO4 + HNO3 → NO2+ + H2O + HSO4-

• HSO4- + H+ → H2SO4

What is alkylation and what are the reagents and conditions required

• benzene + haloalkane → alkylbenzene + HHalide

• haloalkane, anhydrous AlCl3

• room temperature

How does AlCl3 act as a catalyst in alkylation

• AlCl3 + C6H5Cl → AlCl4- + CH3CH2+

• AlCl4- + H+ → AlCl3 + HCl

What is acyclation and what are the reagents and conditions required

• benzene + acyl halide → ketone + HHalide

• acyl chloride, anhydrous AlCl3

• reflux at 150ºC

  
  

What are the electron donating groups and their properties

• OH, CH3, NH2

• e- density of ring increases

• easier substitution

• 2,4,6 directing

What are the electron withdrawing groups and their properties

• COCH3, COOH, NO2

• electron density of ring decreases

• harder substitution

• 3,5 directing

What happens to the electron donating/withdrawing groups

• the delocalised π e-s become more nucleophillic

• more susceptible to electrophile attack

What is a phenol

• benzene ring with OH group DIRECTLY attached

Why are phenols soluble in water

• forms hydrogen bonds with water molecules

What is phenols acidity level

• weak acid

What is the equation for phenols with 1+metals (Li) and what is the name of the organic product

• C6H5OH + Li → C6H5O-Li+ + ½ H2

• lithium phenoxide

What is the equation for phenols with 2+metals (Mg) and what is the name of the organic product

• 2C6H5OH + Mg → (C6H5O-)2Mg2+ + H2

• magnesium diphenoxide

What is the equation for phenols with alkalis (NaOH) and what is the name of the organic product

• C6H5OH + NaOH → C6H5O-Na+ + H2O

• sodium phenoxide

What is the equation for phenols with alkalis ( Ca(OH)2 ) and what is the name of the organic product

• 2C6H5OH + Ca(OH)2 → (C6H5O-)2Ca2+ + 2H2O

• calcium diphenoxide

How do phenols react with carbonates

• phenols dont react with carbonates as they are very weak acids

How can you identify a phenol by using observations

• add carbonate: fizzing doesnt occur

• add UI: red/orange

What is the mechanism and equation for FULL bromination of phenol, state conditions and reagants and name organic product

• C6H5OH + 3Br2 → C6H2Br3OH + 3HBr

• 2,4,6-tribromophenol

• bromine water, room temperature

What are the observation when reacting bromine water with phenol

• orange → colourless

• white ppt formed

Explain why phenol reacts with Br2 more readily than benzene

• phenol is more reactive as a lone pair of electrons on oxygen atom in phenol group is delocalised into the benzene ring

• creates a higher π electron density around the ring

• increased e- density polarises Br2 molecules which are more strongly attracted to the benzene ring

What is the equation for nitration of phenol with dilute HNO3 with conditions and what are the mixture of products (electrophillic substitution)

• C6H5OH + HNO3 → C6H4OHNO2 + H2O

• room temperature

• 2-nitrophenol, 4-nitrophenol

What is the equation for nitration of phenol with conc HNO3 with conditions and what is the organic product called

• C6H5OH + 3HNO3 → C6H5OH(NO2)3 + 3H2O

• room temperature

• 2,4,6-nitrophenol