module 6.1 - organic chemistry and analysis: aromatic compounds, carbonyls and acids

Question 1

kekule model of benzene

Answer 1

• alternating double single carbon bonds
• each carbon has 1 electron in p orbital (at right angles to plane of bonded carbon and hydrogen atoms)
• adjacent p-orbital electrons overlap sideways forming a ring of electron density (delocalised pi system above and below ring of molecule)

Question 2

delocalised model of benzene

Answer 2

• allows electrons to be delocalised as carbons are equally spaced
• p-orbitals overlap above and below plane

Question 3

experimental evidence for delocalised model rather than kekule

Answer 3

• lack of reactivity; benzene doesn’t undergo electrophilic addition or decolourise bromine (since kekule’s model shows alternating single double bonds; it could be classified as an alkene; cyclohexene)
• bond lengths; when examined by x-ray diffraction, bond length was different for single and double carbon bonds, but the bonds of benzene are actually all found to be of equal length
• enthalpy of hydrogenation; doesn’t have 3x enthalpy change of cyclohexane (360kjmol) but has 208 which implies that it is not alternating double single bonds

Question 4

electrophilic substitutions of aromatic compounds

Answer 4

concentrated nitric acid + sulfuric acid
halogen + halogen carrier
haloalkane/acyl chloride + halogen carrier

Question 5

mechanisms for electrophilic substitution of aromatic compounds

Answer 5

do questions

Question 6

explanation for relative resistance to bromination of benzene compared with alkenes

Answer 6

• alkene reacts and decolourises bromine (electrophilic addition)
• benzene = delocalised pi system; p orbitals of all carbons overlap above and below carbon ring
• alkenes = localised pi-orbital overlaps between 2 carbon atoms.
• electron density in localised system of alkenes = much greater than the delocalised system in benzene
• greater electron density in alkenes allows a dipole to be induced more readily in bromine and therefore makes alkenes more susceptible to electrophilic attack
• electron density in benzene’s pi system is not significant enough to produce an electrophile; therefore benzene does not readily undergo electrophilic substitution

Question 7

what demonstrates weak acidity of phenols

Answer 7

• neutralisation reaction with NaOH but absence of reaction with carbonates
• phenoxide dissociates weakly to form phenoxide ion + H+ ion
• phenols react with alkalis to form a salt and water