(10) Aromatic Flashcards
(38 cards)
what does aromatic mean?
particular sort of delocalised bonding
what are arenes?
aromatic hydrocarbons; based on benzene rings
benzene state and where it comes from
colourless liquid at RT and pressure
obtained from petroleum (crude oil)
three different bonding compounds of arenes
- cyclohexene (one C=C)
- cyclohexane (all C-C)
- benzene (3 C=C, every other)
what is the kekulé structure
benzene has 3 C-C and 3 C=C
problems with kekule structure
- alternating bonds, C-C and C=C have different lengths —> xray shows bonds are equal lengths
- would be known as cyclohexa-1,2,3-triene; can hydrogenate = enthalpy -120 kJ mol-1 —> enthalpy = -208 kJ mol-1
- undergoes electrophilic substitution (hydrogen replaced) —> undergoes electrophilic addition (pi-bond breaks = new bond)
what is the difference in enthalpy due to between kekule structure and benzene
the stability from the system of delocalised electrons
how many electrons are free to bond in a benzene ring and what are they used for
- 30 (H 1 each, C 4 each)
- 12 C electrons used for 6 C-H sigma bonds
- 12 C electrons used for 6 C-C sigma bonds
explain the pi bonding in a benzene ring
- each C has an electron not used in sigma bonds
- p-orbitals overlap to form pi-bond delocalised* system of electrons above and below plane of hydrocarbon skeleton
*pi-bonds are spread over several atoms
what does delocalisation of electrons do in a benzene ring
- lowers energy if structure (makes more stable)
- difference in energy values = delocalisation energy
- unique electronic structure has dominant influence on properties of molecule
structure of benzene ring
planar, hexagonal molecule with bond angle = 120°
what does the circle in a benzene ring represent ?
the delocalised pi-electron cloud
what type of reaction will alkenes readily undergo? and an example
electrophilic addition; eg. ethene and bromine (test for unsaturation)
why does benzene not undergo electrophilic addition?
unsaturated, doesn’t react with bromine due to stability of pi-delocalised system of electrons
what type of reaction does benzene undergo
electrophilic substitution where pi-electron cloud is preserved
substituted benzene compounds:
- phenol
- methylbenzene
- phenylamine
- iodobenzene
- bromobenzene
- benzoic acid
- methyl benzoate
- methyl-3-nitrobenzoate
- benzene diazonium chloride
- C6H5OH
-C6H5(CH3)
-C6H5NH2
-C6H5I
-C6H5Br
-C6H5COOH
-C6H5COOCH3
C6H4(NO2)COOCH3
-C6H5N2+Cl-
what are the two mechanisms for reactions of benzene
- monohalogenation
- mononitration
main monohalogenation of benzene studied
monobromination of benzene using bromine in presence of iron/iron bromide (due to stability of delocalised system of electrons in benzene catalyst reuqired)
explain monobromination mechanism reaction
- Br2 is polarised as it approaches benzene
- FeBr3 acts as halogen carrier
- δ- Br atom transfers pair of electrons to iron in FeBr3// other Br atom accepts pair of electrons from delocalised cloud of elections in benzene
- temporilary, delocalised pi-electron system stability lost
- electrons in C-H go back to ring to reform electron cloud losing H+ ion
- FeBr4- reacts with H+ —> FeBr3 (catalyst) and HBr
when does mononitration of benzene occur
when one (or more) H atoms on benzene is replaced by nitrogen group (NO2)
what mechanism is mononitration of benzene and how is the ion needed produced
electrophilic substitution, electrophile (nitronium ion) produced through reaction of conc nitric acid and conc sulfuric acid
HNO3 + 2H2SO4 —> NO2+ + H3O+ + 2HSO4-
what is the nitro group and describe its appearance and use when attached to benzene
NO2
nitrobenzene = yellow oily liquid
dyes and explosives
what happens when benzene reacts with halogenoalkanes and acyl chlorides and what is this called
alkylated and acylated benzene products (called friedel-crafts reactions)
catalyst in friedel-crafts reactions
aluminium halides
