medchem 7 , 8 Flashcards
the structure and properties of benzene
- colorless , toxic cariogenic, liquid at boiling point of 80 c
- have 6 carbons and hydrogens attached to each carbon
- c-c bond are equal to: 139 between single and double bond length
- no formal single or double bonds
- molecular formula C6H6
- aromatic
benzene is a — of these resonance structure
resonance hybrid and individual resonance are imaginary and not real
each carbon in the benzene has — orbital and has — with its electrons sideways and each pi cloud contains — electron
2p , aromatic sextet or aromatic cloud , 6 pi electrons
huckles rule of aromaticity states,
to have an aromatic compound it must have —- each atom in the ring has — and there is —
- planer ring
- pz orbital
- (4n + 2) pi n= integer
benzene aromatic stability can be measured by
- heat of hydrogenation ( aka the addition of h2 to a double bond )
- more stable, less reactive due to the release of energy in the aromatic ring
benze has — times the cylohexene value
3 times as there is 3 double bonds
- more bonds , more stability
substation in the benzene can be :
- ortho: groups r close
- meta : groups r one unit apart
- para : groups are 2 units apart
many drugs are — derivatives
benzene and synthesised from Benzene or substuited benzene
reactions of benzene:
elctrophilic substution - electrophiles react w electron rich aromatic ring as:
-chloloration
- beneze + cl2
- catalyst: AlCL3 or FeCl3
- products: chlorobenze and hcl
- fe2 reacts violently without a catalyst
- br2 reacts in the presence of AlBr3
- no light needed , not a free radical mechanism
steps of the chlorination of benzene
- generation of the electrophile
- reaction between the ring and the electrophile aka one bond forming and the other is breaking
- loss of h+ in the carboncation
- regeneration of the catalyst.
is carbonation aromatic
NO
AlCl3 is a – and cl- is —-
Lewis acid aka elctron acceptor
Lewis base electron pair donor
( bf3 and group 3 are all lewis acids since they have 6 electrons )
the driving force in the loss of H to remove the carbonation is from
regeneration of aromaticity which accounts for the mechanism pathway
reactions of alkylation in benzene don’t occur in
cholorbezene or halides that are directly attached to the carbon
example of alkylation of benzene:
benze + ch3-cl
- generetion of an electrophile = ch3-
nitration of benzene
- sulphuric acid H2SO4 will — the nitric acid HONO2
- the nitric acid will lose — and become — aka an —
- protonate
- h20
- nitronium ion
- electrophile
- its isolectronic w/ c02
bromination of benzene steps
- generation of electrophile ( AlBr3 + br-br ) –> br- + AlBr4 +
- formation of non aromatic carbonation
- aromaticity is gained
- albr4 - + h+ –> albr3 + hbr
the carboncation resonances forms differ in
- position of the +ve charge and the double bond
- this gives the carboncation the stability but it’ll still lose the proton
- more stability is gained by the formation of the aromatic ring again
the number of resonance structures in carboncation
- 3
- and they are formed by the spread of the +ve charge
substuited benzes can undergo
electrophilic substuited reactions
the rate of reaction, products, and what position the ring depends on
the X that is substuited in the ring
electron — groups make the ring more electron rich , more attractive to electrophile and is faster
donating
example of electron donating groups
ch3
another name for electron donating groups
activators
electron — groups make the ring more electron poor and less reactive and slower
withdrawing
another name of electron withdrawing groups
deactivators
cl and no2 are an example of
electron withdrawing groups
why are oh and nh2 groups strongly activating
- bc of the lone pair overlaps with the aromatic ring making it more electron rich
- aka they are electron donating groups
electron donating groups are —- directors
ortho, para
- the +ve sign needs to be next to the original substituent
electron dontating stabilise the
2 intermediate of the carboncation in ortho or para position and teriatry is more stable than secondary
electron — groups are meta directing
withdrawing
electron withdrawing destabilise the carboncation in — and – position but favours —
- ortho, para
- meta
— are electronegative and are electron withdrawing but they are able to donate a pair or electrons and form — . they are — by conjunction and — by inductive effect
- halogens
- resonance
- donating
- withdrawng
- favours ortho, para
all ortho and para are activating groups and are electron donating except
halogens
ch3 and alkyls are
weak activators
halogens are
deactivators
nh2 and oh are
strong activators
n02 , s03h , CF3 , NH3 , C triple bond N , cooh , coor are
strong deactivators as they r elctrongevative or have +ve charge
in halogenation,
Tulane + cl2 with no catalyst will become
- ch2cl + hcl
- toluene here acts as a substuieted alkane and free radical reaction so its not electrophilic subsetution
check slide 20
the type of reactions bezene undergo is —
electrophilic substitution
