Flashcards
chem 25
Cards in this deck (24)
- 1physical properties of benzenecolourless
sweet smell
highly flammable liquid
found naturally in crude oil
can cause cancer - 2define : benzenean aromatic hydrocarbon (arene) and it is unreactive (but it contains multiple bonds so should be very reactive)
- 3define: kekule modelatoms are joined together by alternate single and double bonds
- 4why doesn't the structure explain many chemical/physical properties1. C=C should decolourise bromine but doesn't (benzene doesn't undergo electrophilic addition reactions )
2. length of C-C bonds in benzene are 0.139nm (between the single 0.154 and double 0.134)
3. hydrogenation enthalpy much less than expected (structure is more stable than kekule so proposes delocalised model) - 5molecular properties delocalised model- planar, cyclic, hexagonal hydrocarbon (regular hexagon)
- each carbon bonded to 2Cs+1H
- adjacent p-orbitals overlap above and below plane of C atoms to form a ring of e- density which creates system of pi bond (delocalised e-) - 6define : monsubstitutedaromatic compounds with 1 substituent group (alkyl group, halogen, nitro group)
- 7nitration of benzenereacts slowly with nitric acid
catalysed by sulfuric acid
heated to 50*C
waterbath used to maintain temperature - 8halogenation of benzenehalogens don't react without a catalyst (halogen carrier)
1.bromination
(room temp and pressure with AlBr3)
2. chlorination
(room temp and pressure with AlCl3) - 9define : alkylationsubstitution of an H atom by an alkyl group
- 10define : acylationreacts with acyl chloride
an aromatic ketone is formed - 11reactivity of alkenes with brominealkenes decolourise bromine by electrophilic addition
because alkenes contain localised electrons above and below the plane of the 2C atoms in the double bond
this produces an area of high electron density, the localised e- in the pi bond induce a dipole in the non polar Br2 molecule - 12reactivity of benzene with brominebenzene doesn't react with bromine unless a halogen carrier catalyst is present
benzene has delocalised electrons so the e- density is much less than that in a C=C bond in an alkene
so when a non polar molecule Br2 approaches there's insufficient pi electron density around any 2 C atoms to polarise the bromine molecule
this prevents any reaction - 13define : phenolstype of organic chemical containing a hydroxyl -OH group bonded directly to an aromatic ring
phenols are aromatic so undergo electrophilic substitution reactions - 14phenols' solubilityphenols are less soluble in water than alcohols (because of non polar benzene ring)
it partially dissociates to produce protons - it's a weak acid which can be seen by comparing the acid dissociation constant Ka - 15bromination of phenols(forms a white precipitate and decolourises bromine water)
OH -- @ + 3Br2
no halogen carrier catalyst
at room temperature
Br OH Br
\ l /
@
l
Br - 16nitration of phenol(forms water)
OH -- @ + HNO3
room temperature
with dilute HNO3
OH NO2
l /
@ - 17why does phenol react better with Br2/HNO3 than benzenephenol has a lone pair of e- from the oxygen's p-orbital (of the OH group) being donated to the pi system of phenol
so electron density in phenol ring is increased and attracts the electrophiles more strongly - 18define : further substitutionmany substituted aromatic compounds can undergo a second substitution - DISUBSTITUTION
some reaction happen easily, other require extreme conditions - 19phenylamine + brominebromine reacts with benzene only with a halogen carrier catalyst
but bromine easily reacts with phenylamine
to give a multisubstituted product
Br NH3 Br
\ l /
@
l
Br - 20nitrobenzene + bromenethey need extreme conditions to react
high temp
halogen carrier
NO2
l
@ + HBr
\
Br - 21NH2 and NO2 effect on ringNH2 activates the ring to react more readily with an electrophile
NO2 deactivates the ring to react less readily with an electrophile
diff reaction rates
diff extents of substitutions
diff position of substitutions - 22directing effectsdifferent groups attached to a benzene have different directing effects on any second substituent on the benzene ring
- 232-4 directing groupsortho/para directors
are activating groups
excpet halogens - 243 directing groupsmeta directors
are deactivating groups
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- AS level Chemistry (module 6) Flashcards on chem 25, created by myriam farah on 18/03/2018.
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- myriam farah
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