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Chemistry > 17.4 > Flashcards

17.4 Flashcards

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1
Q

what does acromatic mean

A

delocalised electrons forming pi bonding in a hydrocarbon ring

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2
Q

what is an aliphatic compound

A

all those that are not acromatic

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3
Q

what are the physical properties of benzene

A

colourless liquid
boiling temperature 80*
insoluble in water
carcinogenic
highly unsaturated

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4
Q

what was kekules theory on the structure of benzene

A

benzene has a cyclical structure with alternating double bonds

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5
Q

what were the issues with kekules theory of the structure of benzene

A

benzene does not decolorise bromine water in an addition reaction which should occur because of the C=C bond instead a substitution reaction occurs
there should have been 4 isomers but only 3 were found which suggests the carbon bonds are all the same
the bond length for the carbon bonds in benzene did not match those of C-C or C=C instead it was in between
enthalpy change of hydration was a lot lower than expected

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6
Q

what is the current model for the structure of benzene

A

the electrons all form a delocalised pi bond

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7
Q

why does benzene not undergo an addition reaction with bromine

A

this is because a substitution reaction preserves the stability of the delocalised electrons in the pi bond

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8
Q

explain the hydrogenation of benzene

A

benzene forms a cycloalkane when heated under pressure with hydrogen and a nickel catalyst

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9
Q

what is the equation for the combustion of benzene

A

C6H6 +7.5O2 -> 6CO2 + 3H2O

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10
Q

what will be observed in the combustion of benzene and why

A

smokey flame due to the high carbon to hydrogen ratio therefor there are more carbon particles

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11
Q

when could an addition reaction between benzene and bromine occur

A

in the presence of uv light

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12
Q

what is a halogen carrier

A

a catalyst that helps to introduce a halogen atom into a benzene ring

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13
Q

explain the bromination of benzene

A

heat under reflux with a halogen carrier (acts as catalyst) to produce bromobenzene and hydrogen bromide

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14
Q

why are iron filings often added to bromination of benzene instead of a halogen carrier

A

because the iron filings will react with some of the bromine to form iron(III) bromide

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15
Q

what is the formula of a nitro group

A

NO2

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16
Q

explain the nitration of benzene

A

substitution of a hydrogen atom by a nitro group by warming benzene with nitric and sulfuric acid to produce nitrobenzene and water

17
Q

what is the role of the nitric and sulfuric acid in the nitration of benzene

A

nitric acid is the source of the NO2 group
sulfuric acid acts as a catalyst

18
Q

what are the 2 Friedel-crafts reactions

A

alkylation
acylation

19
Q

what are the general conditions for Friedel-crafts reactions and why

A

catalyst such as aluminum chloride (a halogen carrier) is needed
anhydrous conditions are needed as water would react with the catalyst

20
Q

explain the Friedel-Crafts alkylation reaction

A

substitution of a hydrogen of benzene by an alkyl group (CH3)
reagent - halogenoalkane
products - alkylbenzene (methylbenzene) and hydrogen chloride

21
Q

explain the Friedel-Crafts acylation reaction

A

substitution of a hydrogen of benzene by an acyl group (COR)
reagent - acyl chloride
products - ketone and hydrogen chloride

22
Q

why does benzene attract electrophiles

A

because it is electron rich due to the delocalised electrons in the pi bond above and below

23
Q

explain the steps in the mechanism for electrophilic substitution of benzene

A

the electrophile attracts 2 electrons from the benzene ring forming a covalent bond creating an unstable intermediate, then the H leaves and the 2 electrons restore the delocalised pi bond

24
Q

give some examples of an electrophilic substitution reaction with benzene

A

bromination
nitration
alkylation
acylation

25
Q

what is a phenol

A

a hydroxyl group joined to a benzene ring

26
Q

explain the bromination of phenol

A

occurs at room temperature without a catalyst with bromine water

27
Q

what is observed in the bromination of phenol

A

bromine water is decolourised and a white precipitate is formed

28
Q

why does phenol undergo bromination much more readily than benzene

A

the oxygen in the OH group has a lone pair of electrons which can merge with the delocalised pi bond which increases electron density so the molecule is ‘activated’ because it is much more likely to react towards an electrophile so when a bromine molecules becomes close enough it is polarised and the Br+ electrophile attacks

29
Q

what are the uses of phenols

A

antiseptic in surgerys
manufacturing of polymers and pharmaceuticals

Chemistry (46 decks)

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