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chemistry > C25 - Aromatic compounds > Flashcards

C25 - Aromatic compounds Flashcards

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1
Q

What is benzene

A

C6H6

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2
Q

What are properties of benzene

A

Colourless
Sweet smelling
Highly flammable
Liquid
Carcinogen
Aromatic

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3
Q

What are the two models of benzene

A

Kekule’s model
Delocalised model

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4
Q

What is an arene

A

Aromatic (containing one or more benzene rings) hydrocarbon

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5
Q

What is Kekule’s model

A

Hexagonal carbon ring, joined by alternate double and single bonds

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6
Q

Evidence to disprove Kekule’s model

A

-Lack of reactivity of benzene: it does not undergo electrophilic addition reactions or decolourise bromine suggesting lack of double bonds
-Lengths of C-C bond lengths: bond lengths should be alternating but they are uniform
-Hydrogenation enthalpies: enthalpy should be three times cyclohexane, but isn’t

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7
Q

What is the delocalised model of benzene

A

-Planar, cyclic hexagonal hydrocarbon (C6H6)
-Each C atom uses its available 4 electrons in bonding to 2 other C atoms and 1 hydrogen atom
-Each C atom has 1 electron in a p-orbital, perpendicular to the plane
-Adjacent p-orbital overlap sideways, above and below plane to form ring of e- density
-Overlapping of p-orbitals creates system of pi-bonds, 6 delocalised e- in system of pi-bonds

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8
Q

Naming aromatic compounds with one substituent group

A

-Alkyl groups, halogens, nitro groups are prefixes
-Benzene substituent to alkyl chain (phenyl is prefix)

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9
Q

What is benzoic acid

A
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10
Q

What is phenylamine

A
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11
Q

Naming aromatic compounds with more than one substituent group

A

Alkyl group takes priority (alkylbenzene becomes parent chain)

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12
Q

What is benzaldehyde

A
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13
Q

Benzene electrophilic substitution

A

H atom on benzene ring is replaced by another atom/group of atoms (electrophile)

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14
Q

Nitration of benzene

A

C6H6 + HNO3 –> C6H5NO2 + H2O
Conditions- H2SO4 + 50C

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15
Q

What happens if nitration of benzene occurs at a higher temp

A

Further substitution, forming dinitrobenzene (1,3)

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16
Q

Mechanism of nitration of benzene

A
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17
Q

What does the halogenation of benzene require

A

Require halogen carrier catalyst (can be generated in situ)
Eg. AlCl3, FeCl3, AlBr3, FeBr3

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18
Q

Bromination of benzene

A

C6H6 + Br2 —> C6H5Br + HBr
Conditions- RTP, FeBr3/AlBr3

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19
Q

Bromination of benzene mechanism

A
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20
Q

Chlorination of benzene

A

C6H6 + Cl2 —> C6H5Cl + HCl
RTP, FeCl3/AlCl3

21
Q

Alkylation of benzene

A

Benzene + Haloalkane —> Alkyl benzene + HCl
Conditions - RTP, AlCl3

22
Q

Acylation

A

Benzene + acyl chloride —> Aromatic ketone + HCl
Conditions - AlCl3 RTP

23
Q

Do alkenes decolourise bromine

A

Yes

24
Q

What happens in the reaction of cyclohexene and bromine

A

Pi bond in alkene contains localised electrons above and below the plane of the two carbon atoms in a bond
Produces region of high e- density
Localised e- in pi bond induce a dipole in non-polar bromine
Slightly +ve bromine atom enables the bromine molecule to act like a dipole

25
Q

Does benzene react with bromine

A

No, unless a halogen carrier catalyst is present
Delocalised electrons spread below and above the plane
Lower electron density
Cannot polarise non polar bromine molecule

26
Q

What is phenol?

A

Organic chemical containing hydroxyl group bonded to aromatic ring

27
Q

Are phenols more or less soluble than alcohol

A

Less due to the presence of non-polar benzene ring

28
Q

What happens when phenol is dissolved in water

A

Partially dissociates forming phenoxide ion and proton

29
Q

Is phenoxide a strong or weak acid

A

Weak, it partially dissociates in water

30
Q

Phenols acidity compared to alcohols an Carboxylic acids

A

More acidic than alcohols, less acidic than carboxylic acids

31
Q

What’s Ka

A

Dissociation constant

32
Q

Reactivity of ethanol, phenol and carboxylic acids

A

-Ethanol does not react with NaOH (strong base) or NaCO3 (weak base)
-Phenols react with NaOH (strong base) but not NaCO3 (weak base)
-Carboxylic acids react with NaOH (strong base) and NaCO3 (weak base)

33
Q

What reaction can be used to distinguish between phenol and a carboxylic acid

A

Reaction with NaCO3
Phenol won’t react
Carboxylic acid reacts to produce CO2 gas

34
Q

Reaction of phenol with sodium hydroxide

A

C6H5OH + NaOH –> C6H5NaO + H2O

35
Q

What type of reactions does phenol usually undergo

A

Electrophilic substitution
-Milder conditions than benzene

36
Q

Bromination of phenol

A

C6H5OH + 3Br2 —> C6H2OHBr2 + H2O
Conditions - RTP, no catalyst

37
Q

Nitration of phenol

A

C6H5OH + HNO3 (dilute) —> C6H4OHNO2 + H2O
Conditions - RTP, no catalyst

38
Q

Reactivity of phenol vs benzene

A

Bromine and nitric acid react more readily with phenol
Phenol reacts with dilute nitric acid rather than conc

39
Q

Why is phenol more reactive than benzene

A

Lone pair of e- on p-orbital of O atom is donated into pi system
Inc electron density, attracts electrophiles more strongly

40
Q

What happens when phenols undergo further substitution with nitric acid

A

Form two isomers:
2-nitrophenol
4-nitrophenol

41
Q

Reaction of phenylamine and bromine

A

Occurs more rapidly than benzene

42
Q

Reaction of nitrobenzene and bromine

A

Occurs more slowly than benzene

43
Q

What does it mean to activate

A

A group causes the aromatic ring to react more readily with an electrophile

44
Q

What does it mean to deactivate

A

A group causes the aromatic ring to react less readily with an electrophile

45
Q

What is a directing effect

A

Different groups attached to benzene rings cause further substituents to attach at a specific location, affecting whether it’s an an activating or deactivating group

46
Q

Which directing groups are activating groups

A

2- and 4- directing groups (orthodo and para directors)
EXCEPT halogens

47
Q

Which directing groups are deactivating

A

3 directing groups (meta directing)

48
Q

Directing group table

A

chemistry (20 decks)

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