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A-Level Chemistry > 6.1 Aromatic compounds, carbonyls and acids > Flashcards

6.1 Aromatic compounds, carbonyls and acids Flashcards

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1
Q

benzene general properties

A 
colourless
sweet-smelling
highly flammable liquid
found naturally in crude oil and in cigarette smoke
carcinogen
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2
Q

what benzene is classed as

A

arene or aromatic hydrocarbon

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3
Q

Kekulé model

A

six-members ring of C joined by alternate single and double bonds

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4
Q

evidence to disprove Kekulé model

A

lack of reactivity (benzene doesn’t undergo electrophilic addition or decolorise Br2 despite seeming to have C=C), benzene more stable than expected

length of C-C bonds (all bond lengths (0.139nm) and bond angles are seen to be equal when benzene observed under X-ray diffraction)

hydrogenation enthalpies (much lower (-208 kJ mol^-1) than expected (-360) enthalpy, calculated by 3 times enthalpy of hydrogenation of C=C)

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5
Q

delocalised model of benzene

A

planar, cyclic, hexagonal hydrocarbon (C6H6)
each carbon has 2 electrons used in carbon ring, 1 electron for hydrogen
each carbon has 1 electron in p-orbital perpendicular to plane of bonded carbon and hydrogen atoms
overlapping p-orbitals form delocalised pi-orbital system (delocalised ring of electron density above and below plane of benzene ring)

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6
Q

nitration of benzene

A

NO2^+ ion formed by mixing conc. HNO3 and conc H2SO4, heated between 50°C and 60°C (any higher may form dinitrobenzene)

conc. H2SO4 donates H+ to HNO3
forms intermediate that decomposes to form HNO3

H2SO4(aq) + HNO3(aq) ⇌ HSO4^- + [H2NO3]^+
[H2NO3]^+ ⇌ NO2^+ + H2O

NO2^+ reacts with benzene in electrophilic substitution

NO2^+ + C6H6 -> C6H5NO2 + H^+

H^+ + HSO4^-1 react to form H2SO4, so H2SO4 is catalyst

overall reaction: HNO3(aq) + C6H6(l) -> C6H5NO2(l) + H2O(l)

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7
Q

aldehyde functional group

A

C=O at end of molecule
-al
at least one hydrogen attached to carbonyl group

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8
Q

ketone functional group

A

C=O at middle of molecule (not at the ends)
-one
2 carbon attached to carbonyl group

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9
Q

aldehyde oxidation process

A

under reflux
acidified potassium dichromate and dilute sulfuric acid
forms carboxylic acid
aldehyde + [O] -> carboxylic acid

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10
Q

ketone oxidation process

A

no reaction occurs

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11
Q

nucleophilic addition of ketones and aldehydes

A

C=O bond is polar due to difference in electronegativity of oxygen and carbon
some nucleophiles attracted to and attacks slightly positive carbon atom
different from non-polar C=C (which is electrophilic addition)

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12
Q

reaction of aldehydes with NaBH4

A

reduced to primary alcohols

aldehyde + 2[H] -(NaBH4/H2O)-> primary alcohol + hydroxide ion

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13
Q

reaction of ketones with NaBH4

A

reduced to secondary alcohols

ketone + 2[H] -(NaBH4/H2O)-> secondary alcohol + hydroxide ion

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14
Q

reaction of carbonyl compounds to HCN

A

reacts to form nitrile (increases length of carbon chain)
adds across C=O bond
aldehyde/ketone + HCN -> hydroxynitrile
HCN can be formed with NaCN + H2SO4

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15
Q

mechanism of reaction of carbonyl compound with NaBH4

A

NaBH4 provides hydride ion H- (nucleophile)
H- attracted and donated to partially positive carbon atom (of C=O)
forms dative covalent bond formed between H- and partially positive carbon atom
pi-bond breaks heterolytically, oxygen atom is negative (has 2 electrons)
oxygen donates lone pair of electrons to hydrogen atom in H2O
intermediate is protonated
forms an alcohol and hydroxide ion

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16
Q

mechanism of reaction of carbonyl compound with NaCN/H+

A 
cyanide ion (CN-)attracted and donated to partially positive carbon atom (C=O)
dative covalent bond forms
pi-bond breaks, intermediate has that oxygen becomes negative (has 2 electrons)
intermediate protested by donating lone pair of electrons to hydrogen ion
forms product (usually hydroxynitrile)
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17
Q

NaBH4 full name

A

sodium tetrahydridoborate

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18
Q

conditions for reduction of carbonyls to form hydroxynitriles

A

reducing agent = CN-

cyanide ions need to be acidified (with sulfuric acid, or HCl)

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19
Q

Brady’s

A

2,4-dinitrophenylhydrazine

used to rest for carbonyl group in aldehydes and ketones

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20
Q

testing for carbonyl group in aldehydes and ketones

A

add 5cm^3 of solution of 2,4-dinitrophenylhydrazine (excess) to clean test tube
add 3 drops of unknown compound
leave to stand
if no crystals form, add few drops of sulfuric acid
yellow/orange precipitate = presence of carbonyl group

to identify what carbonyl is present:
purify hydrazone precipiate by recrystallisation
measure melting point of purified orange solid
compare melting point with data table values

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21
Q

Tollen’s reagent

A

Ag(NH3)2OH

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22
Q

Tollen’s reagent test

A

add aqueous silver nitrate to clean test tube
add aqueous sodium hydroxide to silver nitrate until brown precipitate forms (silver oxide)
add dilute ammonia until brown precipitate diseases to form clean colourless solution (Tollen’s reagent)
add unknown solution and Tollen’s reagent into clean test tube
leave to stand in warm water bath
positive if silver mirror formed (aldehyde group present)

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23
Q

acidified potassium dichromate to aldehydes and ketones

A 
ketone = orange solution (no change)
aldehyde = dark green (oxidised to carboxylic acid)
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24
Q

Tollen’s reagent half equation

A

Ag+ (aq) + e- -> Ag(s)
silver ions are reduced to form pure silver
aldehydes/alcohols are oxidised
ketones cannot be oxidised

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25
Q

why Brady’s only reacts with aldehydes and ketones

A

in other compounds e.g. carboxylic acids or amides, Brady’s acts as a base
leaves carboxylate ion negatively charged
unable to be attacked by nucleophile

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26
Q

NaBH4 full name

A

sodium tetrahydridoborate

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27
Q

carboxylic acid group

A

-COOH

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28
Q

aspirin carboxylic acid derivative

A

salicylic acid

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29
Q

solubility of carboxylic acids

A

both C=O and O-H bonds are polar
can form hydrogen bonds with H2O
those with carbon chain of up to 4 are soluble in water
solubility decreases as chain increases as non-polar chain has greater effect on overall polarity of molecule

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30
Q

strength of carboxylic acids

A

weak acids
only partially dissociate
HCOOH (aq) <=> H+(aq) + HCOO- (aq)
HCOOH(aq) + H2O(l) <=> HCOO- (aq) + H3O+ (aq)

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31
Q

carboxylic acids in reactions

A

forms carboxylate salts(ions)

RCOO-

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32
Q

acid reactions of carboxylic acids

A

behaves the same as any other acid

salt produced example: (CH3COO-)2Mg2+

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33
Q

test for carboxyl group

A

carboxylic acids are only common organic compounds sufficiently acidic to react with carbonates
phenols not acidic enough to react with carbonates (helps distinguish between phenol and carboxyl)

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34
Q

benzoic acid solubility

A

fairly insoluble in cold water

soluble in hot water

35
Q

boiling points of carboxylic acids

A

increases with size due to increased London dispersion forces
can also form hydrogen bonds due to C=O and O-H
high MP/BP relative to mass

36
Q

how carboxylic acids can be synthesised

A

oxidation of aldehydes

hydrolysis of esters, acyl chlorides, nitriles, amides

37
Q

ester uses

A

flavourings
solvents (e.g. nail varnish remover)
plasticiser

38
Q

esterification of alcohols and carboxylic acids reaction

A

heated under reflux with strong acid catalyst (conc. sulfuric acid)
alcohol + carboxylic acid -> ester + water

39
Q

why strong acid catalyst is required for esterification

A

strong acid e.g. conc. H2SO4 acts as dehydrating agent
removes water, causing equilibrium to favour forward reaction
increases yield of ester

40
Q

esterification of alcohols and acid anhydride reaction

A

heat under reflux and dry conditions
alcohol + acid anhydride -> ester + carboxylic acid
e.g.
methanol + ethanoic anhydride -> methyl ethanoate + ethanoic acid
irreversible
acid anhydride less toxic than acyl chlorides

41
Q

hydrolysis reaction definition

A

reaction with water or hydroxide ions that breaks compound in two

42
Q

hydrolysis of esters

A

ester + water <=> carboxylic acid + alcohol

43
Q

hydrolysis of esters in alkaline conditions

A

ester + metal hydroxide -> metal ethanoate + alcohol
e.g.
methyl ethanoate + sodium hydroxide -> sodium ethanoate + methanol
water soluble ionic salt used in soap making (saponification)

44
Q

hydrolysis of esters in acidic conditions

A

ester + water -(acid catalyst)-> carboxylic acid + alcohol
e.g.
methyl ethanoate + water -> ethanoic acid + methanol

45
Q

acyl chlorides in RTP

A

clear fuming liquids

very active

46
Q

how to form acyl chloride from carboxylic acid

A

use thionyl chloride SOCl2 with carboxylic acid
replaces -OH group
produces acyl chloride, sulfur dioxide and hydrogen chloride gas
CH3COOH(l) + SOCl2(l) -> CH3COCl(l) + SO2(g) + HCl(g)

47
Q

why carboxylic to acyl chloride reaction good intermediate

A

HCl and SO2 gasses bubble off

only need to separate liquid product from excess liquid starting reagents (using fractional distillation)

48
Q

acyl chloride + water

A

acyl chloride(l) + H2O(l) -> carboxylic acid(aq) + HCl(g)
irreversible
in fume hood as HCl gas is corrosive
very vigorous

49
Q

acyl chloride + alcohol

A

acyl chloride(l) + alcohol(l) -> ester(l) + HCl (g)
room temperature
irreversible

50
Q

why making esters from acyl chlorides is better than from carboxylic acids

A

irreversible

doesn’t need catalyst or additional heat

51
Q

acyl chloride + ammonia

A

acyl chloride(l) + ammonia(g) -> amide(l) + ammonium chloride (l)
primary amide is made
white solid made
vigorous reaction

52
Q

acyl chloride + primary amines

A

acyl chloride(l) + primary amine(l) -> secondary amide(s) + alkylammonium chloride
vigorous reaction
white solid
e.g.
ethanoyl chloride + 2 methyl amine -> N-methylethanamide + methyl ammonium chloride

53
Q

nitration of benzene uses

A

dyes
pharmaceuticals
pesticides

54
Q

why benzene is a nucleophile

A

high electron density around ring due to delocalised pi-orbital system

55
Q

how to draw curly arrows for nitration of benzene and etc.

A

curly arrow from edge of circle to nitrogen on NO2^+
forms intermediate with more than half-circle in benzene ring, and carbocation
curly arrow from carbocation - H bond to carbocation
forms nitrobenzene + hydrogen ion

56
Q

halogenation of benzene

bromination of benzene as example

A

requires presence of halogen carrier (e.g. FeBr3, AlCl3)
electrophile (bromonium ion) generated when halogen carrier reacts with halogen
FeBr3 + Br2 FeBr4^- + Br^+

electrophile (bromonium ion) accepts pair of electrons from benzene ring to form dative covalent bond
organic intermediate unstable
breaks down to form halobenzene (bromobenzene) and hydrogen ion

hydrogen ion reacts with FeBr4^- to regenerate catalyst
H^+ + FeBr4^ FeBr3 + HBr

57
Q

bromination of benzene overall reaction

A

benzene + bromine -[halogen carrier catalyst]-> bromobenze + hydrogen bromide

C6H6 + Br2 -[Fe/AlCl3]-> C6H5Br + HBr

58
Q

conditions of halogenation of benzene

A

room temperature and pressure

presence of halogen carrier

59
Q

alkylation of benzene reaction

A

presence of AlCl3 (halogen carrier catalyst)
benzene + haloalkane -> alkylbenzene + hydrogen halide
e.g.
benzene + chloroethane -[AlCl3]-> ethylbenzene + hydrogen chloride

60
Q

acylation of benzene

A

presence of AlCl3 (halogen carrier catalyst)
benzene + acyl chloride -[AlCl3]-> aromatic ketone + hydrogen chloride
e.g.
benzene + ethanoyl chloride -[AlCl3]> phenylethanone + hydrogen chloride

61
Q

why benzene cannot undergo halogenation unless carrier catalyst present

A

cannot undergo electrophilic addition reactions
benzene has delocalised pi electrons spread above and below ring structure
electron density around any 2 carbon bond much less than in alkenes (that have localised electrons)
benzene unable to polarise the approaching halogen molecule

62
Q

phenol structure

A

contains hydroxyl group directly bonded to benzene ring

63
Q

phenol solubility vs alcohols

A 
less soluble in water than alcohols 
due to presence of non-polar benzene ring
phenol partially dissociates 
forms phenoxide ion and hydrogen ion 
also a weak acid
64
Q

phenol vs alcohol vs carboxylic acid acidity

A

ethanol can’t react with strong base (sodium hydroxide) or weak base (sodium carbonate)
phenol can react with strong base/alkali but not weak bases/alkali
carboxylic acid can react with strong and weak bases/alkali

65
Q

how to distinguish phenol and carboxylic acid

A

react with sodium carbonate

if CO2 gas formed, carboxylic acid present

66
Q

phenol with sodium hydroxide

A

phenol + sodium hydroxide -> sodium phenoxide + water

C6H5OH + NaOH -> C6H5O^-Na^+ + H2O

67
Q

bromination of phenol

A 
phenol + aqueous bromine
forms white precipitate (2,4,6-tribromophenol) and decolourises bromine water (orange to colourless)
smell of antiseptic 
halogen carrier catalyst NOT required 
room temperature 
C6H5OH + 3BR2 -> C6H2Br3OH + 3HBr
68
Q

nitration of phenol

A

phenol + dilute nitric acid at room temperature
forms mixture of 2-nitrophenol (major product) and 4-nitrophenol (minor product) and water
2-nitrophenol is major product as 6-phenol is also made (which is identical to 2-nitrophenol)

69
Q

phenol vs benzene reactivity

A

lone pair of electrons from oxygen p-orbital from hydroxyl group donated to delocalised pi-electron system of phenol
higher electron density (more nucleophilic) so can induce polarity to non-polar molecules
attracts more electrophiles than benzene
e.g.
electron density of phenol ring sufficient to polarise bromine molecules without presence of halogen carrier catalyst

70
Q

activation and deactivation of aromatic rings

A

groups attached to aromatic ring can alter their reactivity
extent and positioning of substitutions also affected
e.g.
-NH2 on phenylamime activates aromatic ring so more substitutions and faster rate of reaction
-NO2 on nitrobenzene deactivates aromatic ring, slower rate of reaction , less substitutions

71
Q

ortho, meta, para

A

functional group at position 1
ortho = pos. 2
meta = pos. 3
para = pos. 4

72
Q

ortho, meta, para directing groups and reactivity of aromatic rings

A

all 2 and 4 directing groups (ortho and para directors) are activating groups (except halogens)
all 3 directing groups (meta-directors) are deactivating groups

73
Q

Friedel-Crafts reaction definition

A

substitution reaction where alkyl or acyl chain replaces hydrogen on a species (e.g. benzene)

74
Q

benzene naming rule exceptions

A

phenylamine = benzene with amine group
benzoic acid = benzene with carboxylic group
benzaldehyde = benzene with aldehyde group

75
Q

benzene naming rules

A

if attached to alkyl group with 7 or more carbons, benzene become substituent, prefix “phenyl” is used
short alkyl chains, halogens, nitro groups attached = “benzene” suffix

76
Q

uses of nitrated benzene

A 
pesticides
dyes
drugs
synthetic rubber
lubricating oils
77
Q

phenol + metal

A

phenol + metal -> metal phenoxide + hydrogen gas

78
Q

why certain groups activate and deactivate aromatic ring

A

if group has a lack of electrons (e.g. positively charged), pulls electrons from delocalised pi electron system, decreases electron density

if group has lone pair, donates electrons to delocalised pi electron system, increases electron density

79
Q

why groups added at pos. 2 and 4

A

resonance structure of phenol at pos. 2-4-6 have partial negative charge
this opens these positions for possible attack by electrophiles

80
Q

why groups added at pos. 3

A

to avoid putting the charge that develops onto the carbon attached to the electron-withdrawing group
incoming electron pole must attach to pos. 3
charge cannot be delocalised onto carbon containing deactivating group

81
Q

how to form acid anhydride

A

condensation of 2 carboxylic acids

82
Q

acid anhydride + phenol

A

acid anhydride + phenol -> phenyl carboxylate + carboxylic acid
e.g.
ethanoic anhydride + phenol -> phenyl ethanoate + ethanoic acid

83
Q

recrystallisation method

A

choose solvent that organic compound will just about dissolve in
warm up solvent and add solute (use smallest amount of solvent possible)
allow solution to cool down and organic compound will precipitate out
leave over night in an oven to dry

A-Level Chemistry (12 decks)

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