6.1 Aromatic compounds, carbonyls and acids Flashcards
benzene general properties
colourless sweet-smelling highly flammable liquid found naturally in crude oil and in cigarette smoke carcinogen
what benzene is classed as
arene or aromatic hydrocarbon
Kekulé model
six-members ring of C joined by alternate single and double bonds
evidence to disprove Kekulé model
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)
delocalised model of benzene
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)
nitration of benzene
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)
aldehyde functional group
C=O at end of molecule
-al
at least one hydrogen attached to carbonyl group
ketone functional group
C=O at middle of molecule (not at the ends)
-one
2 carbon attached to carbonyl group
aldehyde oxidation process
under reflux
acidified potassium dichromate and dilute sulfuric acid
forms carboxylic acid
aldehyde + [O] -> carboxylic acid
ketone oxidation process
no reaction occurs
nucleophilic addition of ketones and aldehydes
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)
reaction of aldehydes with NaBH4
reduced to primary alcohols
aldehyde + 2[H] -(NaBH4/H2O)-> primary alcohol + hydroxide ion
reaction of ketones with NaBH4
reduced to secondary alcohols
ketone + 2[H] -(NaBH4/H2O)-> secondary alcohol + hydroxide ion
reaction of carbonyl compounds to HCN
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
mechanism of reaction of carbonyl compound with NaBH4
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
mechanism of reaction of carbonyl compound with NaCN/H+
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)
NaBH4 full name
sodium tetrahydridoborate
conditions for reduction of carbonyls to form hydroxynitriles
reducing agent = CN-
cyanide ions need to be acidified (with sulfuric acid, or HCl)
Brady’s
2,4-dinitrophenylhydrazine
used to rest for carbonyl group in aldehydes and ketones
testing for carbonyl group in aldehydes and ketones
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
Tollen’s reagent
Ag(NH3)2OH
Tollen’s reagent test
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)
acidified potassium dichromate to aldehydes and ketones
ketone = orange solution (no change) aldehyde = dark green (oxidised to carboxylic acid)
Tollen’s reagent half equation
Ag+ (aq) + e- -> Ag(s)
silver ions are reduced to form pure silver
aldehydes/alcohols are oxidised
ketones cannot be oxidised
why Brady’s only reacts with aldehydes and ketones
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
NaBH4 full name
sodium tetrahydridoborate
carboxylic acid group
-COOH
aspirin carboxylic acid derivative
salicylic acid
solubility of carboxylic acids
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
strength of carboxylic acids
weak acids
only partially dissociate
HCOOH (aq) <=> H+(aq) + HCOO- (aq)
HCOOH(aq) + H2O(l) <=> HCOO- (aq) + H3O+ (aq)
carboxylic acids in reactions
forms carboxylate salts(ions)
RCOO-
acid reactions of carboxylic acids
behaves the same as any other acid
salt produced example: (CH3COO-)2Mg2+
test for carboxyl group
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)
benzoic acid solubility
fairly insoluble in cold water
soluble in hot water
boiling points of carboxylic acids
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
how carboxylic acids can be synthesised
oxidation of aldehydes
hydrolysis of esters, acyl chlorides, nitriles, amides
ester uses
flavourings
solvents (e.g. nail varnish remover)
plasticiser
esterification of alcohols and carboxylic acids reaction
heated under reflux with strong acid catalyst (conc. sulfuric acid)
alcohol + carboxylic acid -> ester + water
why strong acid catalyst is required for esterification
strong acid e.g. conc. H2SO4 acts as dehydrating agent
removes water, causing equilibrium to favour forward reaction
increases yield of ester
esterification of alcohols and acid anhydride reaction
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
hydrolysis reaction definition
reaction with water or hydroxide ions that breaks compound in two
hydrolysis of esters
ester + water <=> carboxylic acid + alcohol
hydrolysis of esters in alkaline conditions
ester + metal hydroxide -> metal ethanoate + alcohol
e.g.
methyl ethanoate + sodium hydroxide -> sodium ethanoate + methanol
water soluble ionic salt used in soap making (saponification)
hydrolysis of esters in acidic conditions
ester + water -(acid catalyst)-> carboxylic acid + alcohol
e.g.
methyl ethanoate + water -> ethanoic acid + methanol
acyl chlorides in RTP
clear fuming liquids
very active
how to form acyl chloride from carboxylic acid
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)
why carboxylic to acyl chloride reaction good intermediate
HCl and SO2 gasses bubble off
only need to separate liquid product from excess liquid starting reagents (using fractional distillation)
acyl chloride + water
acyl chloride(l) + H2O(l) -> carboxylic acid(aq) + HCl(g)
irreversible
in fume hood as HCl gas is corrosive
very vigorous
acyl chloride + alcohol
acyl chloride(l) + alcohol(l) -> ester(l) + HCl (g)
room temperature
irreversible
why making esters from acyl chlorides is better than from carboxylic acids
irreversible
doesn’t need catalyst or additional heat
acyl chloride + ammonia
acyl chloride(l) + ammonia(g) -> amide(l) + ammonium chloride (l)
primary amide is made
white solid made
vigorous reaction
acyl chloride + primary amines
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
nitration of benzene uses
dyes
pharmaceuticals
pesticides
why benzene is a nucleophile
high electron density around ring due to delocalised pi-orbital system
how to draw curly arrows for nitration of benzene and etc.
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
halogenation of benzene
bromination of benzene as example
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
bromination of benzene overall reaction
benzene + bromine -[halogen carrier catalyst]-> bromobenze + hydrogen bromide
C6H6 + Br2 -[Fe/AlCl3]-> C6H5Br + HBr
conditions of halogenation of benzene
room temperature and pressure
presence of halogen carrier
alkylation of benzene reaction
presence of AlCl3 (halogen carrier catalyst)
benzene + haloalkane -> alkylbenzene + hydrogen halide
e.g.
benzene + chloroethane -[AlCl3]-> ethylbenzene + hydrogen chloride
acylation of benzene
presence of AlCl3 (halogen carrier catalyst)
benzene + acyl chloride -[AlCl3]-> aromatic ketone + hydrogen chloride
e.g.
benzene + ethanoyl chloride -[AlCl3]> phenylethanone + hydrogen chloride
why benzene cannot undergo halogenation unless carrier catalyst present
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
phenol structure
contains hydroxyl group directly bonded to benzene ring
phenol solubility vs alcohols
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
phenol vs alcohol vs carboxylic acid acidity
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
how to distinguish phenol and carboxylic acid
react with sodium carbonate
if CO2 gas formed, carboxylic acid present
phenol with sodium hydroxide
phenol + sodium hydroxide -> sodium phenoxide + water
C6H5OH + NaOH -> C6H5O^-Na^+ + H2O
bromination of phenol
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
nitration of phenol
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)
phenol vs benzene reactivity
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
activation and deactivation of aromatic rings
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
ortho, meta, para
functional group at position 1
ortho = pos. 2
meta = pos. 3
para = pos. 4
ortho, meta, para directing groups and reactivity of aromatic rings
all 2 and 4 directing groups (ortho and para directors) are activating groups (except halogens)
all 3 directing groups (meta-directors) are deactivating groups
Friedel-Crafts reaction definition
substitution reaction where alkyl or acyl chain replaces hydrogen on a species (e.g. benzene)
benzene naming rule exceptions
phenylamine = benzene with amine group
benzoic acid = benzene with carboxylic group
benzaldehyde = benzene with aldehyde group
benzene naming rules
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
uses of nitrated benzene
pesticides dyes drugs synthetic rubber lubricating oils
phenol + metal
phenol + metal -> metal phenoxide + hydrogen gas
why certain groups activate and deactivate aromatic ring
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
why groups added at pos. 2 and 4
resonance structure of phenol at pos. 2-4-6 have partial negative charge
this opens these positions for possible attack by electrophiles
why groups added at pos. 3
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
how to form acid anhydride
condensation of 2 carboxylic acids
acid anhydride + phenol
acid anhydride + phenol -> phenyl carboxylate + carboxylic acid
e.g.
ethanoic anhydride + phenol -> phenyl ethanoate + ethanoic acid
recrystallisation method
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
