6.1 aromatic compounds, carbonyls and acids Flashcards
describe kekulé’s for benzene model using 3 points
- sigma bonded ring
- 3 alternating pi bonds
- highly unsaturated
name 3 problems with kekulé’s benzene model
- benzene is resistant to addition reaction and undergoes substitutions. suggests there aren’t double bonds
- enthalpy of hydrogenation - benzene has lower enthalpy than predicted, value for benzene is LESS EXOTHERMIC
- all 6 carbon bonds are the same length so all bonds must be uniform
how many pi electrons are there in the benzene ring?
6 pi electrons
describe the structure/bonding/delocalised system of benzene in 3 points
- each C is sigma bonded to a H, forming a planar hexagonal ring
- p orbitals overlap above and below the ring to make pi bonds
- 6 electrons in pi bonds are delocalised around the ring
which mechanism does benzene undergo?
electrophilic substitution
describe the 3 steps of electrophilic substitution of benzene
- electron from benzene RING is accepted by the electrophile
- electrophile is bonded to a carbon with a hydrogen and the C-H bond is attracted to the plus in the middle of the broken benzene ring
- H+ is lost and remains as a product along with the benzene compound
what is halogenation?
substituting a halogen for a hydrogen on a benzene ring
what condition are required for a halogenation reaction?
halogen carrier catalyst
- FeCl₃
- AlCl₃
give the overall equation for the halogenation between benzene and chlorine
C₆H₆ + Cl₂ –> C₆H₅Cl + HCl
halogenation: give two equations showing the formation of an electrophile and the regeneration of the catalyst
forming electrophile:
Cl₂ + FeCl₃ –> FeCl₄⁻ + Cl⁺
regenerating catalyst:
H⁺ + FeCl₄⁻ –> FeCl₃ + HCl
what is nitration?
substituting a hydrogen for a nitronium ion -NO₂⁺ on a benzene ring
which reagents and conditions are required for the nitration reaction?
reagent: HNO₃
catalyst: concentrated H₂SO₄
give the overall nitration reaction
C₆H₆ + HNO₃ –> C₆H₅NO₂ + H₂O
nitration: give two equations showing the formation of the electrophile and the regeneration of the catalyst
formation of electrophile:
HNO₃ + H₂SO₄ –> NO₂⁺ + H₂O + HSO₄⁻
catalyst regen:
HSO₄⁻ + H⁺ –> H₂SO₄
what is a phenol molecule?
a benzene ring with an -OH attached to a carbon
what are friedel-crafts reactions?
reactions in which hydrogens are substituted with R groups on benzene
what is alkylation?
substituting a hydrogen for an akyl group on a benzene
how does alkylation occur? give the overall equation of the substitution of a methyl group to benzene
occurs like a halogenation reaction with same catalysts but reagent is a haloalkane
C₆H₆ + CH₃Cl –> C₆H₅CH₃ + HCl
give the functional group of an acyl chloride
RC=OCl
how can an acyl chloride be used in friedel-crafts reactions?
as a halogen carrier - the chlorine on the acyl group and the hydrogen on the benzene are removed and form HCl
list phenol, benzene and cyclohexene in order of electron density, from most dense to least dense
most - alkene
phenol
least - benzene
explain how phenol has a greater electron density than benzene
the lone pairs and p-orbital on the oxygen in the -OH group add to the delocalised of benzene and make it more electron dense
how does increased electron density in benzene, phenol and alkenes increase reactivity?
more electron density means the electrophile is more readily polarised and so is more likely to react
which two electrophilic subsitution reactions with pheno are you supposed to learn?
with Br
with DILUTE nitric acid
what conditions are required for both electrophilic substitution reactions of phenol? why are these condition required?
Br- NO catalyst - phenol is reactive enough
HNO₃ - no catalyst and DILUTE acid - phenol is more reactive and so doesn’t need as concentrated acid
what are the products formed from electrophilic substitution of phenol with bromine?
2,4,6-tribromophenol
multiple substitutions occur
what are the products formed from electrophilic substitution of phenol with nitric acid?
mixture of 2-nitrophenol and 4-nitrophenol
phenol is a ____ acid and _______ disassociates in water. phenol is a ____ acid than aliphatic alcohols becasue….
- weak
- partially
- stronger acid than aliphatic alcohols because the ring weakens the O-H bond so H+ is easily lost
give and a reversible equation demonstrating the weak acidity of phenol
C₆H₅OH(aq) —> C₆H₅O⁻(aq) + H⁺(aq)
phenol + NaOH —> ?
—-> C₆H₅O⁻Na⁺ + water
makes salt-sodium phenoxide which is ionic and soluble in water
phenol + Na —> ? how do they react? give a balanced eq
makes ionic salt, reacts vigorously, fizzing, sodium dissovles.
2C₆H₅OH + 2Na —> 2C₆H₅O⁻Na⁺ + H₂
phenol does not react with __________.
metal carbonates
what is a directing group? give 2 examples
a group on benzene that donates lone pairs into the benzene ring, giving a 2,4 directing effect
eg. -OH and -NH₂
what is a withdrawing group? give to examples
a group that withdraws electron from benzene, making substitutions more common on 3 position
eg. -NO₂ and -COOH
what kind of mechanism is the reduction of carbonyls?
nucleophilic addition
what happens during a reduction of a carbonyl?
ketone/aldehyde becomes a secondary/primary alcohol
what is the reagent used for the reduction of carbonyls? what is the nucleophile in the reaction?
what are the conditions?
reagent: NaBH₄
nucleophile is :H⁻ hydride ions
conditions: aqueous or alcoholic solution
describe the mechanism of the reduction of carbonyls
- curly arrow from hydride ion lone pair to C and curly arrow from C=O bond to O.
- dipoles on C=O: C is delta+ and O is delta-
INTERMEDIATE: - curly arrow from lone pair on O⁻⁻ to H on H₂O and curly arrow from H-O bond to O.
- dipoles on H₂O: H is delta+ and O is delta-
what products are formed from the reduction of carbonyls?
alcohol and HO⁻⁻
what is Brady’s reagent and what is it used for? how does it work?
2,4-DNPH
used to identify ketones and aldehydes
forms ORANGE precipitate when reacted with aldehydes and ketones
how can Brady’s be used to determine the exact original compound?
- aldehydes and ketones have similar boiling points
- reacting with Brady’s forms orange precipitates with distinct melting points
- melting point can be compared to known values/database
what is tollens reagent and what is it used for? how does it work?
it is ammoniacal silver nitrate and a mild oxidising agent
used for identifying aldehydes
only oxidises aldehyde and solid silver/silver mirror is formed
what are the carboxylic acid reaction with: metal? metal oxide? metal hydroxide? metal carbonate?
- carboxylate salt + hydrogen
- carboxylate salt + water
- carboxylate salt + water
- carboxylate salt + carbon dioxide
what is the ester functional group?
-COO
C=O
|
O
give the equation and conditions of esterification involving an alcohol
alcohol + carboxylic acid –> ester + water
under REFLUX and concentrated H₂SO₄ catalyst
propanol + ethanoic acid?
propyl ethanoate + water
pentanoic acid + butanol?
butyl pentanoate + water
what is used to prepare esters that isn’t an alcohol or carboxylic acid?
acid anhydride
O=C-O-C=O
| |
R. R..
give the esterificiation equation using the acid anhydride (CH₃CO)₂O and methanol
methanol + ethanoic anhydride –> methyl ethanoate + ethanoic acid
give the general equation of forming esters with acid anhydrides
alcohol + acid anhydride –> ester + carboxylic acid
how does the esterification occur between an acid anhydride and alcohol?
the OH group loses an O to the C in the acid anhydride and forms an ester group
the H on the OH group goes to the O where the anhydride was broken and forms the COOH
what products are obtained in the acid and alkali hydrolyses of esters?
acid: original products (alcohol and carboxylic acid)
alkali: carboxylate salt (COO⁻) forms and original alcohol
what is the functional group of an acyl group?
COCl
give the general equation of the formation of acyl chlorides
carboxylic acid + SOCl₂ –> acyl chloride + SO₂ + HCl
describe how an acyl chloride formed
- Cl on SOCl₂ swaps with the OH on the COOH group
- the O from the OH goes to SO₂
- the H from the OH and the Cl go to the HCl
how do you name acyl chlorides?
longest chain length + oyl chloride
eg. ethanoyl chloride or butanoyl chloride
acyl chloride + water?
carboxylic acid + HCl
acyl chloride + alcohol?
ester + HCl
acyl chloride + amine?
primary amide + NH₄Cl or HCl
acyl chloride + primary amine?
secondary amide + NH₄Cl or HCl
what can phenol not do with regard to esterification?
phenol cannot react with carboxylic acids to form an ester
give the esterification general equation involving phenol
acyl chloride + phenol –> phenol ester + HCl
more vigorous than carboxylic acid + alcohol
are electrophiles + or - ?
electrophiles = positive
are nucleophiles + or - ?
nucleophiles = negative
