18 - Organics III Flashcards
what is the Kekulé model of benzene
- 6 carbon ring with 3 double bonds
what is the delocalised model of benzene
- overlap of 6 p orbitals to form a pi system
evidence of the delocalised model
- through x ray diffraction, found to be a perfect hexagon so can’t be Kekulé as double bonds are shorter than single
- enthalpy of hydration of benzene is less exothermic than expected, due to its extra stability due to the delocalised electrons increasing its entropy
benzene and bromination
- resistant to electrophilic addition and instead does electrophilic substitution
- delocalised pi bonds in benzene vs dlocalised electron density in alkenes
melting points of benzene compared to methylbenzene
methylbenzene much lower as the methyl group prevents good packing
reaction of benzene with oxygen in air
combustion, gives a very sooty flame as the C:H ratio is very high
mechanism of benzene with bromine
- check mechanism
- AlBr3 catalyst
- eventually goes from orange to colourless
why is AlBr3 a good catalyst
aluminium is electron deficient as it has only 6 valence electrons
mechanism of benzene with nitric acid and sulfuric acid
- check mechanism
- T < 50°C prevents too many substitutions and TNT is explosive
mechanism of Friedel-Crafts alkylation
- check mechanism
- bromoalkane and AlBr3 catalyst
mechanism of Friedel-Crafts acylation
- check mechanism
- acyl chloride and AlCl3 catalyst
why does benzene undergo electrophilic substitution
- benzene has high electron density above and within the ring
- can easily donate electrons from the pi system
relative reactivity of phenol compared to benzene
- phenol more reactive as the two lone pairs from the Os are donated into the ring
- creates more electron repulsion in the ring so more likely to be donated
relative stability of phenol compared to benzene
- phenol more stable as the lone pairs on the Os delocalise into the ring, spreading the charge
phenol acid
- draw dissociation
- phenoxide ion more stable due to delocalisation of negative charge
- weak and only reacts with OH-
mechanism of bromination of phenol
- check mechanism
- solid product as it’s a large molecule and breaks more hydrogen bonds than it makes
bromination of phenol vs benzene
phenol: - fast - bromine WATER - room temp 3 substitutions benzene: - slow - need to heat and add AlBr3 catalyst - one substitution
what is the amine functional group
N in a carbon chain
solubility and boiling points of primary, secondary and tertiary amines
1° and 2° have higher boiling points and solubility as they can hydrogen bond
3° still soluble as they have lone pair
bad way to make an amine
R - Br + NH3 –> R-NH2 + HBr
- nucleophilic substitution
- check mechanism
- can’t prevent multiple substitutions
good way to make an amine
- reduction of a nitrile
- LiAlH4 in dry ether
reduction of nitrobenzene with equations
- conc, HCl (aq)
- Sn
- heat under reflux
- add NaOH
- check equation
amine + water equation
- forms alkaline solution
- reversible reaction
- NH2 part + H+ –> NH3+ group
- very basic so reacts with any acid
amine + HCl
check salt
amine + water
check salt
amine + propanoic acid
check salt
how to make an amide
amine + acyl chloride –> amide + HCl
amine + ethanoyl chloride
check reaction
amine + halogenoalkane
check reaction
amine + Cu(II) ions
check complex ion
relative basicities of ammonia, primary amines and aminobenzene
- aminobenzene weakest as the lone pair delocalises into ring, making it less likely to donate to H+
- NH3 middle
- primary amine strongest as R group pushes electrons towards the N lone pair, more repulsion so more likely to donate
what is the amide functional group
- C = O
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N
melting and boiling points of amides
higher than carboxylic acids and very high, two lone pairs on O and lone pair on N can do hydrogen bonding
chemistry of amides, bonds
lone pair on N delocalises into C = O, reducing δ+ on C and strengthening pi bond
formation of polyamides
- condensation polymerisation
- always release HCl or H2O
draw repeat unit of dicarboxylic acid and diamine
check
draw repeat unit of diacyl chloride and diamine
check
draw repeat unit of amino acid polyamide
- check
- form peptide bond
functional group of an amino acid
craboxylic acid with an amino side group
acidity and solubility of amino acids (draw)
- H from OH dissociates and joins NH2 to make NH3+ and OH- in solution (draw)
- form zwitterions
- gives them acidic and basic properties
- very high aqueous solubility as water can hydrate the +ve and -ve charges
- form ionic compounds, white crystalline solid with high melting point
draw amino acid + NaOH and HCl
check
charge of amino acids in solution
mixture of both ions, so never neutral
chirality of amino acids
- all chiral unless R group is H
- rotate plane polarised light in equal angles in opposite directions
hydrolysis of peptide bond (draw)
- heat under reflux
- HCl (aq) or NaOH(aq) and water
- amino acids produced which can be separated by chromotography
- check products with HCl and NaOH
