Aldehydes & Ketones, Amines, Optical Isomerism Flashcards
Nucleophilic Addition ( addition of NaBH₄)
slightly acidic, aqueous
Reagent - NaBH
reaction -> REDUCTION
(does not work for carboxylic acids because needs a stronger reducing agent because it has OH & OR groups pushing electrons towards the carbon which makes it more stable)
Nucleophilic Addition (addition of HCN)
- acidic conditions (for H⁺)
- aqueuous
product: Hydroxynitriles
reagent - KCN + dilute acid
Reduction of Aldehydes and Ketones
Formation of Racemic Mixtures of Carbonyl Compounds
- When aldehydes and unsymmetrical ketones react with Cyanide ( i.e KCN) followed by dilute acid, they form mixtures enantiomers.
- When there is an equal amount of emanations a racemic is formed.
- this is because the C=O group is planar so the :CN- ion can attack from either above or below the molecule.
Making Esters
- Esters are formed by the reaction of a caboxylic acid and alcohol.
e.g Ethanoic acid + ethanol ⇌ ethyl ethanoate + water
( CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O) - the esters formed are less soluble in water than the carboxylic acid and alcohol, less dense and can be separated via funnel.
Ester Hydrolysis
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- when an ester is heated under reflux with dilute acid, a carboxylic acid and alcohol are formed.
- the reaction is reversible and the yield of products is poor
- when an ester is heated under reflux with dilute alkali (e.g NaOH), an alcohol and carboxylate salt are formed.
What is an ester link?
the formation of bonds between glycerol and fatty acids
What caused the properties of soap?
- salt contains a hydrophobic ( fatty acid chain) end and a hydrophilic end (carboxylate ion).
- the hydrophobic end mixes with grease and the hydrophilic end mixes with water
- allowing for both the water and grease to be washed away together.
What are the Acyl groups?
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Why are acid anhydrides less reactive than Acyl Chlorides?
- the ester group attached to the C is not as electronegative as the chloride group for an acyl chloride
Polarisation of Acyl groups
- The oxygen atom is more electronegative than the carbon atom
- therefore permanent dipole-dipole forces in each acyl molecule
- the oxygen atom attracts the electron pair to itself more strongly, becoming more negative
- and the C atoms form delta positives
- the delta positive C is attacked by nucleophiles
- the degree of polarisation, hence the extent of nucleophilic attacks, depends on the groups around C atom in in the acyl group
Products formed from Nucleophilic Addition Elimination
- ## the reactions with acyl chlorides occur at cold or room temperatures and are very vigorous/violent. The reactions with acid anhydrides are less vigorous and may need gentle heating.
Which is a stronger base, propylamine or phenylamine?
- Propylamine
- Phenylamine leads to a reduction in electron density around the N atom
- therefore lone pair on the phenylamine N is partially delocalised into benzene ring
Which is a stronger base, ammonia or phenylmethylamine?
- Phenylamine
- even though there is a phenyl present, It is still a primary amine (draw the structure to review) whilst ammonia is not
- meaning it has a greater inductive effect caused by the alkyl groups on the N which increase the electron density around the N
Compare the Previous Theoretical Model of Benzene (Cyclohexa-1,3,5-triene) to the Updated Version of Benzene
Expected Characteristics of Benzene
- Readily undergoes electrophilic addition reactions (e.g decolorising bromine water)
- very reactive
- has three C=C bonds of length 0.134nm
- has three C-C bonds of length 0.154nm
Reality
- Very Stable
- resistant to undergoing electrophilic addition reactions (e.g no visible reaction with bromine water)
- prefers to undergo electrophilic substitution reactions
- has all identical C-C bonds of all lengths 0.140nm (intermediate of the C-C bond length and C=C bond length, due to the the electron ring)
What is the Thermochemical Evidence for cyclohexa-1,3,5-triene NOT being Benzene?
- The enthalpy of hydrogenation of cyclohexene = -120kJ/mol
thus the enthalpy hydrogenation of cyclohexa-1,3,5-trines must be x3 due the 3 C=C bonds so = -360kj/mol
Benzene has an enthalpy of hydrogenation of =-208kJ/mol
it is less exothermic and more stable by a difference of 152kJ/mol in comparison
this proves they cannot be the same thing
Structure of Benzene
- Planar
- Cylic
- each carbon has a singular hydrogen atom
- each carbon atom has a remaining unhybridised p orbital which overlap to form a ring of delocalised electrons above and below the molecule (and thus ring makes the benzene stable)
- each C-C bond is identical and an intermediate between a single and double bond
General Mechanism for Electrophilic Substitution
- Electrons in the delocalised benzene ring are attracted to the electrophile and an electron pair
form a bond with the electrophile - the delocalised ring is disrupted, the electron pair from the hydrogen bonded to the C atom are returned to the ring
- the delocalised ring is then restored
Formation of TNT (trinitrotoluene)
- to form TNT benzene must be nitrated
- this reaction forms both the explosive TNT and amines which can be used to make dyes and pharmaceuticals
STEPS
- concentrated sulfur acid reacts with concentrated nitric acid
H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄⁻
- H₂NO₃⁺ breaks up to form a nitronium ion (NO₂⁺) and water
H₂NO₃⁺ → NO₂⁺ + H₂O
- Nitronium ion acts as an electrophile in the nitration reaction
(its attracted to the high region of electron density) - Sulfuric Acid is reformed
It bonds with the H atom
Friedel-Crafts Acylation Steps and Mechanism
- Form the electrophile
- use an acyl chloride and “halogen carrier”, aluminium chloride
RCHOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
- the carbocation electrophile substituted into benzene ring
- the AlCl4- ion is attracted to the +ve benzene ring and removes the hydrogen atom bonded to the C atom, reforming the AlCl3
- overall this forms a phenylketone, HCL and AlCl3
conditions:
aluminium chloride catalyst
heated under reflux in a non-aqueous solvent
Why may we use processes like Friedel-crafts Acylation?
- benzene rings are relatively stable and interactive so adding the acyl group can make benzene more reactive
