6.1.2, 3 Carbonyl compounds, Carboxylic Acids and esters Flashcards
What are carbonyls and how are they identified (5)
- Aldehydes and Ketones: CHO/ CO
- Test:
Brady’s reagent (2,4- DNP) which is an oxidising agent
When positive for carbonyls: orange crystals are formed which are hydrozones - To further detect what carbonyl, the crystals are filtered and dried, with its melting point detected.
- The melting point is compared to a database to identify the compound
Test to distinguish Aldehydes from Ketones? (3)
- Tollen’s reagent (oxidising agent) Silver nitrate is reacted with NaOH and dissolved with ammonia
The presence of silver mirrors indicate aldehyde whereas no mirrors will indicate a ketone
Ag+ ions are reduced to silver which forms the mirrors
Describe the reduction of a carbonyl with NaBH4 (6)
- Nucleophillic addition
- Reactant:
Warm and aqueous NaBH4 + carbonyl - Nucleophlie formed: Hydride ion H-
- Reduction of Aldehyde forms primary alcohol
- Reduction of Ketone forms secondary alcohol
- Overall reaction:
Carbonyl + NaBH4 = Primary/secondary alcohol + OH-
Describe the reduction of carbonyls using HCN (5)
- Nucleophillic addition
- HCN is very hazardous therefore NaCN and H2SO4 is used
- CN group extends the alkyl chain which is useful for fuels
- Nucleophile: CN-
- Overall reaction:
Carbonyl + HCN= hydroxynitriles + OH-
Describe the bonding in the carbonyl group (C=O) (4)
- The C=O bond is polar due to O being more electronegative
- The double bond has a sigma bond between C and O each orbital containing an electron
- Pi bonds due to the sideways overlapping of p orbitals above and below C and O atoms
- The shape of the C=O forms a trigonal planar with a greater angle between C and O (121) due to the repulsion of Oxygen’s lone pair of electrons
Describe the oxidation of carbonyls (4)
- Oxidising agent:
Acidified K2Cr2O7 and H2SO4 catalyst
(Cr2O72-, H+) - Aldehyde is oxidised to a carboxylic acid
Colour change of orange to green is observed - Ketone cannot be oxidised
How do you test for alkenes? (2)
- Bromine water is added which is originally orange.
- Alkene decolourises bromine as bromine is added across the double bond of the Alkene
How do you test for carboxylic acids?
- Add aqueous sodium carbonate or sodium hydrogencarbonate.
- Effervescence is observed in the presence of the acid.
- Blue litmus paper also turns red which indicates the pH is less than 7, therefore an acid.
Explain the solubility of alcohols and carbonyls compared to carboxylic acids.
- Carboxylic acids> Alcohols> Carbonyls
- Carboxylic acids can make the most H-bonds with water. It contains a polar O-H group AND a polar C=O group
- Alcohols are more soluble than carbonyls due to the polar OH group can make more H-Bonds than C=O in carbonyls
What is formed when a carboxylic acid reacts with:
- Metals
- Alkali
- Metal carbonate
- Metal: forms a salt and hydrogen gas
- Alkali: Forms a salt and water.
- Metal carbonate: Forms a salt, carbon dioxide and water.
What is a carboxylic acid derivative?
- Any compound that can be hydrolysed to form the parent carboxylic acid
All contain:
- An alkyl group/ H
- Bonded to C=O
- Bonded to another alkyl group
Includes: Esters Acyl chlorides Acid anhydrides Amides
What is an Acyl group?
Formed when a carboxylic acid reacts with thionyl chloride(SOCl2)
This reaction forms SO2 and HCl in gaseous form, which is dangerous, therefore the reaction has to be taken place in a fume cupboard.
Describe the reaction of Acyl chloride with alcohols.
Forms an ester and hydrogen chloride
Describe the reaction of Acyl chlorides and phenol.
Produces an a phenyl ester and hydrogen chloride
Describe the reaction between water and an Acyl chloride.
Forms a carboxylic acid and hydrogen chloride in a hydration reaction.
The reaction is very vigorous and produces very dense HCl fumes
