Carbonyl compounds Flashcards
what are carbonyls ?
compounds with C=O bond
- unsaturated polar bonds
why can’t aldehydes and ketones hydrogen bond with themselves ?
- don’t have a polar OH bond so can’t form H bonds
- instead aldehydes and ketones bond with each other through london forces and permanent dipole- permanent dipole bonds
why can aldehydes and ketones bond with water ?
- lone pairs of electrons on the O atoms of carbonyl group can form H bonds with hydrogen on water molecule
Describe a test for carbonyl groups?
- Brady’s reagent: made of methanol, sulphuric acid and a solution of 2,4- dinitrophenylhydrazine (2,4-DNP)
- orange yellow solution
- forms an bright, orange ppt
(only ketones and aldhydes)
.
Describe the Tollens’ reagent test
- test for aldehydes
- add NaOH to silver nitrate solution until it forms a brown ppt
- add dilute NH3 drop by drop until ppt redissolves
- tollen’s reagent oxidising reagent , so can oxidise aldehydes but not ketones.
- when tollen’s reagent react w. aldehydes, forms a silver mirror
Describe the fehlings solution/benedict’s solution test?
- fehling’s = blue solution of complex copper (II) ions dissolved in NaOH
- if heated with an aldehyde, copper ions are reduced to a red-brick ppt
- benedicts: copper ion dissolved in sodium carbonate instead
Describe the iodoform reaction.
- used to detect ethanal and methyl ketones
- (can also be secondary alcohols, and ethanol)
- add test compound to a mixture of iodine and aqueous sodium hydroxide
- forms a yellow ppt of triiodomethane (CHI3), gives off an antisceptic smell.
- only detects METHYL carbonyl groups
Describe the formation of cyanide ions for nucleophillic addition with carbonyls
- HCN is made in situ
- need to mix H2SO4 with KCN , to make the weak acid HCN.
-KCN added in excess, acts as a good source of nucleophile.
What is the nucleophillic addition rtn between HCN and carbonyls known as?
- known as the chain extension rtns
- increases no. of carbons by 1
why can small aldehydes and ketones only dissolve in water ?
- larger aldehydes and ketones have longer carbon chains which can’t form H bonds with water (they disrupt the H bonding between water molecules)
why do aldehydes and ketone have low boiling points in comparison to alcohols?
-aldehydes and ketones have low boiling point than their equivalent alcohols due to only having London forces and dipole - dipole forces (alcohols have stronger hydrogen bonds)
after adding bradys reagent how would identify specific ketones/aldehydes containing carbonyls groups ?
- the orange ppt formed is a derivative of the carbonyl compound, which can be purified via crystallisation.
- each diff. carbonyl gives a crystalline derivative with a diff. melting point
- measure the melting point of the crystals and compare it the data booklet, and identify the carbonyl compound
