aldehydes and ketones Flashcards
explain the solubility
soluble due to being able to form hydrogen bonds with water
solubility decreases with length of carbon chain as the long non polar hydrocarbon chains hinger hydrogen bonds
explain boiling points
influenced by the permanent dipole-dipole forces between molecules
as they might have similar rfm and therefore electrons and therefore van der waals
branched have lower bp as there is less effective permanent dipole-dipole attraction as branching reduces contact between molecules
how are aldehydes and ketones prepared
from their corresponding primary or secondary alcohol
eg
primary alc + [o] > aldehyde + h2o
secondary alc + [o] > ketone + h2o
what are the conditions for preparing aldehyde or ketone from alcohol
heat with acidified potassium dichromate (vi) solution (relux for carboxylic acid) (heat and distill off for aldehyde)
orange solution changes to green
describe the conditions and type of reaction when a+k react with HCN
cond: add DILUTE hcl to an aqueous solution of KCN to generate HCN in situ
carried out at room temp
nucleophillic addition
explain why hydroxynitriles are optically inactive
aldehydes and unsymmetrical ketones produced a racemic mixture
the carbonyl bond is planar and attacks the carbon atom equally from both sides
one enantiomer rotates the plane of plane polarised light left and the other right thus cancelling eachother out making them optically inactive
recall reaction of a + ketones with 2,4-dinitrophenolhydrazine
condensation reaction(as water is removed) hydrazone derivative is formed
cond: room temp, solution of 2,4-dinitrophenylhydrazine (add dilute H2SO4 if ppt doesnt form)
observations: orange ppt
DNPH (bradys reagent) formula C6H6N4O4
recall how a + k can be distinguished
-oxidation
-fehlings solution Cu+ (aldehyde change blue-> red) (ketone remain blue)
-tollens reagent Ag silver mirror test aldehyde forms silver mirror inside test tube, ketone colourless
