Organic Chemistry 2 Flashcards
optical isomerism is a result of
chirality in molecules with a single chiral centre
chiral carbon
- carbon atom bonded to four different groups or atom
- 3D molecule with no plane of symmetry
- non-superimposable on its mirror image
what are optical isomers
non-superimposable mirror images
optical activity
Rotates plane-polarised monochromatic light in opposite directions
-single enantiomer
racemic mixture
an equal number of each enantiomer
Simple substances which show optical isomerism exist as two isomers known as
enantiomers
achiral
carbon that has a plane of symmetry
aldehyde functional group
R-COH c=o with hydrogen
ketone functional group
R-CO-R c=o
name ending of an aldehyde
-al
ketone name ending
-one
intermolecular forces ketones and aldehydes can form
-london forces
-dipole-dipole, polar c=o bond
no hydrogen bonds:don’t have any hydrogen atoms attached directly to the oxygen
aldehyde and ketone solubility in water
soluble, but falls with chain length
can form hydrogen bonds with water (slightly positive hydrogen)
test for a carbonyl group
2,4-dinitrophenylhydrazine (Brady’s reagent)
room temperature yellow producing orange crystals
oxidisation of carbonyl with Tollen’s reagent
Heat with ammoniacal silver nitrate
for aldehyde Silver mirror forms
for ketone no reaction occurs
oxidisation of carbonyl with Fehling’s solution
heat
aldehyde Turns from a blue solution to form a red precipitate
oxidisation of carbonyl with Dichromate solution
Heat with a mixture of potassium dichromate (vi) solution and sulphuric acid
Turns from an orange solution to a green solution
Carboxylic acid functional group
C=O-OH
RCOOH
Why may a substance have a chiral carbon but not be optically active
Equal amounts of the two enantiomers so the two rotations of plane-polarised light cancel out = racemic mixture
How to get a pure sample or maximise the yield of an aldehyde
Distill off as it’s formed to prevent further oxidation
Carbonyl compounds react with iodine
In presence of an alkali
Pale yellow precipitate of triiodomethane (iodoform) given by ketone or aldehyde with CH3-C=O-R
