carbonyls Flashcards
carbonyl functional group
c=o
why is the carbonyl group polar
polar as O is more electronegative than C
which molecules contain carbonyl groups
Aldehydes and ketones
shape around the C in a carbonyl group
trigonal planar- 120 degrees
3 sigma bonds and 1 Pi bond above and below the c=o
naming aldehydes
aldehyde group is always C1
suffix -al
find and name longest chain
exception is benzaldehyde + phenyl compounds
naming ketones
use suffix -one
find longest chain with ketone attached
exception is phenylethanone - has no phenyl group just benzene
aldehyde functional group
R
C=O
H
ketone functional group
R1
C=O
R2
preparation of aldehydes and ketones
aldehydes- oxidation of primary alcohol by distillation using oxidising agent
ketones- oxidation of secondary alcohol by distillation or reduction using oxidising agent
reduction of aldehydes
reagents: reduced using reducing agent-[H]
Sodium tetradiodobromate- provides H- ion (from BH4-)
conditions: in aqueous solution, warm
reaction type: nucleophilic addition
overall reaction- RCHO + 2[H] –> RCH2OH (primary alcohol)
mechanism: arrow from :H- to C with = O attached, then arrow from double bond to O
O and C should have dipoles
draw intermediate- same as before but single bond :O-
then arrow from :O- to H on H2O molecule
(showing dipoles) and arrow from bond between H and O in h2o molecule to O
draw product + :OH-
reduction of ketones
reagents: reduced using reducing agent-[H]
Sodium tetradiodobromate- provides H- ion (from BH4-)
conditions: in aqueous solution, warm
reaction type: nucleophilic addition
overall reaction- R1COR2 + 2[H] –> R1CHOHR2 (secondary alcohol)
mechanism: arrow from :H- to C with = O attached, then arrow from double bond to O
O and C should have dipoles
draw intermediate- same as before but single bond :O-
then arrow from :O- to H on H2O molecule
(showing dipoles) and arrow from bond between H and O in h2o molecule to O
draw product + :OH-
reaction of aldehydes and ketones with HCN or safer NaCN
Reagent- NaCN/HCN + H+
reaction type: nucleophilic addition
mechanism: arrow from :CN- to C with = O attached, then arrow from double bond to O
O and C should have dipoles
draw intermediate- same as before but single bond :O-
then arrow from :O- to H on H2O molecule
(showing dipoles) and arrow from bond between H and O in h2o molecule to O
draw product + :OH-
Can use H+ instead of H2O - no biproducts
forms a hyroxynitrile, -CN accounts for nitrile
reaction of nitrile groups
readily hydrolysed by acid hydrolysis to form carboxylic acid
useful in synthesis : heat under reflux with aqueous acid
Chemical test for an aldehyde
Tollen’s reagent (ammoniacal silver nitrate)
only oxidises aldehydes, ketones cant be oxidised
add equal vol of reagent and compound in boiling tube to beaker of hot H2O (60 degrees) for 5-10 mins
positive - silver mirror
reactions: Ag+ + e- –> Ag
RCHO + [O] —> RCOOH
chemical test for any carbonyl
2,4 DNPH - 2,4-dinitrophenyldrazine
yellow liquid
add excess to a sample
if positive then forms Orange precipitate- 2,4-dinitrophenyldrazone
identify different carbonyls from DNPH test
purify yellow/orange precipitate by recrystallisation to remove impurities
find the melting point of the purified derivative
compare melting point with known data to identify the aldehyde / ketone formed
pure 2,4DNPH have sharp well defined melting points
impurities lower MP
