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Chemical Reactions of Aldehydes + Ketones - Coggle Diagram
Chemical Reactions of
Aldehydes + Ketones
Oxidation used to distinguish
between Aldehydes + Ketones
Aldehydes
are oxidised
to corresponding
carboxylic acid
(mild oxidation)
sequence
Ketones
cannot be
oxidised
sequence
using acidified potassium dichromate (VI) solution
aldehydes
oxidation occurs
colour change
ORANGE to GREEN
ionic equation
Cr is reduced (even though oxidising agent)
gains electrons
oxidation state goes down
ketones
remains green
using Tollens' reagent
(the silver mirror test)
Tollens' reagent contains complex
(Ag(NH3)2)+
formed by
adding sodium hydroxide solution
to silver nitrate solution
forms brown precipitate silver (I) oxide
add ammonia solution (dropwise)
until precipitate dissolves
forms a colourless solution
method
add few drops unknown solution
to 1cm3 of fresh Tollens' solution
in test tube
warm in hot water bath
aldehydes
produce silver mirror on test tube
(oxidation occurs to carboxylic acid)
(
silver ions reduced
in Tollens to
silver
)
production of silver mirror quicker for
shorter chained aldehydes
ethanal (equation)
ketones
no reaction
solution remains colourless
used to
distinguish between Aldehyde + 1° Alcohol
(Tollens' does not react w alcohols)
using Fehlings solution
Fehlings solution
blue solution
containing copper (II) complex ion
in alkaline solution
method
add few drops of unknown solution
to 1cm3 fresh Fehlings solution
in test tube
warm in hot water bath
aldehydes
produce red precipitate
(oxidised to carboxylic acid)
(Cu2+ in Fehlings reduced to form
red precipitate of copper (II) oxide)
ketones
no reaction
solution remains blue
equation
Reduction
both A+K can
be reduced
carbonyl compound
reduced to alcohols
method
WARMING
with reducing agent
lithium tetrahydridoaluminate (III)
(LiAlH4)/lithal
in solution of ethoxyethane (dry ether)
(why dry? - hydride reacts w water)
aldehydes
reduced to primary alcohols
ketones
reduced to secondary alcohols
carboxylic acids
also reduced to primary alcohols
using lithal
Nucleophilic Addition of HCN
A+K
react w hydrogen cyanide
in nucleophilic addition @ room temp
why
? due to presence of double bond
in carbonyl + polarity of bond
rate of nucleophilic attack depends on
SIZE of δ+ CHARGE on CARBON
(∴ A more reactive than K)
Nucleophile
ion or molecule with lone pair of e
that attack region of low e density
(cyanide ion acts as nucleophile attacking
the δ+ carbonyl carbon)
hydrogen cyanide produced
in situ
dissolving potassium cyanide
in dilute HCl
hydroxynitrile is produced
equation....
why important?
increases C chain by 1 C atom
forms nitrile group (-CN) which
can be:
reduced to amine
converted to carboxylic acid
aldehyde
2-hydroxypropanenitrile
equation...
optical isomerism
products form racemic mixture
ethanal is planar
nucleophile can
attack from above
or
below
forming 2 different products
works for all
unsymmetrical
aldehydes + ketones (not methanal)
ketone
2-hydroxy-2-methylpropanenitrile
equation...
symmetric
ketones only
produce
single
product
unsymmetrical
ketones
produce
racemic
mixture
Condensation Reactions of
2,4-dinitrophenylhydrazine
DNPH
structural (drawing)
formula
molecular formula
structural formula
RMM
= 198
condensation reaction
D: adding 2 small molecules to form large molecule followed by loss of small mol (H2O/HCl)
double bond forms
between carbon + nitrogen
conditions
room temp
2,4-DNPH solution in acid
observations
orange precipitate produced
aldehyde example
ethanal
+ 2,4-DNPH ---> ethanal-2,4-dinitrophenylhydra
zone
+
water
ketone example
propanone
+ 2,4-DNPH ---> propanone-2,4-dinitrophenylhydra
zone
+
water