aldehydes and ketones can be reduced to primary and secondary alcohols respectively using reducing agents like sodium borohydride (NaBH4) dissolved in a mixture of water and methanol
carbonyl compounds undergo nucleophilic addition reactions because the carbonyl carbon is electrophilic due to the polarisation of the C=O double bond, which makes it susceptible to attack by nucleophiles
carbonyls are compounds with a C=O bond. they can be either aldehydes or ketones
when 2,4 DNPH is dissolved in methanol and concentrated sulfuric acid, it reacts with carbonyl groups (C=O) found in aldehydes and ketones to form a brightorange precipitate
aldehydes have their carbonyl group at the end of a carbon chain
ketones have their carbonyl group within the carbon chain
the 2,4-DNPH reaction only occurs with aldehydes and ketones due to their C=O group
other carbonyls like carboxylic acids (COOH) and esters (COO) do not react
tollens' reagent formed by mixing aqueousammonia and silvernitrate. the active substance is the complex ion of [Ag(NH3)2]2+
conditions: heat gently
reaction of tollens' reagent: aldehydes only are oxidised by tollens' reagent into a carboxylic acid. the silver (I) ions are reduced to silver atoms
observation of tollens' reagent: with aldehydes, a silver mirror forms coating the inside of the test tube. ketones result in nochange
aldehydes will undergo oxidation to form a carboxylic acid
the reagents are potassium dichromate K2Cr2O7 and sulfuric acid H2SO4
reagents react in situ to form the oxidising species, Cr2O7 2- and H+
balanced ionic equation for the oxidation of ethanal to ethanoic acid is
what is the simplified balanced symbol equation of oxidation of ethanal to ethanoic acid
CH3CHO + [O] -> CH3COOH
in the laboratory, the oxidation of ethanal to ethanoic acid is gently heated under reflux
as the reaction proceeds, a colour change is observed, from orange to green due to the oxidation state of the chromium changing
NaBH4 is used commonly in organic synthesis
this compound is made of a BH4- ion which acts as a source of hydride ions H-
the hydride ion is the species that is involved in the electrophilic addition and reduction of carbonyl compounds to alcohols
simplified balanced chemical equation for the reduction of pentane-2-one
CH3COC3H7 + 2[H] -> CH3CH(OH)C3H7
hydrogencyanide (HCN) is a weak acid that will partially ionise in solution
a cyanide nucleophile with a negative charge on the carbon atom is formed
HCN + H2O ⇌ CN- + H3O+
hydrogen cyanide
the cyanide ion cannot react directly with carbonyl compounds. but when the reaction is acidified, the carbonyl functional group becomes more reactive as polarity of the C=O bond is increased
hydrogen cyanide
addition of cyanide allows further carbon atoms to be added to the organic molecule
in this reaction a hydroxynitrile is formed
brady's reagent is an orange transparent mixture of methanol, sulfuric acid and a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH)
when brady's reagent added to an aldehyde or ketone a yellow/orange precipitate of 2,4 DNPH derivative is seen
no precipitation is observed with a carboxylic acid or an ester, despite these compounds also having the C=O bond
the 2,4-dinitrophenylhydrazone derivative precipitate can be collected by filtration and purified using recrystallisation
after drying the accurate melting point of the pure product can then be measured through experiment
the aldehyde or ketone can be identified by comparing the melting point of the 2,4-dinitrophenylhydrazone derivative precipitate with a database
the database is a list of accurately measured melting points of 2,4-dinitrophenylhydrazone derivatives, listed against the aldehyde or ketone that made it
tollens' reagent
sodium hydroxide solution is added to silver nitrate solution until a brown precipitate is formed
dilute ammonia is added drop-wise until the brown precipitate redissolves
tollens reagent can be used to distinguish between an aldehyde and a ketone
it is a weak oxidising agent and can react with the carbonyl functional group in an aldehyde but not a ketone
when tollens reagent is added to a ketone there is no reaction, this is because ketones cannot be oxidised further
when tollens reagent is added to an aldehyde a silver mirror is observed
when aldehydes react with tollens reagent a redox reaction occurs
the silver ions are reduced and the aldehyde functional group is oxidised
balanced ionic equation for the oxidation of the silver ions
Ag+ (aq) + e- -> Ag (s)
the oxidation of silver ions causes silver metal to be precipitated out and this is observed as a silvermirror effect on the inside of the reaction vessel
simultaneously the aldehyde functional group is oxidised to a carboxylic acid, oxidising agent is summarised as [O]