organics - aldehydes and ketones Flashcards
what is the functional group of aldehydes and ketones
carbonyl functional group, C=O
This is why aldehydes and ketones are also known as carbonyls
how are the functional groups different in an aldehyde compared to a ketone
-The carbonyl group in an aldehyde is always situated at the end of the chain
When naming aldehydes, you do not include the ‘1’ in the name, the carbonyl carbon is always number 1 on the chain
The simplest aldehyde is methanal, HCHO, with the only carbon being that of the carbonyl group
-The carbonyl group in a ketone is always located within the chain
The simplest ketone is propan-2-one, CH3COCH3, as you need an alkyl group either side of the carbonyl carbon in a ketone
how do you poduce an aldehyde from a 1* alcohol
oxidising agent - K2Cr2O7 and H2SO4 - and distill off product
how do u produce a carboxylic acid from an aldehyde
oxidising agent - K2Cr2O7 and H2SO4 - heat or reflux
or tollens reagent-heat
or fehlings solution-heat
how to produce a carboxylic acid from a 1* alcohol
oxidising agent - K2Cr2O7 and H2SO4 - reflux
The aldehyde would still be produced, but as it evaporates it would condense and drop back into the reaction mixture, to be further oxidised to the carboxylic acid
how to produce a ketone from a 2* alcohol
oxidising agent - K2Cr2O7 and H2SO4 - heat or reflux
how to produce 1* alcohol from an aldehyde
NaBH4 aq
how to produce a 2* alcohol from a ketone
NaBH4 aq
why can no further oxidation take place from 2* alcohols
This is because ketones do not have a readily available hydrogen atom, in the same way that aldehydes (or alcohols) do
An extremely strong oxidising agent would be needed for oxidation of a ketone to take place
The oxidation will likely oxidise a ketone in a destructive way, breaking a C-C bond
describe the nucleophilic addition of aldehydes and ketones to produce primary and secondary alcohols
The carbonyl group -C=O, in aldehydes and ketones is polarised
The oxygen atom is more electronegative than carbon drawing electron density towards itself
This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged
The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion
the reagent is NaBH4 and acts as a source of hydride ions and water provides H+
the conditions are aqueous
H- to carbon
arrow from middle of C=O to O
arrow from O to H+
describe nucleophilic addition with CN
reagent is KCn with dilute acid - this is bc KCN is toxic so HCN is generated instead
In step 1, the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
In step 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,
e.g. 2-hydroxypropanenitrile
how does nuclophilic addtion of CN produce racemic mixture
due to chiral carbon and planar nature
the CN- ion can attack C from above or below with equal possibility
-producing racemic
how do u distinguish bwt aldehydes and ketones
Weak oxidising agents can be used to distinguish between an aldehyde and a ketone
The aldehyde will be oxidised to a carboxylic acid, but the ketone will not undergo oxidation
There are a number of tests that can be used to distinguish between aldehydes and ketones
You specifically need to know the following methods to distinguish between an aldehyde and a ketone:
Tollens’ reagent (this is the most commonly used method)
Fehling’s solution
Heating with acidified potassium dichromate could also be used to distinguish between an aldehyde and a ketone
The aldehyde would be oxidised, and you would see an orange to green colour change
The ketone would not be oxidised, so you would see no colour chang
how to carry out tollens silver mirror test
Tollens’ reagent contains the silver(I) complex ion [Ag(NH3)2]+
This is formed when aqueous ammonia is added to a solution of silver nitrate
Tollens’ reagent is also known as ammoniacal silver nitrate
If gently warmed with Tollens’ reagent, an aldehyde will become oxidised
The silver(I) complex ion solution, [Ag(NH3)2]+, is colourless
As the aldehyde is oxidised, it causes the [Ag(NH3)2]+ ions to become reduced to solid metallic silver, Ag
This is why a positive test result is called a “silver mirror”
Positive Test Result:
When Tollens’ reagent is gently warmed with an aldehyde, the silver mirror is formed
This is the positive test result
When Tollens’ reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens’ reagent, so no reaction takes place
This is a negative test result
how to carry out fehlings test
Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent
If an aldehyde is warmed with Fehling’s solution, the aldehyde will be oxidised and a colour change will take place
Fehling’s solution is blue, because of the copper(II) complex ions present
During the reaction, as the aldehyde is oxidised to a carboxylic acid, the blue Cu2+ ions are reduced to Cu+ ions and a brick red precipitate is formed
The brick red precipitate is copper(I) oxide
If a ketone is warmed with Fehling’s solution, no reaction takes place as the ketone will not be oxidised, so the solution will remain blue
