Chem. Unit 4, Chapter 5 - Compounds containing the carbonyl group

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Preview of Chem. Unit 4, Chapter 5 - Compounds containing the carbonyl group

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Ajay Rai
Compounds containing the carbonyl group:
Aldehydes and ketones both contain a carbonyl group (>C=O).
A solution of potassium dichromate(VI) acidified with dilute sulphuric acid will oxidise primary
alcohols (RCH2OH). The orange Cr2O72 ion is reduced to the green chromium(III) cation (Cr3+)
during the reaction, indicating that oxidation has occurred.
The oxidant [Cr2O72 + H+] is usually represented by [O] in equations.
Prolonged heating under reflux with excess acidified dichromate completely oxidises an alcohol
to a carboxylic acid, otherwise it remains an aldehyde.
In order to isolate an aldehyde it must be immediately distilled from the reaction mixture. This is
possible as aldehydes have relatively lower boiling points as molecules are bound by weak
dipoledipole IMF compared to the stronger hydrogen bonding between alcohol and acid
molecules.
The oxidation of secondary alcohols by warm, acidified potassium dichromate(VI) produces
ketones (R1C=OR2), R1 and R2 representing alkyl/aryl groups.
Tertiary alcohols don't oxidise easily. Acidified dichromate(VI) remains orange when heated with
a tertiary alcohol. As there's no H atom to be removed from the carbon to which the OH group is
attached, the oxidant [O] can't remove 2 H atoms to form H2O and the carbonyl group.
However, a stronger oxidant such as acidified potassium manganate(VII), KMnO4, can break CC
bonds to oxidise the alcohol. This produces a mixture of compounds, each with shorter chains.
Reduction reactions:
Fehling's solution ­ a blue solution, containing the copper(II) complex ion in alkaline solution.
When warmed with an aldehyde the aldehyde's oxidised to a carboxylic acid, the blue copper(II)
reducing to a brick red precipitate of copper(I) oxide. Ketones don't react.
Tollen's reagent ­ dilute ammonia solution added to silver(I) nitrate solution containing the
silver(I) complex ion, diamminesilver(I), [Ag(NH3)2]+. When gently warmed with an aldehyde the
colourless diamminesilver(I) ion reduces to metallic Ag(0), causing a silver mirror to appear.
Oxidation reactions:
The oxidation reactions to make aldehydes and ketones can be reversed with a strong reducing
agent such as LiAlH4, lithium tetrahydrodialuminate, used in non aqueous solvents. Alternatively
NaBH4, sodium tetrahydrodiborate, can be used in water. These agents, represented by [H],
reduce aldehydes to primary alcohols and ketones to secondary. Carboxylic acids can also be
reduced to primary alcohols as they're reduced to aldehydes first.
Compounds containing the carbonyl group

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Ajay Rai
Both the reduction and oxidation reactions are of the nucleophilic addition mechanism. NaBH4
provides hydride ions (:H).
Carboxylic acids with fewer than 6 carbon atoms per molecules are water soluble. In aqueous
solution they're only slightly ionised to give low concentrations of hydrogen and alkanoate ions.
The partial ionisation means that carboxylic acids are weak acids.
Esters are formed when an alcohol and a carboxylic acid react together in the presence of a
concentrated, strong acid catalyst, such as HCl or H2SO4.…read more

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