3.3.8 Aldehydes and Ketones
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Can aldehydes hydrogen bond with other aldehyde molecules?
No
Which molecules can aldehydes hydrogen bond to?
Alcohols and water
What are the differences between aldehydes when the chain length gets longer?
Less polar Increased vdw forces
How is an aldehyde formed from an alcohol?
Alcohol + hot potassium dichromate + sulfuric acid
Why is a numbering system not need for an aldehyde?
It always occurs at the end of the chain
How do ketones form hydrogen bonds with water?
The hydrogen bonds form between the electronegative oxygen atom on the ketone and the positive hydrogen atom on the water molecule
How does the C=O bond affect the rest of the aldehyde / ketone?
It is a very strongly polar bond with the carbon being positive so there are permanent dipole-dipole forces between the molecules
Are aldehydes and ketones soluble?
Shorter chains are completely soluble because they form hydrogen bonds
How reactive are carbonyl compounds?
It is strong but almost all the reactions of carbonyl compounds involve the C=O bond This is because the difference in electronegativity makes the bond very polar Nucleophilic reagants attach the positive carbon bond Addition reactions are also possible
What are carbonyl compounds?
Compounds with a C = O bond
What are the reactants for the addition of hydrogen cyanide?
Sodium or potassium cyanide Dilute hydrochloric acid
What is the reaction mechanism for the addition of hydrogen cyanide to a ketone?
See card
What is the reaction mechanism for the addition of hydrogen cyanide to an aldehyde?
See card
What are hydroxynitriles?
Aldehyde that have undergone the addition of hydrogen cyanide
How are enantomers produced during the addition of hydrogen cyanide to aldehyde and ketones?
When carried out with an aldehyde or an unsymmetrical ketones, the CN ion may attack from above or below the C=O group
Can aldehydes be oxidised?
Aldehydes can be oxidised to carboxylic acids [O] = oxidising agent The oxidising agent is acidified potassium dichromate
Can ketones be oxidised?
No because a C-C bond must be broken
What is the Fehling’s test?
Fehling’s A contains Cu2+ ions and is blue Fehling’s B contains an alkali and a complexing agent When an aldehyde is warmed with Fehling’s solution, a red precipitate of copper (1) oxide is produced as the copper (2) oxide oxidises the aldehyde to a carboxylic acid and is itself reduced to copper (1) Ketones don’t react
What is the silver mirror test?
Tollens’ reagent contains the complex Ag(NH3)2 When an aldehyde is warmed with Tollens’ reagent Aldehydes are oxidised to carboxylic acids Ag+ is reduced to metallic silver
How do you name an aldehyde?
Count the carbon as part of the root e.g. CH2O = methanal
What intermolecular forces occur between aldehyde molecules?
The C = O bond is highly polar so dipole - dipole forces are formed which gives a higher melting point than an alkane of the same length