4.4 Aldehydes and Ketones Flashcards
Discuss the structure and naming of aldehydes and ketones:
-Aldehydes and ketones both have a C=0 group where the carbon is delta positive and the oxygen is delta negative (due to oxygens higher electronegativity thus pulling more electrons)
- The double bond has a sigma and a pie bond above and below the plane of the sigma bond (caused by a p-p orbital overlap)
-In an aldehyde the carbon atom is bonded to at least one hydrogen atom.
- An aldehyde has the suffix -al the end of the name and ketones end in -one.
How are aldehydes and ketones formed from alcohols?
Alcohols are oxidised using an oxidising agent of acidified potassium dichromate.
- Primary alcohols are oxidised to an aldehyde and then, on further oxidation, to a carboxylic acid.
- Secondary alcohols are oxidised to a ketone.
How can we distinguish between aldehydes and ketones and why is this possible?
It’s possible because aldehydes and can be further oxidised to carboxylic acid thus reducing the oxidising agent, whereas ketones cannot be further oxidised thus not reducing the oxidising agent.
A simple test used to distinguish between the two is Tollens reagent. It is made by adding aqueous sodium hydroxide (NaOH) to a silver nitrate solution until a brown precipitate of silver(I)oxide is formed. Aqueous ammonia is then added to this reagent and the tube is gently warmed. If the compound is an aldehyde, a silver mirror coats the inside of the tube as the Ag+ ions are reduced to silver. A ketone will not reduce tollens reagents Ag+ thus no silver mirror will form.
Another mild oxidising agent that will react with an aldehyde but not a ketone is Fehlings reagent. The test works as two solutions fehlings A and B are mixed making a deep blue solution containg copper ions. An aldehyde will reduce the copper ions and the solution will be replaced by an orange red precipitate.
How does the reduction of aldehydes and ketones work?
Aldehydes and ketones can be reduced to primary and secondary alcohols respectively. The reducing agents used are NaBH4 or LiAlH4. Of these two reagent NaBH4 is preferred as it is safer and can be used in aqueous conditions.
Why are aldehydes and ketones susceptible to nucleophilic attack and thus nucleophilic addition reactions?
Aldehydes and ketones contain a polar carbonyl bond C delta plus=O delta minus. The relatively electron deficient carbon atom (+) can be attacked by nucleophiles (-) such as a cyanide ion -CN.
What is made in reaction of propanone with hydrogen cyanide and what is this reaction called?
The reaction is a nucleophilic addition reaction where the hydrogen cyanide breaks the C=0 bond and adds a C triple bond N to the C and a H to the O creating 2-hydroxy-2-methylpropanenitrile.
What are aldehydes and ketones at room temp and pressure?
Liquids
How are aldehydes and ketones identified?
By finding out their boiling point, this is difficult as some of the boiling points are too similar to each other or too low or too high. The way to overcome this is to react the aldehyde and the ketone with a suitable reagent to give a solid with a definite melting temperature within an acceptable temperature range. One reagent that is used to produce a suitable solid is 2,4- dinitrophenylhydrazine. This material dissolved in acid gives a simple 2,4-DNPH. Mixing with a suspected aldehyde or ketone gives an orange red solid, which is filtered off and purified. The formation of said solid shows that it is either an aldehyde or a ketone and the melting point of the solid is compared to a table of known values to identify the aldehyde or ketone present.
What is the triiodomethane (iodoform) reaction?
Triiodomethane (traditional name for iodoform), CHI3 is a yellow solid. The formation of this yellow solid is used to identify the presence of a CH3C=O group (methyl carbonyl group) or can be used to identify a CH3CH(OH) group in a molecule. It works as the organic compound is warmed with a colourless solution of iodine in aqueous sodium hydroxide (NaOH) to produce iodoform (CHI) which gives the observation of a yellow solid.
What are some common compounds that undergo the iodoform test?
Common compounds that undergo the triiodomethane test are as follows,
Ethanol,ethanal,propan-2-ol,propanone, butanone, butane-2-ol and phenylethanone. These all produce CHI3 (yellow solid) when reacting with iodine in aqueous sodium hydroxide.
Ones that are common that do not react to give a yellow solid are- methanol,propane-1-ol and propanal.
