Carbonyl Compounds

Addition of HCN

The :CN- ion then uses its lone pair of electrons to form a bond with the carbon of the carbonyl group. Simultaneously the C=O double bond is broken heterolytically with oxygen gaining an electron and becoming negatively charged .The oxygen then uses its lone pair of electrons to form a covalent bond with a hydrogen atom from a HCN molecule and a hydroxynitrile is formed. Reagents - Potassium/sodium cyanide and water Conditions - mixed with a solution of sodium or potassium cyanide in water to which a little sulphuric acid has been added Product - A hydroxynitrile Reaction Type - Addition These hydroxynitriles are useful as they can be converted easily into carboxylic acids by hydrolysis

What Are Carbonyl Compounds?

Carbonyl compounds are substances that contain a carbon-oxygen double bond - aldehydes and keytones

Using sodium tetrahydridoborate

Solid sodium tetrahydridoborate is added to a solution of the aldehyde or ketone in an alcohol such as methanol, ethanol or propan-2-ol. It is either heated under reflux or left for some time around room temperature. This varies depending on the nature of the aldehyde or ketone. At the end of this time, a complex similar to the previous one is formed. In the second stage of the reaction, water is added and the mixture is boiled to release the alcohol from the complex. Again, the alcohol formed can be recovered from the mixture by fractional distillation.

Oxidation of Aldehydes and Ketones

The presence of the hydrogen atom attached to the C=O bond in aldehydes makes the easily oxidised so are strong reducing agents. Ketones on the other hand, are resistant to oxidation. this is how we distinguish between the two. Under acidic conditions, the aldehyde is oxidised to a carboxylic acid. Under alkaline conditions, this couldn't form because it would react with the alkali. A salt is formed instead. Reagents - Acidified potassium dichromcate solution and sulfuric acid Conditions - Warmed gently Product - Carboxylic acid Reaction Type - Redox 3RCHO + Cr2O7(2-) + 8H+ ===> 3RCOOH + 2Cr(3+) + 4H2O An aldehyde will reduce dichromate ions, causing a colour change from orange to green. With a keytone the dichromate ions will not be reduced so the solution remains orange.

Physical Properties

These molecules are polar due to the difference in electronegativity between the carbon and oxygen of the C=O bond not being cancelled out. Polarity means Van der Waals forces are present causing a high amount of energy in the form of heat to break down the molecule, high melting and boiling points

Triiodomethane (iodoform) reaction to identify the presence of a CH3CO group in aldehydes and ketones.

Using iodine and sodium hydroxide solution - Iodine solution is added to a small amount of aldehyde or ketone, followed by just enough sodium hydroxide solution to remove the colour of the iodine. - A positive result is the appearance of a very pale yellow precipitate of triiodomethane (previously known as iodoform) - CHI3. Using potassium iodide and sodium chlorate(I) solutions - Potassium iodide solution is added to a small amount of aldehyde or ketone, followed by sodium chlorate(I) solution. - The positive result is the same pale yellow precipitate as before. What the triiodomethane (iodoform) reaction shows A positive result - the pale yellow precipitate of triiodomethane (iodoform) - is given by an aldehyde or ketone containing the grouping: "R" can be a hydrogen atom or a hydrocarbon group (for example, an alkyl group), if "R" is hydrogen, then you have the aldehyde ethanal, CH3CHO. Ethanal is the only aldehyde to give the triiodomethane (iodoform) reaction. If "R"is a hydrocarbon group, then you have a ketone. Lots of ketones give this reaction, but those that do all have a methyl group on one side of the carbon-oxygen double bond. These are known as methyl ketones.

Reaction with 2,4-dinitrophenylhydrazine (test for the C=O bond)

2,4-dinitrophenylhydrazine is hydrazine with a benzene ring of C6H5 (phenyl) and two nitro (NO2) groups attached. You need to dissolve 2,4-dinitrophenylhydrazine in methanol and concentrated sulfuric acid, which is known as Brady's reagent. And a few drops of the ketone or aldehyde to the reagent and a bright orange precipitate will form, indicating the presence of a carbonyl group.

Oxidation using Fehling's or Benedict's

A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes. Ketone - No change in the blue solution. Aldehyde - The blue solution produces a dark red precipitate of copper(I) oxide. Aldehydes reduce the complexed copper(II) ion to copper(I) oxide. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

Ketones

A ketone does not have a hydrogen bonded to its C=O group, but instead two R groups, these can be any compound containing carbon.

Making Ketones

A ketone is formed by the oxidation of a secondary alcohol, by heating the alcohol under reflux with acidified potassium dichromate(VI) and sulfuric acid. C3H7OH + [O] ===> CH3COCH3 + H2O

Making Aldehydes

Aldehydes are made by oxidising primary alcohols using acidified potassium dichromate(VI) and sulfuric acid. There is, however, a problem. The aldehyde produced can be oxidised further to a carboxylic acid by the acidified potassium dichromate(VI) solution used as the oxidising agent. In order to stop at the aldehyde, you have to prevent this from happening. To stop the oxidation at the aldehyde, you . . . - Distil off the aldehyde as soon as it forms. Removing the aldehyde as soon as it is formed means that it doesn't stay in the mixture to be oxidised further. - Use an excess of the alcohol. That means that there isn't enough oxidising agent present to carry out the second stage and oxidise the aldehyde formed to a carboxylic acid. CH3CH2OH + [O] ===> CH3CHO + H2O

Reduction

Aldehydes can be reduced by LiAlH4 or NaBH4 to form a primary alcohol CH3CHO + 2[H] ===> C2H5OH Ketones can also be reduced by LiAlH4 or NaBH4 but form secondary alcohols. C3H6O + 2[H] ===> CH3CHOHCH3

Addition-Elimination Reaction

All carbonyls react with compounds containing the NH2 group. The lone pair of electrons on the nitrogen acts as a nucleophile bonding to the 𝛿+ carbon atom The intermediate with the O- ion then loses a water molecule and a double bond is formed, C=N C=O + H2N-X -> C=N-X + H2O The reaction with aldehydes and asymmetrical ketones produces geometric isomers

Aldehydes

In aldehydes, the carbonyl group has a hydrogen atom attached to it together with most commonly, a hydrocarbon group which might be an alkyl group or one containing a benzene ring. General formula of an aldehyde - CnH2n+2.CHO

Using Lithium tetrahydridoaluminate

Lithium tetrahydridoaluminate is much more reactive than sodium tetrahydridoborate. It reacts violently with water and alcohols, and so any reaction must exclude these common solvents. The reactions are usually carried out in solution in a carefully dried ether such as ethoxyethane (diethyl ether). The reaction happens at room temperature, and takes place in two separate stages. In the first stage, a salt is formed containing a complex aluminium ion. The following equations show what happens if you start with a general aldehyde or ketone. R and R' can be any combination of hydrogen or alkyl groups. The product is then treated with a dilute acid (such as dilute sulphuric acid or dilute hydrochloric acid) to release the alcohol from the complex ion. The alcohol formed can be recovered from the mixture by fractional distillation.