In A-Level Chemistry - Organic chemisrtry, we look at different functional groups in hydrocarbons. Let's look at the homologous series of ketones and their properties here.
Carbonyl Compounds:
- Ketones are a carbonyl compound.Carbonyl compounds contain the >C=O group. The two types of carbonyl compounds are aldehydes and ketones.
- The carbon atom is bonded to the oxygen by a σ-bond and a π-bond.
- Oxygen is more electronegative than carbon, so the bonding electrons are pulled towards the oxygen atom making it δ- and the carbon δ+.
- The polar nature of the bond makes the carbon susceptible to attack by nucleophiles.
Ketones
Ketones do not have a hydrogen atom bonded to the carbon of the group. The formulae of ketones can be represented as RCOR’, whereR and R’ are alkyl or benzene ring groups. The simplest ketone is propanone(has 3 C’s)
Polarity
The carbon atom has two single bonds, one double bond and no lone pairs, so the electrons in the three bonds repel each other and take up the position of maximum separation. This is a
planar triangular shape with bond angles of 120°. This planar shape makes it easy for nucleophiles to attack the carbon atom from either above or below, and so a single optical isomer is never obtained by addition to carbonyl compounds. The polarity of the >C=O group is not cancelled out by the other two groups attached to the carbon atom, therefore ketones are polar.
Boiling points
The general increase in boiling points is due to permanent dipole-dipole forces (as they are polar molecules) and instantaneous induced dipole-dipole forces. H bonding is not possible in carbonyl compounds because ketones have a hydrogen atom that is sufficiently δ+, so their
boiling points are lower than those of alcohols. In comparison with alkanes and alkenes, they are not polar so their intermolecular forces are weaker and their boiling points lower than those of ketones with the same number of electrons in the molecule.
Solubility
Small ketones are soluble in water due to H bonds between a lone pair of the carbonyl group and hydrogen of water. However, as size increases, the solubility decreases due to interference in H bonding by hydrocarbon ‘tails’ of ketones.
Identification
1. To test for a carbonyl group in a compound, add a solution of 2,4-dinitrophenylhydrazine (Brady’s reagent) to the suspected carbonyl compound:
· Aldehydes and ketones give yellow precipitates
· Aromatic aldehydes and ketones give orange precipitate
Ketones react with compounds containing an H2N group. The lone pair of electrons on the nitrogen atom acts as a nucleophile and forms a bond with the δ+ carbon atom in the C=O group. However, instead of an H+ ion adding on to the O- formed, the substance loses a water molecule and a C=N bond is formed. Since a water molecule is lost, this reaction is a condensation reaction. This reaction produces an insoluble product, which can be used to test for the presence of a carbonyl group.
>C=O+ H2N ⇨ >C=N-X + H2O
Identifying a speicific compound
The identify of the carbonyl compound can be found by:
React the carbonyl compound with a solution of 2,4-dinitrophenylhydrazineFilter off theprecipitateRecrystallise the precipitate using the minimum amount of hot ethanolDry the purified product and measure its melting temperature. Refer to a data book and compare this melting temperature with those of 2,4-dynitrophenylhydrazine derivatives of aldehydes and ketones.
Oxidation
Ketones CANNOT be oxidised.
If oxidation is attempted using oxidation agents, Fehling’s solution remains blue and Tollens’ reagent remains colourless:
Reduction
Ketones can be reduced by lithium tetrahydridoaluminate (III), LiAlH4. This compound acts as a source of H- ions. The reaction takes place in two distinct steps:
· The first step is the addition of H- to the δ+ carbon atom. In this step the reagents must be kept dry. It is carried out in ether solution.
· The second step is the addition of an aqueous solution of an acid, which protonates the O- formed previously
· The result is that the carbonyl compound is reduced to an alcohol.
· If ketones are reduced secondary alcohols are formed.