In A-Level Chemistry - Organic chemisrtry, we look at different functional groups in hydrocarbons. Let's look at the homologous series of aldehydes and their properties here.
Carbonyl Compounds:
- Aldehydes 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.
Aldehydes
Aldehydes have a hydrogen atom bonded to the carbon of the C=O group. The formulae of aldehydes can be represented by RCHO, where R isa hydrogen atom, an alkyl group or benzene ring. Examples include:
These all have exactly the same end to the molecule. All that differs is the complexity of the other group attached.
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 aldehydes 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 aldehydes 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 aldehydes with the same number of electrons in the molecule.
Solubility
Small aldehydes 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 aldehydes.
Identification
1. To test for a carbonyl group in a compound, add a solution of 2,4-dinitrophenylhydrazine (Brady’s reagent) to the suspected carbonylcompound:
· Aldehydes and ketones give yellow precipitates
· Aromatic aldehydes and ketones give orange precipitate
Aldehydes 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-dinitrophenylhydrazine
- Filter off theprecipitate
- Recrystallise the precipitate using the minimum amount of hot ethanol
- Dry 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
Aldehydes are readily oxidized. If the reaction is carried out in acid or neutral solution the product is a carboxylic acid:
RCHO + [O] ⇨ RCOOH
If the reaction is carried out in alkaline solution (Fehling’s solution or Tollens’ reagent) the product is the carboxylate anion:
RCHO + [O] +OH- ⇨ RCOO- + H2O
There are a number of suitable oxidizing agents. If these tests are carried out on a ketone, Fehling’s solution remains blue and Tollens’ reagent remains colourless:
· Tollen’s reagent: Reduced to give a silver mirror on warming with an aldehyde
· Fehling’s Solution: Reduced from a deep-blue solution to a red precipitate of copper (I) oxide, Cu2O, when warmed with an aldehyde.
· Acidified dichromate VIions: Orange potassium dichromate (VI) in acidic solution is reduced to green Cr (III) on heating with an aldehyde.
Reduction
Aldehydes 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. E.g. ethanal is reduced to ethanol:
CH3CHO+ 2[H] ⇨ CH3CH2OH
· If aldehydes are reduced primary alcohols are formed. If ketones are reduced secondary alcohols are formed.
Drafted by Eunice (Chemistry)
References:
https://www.sciencedirect.com/topics/neuroscience/aldehydes
https://chemistry.stackexchange.com/questions/18481/why-doesnt-the-bradys-test-2-4-dnph-work-on-carboxylic-acids