3.3.7 & 3.3.8 - Aldehydes, Ketones and Optical Isomerism - Chemistry Alevel
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33 Terms
Empirical formula - definition
The simplest whole number ratio of atoms of each element in a compound
Molecular formula - definition
The actual number of atoms of each element in a compound.
General formula - definition
The simplest algebraic formula of a member of a homologous series.
Structural formula - definition
The arrangement of atoms and, where needed, the bonds between them.
Displayed formula - definition
The arrangement of all atoms in a molecule and all of the bonds between them.
Skeletal formula - definition
The chain with all the hydrogen atoms removed and just a carbon skeleton with functional groups attached.
Characteristics of a homologous series
Similar chemical properties/gradual change in physical properties/general formula/same functional group.
Structural isomer - definition
Same molecular formula but different structural formula.
Stereoisomerism - definition
Same structural formula but bonds occupy different positions in space.
Why does E-Z isomerism occur?
Two carbons with a double bond with restricted rotation each bonded to two different substituents.
E means
Highest priority on different sides.
Z means
Highest priority on same side.
Aldehydes - oxidised
Aldehydes are readily oxidised to carboxylic acid.
Fehling's solution - Procedure
Warm the unknown compound with Fehling's solution.
Fehling's solution - aldehyde
A brick red precipitate forms.
Fehling's solution - ketone
Solution remains blue.
Fehling's solution - Background
Fehling's solution contains Cu²⁺ giving it a blue colour/the Cu²⁺ is reduced to form copper(I) oxide (a brick red precipitate)/the aldehyde is oxidised to form a carboxylic acid.
Tollens' reagent - Procedure
Add Tollens' reagent to the unknown compound and then warm it.
Tollens' reagent - aldehyde
Silver mirror forms.
Tollens' reagent - ketone
Solution remains colourless
Tollens' reagent - Background
Tollens' reagent contains [Ag(NH₃)₂]⁺. It is reduced to form a silver mirror of metallic silver/the aldehyde is oxidised by Tollens' reagent to form a carboxylic acid.
HCN reaction - use
It is useful synthetically to increase the length of the carbon change.
HCN - dangers
It is a gas at room temperature which, if inhaled, will lead to poisoning of the central nervous system (deadly).
HCN used - what does it contain?
Some dissolved HCN and some dissolved CN⁻ ions so that the reaction is slower but more safe.
Optical isomerism - definition
Two non-superimposable mirror images owing to four different substituents bonded to one carbon.
Why can optical isomerism occur?
As a result of chirality in molecules, limited to molecules with a single chiral centre.
Chiral - definition
A carbon atom bonded to four different groups is asymmetric or chiral.
Enantiomers - definition
Two molecules which are optical isomers of each other.
Racemic mixture/Racemate - definition
A mixture of equal amounts of enantiomers (two isomers).
Optical isomers - how to identify
Optical isomers have identical chemical and physical properties but cause plane polarised light to rotate in different directions.
Racemic mixtures - polarised light effect
They are optically inactive as the enantiomers cancel out the effect of each other.
How is a racemic mixture formed?
There is equal chance of attack from above and below of the planar carbonyl group.
Optical isomerism - drug industry problems
One may be an effective drug but the other could be toxic/separation of enantiomers is difficult and expensive as they have similar properties/selling the mixture is wasteful as half is inactive/you have to design an alternative synthesis that makes only the required isomer.