Unit 4- Further organic chemistry Flashcards
What are the two forms of stereoisomerism?
geometric and optical isomers
what are geometric isomers?
isomers which have the same gruop of atoms but in a different spatial arrangement
How does geometric isomer occur?
due to the lack of rotation by the carbon carbon double bond
What is chirality?
when two objects are non superimposable and are mirror images of eachother, they are chiral. This feature is exclusive to asymmetrical objects
How is a molecule chiral?
if there are four different groups attached to a central carbon where the molecules are non superimposable and are mirror images
If the isomers are mirror images of eachother, what are they reffered to as?
enantiomers
How to tell between two enantiomers?
different effect on plane of polarised light where one enantiomer rotates the plane of light clockwise while the other anticlockwise
What type of wave is the plane of polarised light?
transverse
When an enantiomer is placed between two polaroids, what happens?
the second polaroid must be rotated to allow the light to pass through, this is the angle of rotation
What mixtures have no effect on the plane polarised light?
racemic mixtures
why do racemic mixtures have no effect on plane polarised light?
they are formed in a 1:1 ratio therefore the clockwise rotation is cancelled by the anticlockwise rotation
What is the functional group of an aldehyde?
a bonded hydrogen atom and a double bonded oxygen atom to a carbon atom
H-C=O
What is the functional group of a ketone?
A double bonded oxygen atom to a carbon
-C=O
what is the C=O functional group referred to as?
a carbonyl group
what are the properties of the carbonyl group?
- greater boiling and melting temperature in comparison to alkanes but lower than alcohol
- straight chain isomers are more packed therefore stronger than branched chain
- aldehydes are slightly more volatile than ketones
- aldehydes and ketones are miscible with water but this decreases as chain length increases
Why are aldehydes and ketones soluble in water?
Due to the ability to hydrogen bond with water
What are the effects of a carbonyl group?
- pi and sigma bonds present
- the bond is permanently polarised
- cannot hydrogen bond between themselves
- permament dipole-dipole interactions
Describe the oxidation reaction of aldehydes and ketones
- aldehydes oxidise due to the hydrogen atom
- oxidises to carboxylic acid
- warming with potassium dichromate (VI) acidified with sulfuric acid
- ketone does not readily oxidise
Describe the reaction with fehlings and benedicts
- If an aldehyde is warmed, it is oxidised to a carboxylic acid
- This reduces the fehlings giving a red-brown precipirate
- precipitate is of copper(I) oxide
- ketone does not react
How is tollens reagent formed?
- produced by dissolving nitrate in water and adding aqeuous ammonia
- silver (I) oxide is formed but dissolves as more ammonia is added
- forms the complex diammine silver (I) ion
Describe the reaction with tollens reagent
- Tollen is warmed gently with aldehyde
- the aldehyde is oxidised
- silver (I) ions are reduced forming silver metal
- produces a silver mirror
- ketone does not react
What are the oxidation reactions of aldehydes and ketones?
- oxidation by potassium dichromate (VI)
- With fehlings and benedicts
- With tollens reagent
What is the reduction reaction of aldehydes and ketones?
- aldehydes can be redcued to primary alcohols
- ketones can be reduced to secondary alcohols
- Lithium tetrahydridoalumate (LiAlH4) dissolved in dry ether can be used
- sodium tetrahydridoborate (NaBH4) dissolved in ethanol can be used
What is the addition reaction of aldehydes and ketones?
- addition reaction with hydrogen cyanide (HCN)
- produces hydroxynitriles
- occurs in presence of cold alkali and potassium cyanide
- the equilibrium K+ + CN- + H2O à K+ + HCN + OH- is formed
