17 - Organic Chemistry II Flashcards
What are stereoisomers?
Molecules with:
- same structural formula
- different spatial arrangement of atoms
When does E/Z (geometrical) isomerism arise?
- Alkenes
- restricted rotation about the C=C double bond.
- Two different groups attached to each of the carbons in the double bond.
What is the priority group in E/Z isomerism?
The atom/group with the higher atomic/molecular mass.
How do you name E/Z stereoisomers?
- E (opposite): if the priority groups are on opposite sides of the double bond.
Z (together): if the priority groups are on the same side of the double bond.
When does optical isomerism occur?
Occurs in carbon compounds that have 4 different groups attached to a carbon (asymmetric carbon).
What do you call a carbon atom with 4 different groups attached?
Chiral (asymmetric) carbon atom.
What are the characteristics of optical isomers?
- They have similar physical and chemical properties.
- they rotate plane polarised light in opposite directions.
- they are mirror images of each other.
- they are non-superimposable (don’t overlap).
How much does one optical isomer rotate plane polarised light by compared to the other?
the two optical isomers rotate plane polarised light by the same amount in opposite directions.
What do you call the optical isomer that rotates plane polarised light in the clockwise direction?
dextrorotatory isomer.
What do you call the optical isomer that rotates plane polarised light in the anti-clockwise direction?
laevorotatory isomer.
What is a racemic mixture / racemate?
- A mixture containing equal amounts of the two enantiomers / optical isomers.
What is an enantiomer?
Two compounds that are optical isomers of each other are called enantiomers.
Can racemic mixtures / racemates rotate plane-polarised light?
- No.
- the rotation caused by the dextrorotatory isomer is cancelled out by the rotation caused by the laevorotatory isomer.
Why does a racemic mixture form in the nucleophilic addition of HCN to aldehydes and unsymmetrical ketones?
- carbonyl is planar.
- CN nucleophile is equally likely to attack either side of the molecule.
- this results in different enantiomers being formed, and therefore a racemate forms.
- Since the product is a racemate, the product will not be optically active (cannot rotate plane-polarised light).
What happens in the SN1 mechanism?
- The halogen breaks away from the haloalkane to form a planar carbocation intermediate.
- The OH- ion can attack the planar carbocation intermediate from either side of the molecule resulting in different enantiomers.
- a racemate forms.
- since a racemate forms, the product is not optically active.
What happens in the SN2 mechanism?
No intermediates form, reaction occurs via a transition state.
- OH- ion attacks the chiral carbon atom, electrons are being transferred to the halogen, which is about to break off.
- In the transition state, the chiral carbon atom should have 5 bonds, 2 of which are partial bonds with OH and the halogen. The transition state has a negative charge.
- In the final product, the 3 groups that remained attached throughout the whole process are now inverted (like an opposite enantiomer).
- The product will rotate plane polarised light in the opposite direction to the reactant (since it is inverted).
How might a racemate form in the reaction of the electrophilic addition of HBr to an unsymmetrical alkene?
- There are two possible ways the H and Br ions can be added onto the alkene.
- One of the possibilities leads to the bromide ion attacking the planar carbocation from both sides, resulting in two enantiomers from forming.
- Therefore a racemate forms.
- The pathway where a racemate forms is the major product.
Are aldehydes and ketones soluble in water?
- The smaller carbonyls are soluble in water as they can form H bonds with water molecules.
- Larger carbonyls are not soluble in water because more H bonds are broken between water molecules (to fit the long alkyl chain of the carbonyl), than are made between the water molecules and carbonyl molecules.
What kinds of intermolecular forces are there in pure carbonyls?
- Pure carbonyls cannot hydrogen bond because the O atom is not bonded to a H atom.
- Instead, pure carbonyls by permanent dipole bonding.
Why can carbonyls undergo nucleophilic addition reactions?
- The C=O bond is polarised because O is more electronegative than carbon. The delta positive carbon atom attracts nucleophiles.
Oxidation of aldehydes
- potassium dichromate (VI) and dilute sulfuric acid.
- heat under reflux.
- carboxylic acid forms.
- observation: orange to green colour change.
Can ketones be oxidised?
No.
How else can aldehydes be oxidised?
- Fehling’s solution
- Tollen’s reagent
Aldehydes and Tollen’s reagent
Used to test for aldehyde functional group.
- Tollen’s reagent formed by mixing aqueous ammonia and silver nitrate.
- heat gently.
- observation: silver mirror forms.
Ketones will result in no change/reaction as they cannot be oxidised.
Aldehydes and Fehling’s solution
Used to test for aldehyde functional group.
- Fehling’s solution contains blue Cu2+ ions.
- heat gently.
- observation: red precipitate forms (of Cu2O copper (I) oxide).
Ketones will result in no change/reaction as they cannot be oxidised.
Reduction of aldehydes and ketones
The carbonyls will be reduced to alcohols.
- LiAlH4.
- Dry ether.
- Room temperature and pressure.
- When writing equations, reducing agent is represented as [H]. think of how many hydrogens need to be added on (2) to make the carbonyl into an alcohol, and this is how many moles of the reducing agent you need.
