Nucleophilic addition Flashcards
What is an imine?
A C=N bond - there can be primary and secondary imines based on the number of carbons bonded to the C=N
What is the polarity of an imine?
Imines are polar due to difference in electronegativity but they are less polar compared to a carbonyl group
What is oxidation?
Gain of C-O bonds
Loss of C-H bonds
Loss of electrons
What is a strong oxidising agent example?
H2CrO4 - chromic acid
What is a weak oxidising agent example?
PCC
What happens when you oxidise a primary alcohol with H2CrO4?
Forms a carboxylic acid
What happens when you oxidise a primary alcohol with PCC?
Forms an aldehyde
What happens when you oxidise a secondary alcohol with H2CrO4?
Forms a ketone - no further oxidisation
What happens when you oxidise a ketone using H2CrO4?
Nothing - ketones are not able to be oxidised
What happens when you oxidise a tertiary alcohol with H2CrO4?
Nothing - tertiary alcohols are unable to be oxidised
What is the rate determining step of a nucleophilic addition reaction?
The rate determining step of a nucleophilic addition reaction includes both the nucleophile and the carbonyl compound (polar compound)
What happens to the rate of nucleophilic addition as the carbonyl compound becomes more substituted?
Substituted groups of a carbonyl compound donate a small amount of negative charge towards the carbon of the C=O bond reducing its partial positive charge and hence slows down the rate of nucleophilic attack
Also, more substitution and the size of the substituted groups means there is more steric interference as the nucleophile tries to attack the partial positive carbon thus also slowing down the rate of reaction
What is the reagent for a nucleophilic addition reaction?
Any strong nucleophile (e.g. H-) for the attack on the carbon :
NaBH4 (weaker source of H-)
LiAlH4 (stronger source of H-)
Grignard reagent (RMgX)
Then an acid H30+ for the movement of lone pair of electrons from O- to H+
If the nucleophile and acid are added at the same time they will react together
What is grignard addition?
Use of grignard reagents to allow a wider range of R groups to be added to a nucleophile
How are grignard reagents made?
By adding Mg, ether to an alkyl halide
RX -> RMgX
Because the magnesium has a much lower electronegativity compared to the carbon the carbon gains a very strong partial negative charge and therefore can be considered a carbanion
Can CO2 undergo nucelophilic addition?
Yes, addition of a grignard reagent to CO2 will result in nucleophilic addition on a single C=O bond
Can alcohols act as the nucleophile for a nucleophilic addition reaction?
Yes, however as they are weak nucleophiles the electrophile (C=O group) must be activated by addition of H+
What is an acetal?
Functional group in which there are two ether groups bonded to the same carbon
How do you form an acetal?
Add two molar equivalents of alcohol with acid to a ketone
A hemiacetal forms first and then further nucleophilic substitution occurs to form the acetal
How do you form an imine?
Add an amine nucleophile to a ketone to form a hemiaminal C(OH)(NHR)R1R2 then remove a water molecule from this compound via elimination
Can imines and nitriles undergo nucleophilic addition?
Yes, they can undergo nucleophilic addition with strong nucleophiles and also weaker oxygen nucleophiles such as water
How do you form a ketone from a primary imine?
Add water with acid in a nucleophilic addition reaction to form hemiaminal. Then add water with acid again to form ketone from elimination reaction
How do you form a carboxylic acid from a nitrile?
Add acid with water to form an alcohol substituted imine and amide in equilibrium - then add water with acid to substitute NH2 group to form carboxylic acid
What effect does nucleophilic addition have on stereochemistry?
If the product is chiral - it will be a racemic mixture as the ketone is a sp2 hybridised molecule
Name the reagents for aldehyde/ketone to alcohol
- NaBH4 or LiAlH4
2. H3O+
Name the reagents for aldehyde/ketone to alcohol with R group substitutent
- RMgX (grignard reagent)
2. H3O+
Name the reagents for aldehyde/ketone to an yn-ol
- RCtriplebondC(-)
2. H3O+
Name the reagents for aldehyde/ketone to cyanohydrin
- NaCN
2. H3O+
Name the reagents for aldehyde/ketone to hemiacetal
ROH / H+
Name the reagents for hemiacetal to acetal
ROH / H+ (substitution)
Name the reagents for aldehyde/ketone to imine
NH2R (no need for acid as amine strong enough nucleophile)
Addition will occur first forming a hemiaminal then elimination to form imine
Name the regaents for imine to amine
- LiAlH4
2. H3O+
How do you form a grignard reagent?
React an alkyl halide with Mg to form R-MgX
