Nucleophillic addition to carbonyls Flashcards
What is the LUMO?
Lowest unoccupied molecular orbital
What is the HOMO?
Highest occupied molecular orbital
What is the key difference between c=o and c=c?
C=O is polarised
Why is there polarity in C=O
The oxygen is more electronegative than carbon which means it has a delta negative charge, leaving carbon with a delta positive
Why is nucleophilic attack able to happen?
The carbon is delta positive
What is the oxidation level?
The number of bonds from carbon to atoms other than C or H
What is found at the CO2 oxidation level?
Carbon dioxide
Carbonate
Tetrachloromethane
What is found at the carboxylic acid level?
Carboxylic acids Ester Acid chloride Amide Acid Anhydride Orthoester
How many bonds to heteroatom does the CO2 oxidation level have?
4
How many bonds to heteroatom does the carboxylic acid level have?
3
What is found at the aldehyde oxidation level?
Aldehyde Ketone Imine Hydrate Hemiacetal Acetal Enol
How many bonds to heteroatom does the aldehyde oxidation level have?
2
What is found at the alcohol oxidation level?
Alcohol Ether Amine Alkyl chloride Alkene
How many bonds to heteroatoms does the alcohol oxidation level have?
1
What is a pi bond equivalent to?
one bond to a heteroatom
How many bonds to heteroatom does the alkane oxidation level have?
0
What must happen if you make a bond to carbon?
You have to break one
What are the two steps in the nucleophilic addition mechanism?
- Nucleophilic attack at the carbonyl carbon
2. Protonation of the oxygen anion forming an OH group
What are the two reasons for the carbonyl carbon being electrophillic ?
- Bond polarity
2. Orbitals
How do orbitals cause an electrophilic carbonyl carbon?
Electrons are transferred from the homo of the nucleophile into the lumo of the carbonyl compound
What two orbitals make up C=O?
C-O Pi bonding orbital
C-O Pi star anti bonding orbital
What are the characteristics of the C-O pi bonding orbital?
Smaller on carbon
Larger on oxygen
What are the characteristics of the C-O pi star anti bonding orbital?
Bigger on carbon
Smaller on oxygen
What type of reaction is cyanohydrin formation?
Equilibrium
Reversible
What is cyanohydrin formed from?
Ketone or aldehyde
In cyanohydrin formation, where does the equilibrium lie?
Towards cyanohydrin
Why does it turn into cyanohydrin more easily?
Steric effect
Larger R groups
Reduction in angle when converting sp2 to 2p3
What does the hydride ion (H-) act as?
Base
Due to high charge density
What is used as an alternative source of nucleophilic hydrogen ?
Sodium borohydride
What is NaBH4 able to reduce?
Aldehydes and ketones
What can NaBH4 not reduce?
Less reactive carbonyl compounds such as esters and amides
What s a more powerful hydride donor than NaBH4?
LiAlH4
What can’t you use LiAlH4 with and why?
Water or alcohols
It is much more reactive and causes a vigorous and violent reaction
What type of solvents is LiAlH4 used in? Examples?
Aprotic
Diethylether / THF
What type of bonds do organometallic compounds have?
Carbon - metal bond
What are organomagnesium compounds called?
Grignard reagents
What is formed when the organometallic reagents have reacted with the C=O?
Alcohol with the alkyl group found in the reagent
What are the two steps of reagents used for organometallic reactions?
- Organometallic reagent eg. MeLi, Et2O
2. H2O
When you add water to C=O of aldehydes and ketones, what is formed?
a 1,1-diol (hydrate)
What does the position of equilibrium depends on?
Nature of the R groups
What is the steric argument for the addition of water?
The larger the R groups, the less favourable the equilibrium
What is formed when alcohols are added to aldehydes and ketones?
Hemiacetals
Acetals
What is the role of the ethanol in the addition of water to aldehydes and ketones?
Shuttles the proton
How do strong acids and bases increase the rate of formation of hydrates and hemiacetals?
Allowing proton transfer to occur before addition to C=O
In acid catalysis of hydrate and hemiacetal formation, what catalyst can be used?
HCl
In acid, why is it good to protonate to carbon first before attack of C=O?
It becomes more delta positive and becomes more electrophilic
In base catalysis of hydrate and hemiacetal formation, what catalysis can be used/
NaOH
What happens in base catalysis of hemiacetal and hydrate formation?
The nucleophile is deprotonated first and this makes the alcohol more nucleophilic
Where is the lumo in the C=O?
Pi star
Where is the homo in the C=O?
Pi
What reagents/solvents are used to form an alcohol from a ketone/aldehyde?
- LiAlH4, THF
2. H2O
What solvents do you use Grignard reagents with?
THF
H2O
