Elimination reactions Flashcards
What is an elimination reaction?
the loss of a leaving group and the formation of an alkene
What does the nucleophile act as in an elimination reaction?
base
How many molecules are involved in the rate determining step in E1?
1
Why is HSO4- a poor nucleophile?
It is a very weak base
It is stable as the negative charge is spread across the 3 atoms
Which step is the rate determining step in E1?
The slow step
the kicking out of the leaving group
How is the carbenium ion stabilised in E1?
Sigma conjugation
electron density is donated from the C-H bond which breaks to the carbenium ion
Why is a tertiary carbenium ion the most stable?
There are 3 lots of sigma conjugation which means it is more stable
What is an intramolecular electron source?
The C-H bond which is adjacent to the carbenium io
Why does an intramolecular substitution just happen (E1) ?
It wants to lower the energy of the system as it has a positive charge
How many molecules are involved in the rate determining step for E2?
2
Why is the nucleophile acting as a base?
It is attacking the hydrogen and not the carbon
What are the steps of an E1 reaction?
Leaving group is protonated to make it a better leaving group
L.G kicked out leaving a carbenium ion
Nucleophile attacks the carbenium ion
For E1 reactions, what is the order of reactivity related to?
Stability of the intermediate carbenium ion
The more stable, the better the E1 will be
When is elimination not possible?
When there are no hydrogens β-to the leaving group
What type of solvents favour E1 reactions and why?
Polar
They can stabilise the charged, ionic intermediate
What is the issue with using H2O as a solvent?
It is too polar, organic molecules don’t dissolve
What are some examples are polar solvents?
ROH
DMSO
DMF
What are some examples of mid-polar solvents?
THF
Et2O
DCM
EtOAc
What are some examples of non-polar solvents?
Hexane
Petrol
Toulene
Benzene
How does the leaving group influence the rate of reaction?
It is involved in the rate determining step so the better the leaving group the faster the reaction
Why is the E1 reaction for primary or secondary alcohols slow?
The secondary carbenium ion is not stable
Why would an E2 reaction for primary or secondary alcohols not happen?
A strong base would deprotonate the OH rather than the hydrogen adjacent to the OH group
The stronger the base…
The more likely it is to take only the most acidic hydrogen
What is alcohol activation?
Converting OH into a better leaving group which is usually done through the formation of a tosylate
What is stereoselectivity?
Gives one stereoisomer of the product in preference
Which elimination reaction can be stereoselective?
E1
Which isomer, E or Z, will be more stable and preferred and why?
E
Steric interactions, more steric repulsions in a z isomer as the large groups are close together
Which isomer, E or Z, forms fastest and why?
E
Lower activation energy
What will the major product be of an E1 reaction?
The more substituted alkene as it is more stable
Why are more substituted alkenes more stable
There is increased σ-conjugation which involves σ* orbitals
The more σ* orbitals = more spread out and therefore more stable
What is regioselectivity?
Preference of a chemical reaction to form one positional isomer over another
What do E2 reactions rely on?
100% orbital reliance
Where must the two groups being eliminated in an E2 reaction lie?
In the same plane
What are the two conformations which may be produced from an E2 reaction?
Synperiplanar
Antiperiplanar
Which is the preferred product for E2, synperiplanar or antiperiplanar?
Antiperiplanar
Why is aniperiplanar preferred over synperiplanar?
The orbitals are more in plane so they have better overlap and are closer to one another as the large lobe overlaps with a large lobe
In a newman projection, where do the dashes lie?
Point into page
Left hand side
In a newman projection, where do the wedges lie?
Point out of the page
Right hand side
Which conformation, synperiplanar or antiperiplanar, is lower in energy? Why?
Antiperiplanar
The transition state is a staggered conformation which is preferred
Less steric hinderance = more stable
What is the π bond of the alkene formed from?
σ-orbital of the C-H bond and the σ*-orbital of the C-X bond
What happens during the transition state?
Electrons move from the filled σ-orbital to the empty σ*-orbital
What types of reactions can the E2 reactions be when there is more than one hydrogen adjacent to the leaving group?
Regioselective
Stereoselective
What is the regioselectivity due to in E2 reactions?
Formation of the more stable alkene through the lower transition state
What is the stereoselectivity due to in E2 reactions?
Elimination occuring from the lower energy of the two possible antiperiplanar conformations
In E2 reactions, what type of interactions destabilise the Z isomer?
Gauche interactions
What are gauche interactions?
Steric interactions
More hindered
When is an E2 reaction stereospecific?
When both groups being eliminated are attached to stereocentres
In E2 reactions, what does each diastereoisomer of the starting material give?
A different diastereoisomer of the alkene product
What is the origin of stereospecificity?
In both cases, the E2 reaction must occur through the antiperiplanar transition state
What are diastereomers?
Stereoisomers that are not mirror images
How do you work out the amount of diastereomers?
2ⁿ
n is the number of chiral centres
Explain the regiochemistry of E2?
Transition state and the base reacting with the hydrogen are the two factors
What happens in E2 reaction if the base is large? (regiochemistry)
The product will arise from reacting at the least hindered hydrogen on a carbon adjacent to the leaving group
What happens in E2 reaction if the base is small? (regiochemistry)
The product will be the most stable alkene
The most stable alkene is the most substituted and usually arises from removal of a more hindered hydrogen adjacent to the leaving group
