Organic Chemistry Flashcards
What is a nucleophile?
An electron rich species that has a pair of electrons in a high energy filled orbital (HOMO).
The nucleophile may have a negative charge.
What is an electrophile?
An electron poor species that either has an empty atomic orbital, molecular orbital or a low energy antibonding orbital (LUMO).
They may have a positive charge.
What does an SN2 reaction profile look like?
A single hump.
What does the transition state look like for an SN2 reaction?
There are partial bonds to the nucleophile and the leaving group.
The nucleophile and leaving group are at 180° to each other.
The reacting centre is flattened (sp²) which leads to inversion of stereochemistry.
What is the rate equation for an SN2 reaction?
Rate = k [Nu][Ep].
Which orbital does the nucleophile donate two electrons to in an SN2 reaction?
The empty sigma antibonding orbital of the electrophile.
What makes a good leaving group?
The more stable the leaving group, the better. A lower pKa value indicates anion stability and suggests a good leaving group.
What is order of leaving group suitability for the halogens?
I- is the most stable and F- is the least stable.
What makes a good nucleophile?
Generally a higher pKa indicates a better nucleophile.
What are the features of an SN1 reaction?
The leaving group leaves first which forms a carbocation. The nucleophile then adds onto the carbocation.
Stereochemistry is destroyed during this reaction which leads to the formation of a racemic mixture.
What does the reaction profile look like for an SN1 reaction?
Two humps.
The first hump is larger than the second hump.
What is the rate equation for an SN1 reaction?
Rate = k [Ep].
The rate only depends on the concentration of the electrophile.
How can the stability of a carbocation be increased?
The carbocation can be more substituted. This is because electron density is donated towards the carbocation which stabilises it. This is known as hyperconjugation.
Where are SN1 reactions favoured?
Where there is a tertiary electrophile (stabilised carbocation).
There is no reaction with primary electrophiles.
When are SN2 reactions favoured?
For primary and secondary electrophiles.
Not favoured for tertiary electrophiles due to steric block for the nucleophile approach.
What is the rate equation for an E1 reaction?
Rate = k [R₃LG]
The rate only depends on the substance that has the leaving group.
What is the reaction profile for an E1 reaction?
Two humped.
Is the E or Z isomer favoured during an E1 reaction?
The E isomer. This is because bulkier groups are further apart which minimises steric repulsion.
Is an internal alkene or terminal alkene more likely to form?
An internal alkene as it has a greater opportunity for hyperconjugation.
What is the rate equation for an E2 reaction?
Rate = k [RLG] [Base]
What does the reaction profile look like for an E2 reaction?
A single hump.
What is the conformation requirement for an E2 reaction?
The leaving group must be antiperiplanar to hydrogen in which the base reacts with.
Why is the antiperiplanar conformation required in an E2 reaction?
An E2 reaction requires alignment of:
- filled sigma orbital of C-H bond being broken by base.
- Empty sigma* orbital of C-LG bond.
This leads to the formation of a new pi bond.
How can the terminal alkene be favoured?
A bulky base can be used.
This is because the steric hindrance slows down the removal of the internal proton.
What is a carbonyl group?
A C=O group.
What are examples of carbonyl containing groups?
Aldehydes, ketones and amides.
What are the structural features of a carbonyl group?
It is planar.
The bond angles are 120°.
