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°.
What is carbonyl reactivity dictated by?
The presence of the electronegative oxygen atom forming a dipole.
How does a nucleophile add to a carbonyl group?
It follows the Burgi-Dunitz trajectory.
The nucleophile approaches at an angle of 107°.
The approach is dictated by the LUMO and repulsion from the attached carbonyl substituents.
How is a Grignard reagent formed?
From the reaction between magnesium and alkyl bromides/iodides.
What is a Grignard reagent?
A reagent (Me-MgBr) that is reactive due to the electropositive magnesium atom. The attack comes from the Me-Mg bond.
What is the structure of NaBH₄?
[H3B-H]⁻ Na⁺
The attack comes from the B-H bond as B is electropositive.
What factor affects the reactivity of carbonyl groups?
The atom(s) attached to the carbon atom.
If the atom is electron withdrawing, the LUMO energy is lowered so it is closer to the HOMO which results in a faster reaction.
If the atom is electron donating, the LUMO energy is raised so it is further away from the HOMO. This results in a slower reaction.
Why is lithium aluminium hydride a better reducing agent than sodium borohydride?
There is a greater difference in electronegativity between Al-H than B-H.
What does sodium borohydride reduce?
Carbonyls more or equal in reactivity than ketones.
Oxonium, Acyl chlorides, aldehydes, ketones.
What can lithium aluminium hydride reduce?
All carbonyls.
Why do aldehydes react faster than ketones?
In ketones, the two alkyl groups hinder the approach of the nucleophile. They also make the dipole weaker as they have an inductive effect.
How is an amide formed?
By reacting an amine and an acid chloride.
Why doesn’t reacting an amine with a carboxylic acid produce an amide?
NH3+ is a poor nucleophile and COO- is a poor electrophile.
A salt is produced instead.
What is a lactone?
A cyclic ester.
What is a lactone reduced to?
An open chain diol.
What is a lactam?
A cyclic amide.
What are lactams reduced to?
Cyclic amines.
What is a proton transfer?
The transferring of protons between the organic molecule and water (or hydronium ions).
They are rapid and reversible in polar solvents.
When is a hemiacetal formed?
When a ketone reacts with an alcohol in the absence of acid.
When is an acetal formed?
When a ketone reacts with an alcohol in the presence of acid.
When is an imine formed?
When a ketone reacts with a primary amine.
When is an enamine formed?
When a ketone reacts with a secondary amine.
What is tautomerisation?
An intramolecular proton transfer.
What is the catalyst for keto-enol tautomerism?
Acid or base.
What is an enolate?
The conjugate base of an enol.
What is an enol?
A compound that contains an OH group and an adjacent C=C.
Why are 1,3-dicarbonyls more acidic than standard carbonyls?
There is extra stabilisation due to resonance with adjacent carbonyl.
Why are enamines used to make enolates?
Aldehydes are too reactive to use.
What are some trends for acidity for inorganic compounds?
The further down the periodic table, the greater the acidity.
Oxygen acids are stronger than nitrogen acids.
What is the pKa value of methane?
48
What is the pKa value of ammonia?
33
What is the pKa value of water?
16
What is the pKa value of HF?
3
What happens to acidity going from HF to HI?
Acidity increases despite the decrease in electronegativity.
This is due to the weaker bond strength and the greater ion size.
What happens to acidity going from HClO to HClO₄?
Acidity greatly increases.
This is due to the negative charge being delocalised across the oxygen atoms.
What is the pKa for the deprotonation of phenol?
10
What is the pKa for the deprotonation cyclohexanol?
16
What is the pKa for the protonation of alcohols?
0
What is the pKa for the deprotonation of carboxylic acids?
5
What is the pKa for the deprotonation of alcohols?
15
How are Grignard reagents made?
By reacting magnesium turnings with alkyl halides in ether solvents.
What is the pKa of a ketone?
20
What is the pKa of an ester?
25
