Lecture 9 and 11. Basic Mechanisms and Stereochemistry Flashcards
What is a saturated carbon ?
Has bonds to four different atoms
What happens in a nucleophilic substitution reaction ?
One of the atoms or groups bound to a carbon is replaced by another
What are the two types of mechanisms for nucleophilic substitution reactions ?
- Stepwise
2. Simultaneous
What does Sn1 mechanism mean ?
Stepwise reaction
What are the kinetics of a stepwise mechanism ?
Unimolecular
What is meant by unimolecular ?
An elementary reaction in which the rearrangement of a single molecule produces one or more molecules of product
What does the stepwise mechanism rate depend on ?
The electrophile only
What is the rate equation for a stepwise mechanism ?
r = k[R - X]
What type of reaction is the stepwise mechanism ?
1st order reaction
What is a first order reaction ?
Concentration depends on only one reactant
Would you describe a stepwise mechanism a nucleophilic or electrophilic reaction ?
Nucleophilic
What is the rate limiting step of a stepwise mechanism ?
Slow, reversible dissociation of leaving group to form carbocation
What has no effect on the rate of the stepwise process ?
Adding more nucleophiles
In a stepwise mechanism, what is the formation of the cation followed by ?
Relatively fast attack by nucleophile
How can we show the Gibbs free energy change over the course of a reaction ?
A reaction coodinate diagram
What is a transition state ?
They are not stable species and cannot be isolated
What type of kinetics are simultaneous ?
Bimolecular
What is a bimolecular kinetic ?
The chemical combination of two molecular entities in a reaction that is either reversible or irreversible
What is the rate equation for a simultaneous mechanism ?
r = k[R - X][Nu]
What are simultaneous mechanisms rates dependent on ?
Electrophile and nucleophiles
What type of reactions are simultaneous mechanisms ?
2nd order
What features in the rate limiting step of the simultaneous mechanism ?
Nucleophiles and electrophiles
What does not feature in the simultaneous mechanism ?
Intermediate species
What angle does the nucleophile attack at in a simultaneous reaction ?
180
What do stepwise reactions proceed via ?
Carbocation intermediates
What will speed up a stepwise mechanism ?
Structural features which stabilise carbocations
What do factors that stabilise a chemical species also stabilise ?
The transition states that lead to and from the stabilisation of the chemical species
What does the stabilisation of the cation also lower ?
The activation energy
What are tertiary carbocations well - stabilised by ?
Hyperconjugation
What is hyperconjugation ?
The donation of electron density from aligned C-H or C-C bonds
When does a stepwise mechanism become more favorable ?
More groups binding to the reacting carbon
What is a simultaneous reaction sensitive to ?
Steric hindrance
What is steric hindrance ?
Other atoms getting in the way
What does the increasing steric hindrance block in a simultaneous reaction ?
The approach of the nucleophile at 180 degrees
What is the only mechanism for tertiary centres ?
Stepwise mechanism
What is the only mechanism for substitution at methyl and primary centres ?
Simultaneous mechanisms
What can secondary electrophiles react by ?
Either mechanism
What is the rate of reaction sensitive to ?
The reactivity of the nucleophile
What are better leaving groups ?
Those with lower pKas
How would you convert HO- to a better leaving group ?
Protonation with strong acid
When does nucleophilicity increase ?
As the attacking atom comes from further down the periodic table
What do wedged bonds show ?
Substituents that stick out from the paper
What do hashed bonds show ?
If they stick into the paper
What is a chiral molecule ?
If it cannot be superimposed on its mirror image
What is meant by achiral ?
Not chiral
What happens in a simultaneous reaction, when the nucleophile approaches at 180 degrees to the leaving group ?
The stereochemistry of the reaction centre is inverted
What happens in a stepwise mechanism, when the planar carbocation intermediate is attacked from either side ?
A 50:50 mixture of both enantiomers (a racemic mixture)
What is a enantiomer ?
Mirror image form
What is an example of a natural enantiomer ?
Alanine
Are enantiomers any different from there original form ?
No
How can enantiomer’s be differentiated ?
By their ability to rotate plane polarised light
How can enantiomer’s be differentiated ?
By their ability to rotate plane polarised light
How can enantiomer’s be differentiated ?
By their ability to rotate plane polarised light
What is optical rotation ?
The ability to rotate plane polarised light
What is meant by dextrorotatory ?
If a chiral compound rotates plane polarised light to the right
What is meant by laevorotatory ?
If a chiral compound rotates plane polarised light to the left
What are the rules in Cahn-Ingold-Prelog ?
- Assign a priority to each substituent at the chiral centre using atomic number
- Orient the molecules so that the lowest priority group is at the back
- Are the substituents linked by clockwise or anticlockwise rotation
What is meant by clockwise ?
R - enantiomer (rectus)
What is meant by anticlockwise ?
S- enantiomer - sinister
What can stabilise the carbocation using the alignment of C-H or C-C on adjacent carbons ?
Hyperconjugation
What can C=C double bonds be stabilised by ?
Conjugation
What is delocalisation ?
Spreading out of charge which is a stabilising effect
What is a consequence of pi bond stabilisation of carbocations ?
Carbocations from two isomeric species can be the same
What are electrons in pi bonds higher in energy than ?
Electrons in sigma bonds
What are electrons in pi bonds higher in energy than ?
Electrons in sigma bonds
As electrons in pi bonds are higher in energy than those in sigma bonds, what is the result of this ?
They are more reactive
What do alkenes react with ?
Electrophilic species
What is regioselectivity ?
Preference of chemical bonding or breaking in one direction over all possible other directions
What does the electrophilic addition of HBr start with ?
The protonation of the alkene resulting in a carbocation intermediate
Where can protonation occur on an alkene ?
Either fae
Where can nucleophilic attack on a carbocation occur ?
On either face
How can we define the prochiral faces of the alkene or carbocation ?
Cahn-Ingold-Prelog
Does adding more hydroxide to a stepwise mechanism speed it up ?
No but it can cause an elimination reaction to begin
What is an E2 reaction ?
An elimination, simultaneous reaction
What isn’t hydroxide strong enough to do ?
To deprotonate a C-H to form a carbanion
Why does an E2 reaction proceed in the presence of hydroxide ?
Because there is a leaving group able to accept the electrons, allowing a double bond to form
What is an E1 reaction ?
An elimination reaction proceeding via a carbocation intermediate - stepwise
Why doesn’t a Sn1 reaction in the presence of H2SO4 ?
Its conjugate base is a terrible nucleophile
What do strong bases favour ?
Deprotonation, making elimination more likely
At nucleophilic dependent elimination reactions, where does it preferentially attack ?
At the Hydrogen base
At nucleophilic dependent substitution reactions, where does it preferentially attack ?
At the carbon nucleophile
What does neutral ethanol attack preferentially by ?
Sn1
What does charged ethoxide attack preferentially by ?
E2
What else can effect whether a reaction is substitution or elimination ?
The size of the nucleophile/base
What does nucleophilic substitution require ?
The nucleophile to push towards a four coordinate carbon atom - more crowded than deprotonation
What is a trans molecule ?
On opposite sides
What is a cis molecule ?
On same sides
What letter do we use for trans molecules ?
E - entegegen
What letter do we use for cis molecules ?
Z- zusammen
What is the Re face ?
Clockwise
What is the Si face ?
Anticlockwise
