Lecture 11

Question 1

Substitution reactions

Answer 1

• It can be nucleophilic or electrophilic substitution.
• In a nucleophilic substitution, a nucleophile replaces a leaving group from a carbon atom, using its lone pair of electrons to form a new bond to the carbon atom.
• In a electrophilic substitution, an electrophile (generally a hydrogen atom) replaces a leaving group from the carbon atom. Used mostly in aromatic compound substitution reactions

Nucleophilic Substitution Mechanisms
Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). An sp3-hybridized electrophile must have a leaving group (X) in order for the reaction to take place

Question 2

SN1 + SN2

Answer 2

Nucleophilic substitution at Saturated Carbon
SN1 mechanism: the leaving group goes first and the nucleophile comes in later
SN2 mechanism: the two events happen at the same time

The second mechanism shows that the only way the carbon can accept electrons is if it loses electrons at the same time.
SN1 and SN2 reactions typically involve alkyl halides or alcohol

Question 3

Nomenclature

Answer 3

In a SN1 reaction the leaving group leaves the substrate, and then the nucleophile forms a bond.
Substitution, nucleophilic, 1 molecule in the rate determining step.

In a SN2 reaction the nucleophile forms a band to the substrate at the same time the leaving group leave.
Substitution, nucleophilic, 2 molecules in the rate determining step.

The evidence that convinced chemist about these two mechanisms (SN1 & SN2) is kinetics.

Question 4

SN1 reaction mechanism

Answer 4

The SN1 reaction proceeds stepwise. The leaving group first leaves, a carbocation forms that is attacked by the nucleophile.

The rate is dependent on the formation of the carbocation

Carbocations: cations where the positive charge is on the carbon atom

The formation of the carbocation is the rate-determining

Question 5

Standard Nucleophilic Substitution

Answer 5

SN1 reaction sequence
In an SN1 reaction, a planar carbocation species is formed first, which then reacts further with the nucleophile who is free to attack from either side.

Carbocation formation is significantly slower than nucleophilic substitution

Rate dependent on slow
carbocation formation – Rate
determining step

Question 6

SN1 reactions

Answer 6

 Rate determining step is how fast the
carbocation can be formed
 First Order Rate Kinetics
 Substitution at more than one position
possible

Question 7

SN2 reaction mechanism

Answer 7

The SN2 reaction is concerted. That means an SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step.

Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile