Mechanisms and Processes

Question 1

Mechanism

Answer 1

• a graphical depiction using arrows to describe how a chemical reaction occurs, the directionality and flow of electron density between atoms directly involved in the reaction and all relevant intermediates and non-zero formal charges

Question 2

Nucleophiles vs electrophiles

Answer 2

Nucleophiles:

• atom or species that donates a share in electron density
• they share electron density with electron deficient species (electrophiles)
• lone pairs or pi electrons can be used nucleophilically
• they can be neutral or negative

Electrophiles:

• an atom or species that accepts a share in electron density
• electrophiles accept electron density from nucelophilic species
• carbocations, atoms with empty p orbs, and atoms with partial positive charges can be electrophilic

Question 3

Heterolytic bond cleavage

Answer 3

• the unequal breaking of a sigma bond between two atoms
• electron density that is covalently shared between two atoms is accommodated by one atom while the other atom loses a share in electron density

Question 4

Hetero bond formation

Answer 4

• the formation of a sigma bond between two atoms involving only one of the atoms contributing two electrons into the covalent bond
• two electrons are shared from one atom to another to form a covalent interaction

Question 5

Hetero bond formation

Answer 5

• the formation of a sigma bond between two atoms involving only one of the atoms contributing two electrons into the covalent bond
• two electrons are shared from one atom to another to form a covalent interaction

Question 6

Homolytic bond cleavage

Answer 6

• the equal breaking of a sigma bond between two atoms results in a pair of radical species
• one electron goes with each atom that was formally bonded (single barbed arrows are used)
• this is common in reactions involving radical species

Question 7

Homo bond formation

Answer 7

• the formation of a new sigma bond involving two radical species each sharing an electron to covalently bond
• an electron is contributed by each radical species towards the new sigma bond
• single barbed arrows should point at each other

Question 8

Proton transfer

Answer 8

• the transfer of a proton (H+) from an acidic species to a species that is acting as a base, creating a conjugate acid/base species
• density is shared from the base to the acidic proton, forming a covalent bond and the acidic hydrogen bond is broken and the 2 electrons that were covalently shared are left on the acidic atom

Question 9

electrophilic addition

Answer 9

• the sharing of pi-electrons with an electrophile to make a new sigma bond
• the pi electron density is shared nucleophilically from the pi bond to the electrophilic species causing the pi bond to break and adding of the electrophilic species into the structure

Question 10

Nucleophilic addition

Answer 10

• a nucleophile attacks and adds into a structure, forming a new sigma bond
• 2 electrons are shared by a nucleophile to an electrophilic atom, resulting in the addition of the nucleophilic species into the compound
• pi systems are often broken (as examples)

Question 11

nucleophilic elimination

Answer 11

• electrons in a tetrahedral structure are collapsed to form a pi bond, simultaneously expelling a leaving group
• the atom collapsing its electrons can be neutral or formal negative
• the propensity of the tetrahedral to collapse depends on leaving group availability

Question 12

Sn2

Answer 12

• concerted, simultaneous substitution of one atom/group (no intermeiiate)
• Nu attacks the electrophile and adds density to the sigma * orb and the bonding interaction between the carbon and the leaving group breaks
• all bond breaking and making occurs at the same time

Question 13

Sn1

Answer 13

• substitution of one atom/group for another proceeding through a carbocat intermediate
• LG departs, carbocat is formed (heterolytic bond cleavafe), then NU will add to carbocat (hetero bond formation) and if the Nu is neutral then there is a 3rd deprotonation step

Question 14

E2

Answer 14

• concerted, simultaneous beta elimination of a H to form a pi bond with the expulsion of a LG
• a base removes a proton and the electrons from the C-H bond are used to form a new pi bond between the carbon and the LG
• the LG and beta hydrogen must have an anti relationship

Question 15

electrophilic elimination

Answer 15

• a species acts as a base to remove a hydrogen beta to a carbocat and then form a pi bond
• base remvoes the B-H, pi bond forms between C bearing the H and the adjacent carbocat
• the base can be neutral or formal neg