Organic Reactions and Mechansims

Question 1

DKS mechanistic priority rules

Answer 1

1. Electron flow must start from an electron rich site
2. Electron flow must move towards an electron poor site
3. Electron flow must end up somewhere good
4. Charge is conserved
5. This may not be the end of the process

Question 2

Electron rich sites

Answer 2

1. Negative charge (Cl⁻, OH⁻)
2. Lone pair (R-NH₂, H₂O)
3. Double bond (alkene, aromatic ring
4. singe bond, especially if weak (O-O)

Question 3

Electron poor sites

Answer 3

1. Carbocation or H⁺
2. Atom adjacent to positive heteroatom (positive hetero atoms cannot be attacked directly - alrady have 8e-)
3. Onwards towards a positive charge (e.g. if carbocation cannot be attacked directly, hetero atom attacked and electrons from bond pushed onto carbocation)
4. Partially positive charge
5. Partially positive charge at an alternating distance (may be dictated by different reagents)

Question 4

‘Good’ areas for electron flow to end up

Answer 4

Electronegative atoms

Question 5

Nucleophile

Answer 5

Electron rich species that will react with C⁺ or Cδ⁺

Question 6

Base

Answer 6

Electron rich species that will react with H⁺ or Hδ⁺

Question 7

Electrophile

Answer 7

Electron poor atom that will react with a nucleophile.
Often C⁺ or Cδ⁺.

Question 8

Acid

Answer 8

Electron poor species that will react with a base.
Often H⁺ or Hδ⁺.

Question 9

Chemoselectivity

Answer 9

When several different sites could react, one does so in preference

Question 10

How does electronegativity affect nucleophile reactivity?

Answer 10

Less electronegative is better - the less electronegative the atom, the more likely it is to donate its electrons in nucleophilic attack

Question 11

How do mesomeric effects affect nucleophile reactivity?

Answer 11

More resonance is worse - electrons that are shared in resonance conjugation are not so easily donated during nucleophilic attack

Question 12

How do sterics affect nucleophilic reactivity?

Answer 12

Steric hindrance reduces nucleophile reactivity as is prevents donation of electrons

Question 13

Why are bases less suscpetible to steric hindrance than nucleophiles?

Answer 13

Bases attack exposed hydrogens, rather than carbon atoms (which are more crowded).

Question 14

Impact of inductive effects on electrophile reactivity

Answer 14

+I groups push electron density onto the electrophile (C+ or Cδ+) , reducing reactivity.
-I groups withdraw electron density from electrophile, increasing reactivity
Remember, where mesomeric effects are also present, these will take dominance

Question 15

Impact of mesomeric effects on eletrophile reactivity

Answer 15

+M effect pushes electron density onto electrophile - reduces reactivity (positive charge weakened).
-M effect withdraws electron density from electrophile - increases reactivity (positive charge strengthened).
Remember, resonance conjugation from less electronegative atom (e.g. N) has greater +M effect (less electronegative atom shares electrons in resonance more readily)

Question 16

Regioselectivity

Answer 16

When two different functional groups could react, but one does so in preference to the other.
When the reaction could produce different possible intermediates, the reaction pathway with the most energetically accessible intermediates will be preferred.

Question 17

How to determine the most acidic site on a molecule.

Answer 17

The most acidic site generates the most stable conjugate base.
- Negative charges on electronegative atoms
- Product stabilised by resonance where possible

Question 18

How to determine the most basic site on a molecule.

Answer 18

Most basic site has the greatest electron density and is most able to donate the electron density.
- lone pairs / negative charges localised (no resonance)
- electron density on less electronegative atoms (can donate more easily)