Fundamentals 3: Reactivity, intermediates and mechanism

Question 1

what is activation energy?

Answer 1

the energy required by molecules in collision to overcome any natural repulsion.

Question 2

do the interacting orbitals need to be closer together or further apart to be more stabilised?

Answer 2

closer together.

larger stabilisation helps to form sigma bond interactions.

Question 3

what do nucleophiles do with regards to electrons?

Answer 3

donate from filled, high energy HOMO.

Question 4

what do electrophiles do with regards to electrons?

Answer 4

accept electrons into empty, low energy LUMO.

Question 5

what types of nucleophiles are there?

Answer 5

ones with a lone pair.

ones with a negative charge.

ones with a C=C double bond.

ones with a σ bond between electropositive atoms.

Question 6

what types of electrophiles are there?

Answer 6

ones with an empty AO.

ones with a single bond to an electronegative atom.

ones with a double bonded electronegative atom.

Question 7

attacking carbonyl bonds:

C=O

Answer 7

nucleophile electrons add into empty π* orbital.

new bond formed at carbonyl carbon because it is electrostatically attractive, gives best orbital overlap.

nucleophile must approach from 107⁰.

Question 8

what are carbocations?

Answer 8

high energy, electron deficient species.

positive charge located at the carbon centre.

important in Sn1 and E1 reactions.

Question 9

what is the order of stability for alkyl-substituted carbocations?

Answer 9

(most stable)

tertiary
secondary
primary
methyl

(least stable)

Question 10

what is hyperconjugation?

Answer 10

overlap of a σ bond with an adjacent empty/partially filled orbital.

stabilises the molecule by delocalising electrons and distributing charge more evenly.

makes it less reactive.

Question 11

what is the difference between hyperconjugation and the inductive effect?

Answer 11

the inductive effect occurs through the polarisation of sigma bonds, rather than through the overlap of sigma bonds with p orbitals.

Question 12

how do resonance structures arise?

Answer 12

when 2+ Lewis structures can be drawn for a molecule, with different arrangements of the electrons, but identical arrangements of the atoms.

higher number of resonance contributors = greater stability via electron delocalisation.

Question 13

how does lone pair donation occur and what are the implications of it?

Answer 13

neighbouring LPs can stabilise carbocations.

only happens if the cation p-orbital can overlap with the lone pair orbital.

Question 14

what are carbanions?

Answer 14

high energy, electron rich species, so called because they have a negative charge located at the carbon centre.

Question 15

what is the order of stability in carbanions?

Answer 15

(most stable)

methyl
primary
secondary
tertiary

(least stable)

Question 16

what factors affect a carbanions stability?

Answer 16

delocalisation through an adjacent C=C π bond.

more resonance structures = more stable.

a lower pKa is indicative of a more stable anion.

Question 17

why does a lower pKa indicate a more stable anion?

Answer 17

low pKa = stronger acid = more likely to donate proton to base.

negative charge on conjugate base is more evenly distributed, hence more stable.

Question 18

what are radicals?

Answer 18

species which contain an unpaired electrons.

highly reactive.

formed through homo/heterolytic cleavage.

Question 19

what is homo and heterolytic cleavage?

Answer 19

homo = bond broken so each atom retains one of the two electrons that it shared, initiated through heat or light.

hetero = bond broken so one atom retains both electrons, forms 2 ions (+ and -).

Question 20

why are tertiary radicals more stable than primary and secondary?

Answer 20

greater degree of hyperconjugation.

tertiary has 3 neighbouring sigma bonds that can participate in hyperconjugation,

unlike secondary and primary which have 2 and 1 respectively.