organic chemistry - essential reaction mechanisms

Question 1

homolytic mechanism

Answer 1

each fragment possesses one of bonding e- = radicals

bond broken = radicals

Question 2

heterolytic mechanism

Answer 2

one of the fragments retain both bonding e- = ions

bond broken = ions

Question 3

homogenic mechanism

Answer 3

radical - e- donated by each fragment

bond making

Question 4

heterogenic mechanism

Answer 4

polar - 2 e- donated by one fragment

bond making

Question 5

agents

Answer 5

radicals - unpaired e- - Cl.

polar - nucleophile and electrophile

Question 6

nucleophile

Answer 6

e- pair introduced into e- deficient substrate
anion (H-,OH-)
neutral molecule - NH2, HOH

Question 7

electrophile

Answer 7

e- deficient binds to substrate centre with higher e- density
cation - Br+
neutral molecule - Lewis acids , AlCl3

Question 8

nucleophilic substitution

Answer 8

typical of haloalkane
Nu- -> HS-, HO-, Cl-
2 mechanisms - Sn1 and Sn2

Question 9

Sn2

Answer 9

substitution + Nu- biomolecular

single step and no intermediate - Nu- react with alkyl halide/ substrate from direction opposite of leaving group

Question 10

Sn2 results and equation

Answer 10

inversion of configuration

rate = K[R-Hal][Nu]

Question 11

Sn1 steps

Answer 11

2 steps - unimolecular Nu- sub 1 reaction

• leaving group dissociate = carbocation intermediate - need more energy - slow step - rate limiting step
• carbocation - structure e- deficient - not fulfil octane - unstable and v. reactive - fast step

Question 12

Sn2 results and equation

Answer 12

racemisation of configuration

rate = K[R-Hal]

Question 13

addition - electrophilic addition

Answer 13

E form covalent bond - attach e- rich unsaturated C=C bond

typical of alkene and alkynes

Question 14

nucleophilic addition

Answer 14

Nu- attach C=C and Y= O,S,N etc
double bond goes to Y and Nu attaches to C=C
Y attaches to E

Question 15

nucleophilic addition - compounds with C=O

Answer 15

carries partial charges and Nu - H2O, OH, carbanion

Question 16

example of nucleophilic addition

Answer 16

aldehyde/ketone
hemi-acetyl formation - acid/ base catalysed (has mechanisms)
in nature - hemi-acetyl formed in monosaccharides - open chain D glucose B-D- glucose

Question 17

elimination

Answer 17

beta elimination
e.g. glycolysis - 2-phosphoglycerate phosphoenol-pyruvase + water
3 mechanisms - E1, E2, E1cB

Question 18

E2

Answer 18

one step bimolecular reaction mechanism

Question 19

E1

Answer 19

2 step unimolecular reaction mechanism involve carbocation intermediate step wise action

Question 20

E1cB

Answer 20

2 step reaction mechanism involving carbanion intermediate

step wise action - most biologically relevant

Question 21

rearrangement

Answer 21

often migration of H atom, changing position of double bond

Question 22

rearrangement - Keto-erol tautomerism of carbonyl compounds and example

Answer 22

equilibrium between keto and erol form

e.g. isomerisation of monosaccharide occur via enol form - glucose fructose