CHAPT 4.2: Nucleophilic substitution Flashcards Preview

CHEM III organic chemistry > CHAPT 4.2: Nucleophilic substitution > Flashcards

Flashcards in CHAPT 4.2: Nucleophilic substitution Deck (13)
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1

good LG and bad LG

best LG shd be the weakest base as it stabilises the negative charge better
good LG: R-OTS, R-BR, R-Cl, R-I
bad LG: R-OH , R- F , R-NH2, R-OR ( need protonation )

2

best solvent for SN2

polar aprotic solvents
eg. DMF, DMSO, DMA, Acetone
polar aprotic solvents only solvates cations and leaves nucleophile( anion) 'naked', thus nucleophile is more reactive and causes a faster reaction

3

Best solvent for SN1

Polar protic solvents
eg. water, alcohol ,NH3, COOH
stabilises ion by separating intimate ion pair and solvating both the cation and anion.
Carbocation becomes more stabilised as the positive charge is delocalised.
Nucleophile is also more stabilised and less reactive but since it is not in the rate law, it does not affect the rate

4

nucleophilicity in protic solvents

Bigger nu have better nucleophilicities in polar protic solvents
I- > Br- > Cl- > F-

5

nucleophilicities in aprotic solvents

smaller nu have better nucleophilicities in aprotic solvents
F->Cl->Br->I-

6

what makes a nucleophile strong?

strong base= strong nu
- nu increases down the grp as atomic radii increases down the group , electrons will be bound less tightly and donates electrons more easily
- -ve charge = stronger nu because richer in electron density

7

eg of strong and weak nu

strong -OH AND -NH2 AND RO- AND -NR2
weak H20 AND NH3 AND ROH AND PR3( need deprotination )

8

how to differentiate solvents

polar protic solvents : contain H bonded to NOF
polar aprotic solvents: do not contain H bonded to NOF
non- polar aprotic; hexane, CCL4, benzene

9

SN2 substrate

CH3 > PRIMARY > SECONDARY
tert substrates no no SN2
- depends on steric hindrance, /bulky groups around alpha carbon

10

SN1 substrate

TERT > SEC > PRI > METHYL
- depends on stability of carbocation , more stable carbocation = faster rxn

11

stereochemistry of SN2 and SN1 Rxn

SN2: 100% inversion of products ( backside attack)
SN1 : major inversion, minor retention of products

12

mechanism of SN2 and SN1

SN2: 1 STEP , HAVE TS
SN1 : 2 STEP , HAVE INTERMEDIATE
1. formation of carbocation
2. nu attacks alpha carbon

13

physical properties of Halogenoalkanes

1. BP: small amt of energy is required to overcome the permanent dipole - permanent dipole forces of attraction between alkyl halide molecule
2. polarity: Insoluble in water, soluble in organic solvents ( CCl4 or benzene )