Halogeno-compounds Chapter 33

Structures Halogenoalkanes: X bond to sp 3 carbon RCH H X RCH R X RCR R X 1 o Primary 2 o Secondary 3 o Tertiary

Structures Halobenzene: X bond to benzene, sp 2 carbon X

Reactions of Halogenoalkanes Two major types: Nucleophilic Substitution (S N ) Elimination (E)

Nucleophilic Substitution (S N ) RC + H H X - :Nu - RCH H Nu + X - Polar C-X bond C + is attacked by :Nu - C-X bond is broken to give out X -

Bimolecular Nucleophilic Substitution (S N 2) H C + H CH 3 Br - HO - HOC Br H H CH 3 - HOC H H CH 3 + Br - If C is chiral, completed stereochemical inversion.

Bimolecular Nucleophilic Substitution (S N 2) OH - + CH 3 CH 2 Br CH 3 CH 2 OH + Br - HOC Br H H CH 3 - Rate law: Rate = k [OH - ][CH 3 CH 2 Br] (Bimolecular, 2 nd order)

Unimolecular Nucleophilic Substitution (S N 1) R C + R R Br - (rds) C+C+ R RR + Br - (sp 2, trigonal planar) C+C+ R RR + H 2 O: -H + R C R R OH

Unimolecular Nucleophilic Substitution (S N 1) R 3 C-Br + H 2 O:R 3 C-OH + HBr R 3 C + + Br - + H 2 O R 3 C---Br R 3 C---OH 2 + Rate law: rate = k [R 3 CBr] (1 st order, Unimolecular)

Relative rates of S N 1 and S N 2 CompoundSN1SN1SN2SN2 CH 3 X130 CH 3 CH 2 X1.61 (CH 3 ) 2 CHX320.2 (CH 3 ) 3 CX

Factors affecting relative rates Structure - Steric Factor The size of atoms or groups at/near the reactive site affects S N 2. Bulky groups (-R) at the C-X site slow down S N 2 reaction.

Factors affecting relative rates Structure - Stability of carbocation R 3 C + > R 2 CH + > RCH 2 + > CH 3 + (R group is e - donating) Stable carbocation favours S N 1 mechanism.

Factors affecting relative rates (By electronic factor) S N 1 Inc. stability of carbocation R 3 C-X R 2 CH-X RCH 2 -X CH 3 -X (3 o ) (2 o ) (1 o ) (methyl) S N 2 Inc. easy of access (By steric factor)

Effect of nucleophile S N 2 Strength and concentration have effect RO:- > :OH - > ROH > H 2 O: S N 1 No effect Factors affecting relative rates

Effect of leaving groups Relative rate of substitution C-I > C-Br > C-Cl Explanation : Bond energy C-I 238 C-Br 276 C-Cl 338 (*exp.1 p.235)

Factors affecting relative rates Effect of solvent: Polar solvent stabilize the carbocation and hence favour S N 1 reaction Increase in polarity: CH 3 COCH 3 << R-OH < H 2 O

Synthetic applications Nitrile Formation ethanol, reflux R-Br + KCN R-CN + KBr H + 1.LiAlH 4 R-CN RCOOH RCH 2 OH 2.H 2 O (Increase carbon chain length by one carbon)

Synthetic applications Formation of C-O bond R-Br + NaOH ROH R-Br + RO - Na + ROR Formation of amine RI + NH 3 R-NH 2

Elimination H H HH H C C X HO: - H HH H C C + H 2 O + X -

Competition between S N and E H H HH H C C X Nu: - ESNSN Good Nu: - are also good B: - (S N always competes with E)

1.Highly substituted haloalkanes is more likely to undergo elimination (Steric Effect) Favor S N 3 o RX 2 o RX 1 o RX Favor E Conditions favour E

2. Use less polar solvent e.g. 75% ethanol + 25% water is better than 25% ethanol + 75% water Polar solvent favors the formation of highly concentrated charged particles. T.S. of S N 2 reaction is Nu - ….R….X - is more concentrated than Nu - …H – C - C….X -

Conditions favour E 3. Higher temperature and prolonged refluxing Breaking of C-H bond and C-X bonds require greater Activation Energy. CH 3 CHBrCH 3 NaOH C 2 H 5 OH, H 2 O 45 o C 100 o C CH 3 CH=CH 2 + (CH 3 ) 2 CH-OC 2 H 5 (or OH) (53%) (47%) (64%) (36%)

Conditions favour E 4. Stronger base: RO - > ROH CH 3 CBr 25 o C C 2 H 5 OH C 2 H 5 O - /C 2 H 5 OH (CH 3 ) 2 C=CH 2 (19%) (93%)

Applications of Elimination Preparation of Alkenes e.g. C 2 H 5 O - Na + /C 2 H 5 OH C 2 H 5 Br C 2 H 5 OC 2 H 5 + CH 2 =CH 2 heat 99% 1% C 2 H 5 O - Na + /C 2 H 5 OH (CH 3 ) 2 CHBr C 2 H 5 OCH(CH 3 ) 2 + CH 2 =CHCH 3 heat 21% 79%

Applications of Elimination Preparation of Alkenes e.g. C 2 H 5 O - Na + /C 2 H 5 OH (CH 3 ) 3 CBr (CH 3 ) 2 C =CH 2 heat 100%

Applications of Elimination Preparation of Alkynes e.g. Br 2 CH 3 CH=CHCH 3 CH 3 CHBrCHBrCH 3 C 2 H 5 O - Na + /C 2 H 5 OH CH 3 C CCH 3 heat

Uses of Halogeno-compounds Please refer to Section 33.6 on p.253