Halogeno Compounds

Halogeno-compounds Chapter 33

Structures Halogenoalkanes: X bond to sp3 carbon H R

C

R H

X 1o Primary

R

C

R H

R

C

R

X

X

2o Secondary

3o Tertiary

Structures Halobenzene: X bond to benzene, sp2 carbon X

Reactions of Halogenoalkanes Two major types: •Nucleophilic Substitution (SN) •Elimination (E)

Nucleophilic Substitution (SN) :Nu-

H R

Cδ+ H

•Polar C-X bond

Xδ-

•Cδ+ is attacked by :NuH R

C Nu

H + X-

•C-X bond is broken to give out X-

Bimolecular Nucleophilic Substitution (SN2) HO

H

-

H

H Cδ+ Brδ-

HO

CH3 C

H

Br

H CH 3

H HO

C

-

+ Br-

CH3

If C is chiral, completed stereochemical inversion.

Bimolecular Nucleophilic Substitution (SN2) H HO

C

Br

H CH 3

OH- + CH3CH2Br

CH3CH2OH + Br-

Rate law: Rate = k [OH-][CH3CH2Br] (Bimolecular, 2nd order)

Unimolecular Nucleophilic Substitution (SN1) R

R R

Cδ+ Brδ-  (rds) R

C+

R R 2 (sp , trigonal planar)

R C+ R

+ H2O: R

+ Br-

R

-H+ R

C R

OH

Unimolecular Nucleophilic Substitution (SN1) R3C---Br

R3C---OH2+

R3C+ + Br- + H2O R3C-Br + H2O:

R3C-OH + HBr

Rate law: rate = k [R3CBr] (1st order, Unimolecular)

Relative rates of SN1 and SN2 Compound

S N1

SN 2

CH3X

1

30

CH3CH2X

1.6

1

(CH3)2CHX 32

0.2

(CH3)3CX

0.00001

107

Factors affecting relative rates Structure - Steric Factor The size of atoms or groups at/near the reactive site affects SN2. Bulky groups (-R) at the C-X site slow down SN2 reaction.

Factors affecting relative rates Structure - Stability of carbocation R3C+ > R2CH+ > RCH2+ > CH3+ (R group is e- donating) Stable carbocation favours SN1 mechanism.

Factors affecting relative rates (By electronic factor)

SN 1

R3C-X (3o)

Inc. stability of carbocation

R2CH-X

RCH2-X

CH3-X

(2o)

(1o)

(methyl) SN 2

Inc. easy of access

(By steric factor)

Factors affecting relative rates Effect of nucleophile SN2 Strength and concentration have effect RO:- > :OH- > ROH > H2O: SN1 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 SN1 reaction •Increase in polarity: CH3COCH3 << R-OH < H2O

Synthetic applications Nitrile Formation ethanol, reflux

R-Br + KCN



H+

R-CN

 RCOOH

R-CN + KBr 1.LiAlH4



RCH2OH

2.H2O

(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 + NH3 → R-NH2

Elimination

H HO:

-

H

H

C

C

H

X

H

H

H

H

C

C

+ H2O + X-

H

Competition between SN and E

H E

H

H

C

C

H

X

H

SN

Nu:-

Good Nu:- are also good B:(SN always competes with E)

Conditions favour E • Highly substituted haloalkanes is more likely to undergo elimination (Steric Effect) Favor SN 3oRX 2oRX 1oRX 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 SN2 reaction is Nuδ-….R….Xδ- is more concentrated than Nuδ-…H – C - C….Xδ-

Conditions favour E 3. Higher temperature and prolonged refluxing Breaking of C-H bond and C-X bonds require greater Activation Energy. 45oC

(47%) (53%) NaOH CH3CHBrCH3 CH3CH=CH2 + (CH3)2CH-OC2H5 C2H5OH, H2O 100oC (or OH) (36%) (64%)

Conditions favour E 4. Stronger base: RO- > ROH C2H5OH

CH3 CH3 C Br CH3

25oC

(19%) (CH3)2C=CH2

C2H5O /C2H5OH -

(93%)

Applications of Elimination Preparation of Alkenes e.g. C2H5O-Na+/C2H5OH

→

C2H5Br

C2H5OC2H5 + CH2=CH2

heat

99%

1%

C2H5O-Na+/C2H5OH

(CH3 )2CHBr

→

heat

C2H5OCH(CH3)2 + CH2=CHCH3 21%

79%

Applications of Elimination Preparation of Alkenes e.g. C2H5O-Na+/C2H5OH

(CH3)3CBr

→

heat

(CH3)2C =CH2 100%

Applications of Elimination Preparation of Alkynes e.g. Br2

CH3CH=CHCH3 → CH3CHBrCHBrCH3 C2H5O-Na+/C2H5OH → heat

CH3C≡CCH3

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