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.

Halogeno-compounds

Chapter 33

Structures

Halogenoalkanes: X bond to sp3 carbon

R C H

H

X

R C H

R

X

R C R

R

X

1o Primary 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)

R C+ H

H

X-

:Nu-

R C H

H

Nu

+ X-

•Polar C-X bond

•C+ is attacked by :Nu-

•C-X bond is broken to give out X-

Bimolecular Nucleophilic Substitution (SN2)

H

C+

HCH3

Br-

HO-

HO C Br

H

H CH3

-

HO C

H

H

CH3

+ Br-

If C is chiral, completed stereochemical inversion.

Bimolecular Nucleophilic Substitution (SN2)

OH- + CH3CH2Br CH3CH2OH + Br-

HO C Br

H

H CH3

-

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

Unimolecular Nucleophilic Substitution (SN1)

R

C+

RR

Br- (rds)

C+

R

RR+ Br-

(sp2 , trigonal planar)

C+

R

RR+ H2O: -H+

R

CR

R

OH

Unimolecular Nucleophilic Substitution (SN1)

R3C-Br + H2O: R3C-OH + HBr

R3C+ + Br- + H2O

R3C---Br R3C---OH2+

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

Relative rates of SN1 and SN2

Compound SN1 SN2

CH3X 1 30

CH3CH2X 1.6 1

(CH3)2CHX 32 0.2

(CH3)3CX 107 0.00001

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 slowdown 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)

SN1 Inc. stability of carbocation

R3C-X R2CH-X RCH2-X CH3-X

(3o) (2o) (1o) (methyl)

SN2

Inc. easy of access (By steric factor)

Effect of nucleophile

SN2 Strength and concentration have effect RO:- > :OH- > ROH > H2O:

SN1 No effect

Factors affecting relative rates

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 carbocationand hence favour SN1 reaction

•Increase in polarity:CH3COCH3 << R-OH < H2O

Synthetic applications

Nitrile Formation ethanol, reflux

R-Br + KCN R-CN + KBr H+ 1.LiAlH4

R-CN RCOOH RCH2OH 2.H2O

(Increase carbon chain length by one carbon)

Synthetic applications

Formation of C-O bond

R-Br + NaOH ROHR-Br + RO-Na+ ROR

Formation of amine

RI + NH3 R-NH2

Elimination

H

H

H H

HC C

XHO:-

H

H H

HC C

+ H2O + X-

Competition between SN and E

H

H

H H

HC C

X

Nu:-

E SN

Good Nu:- are also good B:-

(SN always competes with E)

1. Highly substituted haloalkanes is more likely to undergo elimination (Steric Effect) Favor SN

3oRX 2oRX 1oRX

Favor E

Conditions favour 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 highlyconcentrated charged particles.

T.S. of SN2 reaction is Nu-….R….X- ismore 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.

CH3CHBrCH3

NaOH

C2H5OH, H2O

45oC

100oCCH3CH=CH2 + (CH3)2CH-OC2H5

(or OH)

(53%) (47%)

(64%) (36%)

Conditions favour E

4. Stronger base: RO- > ROH

CH3

CH3

CH3C Br 25oC

C2H5OH

C2H5O-/C2H5OH

(CH3)2C=CH2

(19%)

(93%)

Applications of Elimination

Preparation of Alkenese.g. C2H5O-Na+/C2H5OH

C2H5Br C2H5OC2H5 + CH2=CH2

heat 99% 1%

C2H5O-Na+/C2H5OH

(CH3 )2CHBr C2H5OCH(CH3)2 + CH2=CHCH3

heat 21% 79%

Applications of Elimination

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

(CH3)3CBr (CH3)2C =CH2

heat 100%

Applications of Elimination

Preparation of Alkynese.g. Br2

CH3CH=CHCH3 CH3CHBrCHBrCH3

C2H5O-Na+/C2H5OH

CH3CCCH3

heat

Uses of Halogeno-compounds

Please refer to Section 33.6 on p.253