HALOGENOALKANES

OR ALKYL HALIDES

SITES:

http://brakkeibchem.wikispaces.com/file/view/T10D08+-+10.28.11+-+10.5-6+Halogenalkanes+Sn1+Sn2.pdf

Animation: http://www.rod.beavon.clara.net/SN2.htm

Halogenoalkanes contain F, Cl, Br, I bonded to an alkyl group.

CH3CH2F CH3CH2Cl CH3CH2Br CH3CH2I

Bond C-F C-Cl C-Br C-I

Bond Enthalpy 484 338 276 238

(kJmol-1 )

PRIMARY,SECONDARY,TERTIARY HALOGENOALKANES

Primary halogenoalkanes:

In a primary halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.

Secondary: Tertiary:

HALOGENOALKANES UNDERGO SUBSTITUTION.

NUCLEOPHILIC SUBSTITUTION

NUCLEOPHILE

A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else.

Nucleophiles are either fully negative ions, or have a lone pair of electrons. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

SUBSTITUTION NUCLEOPHILIC BIMOLECULAR – SN2

Primary halogenoalkanes undergo SN2:

Because the mechanism of the reaction involves 2 species, it is known as an SN2 reaction.

http://iverson.cm.utexas.edu/courses/310N/ReactMoviesFl05%20/SN2text.html

PAGE 226

+ -CH3

H

BrC

H

-OH

CH3

H

HO C H

Br-

HYDROXIDE ION WITH BROMOETHANE

SN2 Mechanism

ethanol

reaction equation

2(species reacting in the slowest step)

SN2

S (substitution)N(nucleophilic)

CH3

H

BrC

H

HO

-

SN1 – SUBSTITUTION NUCLEOPHILIC UNIMOLECULAR

(CH3)3CBr + OH- => (CH3)3COH + Br-

2 bromo 2 methylpropane reacts with warm dilute aqueous sodium hydroxide solution to form the tertiary alcohol.

The rate of reaction shows that the rate depends only on the concentration of the halogenoalkane and does not depend on the concentration of OH-

GENERAL RULE

Primary and secondary halogenoalkanes tend to undergo SN2 reactions

Tertiary halogenoalkanes tend to undergo SN1 reactions

SN2 reactivity rates follow the trend:

  CH3X > primary > secondary > tertiary      

Why???? PAGE 227

STABILITY OF CATIONS

Alkyl groups tend to push the bonding pair of electrons towards the carbon they are bonded to.

This is known as the positive inductive effect.

The positive charge can be spread over more atoms, increasing the stability of the ion. It will more likely be formed.

SN1 X SN2

DIAGRAM OF COMMON ORGANIC REACTIONS

.

21

ANSWERS

NUCLEOPHILIC SUBSTITUTION

propanenitrile

CH3CH2I (ethanol) + CN-(aq) CH3CH2CN + I-

cyanide ion with iodoethane

cyanide ion with 2-bromo,2-methylpropane

2,2-dimethylpropanenitrile

(CH3)3CBr (ethanol) + CN-

(CH3)3CCN + Br-

(aqueous)

mechanism

+ -CH3

CH3

BrC

CH3

Br-

CN- ION WITH 2-BROMO,2-METHYLPROPANE (SN1)

Nucleophilic substitution mechanism

2,2-dimethylpropanenitrile

1(species reactingin the slowest step)

SN1

S (substitution)N(nucleophilic)

Br-

CH3

CH3

C

CH3

+ CH3

CH3

CNC

CH3

CN-

reaction equation

ELIMINATION

PRINCIPLES OF ELIMINATION

Elimination of a hydrogen halide produces an alkene

In general, in aqueous solution substitution takes place

In ethanolic solution, elimination takes place

MECHANISMS

E1

E2

The particular mechanism depends on the strength of the base and the environment of the halide atom

E2

E1