halogenoalkanes or alkyl halides
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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