S ITES : http://brakkeibchem.wikispaces.com/file/view/T10
D08+-+10.28.11+-+10.5- 6+Halogenalkanes+Sn1+Sn2.pdf Animation:
http://www.rod.beavon.clara.net/SN2.htm
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Halogenoalkanes contain F, Cl, Br, I bonded to an alkyl group.
CH 3 CH 2 F CH 3 CH 2 Cl CH 3 CH 2 Br CH 3 CH 2 I Bond C-F C-Cl
C-Br C-I Bond Enthalpy 484 338 276 238 (kJmol -1 )
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P RIMARY,S ECONDARY,T ERTIARY H ALOGENOALKANES Primary
halogenoalkanes: In a primary halogenoalkane, the carbon which
carries the halogen atom is only attached to one other alkyl
group.
N UCLEOPHILE 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.
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SUBSTITUTION NUCLEOPHILIC BIMOLECULAR SN 2 Primary
halogenoalkanes undergo SN2: Because the mechanism of the reaction
involves 2 species, it is known as an S N 2 reaction.
http://iverson.cm.utexas.edu/courses/310N/ReactMoviesFl05%20/SN2text.html
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P AGE 226
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++ -- CH 3 H Br C H - OH CH 3 H HO C H Br - HYDROXIDE ION WITH
BROMOETHANE SN2 Mechanism ethanol reaction equation 2 (species
reacting in the slowest step) SN2SN2 S (substitution) N
(nucleophilic) CH 3 H Br C H HO -
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SN1 S UBSTITUTION N UCLEOPHILIC U NIMOLECULAR (CH 3 ) 3 CBr +
OH - => (CH 3 ) 3 COH + 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 -
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G ENERAL RULE Primary and secondary halogenoalkanes tend to
undergo S N 2 reactions Tertiary halogenoalkanes tend to undergo S
N 1 reactions S N 2 reactivity rates follow the trend: CH 3 X >
primary > secondary > tertiary Why???? PAGE 227
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S TABILITY OF C ATIONS
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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.
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SN1 X SN2
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D IAGRAM OF COMMON ORGANIC REACTIONS. 21
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A NSWERS
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N UCLEOPHILIC SUBSTITUTION propanenitrile CH 3 CH 2 I
(ethanol)+ CN - (aq)CH 3 CH 2 CN + I - cyanide ion with iodoethane
cyanide ion with 2-bromo,2-methylpropane 2,2-dimethylpropanenitrile
(CH 3 ) 3 CBr (ethanol) + CN - (CH 3 ) 3 CCN + Br - ( aqueous )
mechanism
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++ -- CH 3 Br C CH 3 Br - CN - ION WITH 2- BROMO,2-
METHYLPROPANE (S N 1) Nucleophilic substitution mechanism
2,2-dimethyl propanenitrile 1 (species reacting in the slowest
step) SN1SN1 S (substitution) N (nucleophilic) Br - CH 3 C + CN C
CH 3 CN - reaction equation
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E LIMINATION
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P RINCIPLES OF ELIMINATION Elimination of a hydrogen halide
produces an alkene In general, in aqueous solution substitution
takes place In ethanolic solution, elimination takes place
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M ECHANISMS E1 E2 The particular mechanism depends on the
strength of the base and the environment of the halide atom