Halogenoalkanes
Halogenoalkanes
substitution reactions of halogenoalkanes with hydroxide
https://www.youtube.com/watch?v=hqJ1ciZDXH8&index=25&list=PL73D2B
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Explain why the hydroxide ion is a better nucleophile than water
https://www.youtube.com/watch?v=Do8ugMmvMs&index=29&list=PL73D2BCC23E83D555
Describe and explain rates of nucleophilic substitution
https://www.youtube.com/watch?v=SBxsoWUxQbg&index=30&list=PL73D
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Explain how the rate of Sn1/Sn2 of halogenoalkanes by OH- changes
https://www.youtube.com/watch?v=SBxsoWUxQbg&index=30&list=PL73D
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Animation:
https://www.youtube.com/watch?v=TnY1S5IdVqI
Nucleophilic Substitution RX on primary halogenoalkane:
SN2 Mechanisms
C2H5Cl + OH- 
chloroethane
C2H5OH + Clethanol
Bimolecular Reaction :
rate=k[OH-][CH3CH2Cl] proportional to both reactants= order 2
- Inversion of the arrangement of the atoms around C = Stereospecific.
- Faster reaction favored by aprotic and polar solvent:
OH- is not solvated by solvent and reacts faster.
Reaction profile for an SN2 reaction ( ONE STEP- one Ea )
Nucleophilic Substitution RX on tertiary halogenoalkane:
SN1 Mechanisms
(CH3)3CCl + OH-
 (CH3)3COH + Cl-
2-chloro-2-methylpropane
Steric hindrance:
Difficult for incoming group to
react due to bulky 3 CH3 groups
Unimolecular Rx
2-methyl-propan-2-ol
Positive induction:
Stabilization effect on the carbocation
rate =k[(CH3)3CCl] determined by unimolecular slow step = order 1
OHOH-
Y
Cl
OH-Cl-
During both steps, there is some arrangement of the atoms which causes an energy maximum.
This is called a transition state ( ts1 and ts2 )
The stability of the carbocation intermediate is shown by the fact that there are small activation
barriers to its conversion either into product or back into reactants.
Reaction favored by protic and polar solvent (with OH or NH) able to
stabilize the intermediate carbocation by ion-dipole interaction and the
leaving group ( ex: Cl-) by proton.
Comparison of the rates of nucleophilic substitutions SN1 SN2
Mechanism:
Tertiary C >
SN1
Secondary >
Primary
SN1- SN2
SN2
Leaving halogen:
C-I >
longer- weaker
C-Br
> C-Cl
>
C-F
shorter-stronger
Solvent:
- SN1 favored by polar, protic solvent (with OH or NH). Stabilization of carbocation.
- SN2 favored by polar, aprotic solvent. OH- non solvated (bare)
The faster reactions (SN1) occur with tertiary iodoalkanes in polar, protic solvents.
lower
Rate of reaction
( C-I> C-Br > C-Cl > C-F)
rate = k [OH-] [CH3Cl]
rate = k [CH3Cl]
higher
One more tutorial on :
-SN2
https://www.youtube.com/watch?v=h5xvaP6bIZI
-SN1
https://www.youtube.com/watch?v=JmcVgE2WKBE
-Solvent effect
https://www.khanacademy.org/science/organic-chemistry/substitutionelimination-reactions/sn1-sn2-sal/v/sn1-sn2-solvent-effects
One last animation :
http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html
We
SN1 and SN2