Halogen-oalkanes

Halogenoalkanes

substitution reactions of halogenoalkanes with hydroxidehttps://www.youtube.com/watch?v=hqJ1ciZDXH8&index=25&list=PL73D2BCC23E83D555

Explain why the hydroxide ion is a better nucleophile than waterhttps://www.youtube.com/watch?v=Do8ugMm-vMs&index=29&list=PL73D2BCC23E83D555

Describe and explain rates of nucleophilic substitutionhttps://www.youtube.com/watch?v=SBxsoWUxQbg&index=30&list=PL73D2BCC23E83D555

Explain how the rate of Sn1/Sn2 of halogenoalkanes by OH- changeshttps://www.youtube.com/watch?v=SBxsoWUxQbg&index=30&list=PL73D2BCC23E83D555

Animation: https://www.youtube.com/watch?v=TnY1S5IdVqI

Nucleophilic Substitution RX on primary halogenoalkane: SN2 Mechanisms Nucleophilic Substitution RX on primary halogenoalkane: SN2 Mechanisms

C2H5Cl + OH- C2H5OH + Cl-

chloroethane ethanol

rate=k[OH-][CH3CH2Cl] proportional to both reactants= order

2

Bimolecular Reaction :

- 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 )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 2-methyl-propan-2-ol

rate =k[(CH3)3CCl] determined by unimolecular slow step = order 1

Steric hindrance:Difficult for incoming group to react due to bulky 3 CH3 groups

Positive induction:Stabilization effect on the carbocation

Unimolecular Rx

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.

OH-

OH-

OH-

OH-

YCl

Cl-

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.

Mechanism: Tertiary C > Secondary > Primary SN1 SN1- SN2 SN2

Leaving halogen:

C-I > C-Br > C-Cl > C-F longer- weaker 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.

Comparison of the rates of nucleophilic substitutions SN1 SN2

Rate of reaction rate = k [OH-] [CH3Cl] rate = k [CH3Cl]

lower ( C-I> C-Br > C-Cl > C-F) higher

http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html

One more tutorial on :

One last animation :

We SN1 and SN2

-SN2 https://www.youtube.com/watch?v=h5xvaP6bIZI  -SN1 https://www.youtube.com/watch?v=JmcVgE2WKBE -Solvent effecthttps://www.khanacademy.org/science/organic-chemistry/substitution-elimination-reactions/sn1-sn2-sal/v/sn1-sn2-solvent-effects