• The halogen atom on the alkyl halide is replaced/substituted with another group (Nucleophile). • Since the halogen is more electronegative than carbon, the C-X bond breaks heterolytically and X- anion leaves (with the lone pair of electrons). • The group replacing/substituting X- is a nucleophile, and X- is a leaving group.
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The question is how did the fallen car broken from the train? There are at least 2 possibilities - Use your imagination .…..
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6.8 SN2 Mechanism (2nd order nucleophilic substitution)
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6.13 SN1 Reaction (continued)
SN1 Mechanism
H H H
2
H HO C Br H H HO C
H + H H
H O
C Br
Br
-
Formation of carbocation (slow, rate determining step)
(CH3)3C Br
• Bimolecular nucleophilic substitution. • Concerted reaction: new bond forming and old bond breaking at same time. • Rate is first order in each reactant. • Walden inversion.
Now we go back to finish Chapter 4 ………….. 4.4 Equilibrium constant and free energy (review Gen Chem)
(CH3)3C
+
+ H O H
(CH3)3C O H H
K eq
[ producti ] i [ reactant j ] j • Loss of H+ (if needed)
+
+ (CH3)3C O H + H O H
(CH3)3C O H
+ H3O
• For chlorination Keq = 1.1 x 1019 • Large value indicates reaction “goes to completion.”
H
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4.4 Equilibrium constant and free energy (continued) Gibbs free energy change
• G = free energy of (products - reactants), amount of energy available to do work.
4.4 Equilibrium constant and free energy (continued) Example:
• Negative values indicate spontaneity. • Standard free energy: Go = -RTlnKeq where R = 8.314 J/(Kmol) and T – temperature in kelvins • Since chlorination has a large Keq, the free energy change is large and negative. • Given that -X is -OH, the energy difference for the reaction above is 4.0 kJ/mol. • What percentage of cyclohexanol molecules will be in the equatorial conformer at equilibrium at 25°C?
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4.5A Enthalpy 4.4 Equilibrium constant and free energy (continued) Factors determining G
• Free energy change depends on – enthalpy – entropy • H = (enthalpy of products) - (enthalpy of reactants) • S = (entropy of products) - (entropy of reactants) • G = H - TS Ho = heat released or absorbed during a chemical reaction at standard conditions. • Exothermic, (H0), heat is absorbed. • Reactions favor products with lowest enthalpy (strongest bonds). •
4.5B Entropy
• So = change in randomness, disorder, freedom of movement. • Increasing heat, volume, or number of particles increases entropy. • Spontaneous reactions: large negative enthalpy and large positvie entropy. • In the equation Go = Ho - TSo , the entropy value is often small. Ho has a unit of kcal/mol, while So has a unit of cal/(mol K).
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4.6 Bond-dissociation enthalpies (continued) 4.6 Bond-dissociation enthalpies Which is more likely?
Estimate H for each step using BDE. • Bond breaking requires energy (+BDE). • Bond
