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Chapter 10
Substitution Reactions of Alkyl Halides

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10-00-00CO
Title
Figure 10.5
Caption
Comparison of transition states for SN2 attack on an electrophilic reactant by iodide and by fluoride.
Notes
Since iodide is larger and more polarizable than fluoride, the iodide transition state has more overlap (bonding) between the nucleophile and the carbon reaction site. In protic media, fluoride nucleophile is stabilized relative to the transition state by hydrogen bonding, increasing the activation energy for the fluoride reaction. Iodide does not hydrogen bond to protic solvents, so it is not stabilized by hydrogen bonding relative to its transition state. Furthermore, the iodide transition state is stabilized relative to the iodide ion reactant by increased bonding between iodide and carbon, lowering the activation energy for the iodide reaction. Consequently, iodide is a better nucleophile than fluoride in protic solvents.
Keywords
figure, 10.5, comparison, transition, state, SN2, iodide, fluoride
10-00-03UN
Title
Survival Compounds
Caption
Structures of organohalides made by red algae and sea hares to protect themselves from predators.
Notes
These organohalides are toxic and foul-tasting.
Keywords
survival, compounds, organohalides
10-00-09T01
Title
Table 10.1 Relative Rates of Sn2 Reactions for Several Alkyl Bromides
Caption
Relative rates of SN2 reactions for several alkyl bromides.
Notes
Alkyl halides in which the carbon reaction center is sterically hindered (crowded) react more slowly in SN2 reactions.
Keywords
table, 10.1, rates, SN2, alkyl, bromides
10-01a,b
Title
Figure 10.1
Caption
MO pictures of the interaction between the HOMO of a nucleophile and the LUMO of an electrophile in a front side vs. a back side attack by the nucleophile.
Notes
In Lewis acid–base reactions, electrons flow from the Lewis base to the Lewis acid. Since electrons flow from the HOMO of one species into the LUMO of the other initially, it is normally appropriate to overlap the HOMO of the Lewis base (nucleophile) with the LUMO of the Lewis acid (electrophile). When this is done for SN2 reactions, the overlap is more favorable in the back-side-attack transition state than it is in the front-side-attack transition state.
Keywords
figure, 10.1, MO, HOMO, LUMO, electrophile, nucleophile, front side, back side
10-02
Title
Figure 10.2
Caption
Potential maps of approach of hydroxide ion nucleophile to methyl, primary, secondary, and tertiary halides.
Notes
Back side approach of the nucleophile to the carbon reaction center becomes progressively more difficult in this series, making SN2 reactions progressively slower.
Keywords
figure, 10.2, potential, maps, hydroxide, methyl, primary, secondary, tertiary, halides, SN2
10-02-01UN
Title
SN2 Reactivity
Caption
Relative reactivities of alkyl halides in an SN2 reaction.
Notes
Reactivity decreases as back side access to the carbon reaction site gets more difficult.
Keywords
SN2, reactivity, back side
10-03
Title
Figure 10.3
Caption
Reaction coordinate diagrams for the SN2 reaction of hydroxide nucleophile with methyl bromide and with a more sterically hindered alkyl bromide.
Notes
The transition state for the reaction involving the hindered alkyl halide has lots of atom–atom repulsions due to steric crowding, and is thus less stable than the transition state for the methyl bromide reaction. Thus, the methyl bromide reaction has a lower activation energy, and it is faster.
Keywords
figure, 10.3, reaction coordinate, diagrams, SN2, hydroxide, methyl, bromide, sterically hindered
10-04
Title
Figure 10.4
Caption
Ball-and-stick models showing an SN2 reaction between hydroxide ion and methyl bromide.
Notes
The transition state for this reaction has a trigonal-bipyramidal shape. The nucleophile and leaving group are 180 degrees apart (axial), and the other three groups attached to the carbon reaction site are in the same equatorial plane, 120 degrees apart.
Keywords
figure, 10.4, ball-and-stick, hydroxide, methyl, bromide, SN2
10-04-05UN
Title
Halide Basicities
Caption
Relative basicities of the halide ions.
Notes
Larger anions are more stable (i.e., less basic) than smaller anions.
Keywords
halide, basicities
10-04-06UN
Title
Halide Leaving Group Ability
Caption
Relative leaving abilities of the halide ions.
Notes
Leaving ability parallels anion stability, and therefore runs opposite to basicity.
Keywords
halide, leaving, ability
10-04-07UN
Title
SN2 Reactivities of Alkyl Halides
Caption
Relative reactivities of alkyl halides in an SN2 reaction.
Notes
Alkyl halides with better leaving groups react faster in an SN2 reaction.
Keywords
SN2, reactivities, halides
10-04-08UN
Title
Acid Strengths
Caption
Relative acid strengths of second-row binary acids.
Notes
Binary acids with more electronegative central atoms form more stable (less basic) conjugate base anions, and are therefore stronger acids than binary acids with less electronegative central atoms.
Keywords
acid, strength, second-row, binary
10-04-09UN
Title
Base Strength and Nucleophilicity
Caption
Relative base strengths and relative nucleophilicities.
Notes
Nucleophilicity parallels base strength in protic solvents.
Keywords
base, strength, nucleophilicity
10-05-01UN
Title
Size, Basicity, and Nucleophilicity
Caption
Relationship between size, basicity, and nucleophilicity of the halide anions in protic solvents.
Notes
In protic solvents, sizes and nucleophilicities of the halide ions parallel one another and trend in the opposite direction from basicities.
Keywords
size, basicity, nucleophilicity, halides
10-05-02UN
Title
Ion–Dipole Interactions
Caption
Ion–dipole interactions between water molecules and an anion.
Notes
Ion–dipole interactions between water molecules and nucleophilic anions reduce the nucleophilicities of anions.
Keywords
ion-dipole, interactions
10-05-03T02
Title
Table 10.2 Relative Nucleophilicity Toward CH3I in Methanol
Caption
Relative nucleophilicities of various nucleophiles toward methyl iodide in methanol.
Notes
Relative nucleophilicities in methanol are determined by a combination of basicities and polarizabilities.
Keywords
table, 10.2, nucleophilicities, methyl, iodide, methanol
10-05-07UN
Title
Nucleophile Steric Effects
Caption
Ball-and-stick models of ethoxide ion and tert-butoxide ion.
Notes
Sterically hindered nucleophiles are less nucleophilic than smaller nucleophiles.
Keywords
nucleophile, steric, ball-and-stick, ethoxide, tert-butoxide
10-05-14UN
Title
Synthesis Using SN2 Reactions
Caption
Various types of organic compounds can be prepared using SN2 reactions.
Notes
SN2 reactions can often be irreversible. In these cases, the reaction proceeds preferentially in whichever direction it needs to to consume a stronger nucleophile and produce a weaker nucleophile.
Keywords
synthesis, SN2
10-05-23T04
Title
Table 10.4 Relative Rates Alkyl Bromides SN1 Reaction
Caption
Notes
Keywords
10-06
Title
Figure 10.6
Caption
Reaction coordinate diagram for an SN1 reaction.
Notes
Since the reaction is first-order in electrophile concentration but zero-order in nucleophile concentration, the nucleophile enters the reaction sequence after the rate-determining step, so that its concentration does not affect the speed of the reaction. Thus, the first step, formation of the carbocation, is rate-determining.
Keywords
figure, 10.6, reaction coordinate, diagram, SN1
10-06-01UN
Title
SN1 Reactivity
Caption
Relative reactivities of alkyl halides in an SN1 reaction.
Notes
The reverse reactivity order is seen for primary, secondary, and tertiary alkyl halides in SN1 reactions and SN2 reactions. SN1 reactivity is governed by intermediate carbocation stability (tertiary cations are most stable and primary cations are least stable).
Keywords
SN1, reactivity
10-06-04UN
Title
Leaving Group Influence on SN1 Reactivity
Caption
Relative reactivities of alkyl halides in an SN1 reaction.
Notes
Electrophiles with less basic (more stable) leaving groups will react faster by SN1 because the stability of the intermediate produced by the rate-determining step in the SN1 reaction is a function of both the carbocation stability and the leaving group stability.
Keywords
leaving, group, influence, SN1, reactivity, alkyl, halides
10-06-49UN
Title
Solvation of Anions and Cations by Water Molecules
Caption
Ion–dipole interactions between water molecules and cations and anions.
Notes
Water moleules orient themselves in such a way that their positively charged hydrogen atoms point toward anions and their negatively charged oxygen atoms point toward cations.
Keywords
solvations, anions, cations, water
10-06-50T07
Title
Table 10.7 Benzene ring
Caption
Dielectric constants of some common solvents.
Notes
The higher the dielectric constant, the more polar the solvent. The more polar the solvent, the faster an SN1 reaction goes. Polar solvents stabilize charged transition states of SN1 reactions which resemble a carbocation/leaving group anion intermediate more than they stabilize neutral reactant electrophile molecules. This lowers the activation energy for SN1 reactions.
Keywords
table, 10.7, dielectric, constants, common, solvents
10-07
Title
Figure 10.7
Caption
Reaction coordinate diagram for a reaction in which the charge on the reactants is greater or more localized than the charge on the transition state.
Notes
An SN2 reaction using an anionic nucleophile fits this profile in which a charged (nucleophilic) reactant is used in the rate-determining step, and the transition state has less localized charge. Therefore, polar solvents slow down SN2 reactions.
Keywords
figure, 10.7, reaction coordinate, diagram, charge, transition, state
10-08
Title
Figure 10.8
Caption
Reaction coordinate diagram for a reaction in which the charge on the transition state is greater than or more localized than the charge on the reactants.
Notes
An SN1 reaction fits this profile in which a charged (nucleophilic) reactant is not used until after the rate-determining step, whereas the transition state has more charge than the reactant (neutral) electrophile. Therefore, polar solvents speed up SN1 reactions.
Keywords
figure, 10.8, reaction coordinate, diagram, charge, transition, state
10-08-05P29Sol
Title
Problem 29
Caption
Ka expressions for dissociation of a neutral acid and for dissociation of a cationic acid.
Notes
Neutral acids generate ionic products, so Ka for neutral acids increases in polar solvents which stabilize ionic products more than neutral reactant. Cationic acids generate cationic hydronium ion product, so polar solvents will stabilize reactants and products of cationic acid-dissociation reactions more or less equally. Ka should not change as much with solvent polarity for cationic acids as it does for neutral acids.
Keywords
problem, 29, Ka, dissociation, acid, neutral, cationic

Title
Table 10.5 Comparison of SN2 and SN1 Reactions
Caption
Notes
Keywords

Title
Table 10.6 Summary of the Reactivity of Alkyl Halides in Nucleophilic Substitution Reactions
Caption
Notes
Keywords

Title
Table 10.8 The Effect of the Polarity of the Solvent on the Rate
Caption
Notes
Keywords
10-00-09T01
Title
Table 10.1 Relative Rates of Sn2 Reactions for Several Alkyl Bromides
Caption
Relative rates of SN2 reactions for several alkyl bromides.
Notes
Alkyl halides in which the carbon reaction center is sterically hindered (crowded) react more slowly in SN2 reactions.
Keywords
table, 10.1, rates, SN2, alkyl, bromides
10-05-03T02
Title
Table 10.2 Relative Nucleophilicity Toward CH3I in Methanol
Caption
Relative nucleophilicities of various nucleophiles toward methyl iodide in methanol.
Notes
Relative nucleophilicities in methanol are determined by a combination of basicities and polarizabilities.
Keywords
table, 10.2, nucleophilicities, methyl, iodide, methanol
10-05-23T04
Title
Table 10.4 Relative Rates Alkyl Bromides SN1 Reaction
Caption
Notes
Keywords
10-06-50T07
Title
Table 10.7 Benzene ring
Caption
Dielectric constants of some common solvents.
Notes
The higher the dielectric constant, the more polar the solvent. The more polar the solvent, the faster an SN1 reaction goes. Polar solvents stabilize charged transition states of SN1 reactions which resemble a carbocation/leaving group anion intermediate more than they stabilize neutral reactant electrophile molecules. This lowers the activation energy for SN1 reactions.
Keywords
table, 10.7, dielectric, constants, common, solvents

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