Chem 240: Ch#06 - Ionic Reactions

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Alkyl halides:

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- halogen atom is attached to an sp3 hybridized carbon

- carbon arrangement therefore general tetrahedral

- carbon-halogen bond... is polarized b/c halogen atoms are more electronegative to carbon.

- halogen size increases as you go down the periodic table

- C--X bond length increases and bond strength decreases as you go down periodic table

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Table representing carbon-Halogen bond length and bond strengths:

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What are alkyl halides used for?

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they are used as solvents for relatively nonpolar compounds and are used as the starting materials for the synthesis of many compounds

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Vinylic halides:

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generally refers to a compound in which a halogen is attached to a C that also forms a double bond to another C.

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Phenyl halide:

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compounds in which halogen is attached to a benzene ring.

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are most alkyl and aryl halides soluble in water?

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they have very low solubilities to water.

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Nucelophilic substitution reaction:

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Nucleophile:

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- love the (+) part of an atom

- when reacts w/ an alkyl halid... (+) part (center) that the nucleophile seeks is the c-atom that bears the x atom

- any (-) ion/ any neutral molecule that has @ least one unshared e- pair.

- reacts w/ an alkyl halid (substrate) by replacing the halogen substituent.

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what is the leaving group in a nucleophilic substitution reaction?

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the halogen substituent... departs as a halide ion.

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general reaction for nucleophilic substitution of an alkyl halide by water:

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What makes a good leaving group?

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- substituent that can leave as a relatively stable, weakly basic molecule/ anion.

- halogen atom of an alkyl halide is a good leaving group b/c once departed it is a weak base and stable anion.

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kinetics:

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the rate of a reaction.

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determing the rate of the reaction:

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- reaction rates are known to be temp. dependent.

- can be determined experimentally be measuring the rate at which a compound disappears from the solution or the rate at which an ion or compound appears.

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Bimolecular:

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two species are involved in the step whose rate is being measured.

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molecularity:

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# of species involved in a reaction step.

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what causes an Sn2 reaction?

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- nucelophilic

- substitution

- bimolecular

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a schematic representation of orbitals involved in Sn2 reaction:

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Describe an Sn2 reaction:

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- nucleophile approaches the carbon bearing the leaving group from the backside (the side directly opposite the leaving group)
--- orbital that contains e- pair of the nucleophile (higest occupied molecular orbital, HOMO) begins to overlap with an empty orbital (lowest unoccupied molecular orbital, LOMO) of the carbon atom bearing the leaving group.

-- as the reaction progresses.... bond between C & Nu strengthens... bond between C & LG weakens.

- C-atom has it config. turned inside out (inversion) and LG is pushed away.

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how many steps are in an Sn2 reaction?

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only one step, no intermediates

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transition state:

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- the formation of an unstable arragnement of atoms which proceeds the reaction

- fleeting arrangment of the atoms in which th enucleophile & the leaving group are both partially bonded to the carbon atom undergoing substitution

- extremely brief existance

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concerted reaction:

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bond formation & breaking simultaneously in a single transition

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Exergonic:

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a reaction that proceeds w/ negative free-energy

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endergonic:

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a reaction tat proceeds w/ a positive free-energy.

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a mechanism for Sn2 reaction:

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Free-energy diagram:

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- a quantity that measures the process of the reaction

>> represents the changes in bond orders and bond distances that must take place as the reactants are converted to products.

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Free energy of activation (delta G):

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- height of energy barrier above the level of reactants.

- the difference in free energy between the reactants and the transition state.

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Relationship between the rate of a reaction & the magnitude of the free energy of activation:

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Inversion:

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caused by a Sn2 backside attach.

- turns inside out in much the same way as an umbrella.

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The stereochemistry of an Sn2 reaction:

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What is required for an Sn1 reaciton?

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- substitution

- nucleophilic

- unimolecular

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rate-determining step:

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(rate-limiting step) slow step of the reaction.

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How fast is the step of the formation of a carbocation in an Sn1 reaction?

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carbocation formation, in general... takes place slowly b/c it is usually a highly endothermic process & is uphill in terms of free energy

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mechanism for Sn1 reaction:

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Describe the process of an Sn1 reaction:

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step 1:
requries heterolytic cleavage of the C-X bond
>> high endothermic
>>high free energy of activation
- departure of the halide takes place at all is largely b/c of the ionizing ability of the solvent (water)
>> water molecules surround & stabilize the cation & anion that are produced

step 2:
intermediate cation reacts w/ water rapidly to produce ion.

Step 3:
rapidly transfers a proton to a molecule of water producing an alcohol

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Carbocation:

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- trigonal planar

- sp2 hybridization

- center C is electron deficient
>> only has 6e- in its outer most shell
------ used to form sigma covalent bonds w/ H
------ p orbitals contain no e-s

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Carbocation:

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- trigonal planar

- sp2 hybridization

- center C is electron deficient
>> only has 6e- in its outer most shell
------ used to form sigma covalent bonds w/ H
------ p orbitals contain no e-s

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the relative stability of carbocations:

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the relative stability of carbocations:

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What influences the stability of carbocations?

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hyperconjugation

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What is hyperconjugation?

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it involves e- delocalization (partial orbital overlap) from a filled bonding orbital to an adjacent unfilled orbital.

- unfilled orbital is the vacant p orbital of the carboncation & the filled orbitals are C-H and C-C sigma bonds at the carbons adjacent to the p orbital of the carbocation.

- sharing of e- delocalizes the (+) charge which causes the molecule to stabilize better.

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the stereochemistry of Sn1 reactions:

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complete racemization:

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if the original compound loses all of its optical activity in the course of the reaction

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complete racemization:

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if the original compound loses all of its optical activity in the course of the reaction

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partial racemization:

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if the original compound loses only part of its optical activity.

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racemization:

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takes place whenever the reaction causes chiral molecules to be converted to an achiral intermediate.

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Another diagram of the stereochemistry of an Sn1 reaction:

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solvolysis reaction:

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- an Sn1 reaction

- a nucleophilic substitution in which the nucleophile is a molecule of the solvent. (solvent & lysis: cleavage by the solvent)

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how solvolysis reaction occurs:

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how solvolysis reaction occurs:

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Important factors that affect the relative rate of Sn1 & Sn2 reactions:

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1. the structure of the substrate

2. the concentration & reactivity of the nucleophile (bimolecular reactions only)

3. effect of the solvent

4. nature of the leaving group

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reactivity in Sn2 reactions:

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methyl>primary>secondary>(tertiary-unreactive)

important factor is steric effects.

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Steric effects:

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an effect on relative rates caused by the space-filling properties of those parts of a molecule attached at or near the reacting site.

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Steric effects:

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an effect on relative rates caused by the space-filling properties of those parts of a molecule attached at or near the reacting site.

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Steric hindrance:

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- one kind of steric effect

- the spatial arrangement of the atoms/ groups @ or near the reacting site of a molecule hinders/ retards a reaction.

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Why is it important to form a stable carbocation in an Sn1 reaction?

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b/c it means that the free-energy of activaiton for the slow step of the reaction will be low enough for the reaction to take place @ a reasonable rate (only tertiary halids react by Sn1 mechanism most of the time)