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55 Cards in this Set

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Alkyl halides:
- 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
Table representing carbon-Halogen bond length and bond strengths:
What are alkyl halides used for?
they are used as solvents for relatively nonpolar compounds and are used as the starting materials for the synthesis of many compounds
Vinylic halides:
generally refers to a compound in which a halogen is attached to a C that also forms a double bond to another C.
Phenyl halide:
compounds in which halogen is attached to a benzene ring.
are most alkyl and aryl halides soluble in water?
they have very low solubilities to water.
Nucelophilic substitution reaction:
Nucleophile:
- 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.
what is the leaving group in a nucleophilic substitution reaction?
the halogen substituent... departs as a halide ion.
general reaction for nucleophilic substitution of an alkyl halide by water:
What makes a good leaving group?
- 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.
kinetics:
the rate of a reaction.
determing the rate of the reaction:
- 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.
Bimolecular:
two species are involved in the step whose rate is being measured.
molecularity:
# of species involved in a reaction step.
what causes an Sn2 reaction?
- nucelophilic

- substitution

- bimolecular
a schematic representation of orbitals involved in Sn2 reaction:
Describe an Sn2 reaction:
- 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.
how many steps are in an Sn2 reaction?
only one step, no intermediates
transition state:
- 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
concerted reaction:
bond formation & breaking simultaneously in a single transition
Exergonic:
a reaction that proceeds w/ negative free-energy
endergonic:
a reaction tat proceeds w/ a positive free-energy.
a mechanism for Sn2 reaction:
Free-energy diagram:
- 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.
Free energy of activation (delta G):
- height of energy barrier above the level of reactants.

- the difference in free energy between the reactants and the transition state.
Relationship between the rate of a reaction & the magnitude of the free energy of activation:
Inversion:
caused by a Sn2 backside attach.

- turns inside out in much the same way as an umbrella.
The stereochemistry of an Sn2 reaction:
What is required for an Sn1 reaciton?
- substitution

- nucleophilic

- unimolecular
rate-determining step:
(rate-limiting step) slow step of the reaction.
How fast is the step of the formation of a carbocation in an Sn1 reaction?
carbocation formation, in general... takes place slowly b/c it is usually a highly endothermic process & is uphill in terms of free energy
mechanism for Sn1 reaction:
Describe the process of an Sn1 reaction:
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
Carbocation:
- 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
Carbocation:
- 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
the relative stability of carbocations:
the relative stability of carbocations:
What influences the stability of carbocations?
hyperconjugation
What is hyperconjugation?
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.
the stereochemistry of Sn1 reactions:
complete racemization:
if the original compound loses all of its optical activity in the course of the reaction
complete racemization:
if the original compound loses all of its optical activity in the course of the reaction
partial racemization:
if the original compound loses only part of its optical activity.
racemization:
takes place whenever the reaction causes chiral molecules to be converted to an achiral intermediate.
Another diagram of the stereochemistry of an Sn1 reaction:
solvolysis reaction:
- an Sn1 reaction

- a nucleophilic substitution in which the nucleophile is a molecule of the solvent. (solvent & lysis: cleavage by the solvent)
how solvolysis reaction occurs:
how solvolysis reaction occurs:
Important factors that affect the relative rate of Sn1 & Sn2 reactions:
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
reactivity in Sn2 reactions:
methyl>primary>secondary>(tertiary-unreactive)

important factor is steric effects.
Steric effects:
an effect on relative rates caused by the space-filling properties of those parts of a molecule attached at or near the reacting site.
Steric effects:
an effect on relative rates caused by the space-filling properties of those parts of a molecule attached at or near the reacting site.
Steric hindrance:
- one kind of steric effect

- the spatial arrangement of the atoms/ groups @ or near the reacting site of a molecule hinders/ retards a reaction.
Why is it important to form a stable carbocation in an Sn1 reaction?
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)