Ch 9 Substitution and Elimination Reactions of Alkyl Halides Flashcards
An Sn2 reaction is bimolecular:
both the alkyl halide and the nucleophile are involved in the transition state of the rate-limiting step, so the rate of the reaction depends on the concentration of both of them
An Sn2 reaction has a one-step mechanism:
the nucleophile attacks the back side of the carbon that is attached to the halogen. Therefore, it takes place with inversion of configuration
Because of steric hindrance, the relative reactivities of alkyl halides in an Sn2 reaction are
primary > secondary > tertiary.
Tertiary alkyl halides cannot undergo Sn2 reactions
The relative reactivities of alkyl halides that differ only in the halogen atom are
RI > RBr > RCl > RF in SN2, SN1, E2 and E1 reactions
Basicity is a measure of
how well a compound shares its lone pair with a proton;
nucleophilicity is a measure of
how readily a species with a lone pair is able to attack an electron-deficient atom
Protic solvents
H2O, ROH
Protic solvents have a hydrogen attached to
an O or an N
Aprotic solvents
DMF, DMSO
In general, the stronger base is
a better nucleophile
If the attacking atoms are very different in size and the reaction is carried out in protic solvent the stronger bases are
poorer nucleophiles, because of ion-dipole interactions between the ion and the solvent
An Sn1 reaction is unimolecular;
only the alkyl halide is involved in the transition state of the rate-limiting step, so the rate of the reaction is dependent only on the concentration of the alkyl halide
An Sn2 reaction has a two-step mechanism:
the halogen departs in the first step, forming a carbocation intermediate that is attacked by a nucleophile in the second step.
Sn1 reactions of alkyl halides are solvolysis reactions, meaning
that the solvent is the nucleophile
The rate of an Sn1 reaction depends on the
ease of carbocation formation
An Sn1 reaction takes place with
razemization
The only substitution reactions that primary and secondary alkyl halides and methyl halides undergo are
Sn2 reactions
The only substitution reactions that tertiary alkyl halides undergo are
Sn1 reactions
Removal of a proton and a halide ion is called
dehydrohalogenation
The product of a dehydrohalogenation elimination reaction is an
alkene
An E2 reaction is
concerted, one-step reaction in which the proton and the halide ion are removed in the same step
CH3CH2X + B: -> CH2=CH2 + +BH + X-
E2 reaction is called
a dehydrohalogenation reaction
An E2 reaction is regioselective:
the major product is the more stable alkene, unless the reactants are sterically hindered or the leaving group is poor
The more stable alkene is generally the
more substituted alkene
alkyl substitution increases the stability of a carbocation and
decreases the stability of a caranion
An E1 reaction is a
two-step reaction in which the alkyl halide dissociates, forming a carbocation intermediate. Then, a base removes a proton from a carbon adjacent to the positively charged carbon
CH3CH2X -> CH3CH2+ + X- -> CH2=CH2 + +BH + X-
An E1 reaction is regioselective:
the major product is the more stable alkene
The only elimination reactions the primary and secondary alkyl halides undergo are
E2 reactions
Tertiary alkyl halides undergo
both E1 and E2 reactions
For alkyl halides that can undergo both the E1 and E2 reactions, the E2 is favored by
a strong base and E1 is favored by a weak base
An E2 reaction is stereoselective:
anti elimination is favored
In E2 reaction: if the beta-carbon has two hydrogens
both E and Z stereoisomers are formed: one with the largest groups on opposite sides of the double bond is formed in greater yield because it is more stable
In the E2 reaction: if the beta carbon is bonded to only one hydrogen
then only one alkene is formed. Its structure depends on the structure of the alkyl halide
An E1 reaction is stereoselective:
both E and Z stereoisomers are formed regardless of the number of hydrogens bonded to the beta-carbon
In an E2 reaction, the two groups eliminated from a six-membered ring must both be in
axial positions
Primary alkyl halides undergo Sn2/E2 reactions:
the substitution reaction is favored unless the nucleophile/base is sterically hindered
Secondary alkyl halides undergo Sn2/E2 reactions:
both substitution and elimination products are formed; strong bases, bulky bases, and high temperatures favor the elimination product
Tertiary alkyl halides undergo
E2 reactions with strong bases and Sn1/E1 reactions with weak bases
Benzylic and allylic halides undergo
Sn1, Sn2, E1 and E2 reactions (except tertiary not Sn2)
Vinylic and aryl halides cannot undergo
Sn2, Sn1, or E1 reactions
Vinylic halides can undergo
E2 reactions with a strong base
Polar solvents insulate
opposite charges from one another
Increasing the polarity of the solvent decreases the rate of the reaction if
one or more reactants that participate in the rate-determining step are charged
Increasing the polarity of the solvent increases the rate of the reaction if
none of the reactants that participate in the rate-determining step are charged
In ether synthesis the less hindered alkyl group should be provided by the
alkyl halide
If the two functional groups of a bifunctional molecule can react with each other
both intermolecular and intramolecular reactions can occur
The reaction more likely to occur depends on the concentration of the bifunctional molecule and the size of the ring that would be formed in the intramolecular reaction:
ring size 5-6 is favored for intramolecular reactions
the higher the concentration the more intermolecular reactions
