Reactions of Alkanes, Alkenes, and Alkynes

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17 Dec 2011

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Chemistry 164 Reactions of Interest
Introduction: Substrate + Reagent ----> Products
Substrate: Must be positive for reaction to happen, electrophile, have positive dipole
center, electron sink (low energy empty MO)
Reagent: Must be negative, Electron Donor, Base
SN2: The reaction rate depends on r=kSNu. SN2 speed is based from [Nu] and [S]. There is no
reaction intermediate and only 1 TS. The nucleophile attacks the anti-bonding MO of the C attached to
the LG. This pushes out the LG, and takes the bond with the C. An inversion of stereochemistry
occurs, R to S/S to R if reaction occurs at stereogenic center. If substrate is enantiometically pure,
product will remain so (no racemization, remains optically inactive.
Factors that influence SN2 reactions:
Substrate structure: Methyl>1>2>(3)
SN only works on sp3 C, no vinylic, aromatic, etc.
Nucleophilicity of Reagent: Strong base>>>Weak base
Neg. Nu >> Neutral Nu
Soft Nu>>Hard Nu
Weak base=Good LG
Polar aprotic solvent good for this reaction because is stabilizes TS, faster reaction
-Polar Protic Solvents are capable of H bonding (generally have OH NH bonds)
-Polar Aprotic solvents are polar, but do not H bond (acteonitrile, DMSO, HMPA,
SN1: The reaction is only dependent on the [S]. It is a 2 step mechanism with 1 intermediate present.
The rate limiting step is in which the carbocation is created. Product is racemic because a front or
backside attack can happen to the carbocation.
Factors that influence SN1 reactions:
Stability of carbocation: 3>2>1>>methyl due to hyperconjugation effect
-Allylic and Benzyllic substrates work well in SN1 because the carbocation is stabilized by resonance.
-Elimination rxns at an sp3 C will only occur if a beta-H is available. These reactions will compete
with SN reactions. If more than beta-H, it can happen on either.
-Elimination is favored over substitution
-if B is a strong base or a poor Nu
-if a chemical is bulky or sterricaly hindered it cannot get to the backside attack of C and
therefore will approach the beta-H instead.
-Regioselectivity - From which C is the H removed? Ans. Produce the alkene that is most substituted.
Tetrasubstituted will be most stable, and therefore preferred.
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E2: Similar to SN2 and in competition with SN2. H picked up by base, L leaving, and pi bond forms
all at the same time. The beta-H and LG must be antiperiplanar. To solve, draw as Neumann
projection, with LG and H in same plane. then the reaction can occur.
E1: Similar to SN1 and in competition with SN1. It is a unimolecular TS with a carbocation
intermediate and it is a 2 step mechanism. Formation of carbocation is the RLS. There is NO
ANTIPERIPLANAR REQUIREMENT (in contrast to E2). The most common is the solvolysis
(protic polar solvent) of tertiary haloalkanes. (no other Nu or B must be present).
-Usually at low temperature SN1 is favored, high temp E1 is favored.
-CONTRAST: E2 can happen on a tertiary substrate especially if Nu is a strong, bulky base!!
Primary substrate:
SN2 is favored unless B is strong, and a beta-H is available, then E2 will be favored. (high
temperature favors E2 reaction)
Tertiary substrate:
Mixtures of SN1 and E1 reactions
Solvolysis in protic solvents will mostly form SN1 products (adjust to favor E1 with temp)
If good Nu, weak base = SN1
If poor Nu, weak base =E1
If strong base is present E2 will occur, especially if bulky
Secondary Substrate:
Generally SN2, E2 but SN1 and E1 possible
SN2 and SN1 can be controlled with solvent
A strong bulky base will go to E2
ALKENES: Mostly aromatic unsaturated HC’s
Calculating degree of unsaturation = #H2 needed to get saturation (1 for DB, 1 for ring)
Can be any combination if given MF.
-Presence of heteroatoms: ignore O, Add halogens, subtract N
Nomenclature: ending “ene”
Find longest chain containing all ene’s at lowest #
If more than one ene use diene, triene, tetraene….
new substituent vinyl -CH=CH2
Trans (or E) is more stable due to steric reasons of less crowding
The more spreading of e- density (empty pi anti-bonding MOs interact w/ vicinal sigma bonds) the
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