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Level 192

Substitution & Elimination Reactions


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Free-Radical Halogenation
Replaces H in alkane with halogen
Selectivity of Halogenation
Selectivity = reactivity/# of sites
Substitution Reactions
Sigma bond to sigma bond
Nucleophiles
Nucleus loving
Lewis Base
A substance that can donate a pair of electrons to form a covalent bond.
Electrophiles
Electron loving, electron deficient, positive charge or partial positive charge
Lewis acid
accepts lone electron pair
Nucleophilicity and Basicity Trends
Both increase going left across period
leaving group
The atom or group of atoms that departs during a substitution or elimination. The leaving group can be charged or uncharged, but it leaves with the pair of electrons that originally bonded the group…
Sn2 Reaction
Bimolecular (Second order) Kinetics - rate dependent on electrophile + nucleophile
Sn1 Reaction
Unimolecular (first order) Kinetics - rate dependent on electrophile
Ether Substitution Reactions
R-O-R' is split by strong mineral acid (HCl, HBr, HI) to make R-X + R'-X + H2O
Elimination Reaction
Two sigma bonds become one pi bond (reverse of addition)
E2 reaction
Leaving group comes off as proton is removed. 1 step
Alcohol + Phosphorus Halides
PBr3 or PCl3 - Halogen replaces OH, H3PO3 byproduct
Alcohol + Thionyl Chloride
Alcohol + SOCl2 + nitrogen ring - Cl replaces OH, SO2 gas byproduct
Elimination Reactions of Alcohols
1° alcohol undergoes E2
Zaitsev's Rule
The more substituted double bond will form in larger proportions (and trans(E))
Halogens - Nucleophile or Base
Always act as nucleophiles, no elimination reactions
Hoffman's Rule
Least substituted double bond occurs in elimination reaction if bulky base is use
E1 reaction
1-Heterolysis of c-x bond to form carbocation
SN2
?:CN
E2 High Temp
1° LG and Sterically Hindered Strong Base
SN2/E2 Heat
1°LG and Good Nucleophile/Strong Base
SN2 (Except F)
1° LG and Poor Nucleophile
E2
1° Carbon (Strong Base, Hindered)
E1 (High Temp) or SN1
3° LG and Good Nucleophile/Weak Base
3° LG and Poor Nucleophile
SN1 (Low Temp) or E1 (High Temp)
Good Nucleophiles/Strong Bases
-OH,-OR, -CH2R, -H
Sterically Hindered Bases
LiN(CH(CH3)3)2, KOC(CH3)3, DBU, DBN
Good Nucleophiles/Weak Bases
-X, -CN, -SR, -N3, -OOCR
Poor Nucleophiles
H2O, ROH, RSH
Sn1
Both mechanisms are sensitive to the nature of the leaving group but the _______ mechanism is more sensitive
E1
reaction type that needs a highly substituted substrate and any base
Reaction of Substituted Carbons SN2
1 > 2 > 3 (rarely)
Reaction of Substituted carbons SN1
3 > 2 > 1 (stability of carbocation is main issue)
1st degree
SN2 with inversion of configuration
3rd degree
SN1 with racemization
Sterically hindered base
Hoffmann product of E2
Only Nucleophile
Halides (Cl- , Br- , I-)
Primary alkyl halide, SN1
Reaction does not occur
Primary alkyl halide, SN2
most favoured in polar solvents (water, alcohols, acetone, DMF etc.) and if a good nucleophile is used.
Primary alkyl halide, E1
Reaction does not occur
Primary alkyl halide, E2
Occurs when strong bases are used
Secondary alkyl halide, SN1
Can occur with benzylic and allylic halides i.e. in polar solvents
Secondary alkyl halide, SN2
Occurs in competition with E2 reaction, favoured for good nucleophiles
Secondary alkyl halide, E1
Can occur with benzylic and allylic halides i.e. in polar solvents
Secondary alkyl halide, E2
Occurs in competition with SN2 reaction, favoured when strong bases are used
Tertiary alkyl halide, SN1
Occurs in competition with E1 reaction, favoured in hydroxylic solvents/non-nucleophilic bases e.g. tertiary amines (acidic/neutral conditions)
Tertiary alkyl halide, SN2
Reaction does not occur
Tertiary alkyl halide, E1
Occurs in competition with SN1 reaction, favoured for alcohols and NO base (acidic/neutral conditions)
Tertiary alkyl halide, E2
Highly favoured when bases are present
Primary alkyl halides in general
More likely to undergo SN2 with good nucleophile or by E2 with strong base
Secondary alkyl halides in general
Can usually react by both SN2 and E2 producing a mixture of substitution and elimination products. E2 favoured when strong bases are present.
Tertiary alkyl halide in general
Reacts by E2 if a strong base is used or by a mixture of SN1 and E1 under neutral/acidic conditions