Elimination Reactions

Dehydrohalogenation (-HX) and Dehydration (-H 2 O) are the main types of elimination reactions.

Dehydrohalogenation (-HX)

E1 mechanism

E2 mechanism

Orientation of elimination: Zaitsev’s Rule In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched  -carbon. A. N. Zaitsev Less branched More branched

Product formed from previous slide

Comparing Ordinary and Bulky Bases

1-butene: watch out for competing reactions!

Substitution or Elimination? Factors to Consider: 1.How Basic is the Nucleophile? 2. Steric Hindrance at Reacting Carbon 3. Steric Hindrance at Nucleophile

Summary of Reactivity

Reactions of Benzene

KMnO 4 oxidationno reaction Br 2 /CCl 4 additionno reaction HIadditionno reaction H 2 /Nireductionno reaction Reagent Cyclohexene Benzene

Heats of hydrogenation and combustion are far lower than they should be. cyclohexene + H 2, Ni  cyclohexane Kcal/mole 1,3-cyclohexadiene + 2 H 2, Ni  cyclohexane Kcal/mole (predicted value = 2 X 28.6 = 57.2 Kcal/mole) benzene + 3 H 2, Ni, heat, pressure  cyclohexane Kcal/mole (predicted value = 3 X 28.6 = 85.8 Kcal/mole) Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted!

Chapter 16 Unusual Stability

Facts about benzene: a)Formula = C 6 H 6 b)Isomer number: one monosubstituted isomer C 6 H 5 Y known three disubstituted isomers C 6 H 4 Y 2 known c)Benzene resists addition reaction, undergoes substitution reactions. d)Heats of hydrogenation and combustion are far lower than they should be. e)From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.

Nomenclature for benzene: monosubstituted benzenes: Special names:

Disubsituted benzenes: ortho- meta- para- 1,2- 1,3- 1,4-

Electrophilic Aromatic Substitution (Aromatic compounds) Ar-H = aromatic compound 1. Nitration Ar-H + HNO 3, H 2 SO 4  Ar-NO 2 + H 2 O 2.Sulfonation Ar-H + H 2 SO 4, SO 3  Ar-SO 3 H + H 2 O 3.Halogenation Ar-H + X 2, Fe  Ar-X + HX 4.Friedel-Crafts alkylation Ar-H + R-X, AlCl 3  Ar-R + HX

+ HO-NO 2 + H 2 SO 4  H 2 O-NO 2 + HSO H 2 O-NO 2  H 2 O + NO 2 H 2 SO 4 + H 2 O  HSO H 3 O + HNO H 2 SO 4  H 3 O HSO NO 2 + nitration

resonance

Mechanism for nitration:

Mechanism for sulfonation:

Mechanism for halogenation:

Mechanism for Friedel-Crafts alkylation:

Substituent groups on a benzene ring affect electrophilic aromatic substitution reactions in two ways: 1)reactivity activate (faster than benzene) or deactivate (slower than benzene) 2)orientation ortho- + para- direction or meta- direction Effect of Substituent

-CH 3 activates the benzene ring towards EAS directs substitution to the ortho- & para- positions -NO 2 deactivates the benzene ring towards EAS directs substitution to the meta- position

Common substituent groups and their effect on EAS: -NH 2, -NHR, -NR 2 -OH -OR -NHCOCH 3 -C 6 H 5 -R -H -X -CHO, -COR -SO 3 H -COOH, -COOR -CN -NR 3 + -NO 2 increasing reactivity ortho/para directors meta directors