Alkenes and Alkynes 1 Properties and Synthesis. Elimination Reactions of Alkyl Halides

The (E) and (Z) system for Designating Alkene Diastereoisomers Z = (zusammen) German for together E = (Entgegen) German for opposite

Rules 1. Assign an atom with greater atomic number as high priority on each side of the sp2 carbon

Rules 2.If the two subsitutents attached to sp2 carbon start with the same atom (tie), move outward until tie breaks

Rules If an atom is doubly bonded or triply bonded, the priority systems treats it as if it were single bonded

Example Using E and Z designation for the following

Relative Stability of Alkenes Cis isomer is less stable due to the greater strain from crowding the adjacent alkyl group

Overall Relative Stabilities of Alkenes The greater the number of attached alkyl groups The more highly substituted the carbon atoms of the double bond) the greater the alkene’s stability

Synthesis of Alkenes via Elmination Two methods for alkene synthesis Dehydrogenation of Alkyl halides Dehydrogenation of Alcohol

Dehydrohalogenation 1 step reaction Rate = [Nu:-] [Substrate]

E2 mechanism: conditions 2 o or 3 o alkyl halide should be used if possible When a synthesis must begin with a 1 o alkyl halide, then a bulky base should be used Strong and nonpolarizable base such as alkoxides should be used in high concentration Sodium ethoxide in ethanol and potassium ter-butoxide in tert-butyl alcohol are based typically used in promote E2 reactions Elevated temperature is usually employed because it generally favors elimination over substitution

Zaitsev’s Rule: Formation of the More Substituted Alkene is Favored with a Small Base Whenever an elimination occurs to give the more stable, more high substituted alkene  Zaitsev’s Rule Dehydrohalogenation of many alkyl halides, yields more than one product.

Zaitsev’s Rule: Formation of the More Substituted Alkene is Favored with a Small Base Transition state resemble s how alkene will be substituted

Formation of the Less subsitituted Alkene using a Bulky Base Hoffmann Rule applies when elimination yields the less substituted alkene

The stereochemistry of E 2 Reactions: The Orientation of groups in the Transition State Anti coplanar transition state is more preferrable Staggered conformation Syn coplanar transition state is prefferred only with rigid molecules Eclipsed conformation

A mechanism Where There Two Axial β Hydrogens

E2 Elimination Where the Only Axial β Hydrogen is from a less stable conformer

Example When cis-1-bromo-4-tert-butylcyclohexane is treated with sodium ethoxide in ethanol, it reacts rapidly; the product is 4-tert-butylcyclohexene. Under the same condition, trans- 1-bromo-4-ter-butylcyclohexane reacts very slowly. Write conformational structures and explain the difference in reactivity of these cis-trans isomers.

Acid-Catalyzed Dehydration of Alcohols Removing water molecule to form an alkene Dehydration Favored high temperature The temperature and concentration of acid required to dehydrate an alcohol depend on the structure of the alcohol substrate Some primary alcohol and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration

Mechanism for Dehydration of Secondary and Tertiary Alcohol E1 mechanism Step 1: protonation – a proton is rapidly transferred from acid to one of the unshared electron pairs of alcohol Step 2: The carbon-oxygen bond breaks heterolytically. The bonding electrons depart with the water molecule and leave behind a carbocation Step 3: acid-base protonation

Example Dehydration of 2-propanol occurs in 14M H 2 SO 4 at 100 o C Using curved arrows, write all steps in a mechanism for the dehydration Explain the essential role of performed in alcohol dehydration by the acid catalyst (hints: consider what would have happen if no acid were present?)