Chapter 7 Alkenes and Alkynes I: Properties and Synthesis Elimination Reactions of Alkyl Halides

Heat of Hydrogenation The hydrogenation of alkenes is an exothermic reaction. The relative stabilities of alkenes can be measured using the heat of hydrogenation reactions. Heats of hydrogenation of three butene isomers: ∆H°≈ -120 kJ mol-1

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

Consider the two alkenes 2-methyl-1-pentene and 2-methyl-2-pentene and decide which would be most stable. 2-methyl-1-pentene 2-methyl-2-pentene disubstituted trisubstituted Less stable More stable

Cycloalkenes The rings of cycloalkenes containing five carbon atoms or fewer exist only in the cis- form. The introduction of a trans- form into small rings if it were possible, cause greater strain than the cis- forms. trans-cyclooctane still be stable at room temperature, so it has been isolated.

Synthesis of Alkenes via Elimination Reactions Alkenes are commonly made by Elimination Reactions Dehydrohalogenation (- HX) Dehydration (- H2O) Dehalogenation (- X2) Synthesis of Alkenes via Hydrogenation Reactions of Alkynes

A. Dehydrohalogenation of Alkyl Halides Dehydrohalogenation is elimination (loss) of HX from an alkyl halide using strong base (KOH)

Zaitsev’s Rule: Formation of the Most Substituted Alkene is Favored with a Small Base Some hydrogen halides can eliminate to give two different alkene products. Zaitsev’s Rule: when two different alkene products are possible in an elimination, the most highly substituted (most stable) alkene will be the major product.

Using Zaitsev’s Rule, predict which would be the major product of the following reaction: monosubstituted trisubstituted Less stable More stable According to the Zaitsev’s Rule The product B would be the major product

Acid Catalyzed Dehydration of Alcohols Recall that elimination is favored over substitution at higher temperatures. Typical acids used in dehydration are sulfuric acid and phosphoric acid. The temperature and concentration of acid required to dehydrate depends on the structure of the alcohol. Primary alcohols are the most difficult to dehydrate.

More easier to dehydrates Secondary alcohols usually dehydrate under milder conditions. Tertiary alcohols are usually so easily dehydrated that extremely mild conditions. The relative ease with which alcohols undergo dehydration is in the following order: More easier to dehydrates

Mechanism for Dehydration of Secondary 2° and Tertiary 3° Alcohols: An E1 Reaction E1 mechanism is a stepwise need acidic catalytst nitializing step formation of Protonated alcohol (Oxonium ion) Step 1 formation of Carbonium ion C+ (intermediate) Step 2 formation of Alkene (product) and acid

Energy diagram of E1 mechanism: A two-step reaction with one intermediate

decreasing of stability Carbocation Stability and the Transition State Recall the stability order of carbocations is: The second step of the E1 mechanism in which the carbocation forms is rate determining. The transition state for this reaction has carbocation character. Tertiary alcohols react the fastest because they have the most stable tertiary carbocation-like transition state in the second step. decreasing of stability

In E1 mechanism the intermediate for 2° or 3° carbocation is formed. The relative heights of DG‡ for the second step of E1 dehydration indicate that primary alcohols have a prohibitively large energy barrier. In E1 mechanism the intermediate for 2° or 3° carbocation is formed. The 3° carbocation intermediate has the lowest energy of activation than the 2° one. 1° C+ less stable 3° C+ most stable

Transition state which give an Alkene Mechanism for Dehydration of Primary Alcohols 1° : An E2 Reaction Primary alcohols cannot undergo E1 dehydration because of the instability of the carbocation-like transition state in the 2nd step. In the E2 dehydration the first step is again protonation of the hydroxyl to yield the good leaving group water. initializing step formation of Protonated alcohol Step 1 formation of Transition state which give an Alkene

Energy diagram of E2 mechanism: a one-step reaction with no intermediate

Dehalogenation of vicinal dihalides | | | | — C — C — + Zn  — C = C — + ZnX2 | | X X eg. CH3CH2CHCH2 + Zn  CH3CH2CH=CH2 + ZnBr2 Br Br

Arrange the following alcohols in order of their reactivity toward acid-catalyzed dehydration (start with the most reactive) 3 1-Pentanol 1° alcohol 2-methyl-2-butanol 1 3° alcohol 3-methyl-2-butanol 2° alcohol 2

Synthesis of Alkenes via Hydrogenation Reactions of Alkynes

Synthesis of alkyne By the reaction of calcium carbide with water + 2 H+ OH-

Synthesis of Alkynes via Elimination Reactions Dehalogenation of vicinal dihalides Alkynes can be prepared by elimination reactions of halogen from a vicinal dihalide in the presence of strong base such as sodium amide (NaNH2). step1 formation of Alkene via E2 Step2 formation of Alkyne via E2

Dehydrohalogenation of Vicinal Dihalides Alkenes can be converted to alkynes by bromination and two consecutive dehydrohalogenation reactions. - 2HBr

Dehydrohalogenation of Geminal Dihalides Geminal dihalides can also undergo consecutive dehydrohalogenation reactions to yield the alkyne Show how you might synthesize ethynylbenzene from methyl phenyl ketone

Outline all steps in a synthesis of propyne from each of the following: a) CH3COCH3 b) CH3CH2CHBr2 c) CH3CHBrCH2Br d) CH3CH=CH2

The Acidity of Terminal Alkynes Recall that acetylenic hydrogens are much more acidic than most other C-H bonds. The higher the percent of s-character of the hybrid orbital, the closer the lone pair is held to the nucleus, and the more stable the conjugate base. The relative acidity of acetylenic hydrogens in solution is:

Bond length (pm) 1.53 1.34 1.2 27 27

7.29 p.324 without consulting tables, arrange the following compounds in order of decreasing acidity: Pentane 4 1-pentene 3 1-pentyne 2 1-pentanol 1

The products are called alkynides Acetylenic hydrogens can be deprotonated with relatively strong bases (sodium amide is typical) The products are called alkynides 29

Replacement of the Acetylenic Hydrogen Atom of Terminal Alkynes Sodium alkynides can be used as nucleophiles in SN2 reactions New carbon-carbon bonds are the result Only primary alkyl halides can be used or else elimination reactions predominate

Outline a synthesis of 4-phenyl-2-butyne from 1-propyne

Your goal is to synthesize 4,4-dimethyl-2-pentyne Your goal is to synthesize 4,4-dimethyl-2-pentyne. You have a choice of beginning with any of the following:

7.31 p.324 Outline the syntheses of Propene from each of the following: