Ch 7- Alkenes and Alkynes I

Division of Material

• Alkenes and Alkynes are very versatile molecules in Organic Chemistry

• As a result, there is a lot of information we need for them.

• To make it manageable, the information is divided into two chapters.

• In this chapter, we will focus on their general properties and how to make them

• In chapter 8, we will focus on the reactions they participate in.

E/Z nomenclature system

• The E/Z system is just like the cis/trans system for double bonds, only the E/Z system is much more versatile!

• Example:

• For the E/Z system, you assign priorities to the two groups bonded to each carbon of the double bond, just like we did with R/S

E/Z nomenclature system

• If the two higher priorities are on the same side, it is designated Z

• If the two higher priorities are on the opposite side, it is designated E

Relative stabilities of Alkenes

• Heats of reactions, or more specifically, Heats of Hydrogenation, can be used to identify the stability of various alkens

• Overall Stabilities:

Synthesis of Alkenes via Elimination Reactions

• Eliminations are most important means of making alkenes

• We will look at two types:

– Dehydrohalogenation of alkyl halides

– Dehydration of Alcohols

Dehydrohalogenation of Alkyl Halides

• It is best to promote E2– Use 2o and 3o alkyl halides– Use bulky base with 1o

– Use high concentration/Strong base– Use alkoxides– Use high temps

Zaitsev’s Rule

• So far, we have seen examples where only one double bond product, or equivalent double bond products, were possible

• Examples:

Zaitsev’s Rule

• In some cases, more than one product is possible

• Example:

• When a small base, such as ethoxide or hydroxide, is used, the most stable alkene will form

Zaitsev’s Rule

• The most stable alkene will be the more substituted alkene, as we saw with the heats of hydrogenation

• Whenever an elimination occurs to give the most highly substituted alkene, it is said to follow Zaitsev’s Rule.

• The reasoning:– T.S. lower energy– Happens faster

Kinetic Control

• In general, the preferential formation of one product because the free energy of activation is lower than another product, therefore, the rate of its formation is faster, is called Kinetic Control of Product Formation.

Hofmann’s Rule

• If a bulky base is used, such a potassium t-butoxide, the formation of the less substituted alkene is favored

• This is due to the steric bulk of the base and its access to the beta hydrogen be eliminated

• Example• When an elimination yields the less substituted

alkene, it is said to follow Hofmann’s Rule

Stereochemistry of the E2 reaction

• Experimental evidence shows that the five atoms in the T.S. of an E2 reaction must lie in the same plane.

• There are two ways this can happen:– Anti-coplanar (preferred)– Syn-Coplanar (only occurs with certain rigid

molecules)

Stereochemistry of the E2 reaction

• Part of the evidence comes from cyclic molecules.

• Consider 1-chlorocyclohexane in the chair:

• Neither an axial-equatorial nor an equatorial-equatorial orientation allows formation of an anti-coplanar T.S.

Stereochemistry of the E2 reaction

• Consider the different behavior of two diastereomers of 2-methyl-1-isopropyl-4-methylcyclohexane:

Acid Catalyzed Dehydration of Alcohols

• Heating most alcohols with a strong Acid causes the alcohol to lose the equivalent of a molecule of water (dehydrate) and form an alkene

• Generic Reaction:

• This reaction is an elimination and is favored at high temps

• The most commonly used acids in labs are Sulfuric Acid and Phosphoric Acid

Important Characteristics of Dehydration Reactions

1) The temperature and concentration of acid required depends on structure of alcohol

-Primary most difficult to dehydrate-Secondary uses milder conditions-Tertiary extremely easy to dehydrate

2) Some primary and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration

Mechanisms for Dehydration of 2o and 3o Alcohols

• Secondary and Tertiary alcohols dehydrate via E1 mechanism

• The slow step, RDS, is the second step, the formation of the carbocation

• This explains the order of reactivity with the tertiary alcohol reacting easiest, due to the tertiary carbocation being the most stable.

Mechanism for Dehydration of 1o Alcohols

• Because primary carbocations are so unstable, primary alcohols dehydrate via E2, not E1

• Example:

Rearrangements

• Understanding the stability of carbocations, we can now explain the rearrangement seen in some dehydrations.

• Example:

– The rearrangment occurs in the second step to form a more stable carbocation.

– 1,2 shift- when atoms or groups rearrange to an adjacent carbon.

Rearrangements

• Rearrangements will always occur when an alkyl group or a hydrogen can shift to form a more stable carbocation!!

• 1,2-methyl shift• 1,2-hydride shift

• Remember, these shifts occur to increase stability so other forms of stability must be considered as well!

Rearrangements

• Only the carbocation rearranges, so dehydration of primary alcohols can not have rearrangements since they are E2 and not carbocation is formed!

• However, as we will see in Ch 8, the alkene product can react with the acid by using its pi electrons to abstract a proton from an acid forming a carbocation.

Rearrangements

• This is technically not a rearrangement!• It is three consecutive reactions! • A dehydration followed by an addition

followed by another elimination!!

Synthesis of Alkynes by Elimination

• Alkynes can be made from alkenes via compounds called Vicinal Dihalides.

• Vicinal Dihalide- a hydrocarbon with halogens on adjacent carbons.

• Vicinal dihalides can be made by the addition of halides to alkenes.– ex

Synthesis of Alkynes by Elimination

• Vicinal dihalides can then undergo two consecutive eliminations to form alkynes.

• If the alkyne formed is a terminal alkyne, then we will have to use 3 equivalents of base and follow with acid to get the product because the acetylenic hydrogen is acidic!

Synthesis of Alkynes by Elimination

• Geminal Dihalides can also be converted to alkynes via a double dehydrohalogenation

• Geminal Dihalide- a hydrocarbon with two halogens on the same carbon

• Geminal dihalides are made from the reaction of ketones with phosphorus pentachloride

• Ex.

Acidity of Terminal Alkynes

• Remember that the Hydrogen on a terminal alkyne is acidic, but not as acidic as alcohols and water.

• Relative Acidity:

• Strength of the conjugate base is the opposite:

Alkylation of Alkynide Ions, revisited

• Earlier, we saw that the alkynide ion is created when a terminal alkyne is mixed with NaNH3 in ammonia

• We also saw that the alkynide ion can react with a methyl halide or a primary alkyl halide with no branching at the beta carbon

• We should now recognize this as an Sn2 reaction with the alkynide ion as the nucleophile and the alkyl halide as the substrate

Alkylation of Alkynide Ions, revisited

• When the alkyl halide is a secondary or tertiary, the alkylnide acts as a base instead and the reaction becomes an E2

Hydrogenation of Alkenes

• Alkenes will react with hydrogen gas in the presence of a variety of metal catalyst to form alkanes.

• In this reaction, you are adding one hydrogen to each carbon of the double bond

• Reactions where the substrate is soluable and the catalyst is not are called Heterogeneous Catalysis

Hydrogenation of Alkenes

• Reactions where both the catalyst and the substrate are soluble are called Homogeneous catalysis

• Rhodium and Ruthenium complexes with a various phosphorus ligands are used in homogeneous catalysis

• As we have seen, these types of reactions are called Hydrogenation and are an example of an addition reaction.

Mechanism for Catalytic Hydrogenation

• The mechanism is quite different for this reaction because it takes place on the surface of the metal

• This reaction is also an example of syn-addition• Syn-addition- an addition that places parts of

the adding reagent on the same side of the reactant

Anti-addition

• The opposite of syn-addition is an anti-addition in which the parts are added to opposite sides of the reactants

• Ex.

• In chapter 8, we will see a number of examples of syn and anti-additions

Hydrogenation of Alkynes

• Typical conditions reduce alkynes to alkanes.

• There are special reagents that will reduce alkynes only to the alkenes.

1) The P-2 catalyst is formed when Nickel acetate is combined with Sodium Borohydride

Hydrogenation of Alkynes

• The P-2 catalyst allows the syn-addition of 1 molecule of hydrogen to an alkyne which results in the cis-alkene.

Hydrogenation of Alkynes

2) Another special catalyst, called Lindlar’s catalyst, also performs syn-addition of only 1 molecule of hydrogen to alkynes

-Lindlar’s catalyst is formed with Paladium deposited on Calcium carbonate in quinoline

Hydrogenation of Alkynes

3) Anti-addition of hydrogen to an alkyne is performed in the presence of lithium or sodium metal in ammonia. The hydrogen gas is not a reactant and the product is the trans alkene.

7.15 Introduction to Organic Synthesis

• We went over this earlier, deals with why we do it, retrosynthesis, etc.

• Read over this section to review

Double bond equivalents

• Dealing with Nitrogen• When a nitrogen is present, we subtract 1H

for each N atom

• Ex. N 1 DBE

End of Chapter material

• Good summaries/reviews:– p 329 Preparation of alkenes and alkynes– p 335 Summary and review tools– p 336 Synthetic Connections and concept map