Organic Chemistry: Alkenes and Isomerism

Questions and Answers

How to use the (E)-(Z) system?

Examine the two groups attached to one carbon atom of the double bond, figure out which of the two has the higher priority. 2. Do the same at the other carbon atom. 3. Compare the group of high priority on one carbon atom with the group of high priority on the other carbon atom. If the two groups of high priority are on the same side, they are Z; if not, they are E.

What does Z mean in the context of the (E)-(Z) system?

Means 'together'; the two groups of higher priority are on the same side of the double bond.

What does E mean in the context of the (E)-(Z) system?

Means 'opposite'; the two groups of higher priority are on opposite sides of the double bond.

Are cis or trans isomers of alkenes more stable?

Trans isomers are generally more stable than cis isomers. Signup and view all the answers

What is hydrogenation?

The addition of hydrogen to an alkene, an exothermic reaction. Signup and view all the answers

What affects the relative stabilities of alkenes?

The more substituted the carbon of the double bond is, the more stable the alkene is. Signup and view all the answers

What is the nature of cycloalkenes with five carbon atoms or fewer?

Cycloalkenes with five carbon atoms or fewer must exist in the cis form. Signup and view all the answers

What are elimination reactions in organic chemistry?

The most important means for synthesizing alkenes. Signup and view all the answers

What are methods for alkene synthesis?

Dehydrohalogenation of alkyl halides and dehydration of alcohols. Signup and view all the answers

What are the best conditions to synthesize an alkene by dehydrohalogenation?

Conditions that promote an E2 mechanism. Signup and view all the answers

What conditions favor an E2 mechanism?

<ol> <li>Use 2° or 3° alkyl halide; 2. Use bulky base; 3. High concentration of strong, nonpolarizable base; 4. EtONa/EtOH and t-BuOK/t-BuOH - common bulky bases; 5. High temperature.</li> </ol> Signup and view all the answers

What is Zaitsev's rule?

Whenever an elimination happens and produces the more stable/more highly substituted alkene. Signup and view all the answers

What happens when you use a small base like ethoxide or hydroxide?

The major product of the reaction will be the more highly substituted alkene. Signup and view all the answers

What is meant by kinetic control of product formation?

When the preferential formation of one product occurs because its free energy of activation is lower than another product. Signup and view all the answers

What happens if you use a bulky base like t-BuOK/t-BuOH?

The major product of the reaction will be the less substituted alkene. Signup and view all the answers

What is Hofmann's Rule?

When an elimination happens and produces the less substituted alkene. Signup and view all the answers

What must be the orientation of five atoms involved in the transition state of an E2 reaction?

The five atoms have to be coplanar, or lie in the same plane. Signup and view all the answers

Why do the molecules have to be coplanar in an E2 reaction?

They have to be coplanar because the orbitals involved in the developing pi bond need to be overlapped properly. Signup and view all the answers

What is the preferred transition state geometry in E2 reactions?

Anti coplanar transition state. Signup and view all the answers

What is a syn coplanar transition state?

Only available when anti coplanar is unavailable, occurs with rigid molecules that cannot achieve anti. Signup and view all the answers

What is dehydration in organic chemistry?

Alcohols undergo this to form an alkene when heated with a strong acid. Signup and view all the answers

What type of reaction is acid-catalyzed dehydration of alcohols?

An elimination reaction favored at higher temperatures. Signup and view all the answers

What do the temperature and concentration of acid required to dehydrate an alcohol depend on?

They depend on the structure of the alcohol substrate. Signup and view all the answers

Which alcohols are the most difficult to dehydrate?

Primary alcohols are the most difficult to dehydrate. Signup and view all the answers

Which alcohols are easier to dehydrate?

Secondary alcohols are a bit easier to dehydrate. Signup and view all the answers

Which alcohols dehydrate most easily?

Tertiary alcohols dehydrate very easily. Signup and view all the answers

What is the order of ease for alcohols undergoing dehydration?

3° > 2° > 1°. Signup and view all the answers

What happens when some 1° and 2° alcohols undergo dehydration?

Some of them undergo rearrangements of their carbon skeleton during dehydration. Signup and view all the answers

What type of reaction is the mechanism for dehydration of 2° and 3° alcohols?

An E1 reaction in which the substrate is a protonated alcohol. Signup and view all the answers

What is the order of stability of carbocations?

3° is most stable, methyl is least stable. Signup and view all the answers

What is the slowest step in the dehydration of 2° and 3° alcohols?

The formation of the carbocation. Signup and view all the answers

What describes the reactions where carbocations are formed from protonated alcohols?

They are really really endergonic. Signup and view all the answers

What stabilizes the transition state and also the carbocation?

Delocalization of the charge - three alkyl groups are present to contribute to electron density by hyperconjugation. Signup and view all the answers

Why does the dehydration of 1° alcohols proceed through an E2 mechanism?

Because the primary carbocation required for dehydration by E1 is pretty unstable. Signup and view all the answers

What mechanism does dehydration of a primary alcohol occur by?

E2 mechanism. Signup and view all the answers

What is a methyl shift?

When a methyl group migrates with its pair of electrons to stabilize a carbocation structure. Signup and view all the answers

What is a 1,2 shift?

Also known as a methyl shift. Signup and view all the answers

How does acid-catalyzed dehydration of alcohols follow Zaitsev's Rule?

Formation of the more stable alkene is favored. Signup and view all the answers

What is a hydride shift?

Can lead to a more stable carbocation. Signup and view all the answers

What is ring expansion?

Favorable if ring strain occurs; expansion can relieve the strain. Signup and view all the answers

What is terminal alkyne acidity?

WAYYYYYY more acidic hydrogen than those bonded to alkenes or alkanes. Signup and view all the answers

From what can alkynes be synthesized?

From vicinal dihalides. Signup and view all the answers

What are vicinal dihalides?

A compound with halogens on adjacent carbons. Signup and view all the answers

How can you form vicinal dibromide?

By the addition of bromine to an alkene; the vic-dibromide can then be put through a double dehydrohalogenation. Signup and view all the answers

What is sodium amide NaNH2?

A very strong base that can be used to cause both dehydrohalogenations needed to synthesize alkyne in one step. Signup and view all the answers

If the product of the dehydrohalogenation reactions is going to be an alkyne, how many molar equivalents of sodium amide are required?

3 equivalents. Signup and view all the answers

What is a geminal dihalide?

Has 2 halogen atoms bonded to the same carbon. Signup and view all the answers

Geminal halides can be converted to alkynes by what process?

Dehydrohalogenation. Signup and view all the answers

How can you turn ketones into alkynes?

Using PCl5 to convert the ketone to gem-dichlorides, then synthesize alkynes. Signup and view all the answers

How can you remove the really really acidic H of a terminal alkyne?

You can use a very strong base like sodium amide (NaNH2) to break it off and form an alkynide anion. Signup and view all the answers

What are alkynide ions good for?

They are good nucleophiles to perform C-C bond-forming reactions with 1° alkyl halides or 1° substrates. Signup and view all the answers

What does alkylation of alkynide ions illustrate?

<ol> <li>Preparation of alkynide ion requires Bronsted-Lowry acid/base chemistry; 2. Alkynide ion = Lewis base and acts as a nucleophile; 3. Alkyl halide = electrophile.</li> </ol> Signup and view all the answers

What is hydrogenation of alkenes?

Alkenes react with H in the presence of metal catalysts to add an H to each C of the double bond. Signup and view all the answers

What is heterogeneous catalysis?

When hydrogenation reactions occur with insoluble Pt, palladium, or nickel catalysts. Signup and view all the answers

What is homogeneous catalysis?

When soluble catalysts to the mixture are involved in hydrogenation reactions. Signup and view all the answers

Catalytic hydrogenation reactions are a type of what?

Addition reactions. Signup and view all the answers

What are saturated compounds?

Compounds with only C-C single bonds, containing the maximum number of hydrogen atoms possible. Signup and view all the answers

What are unsaturated compounds?

Compounds with C-C multiple bonds like alkenes and alkynes; they contain fewer than the maximum number of hydrogens. Signup and view all the answers

Unsaturated compounds can be _ to saturated compounds by __.

Reduced by catalytic hydrogenation. Signup and view all the answers

Even though hydrogenation is exothermic, why is a catalyst needed?

There is a high free energy of activation, and a catalyst is needed to lower it so it can take place at room temperature. Signup and view all the answers

What type of addition reaction is catalytic hydrogenation?

Syn addition. Signup and view all the answers

What is syn addition?

When an addition occurs that puts the parts of the adding reagent on the same side/face of the reactant. Signup and view all the answers

What is anti addition?

When an addition occurs that puts the parts of the adding reagent on opposite sides/faces of the reactant. Signup and view all the answers

What is Lindlar's catalyst?

H2, Pd/CaCO3; metallic palladium deposited on calcium carbonate. Signup and view all the answers

What is Lindlar's catalyst good for?

Making cis-alkenes from disubstituted alkynes. Signup and view all the answers

When does anti addition of hydrogen to the triple bond of alkynes occur?

When the alkyne is treated with lithium or sodium metal in ammonia or ethylamine at low temperatures. Signup and view all the answers

What is dissolving metal reduction?

Anti addition of hydrogen to the triple bond of alkynes occurs when treated with Li or Na metal in ammonia or ethylamine at low temperatures. Signup and view all the answers

What does a dissolving metal reduction produce?

It produces an E or trans-alkene. Signup and view all the answers

What does organic synthesis consist of?

<ol> <li>Reactions that convert one functional group to another; 2. Reactions that make new C-C bonds.</li> </ol> Signup and view all the answers

Study Notes

The (E)-(Z) System

Priority is determined by the groups attached to each carbon of a double bond.
If the higher priority groups are on the same side of the double bond, the configuration is Z; if on opposite sides, it is E.

Definitions of Z and E

Z (zusammen) means "together," indicating the higher priority groups are on the same side.
E (entgegen) means "opposite," indicating the higher priority groups are on opposite sides.

Stability of Isomers

Trans isomers of alkenes are generally more stable than cis isomers due to reduced steric strain from crowding.

Hydrogenation

The addition of hydrogen to an alkene, categorized as an exothermic reaction.

Alkene Stability

More substituted carbons in a double bond lead to increased stability of the alkene.

Cycloalkenes

Cycloalkenes with up to five carbons must adopt a cis configuration.

Synthesis of Alkenes

Key methods include elimination reactions, particularly the dehydrohalogenation of alkyl halides and dehydration of alcohols.

Conditions for Dehydrohalogenation

Favorable conditions include using secondary or tertiary alkyl halides, bulky bases, strong, nonpolarizable bases, and high temperatures.

Zaitsev's Rule

In eliminations, the major product tends to be the more stable, more highly substituted alkene.

Kinetics of Alkene Formation

Kinetic control favors quicker product formation due to lower activation energy.

Mechanisms of Dehydration

Primary alcohols usually undergo dehydration via an E2 mechanism, while secondary and tertiary alcohols typically follow an E1 mechanism.

Carbocation Stability

The order of stability is tertiary > secondary > primary; methyl carbocations are the least stable.

Dehydration Temperature and Acidity

Temperature and acid concentration for alcohol dehydration depend on the alcohol’s structure, with primary alcohols being most difficult to dehydrate.

Rearrangements during Dehydration

Some primary and secondary alcohols may undergo carbon skeleton rearrangements during dehydration.

Shifts in Carbocations

Methyl shifts and hydride shifts can stabilize carbocations by redistributing charge.

Terminal Alkyne Acidity

Terminal alkynes have more acidic hydrogens than alkenes or alkanes, allowing for deprotonation with strong bases.

Alkyne Synthesis from Dihalides

Vicinal dihalides and geminal dihalides can be converted to alkynes through dehydrohalogenation.

Use of Sodium Amide

Sodium amide (NaNH2) acts as a strong base in the synthesis of alkynes, requiring three equivalents for alkyne formation.

Alkynide Ions

Alkynide ions are effective nucleophiles for carbon-carbon bond formation, reacting with primary alkyl halides.

Catalytic Hydrogenation

Hydrogenation of alkenes involves adding hydrogen across the double bond, typically using metal catalysts for reaction facilitation.

Addition Reactions

Hydrogenation is categorized under syn addition, where added components are positioned on the same side.

Lindlar's Catalyst

Used to selectively convert alkynes to cis-alkenes.

Dissolving Metal Reduction

Results in anti addition to alkynes, generating E or trans-alkenes through radical mechanisms.

Organic Synthesis Types

Involves reactions that either convert functional groups or generate new carbon-carbon bonds.

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Description

This quiz covers key concepts related to alkenes, including the (E)-(Z) system for isomerism, stability of isomers, and hydrogenation reactions. Test your knowledge on how the configuration and substitution of alkenes affect their stability and reactivity. Perfect for students studying organic chemistry!