Chemistry Alkenes and Isomerism

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Questions and Answers

How are double bonds numbered in a cyclic alkene?

Using the lowest numbers possible (correct)

From the carbon with the most hydrogens

From carbon 2 to carbon 1 only

In an arbitrary manner

Which statement is true about cis-trans isomerism in alkenes?

Cis isomers have similar groups on opposite sides

All alkenes exhibit cis-trans isomerism

Cis-trans isomerism depends on the arrangement of substituents (correct)

Trans isomers have similar groups on the same side

What must be true for trans cycloalkenes to be stable?

They need at least eight carbon atoms in the ring (correct)

They should be named only if mentioned in IUPAC guidelines

They cannot have any double bonds

They must have fewer than six carbons

What is the significance of the E-Z nomenclature in organic chemistry?

It describes the arrangement of priorities in alkenes Signup and view all the answers

How are the multiplicity of double bonds indicated in alkene names?

By using di-, tri-, or tetra- before the -ene suffix Signup and view all the answers

What is the main factor determining whether cis or trans isomers are formed?

The position of groups on the double bond Signup and view all the answers

Which of the following is an incorrect statement regarding stereochemistry in dienes?

Only the position of one double bond needs to be specified Signup and view all the answers

What characterizes geometric isomerism in alkenes?

The orientation of similar groups relative to the double bond Signup and view all the answers

What type of bond characterizes alkenes?

Carbon-carbon double bond Signup and view all the answers

What is the hybridization of the carbon atoms in alkenes?

sp2 Signup and view all the answers

What is the consequence of the pi bond in ethylene concerning molecular geometry?

The molecule must be coplanar. Signup and view all the answers

How does the length of the C—C bond in alkenes compare to that in alkanes?

It is shorter due to pi bonding. Signup and view all the answers

What is the relationship between elements of unsaturation and the number of hydrogen atoms?

Each element of unsaturation decreases hydrogen count by two. Signup and view all the answers

Which of the following describes the process regarding cis and trans isomers?

They cannot be interconverted without breaking the pi bond. Signup and view all the answers

What is the approximate bond angle between sp2 hybridized carbon atoms?

120° Signup and view all the answers

Which statement about unsaturation is true?

It is indicative of molecular rings or double bonds. Signup and view all the answers

What is the rate-determining step in both E1 and SN1 mechanisms?

Carbocation formation Signup and view all the answers

Which type of base is required for an E2 elimination reaction?

Strong base Signup and view all the answers

What defines the major product in an elimination reaction when more than one product is possible?

Most-stable alkene Signup and view all the answers

What arrangement is required for proper elimination in an E2 reaction?

Anti-coplanar Signup and view all the answers

Which elimination mechanism can lead to the formation of both Zaitsev and Hofmann products?

E2 mechanism with a bulky base Signup and view all the answers

In terms of reactivity among alkyl halides for E2 eliminations, what is the correct order?

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

What is the conformation requirement for E2 reactions occurring on cyclohexanes?

Both leaving group and hydrogen must be axial Signup and view all the answers

What occurs during the first step of the E1 mechanism?

Carbocation is generated Signup and view all the answers

Which of the following statements is true about the boiling points of alkenes?

Boiling points of alkenes increase with mass. Signup and view all the answers

What trend in polarity is observed between cis and trans alkenes?

Cis alkenes exhibit greater polarity than trans alkenes. Signup and view all the answers

How does the stability of double bonds relate to the heat of hydrogenation?

More substituted double bonds generally have a lower heat of hydrogenation. Signup and view all the answers

Which is the least stable configuration among disubstituted alkenes?

Cis isomer Signup and view all the answers

Which of the following statements accurately describes the relationship between stability and heat of hydrogenation?

Higher heat of hydrogenation indicates less stability in alkenes. Signup and view all the answers

What effect do bulky alkyl groups have on the stability of alkenes?

They can enhance stability by increasing the angular separation. Signup and view all the answers

Which characterizes the heat of hydrogenation for alkenes?

It is typically exothermic and indicates alkene stability. Signup and view all the answers

What trend is observed regarding the boiling points of alkenes based on their degree of substitution?

More substituted alkenes have higher boiling points. Signup and view all the answers

What must occur for dehydrohalogenation to take place in an E2 mechanism?

The hydrogen atom and the leaving group must be trans-diaxial. Signup and view all the answers

In which case is elimination favored over substitution?

When the alkyl halide is tertiary. Signup and view all the answers

What characteristic do secondary alkyl halides exhibit in terms of nucleophilic substitution or elimination?

They are influenced by the strength of the nucleophile. Signup and view all the answers

Which step is considered the rate-limiting step in the E1 dehydration mechanism?

Ionization to a carbocation. Signup and view all the answers

According to Zaitsev’s rule, what can be concluded about alkene formation?

The most substituted alkene is preferred. Signup and view all the answers

What role does concentrated H2SO4 play in the dehydration of alcohols?

It protonates the alcohol to form a better leaving group. Signup and view all the answers

When bromine is in an axial position, what configuration allows for the E2 elimination to occur?

The adjacent hydrogen must be axial. Signup and view all the answers

In an E1 mechanism, what common event follows the formation of a carbocation?

Formation of a more stable alkene through rearrangement. Signup and view all the answers

How many unsaturations are present in a compound with the formula C6H7N?

4 unsaturations Signup and view all the answers

When calculating unsaturations, how are halogens treated in comparison to hydrogen atoms?

They replace hydrogen atoms in the count. Signup and view all the answers

In the unsaturation calculation for C4H7Br, what is the reason the final result is one unsaturation?

It accounts for the nitrogen atom in the calculation. Signup and view all the answers

What is the saturated hydrogen count formula applied when there are nitrogen atoms in the compound?

(2 × C) + 2 + N Signup and view all the answers

What must be done with oxygen atoms when calculating unsaturation in a compound?

They are ignored and do not influence the calculation. Signup and view all the answers

In determining the number of unsaturations in C5H8, what is the initial saturated hydrogen count calculated?

12 Signup and view all the answers

What happens to the nomenclature of a compound ending in -ane if it contains a double bond?

It changes to -ene. Signup and view all the answers

What is a key limitation of the unsaturation calculation method discussed?

It cannot differentiate between rings and multiple bonds. Signup and view all the answers

Which statement about the E1 mechanism is correct?

Carbocation formation is the rate-determining step. Signup and view all the answers

What is characteristic of the products formed from an E2 elimination when using a bulky base?

Mixture of products favors the Hofmann product. Signup and view all the answers

In an E2 reaction, what is required for the elimination to occur?

The halide and proton must be anti-coplanar. Signup and view all the answers

Which of the following best describes Zaitsev’s Rule?

The most stable alkene is the major product. Signup and view all the answers

What distinguishes E1 and E2 mechanisms in terms of their reaction steps?

E2 involves concerted formation of alkene and leaving group departure. Signup and view all the answers

Which statement is true regarding the stereochemistry of E2 eliminations?

Most E2 reactions occur with an anti-coplanar transition state. Signup and view all the answers

How does the reactivity of alkyl halides in E2 reactions order according to their structure?

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

In what situation does the Hofmann product predominate in an elimination reaction?

When a bulky base is employed. Signup and view all the answers

What is the minimum number of carbon atoms required for a trans double bond to be stable in cycloalkenes?

8 Signup and view all the answers

Which statement about cyclopropene is accurate regarding its double bond?

The bond angles are compressed to about 60°. Signup and view all the answers

According to Bredt’s Rule, under which conditions can a bridged bicyclic compound have a double bond at a bridgehead position?

If one of the rings contains at least eight carbon atoms. Signup and view all the answers

What is the stability comparison between cis and trans isomers in small cycloalkenes?

Cis isomers are more stable than trans isomers. Signup and view all the answers

Which elimination reaction is described as unimolecular and results in the loss of a hydrogen and a halide?

E1 reaction Signup and view all the answers

In the context of elimination reactions, what characterizes a dehydrohalogenation reaction?

It results in the formation of double bonds. Signup and view all the answers

Which compound is considered stable according to the criteria for cycloalkenes?

Cyclodecene with a trans double bond Signup and view all the answers

What characteristic is unique to E2 elimination reactions compared to E1 reactions?

Requires only one step Signup and view all the answers

How does the sp2 hybridization of carbon atoms influence the molecular geometry of alkenes?

It produces a trigonal planar geometry with 120° angles. Signup and view all the answers

What must occur in order to interconvert between cis and trans isomers of alkenes?

The pi bond must be broken through rotation. Signup and view all the answers

Which of the following statements about elements of unsaturation is accurate?

They decrease the number of hydrogens in the molecule by two. Signup and view all the answers

What is a key factor that allows the formation of a pi bond in alkenes?

The alignment of unpaired electrons in p orbitals of sp2 hybridized carbon atoms. Signup and view all the answers

How do bond lengths in alkenes compare to those in alkanes?

Alkenes have shorter C—C bonds than alkanes due to pi bonding. Signup and view all the answers

What is the correct method to calculate the number of unsaturations in a compound?

Calculate the number of hydrogens expected in a saturated compound and subtract the actual number. Signup and view all the answers

Which statement best describes the relationship between sp2 hybrid orbitals and their s character?

sp2 hybrid orbitals possess more s character than sp3 hybrid orbitals. Signup and view all the answers

What is the significance of the term 'olefins' in relation to alkenes?

It indicates alkenes can form oils under certain conditions. Signup and view all the answers

What is required for the dehydrohalogenation of deuterated 1-bromo-2-methylcyclohexane to occur via the E2 mechanism?

A trans-diaxial relationship between the halide and hydrogen Signup and view all the answers

Which of the following statements best describes the preference for the mechanisms of secondary alkyl halides?

Can promote either SN1 or E2 depending on the strength of the nucleophile Signup and view all the answers

What is the consequence of high temperatures in elimination reactions?

They favor elimination mechanisms Signup and view all the answers

In the E1 dehydration mechanism, which step is considered the rate-limiting step?

Ionization to a carbocation Signup and view all the answers

According to Zaitsev's rule, which statement is accurate regarding the formation of alkenes?

More substituted alkenes are generally more stable and favored Signup and view all the answers

What role does concentrated H2SO4 play in the dehydration of alcohols?

It removes low-boiling alkenes to shift equilibrium Signup and view all the answers

For a biphasic elimination mechanism to work, what condition must be observed in the substrate's conformation?

At least one hydrogen must be anti-periplanar to the leaving group Signup and view all the answers

Which characteristics are typically associated with the use of bulky bases in elimination reactions?

Promotes elimination over substitution Signup and view all the answers

What is the primary reason cis and trans isomers of alkenes cannot interconvert under normal conditions?

The presence of a pi bond restricts rotation. Signup and view all the answers

How do sp2 hybridized carbon atoms differ in character compared to sp3 hybridized carbon atoms?

They possess a higher s character. Signup and view all the answers

When calculating the number of unsaturations in a compound, how should halogens be treated?

They are counted as one hydrogen. Signup and view all the answers

What structural element is characterized as the index of hydrogen deficiency in alkenes?

Carbon–carbon double bonds and cyclic structures. Signup and view all the answers

What is the bond length of a carbon-carbon double bond in alkenes compared to that in alkanes?

It is shorter than C—C bond in alkanes. Signup and view all the answers

What factor contributes to the coplanarity requirement for the formation of a pi bond in ethylene?

The hybridization of the involved carbon atoms. Signup and view all the answers

What is the outcome when high-priority groups on a double bond are replaced with lower-priority groups in terms of E-Z naming?

The nomenclature designates the compound as E. Signup and view all the answers

In a given alkene, if it has two unsaturations and contains nitrogen, how should the initial count of saturated hydrogens be determined?

Using the formula CnH(2n+2) and adjusting for any nitrogen. Signup and view all the answers

Why are trans cycloalkenes typically unstable?

Steric strain occurs due to group orientation. Signup and view all the answers

Which condition is least likely to affect the stable configuration of cyclic compounds containing double bonds?

Overall molecular weight of the compound Signup and view all the answers

What is indicated about the bond angles in sp2 hybridized carbon centers?

They have angles that are roughly 120°. Signup and view all the answers

How does the presence of multiple double bonds in a molecule influence its stereochemistry?

All double bonds must have specified stereochemistry. Signup and view all the answers

When determining priority using the Cahn–Ingold–Prelog rules, which factor primarily dictates the rank of substituents?

Atomic number of the atoms directly bonded to the double bond carbons Signup and view all the answers

What is a common property of geometric isomers in alkenes?

They only differ in spatial arrangement around a double bond. Signup and view all the answers

What is fundamentally unique about the naming convention of alkenes that possess multiple double bonds?

The number of double bonds is denoted with a prefix before the suffix. Signup and view all the answers

Which statement accurately describes the stability of cycloalkenes?

Cis configurations in cycloalkenes are generally more stable. Signup and view all the answers

Which type of alkene has a higher boiling point due to its greater dipole moment?

Cis alkenes Signup and view all the answers

What is the relationship between double bond substitution and heat of hydrogenation?

More substituted double bonds have lower heats of hydrogenation. Signup and view all the answers

Which is true about the stability of alkenes when comparing cis and trans isomers?

Trans isomers generally have lower dipole moments than cis isomers. Signup and view all the answers

How does the density of alkenes compare to water?

Alkenes are less dense than water. Signup and view all the answers

What is the relative stability order of disubstituted isomers in terms of hydrogenation heats?

cis < geminal < trans Signup and view all the answers

What consequence does the presence of bulky alkyl groups have on double bond stability?

They decrease stability due to steric strain experiences. Signup and view all the answers

What does a higher exothermic heat of hydrogenation indicate about an alkene's stability?

The alkene is less stable with higher potential energy. Signup and view all the answers

What effect do alkyl groups have on the pi bond in alkenes?

They donate electron density towards the pi bond, potentially increasing dipole moments. Signup and view all the answers

What factor primarily contributes to the instability of cyclopropene?

Small ring size with high bond angle strain Signup and view all the answers

Which statement is accurate regarding the stability of small cycloalkenes?

The cis isomer tends to be more stable than the trans isomer Signup and view all the answers

What is required for a bridged bicyclic compound to have a double bond at a bridgehead position?

There must be at least eight carbon atoms in one of the rings Signup and view all the answers

Which of the following compounds demonstrates a stable trans double bond?

Cyclodecene Signup and view all the answers

What characterizes the E1 elimination mechanism?

It involves the formation of a carbocation intermediate Signup and view all the answers

Which of the following is a consequence of dehydrohalogenation in alkene synthesis?

Production of alkenes through the loss of HX Signup and view all the answers

Which elimination reaction mechanism is characterized by two groups being lost?

E2 mechanism Signup and view all the answers

What determines whether a given elimination reaction will favor attack at a less substituted or more substituted carbon?

Stability of the resulting alkene Signup and view all the answers

What is the main characteristic of the E1 reaction mechanism?

The rate-determining step involves carbocation formation. Signup and view all the answers

Which elimination product is favored when a bulky base is used in an E2 reaction?

Hofmann product Signup and view all the answers

For an E2 reaction to occur effectively, which arrangement must the hydrogen and halide atoms exhibit?

Anti-coplanar orientation Signup and view all the answers

In the context of elimination reactions, what does Zaitsev’s Rule specify?

The more stable alkene is the major product. Signup and view all the answers

What is the order of reactivity for alkyl halides in E2 eliminations?

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

Which of the following best describes the nature of an E2 elimination reaction?

It is a one-step mechanism. Signup and view all the answers

Which configuration of a cyclohexane is best suited for E2 elimination involving an axial leaving group?

Axial configuration Signup and view all the answers

What is a common post-reaction observation upon carbocation formation in the E1 mechanism?

A stable carbocation is generated. Signup and view all the answers

Study Notes

Alkenes

Alkenes contain carbon-carbon double bonds
They are also called olefins because they form oil-forming gas
The carbon-carbon double bond gives alkenes their reactivity
They have a trigonal planar geometry
The two carbons in the bond are sp2 hybridized, which means they have 120 degree bond angles

Pi Bonds

The unhybridized p orbitals of the carbon atoms overlap to form the pi bond
The pi bond forms when each carbon has one unpaired electron in the p orbital
For the pi bond to form, the two ends of the alkene molecule must be coplanar

Cis-Trans Isomers

These isomers cannot be interconverted
Rotation around the carbon-carbon double bond requires breaking the pi bond
There are two types of isomers: cis and trans
In a cis isomer, the similar groups are on the same side of the double bond
In a trans isomer, the similar groups are on opposite sides of the double bond
Not all alkenes exhibit cis-trans isomerism

Cyclic Compounds

Trans cycloalkenes are unstable unless the ring has at least eight carbons
All cycloalkenes are assumed to be cis unless the trans configuration is explicitly named

E-Z Nomenclature

Cahn-Ingold-Prelog rules are used to assign priorities to groups attached to the carbons in the double bond
In the Z isomer, the high priority groups are on the same side
In the E isomer, the high priority groups are on opposite sides

Commercial Uses of Alkenes

Ethylene has commercial uses as a ripening agent for fruit and a starting material for polymers
Propylene is used in polymers like polypropylene and the production of propylene glycol

Physical Properties of Alkenes

Alkenes have low boiling points.
Branched alkenes have even lower boiling points.
Alkenes are less dense than water.
Alkenes can be slightly polar because the pi bond is polarizable.

Heat of Hydrogenation

Alkenes have different degrees of stability.
Heat of hydrogenation is an exothermic reaction.
More stable alkenes have a lower heat of hydrogenation.
More substituted alkenes are more stable because the bulky alkyl groups are separated by a larger angle

E1 and E2 Reactions

The E1 reaction has two steps:
- Formation of carbocation
- Proton abstraction by a solvent
The E2 reaction is a concerted reaction: the proton is abstracted, the double bond forms, and the leaving group leaves in one step

Zaitsev's Rule

If multiple elimination products are possible, the most substituted alkene is the major product.
The most substituted alkene is the most stable alkene.

E2 Reaction

Order of reactivity for alkyl halides: 3° > 2° > 1°
The E2 reaction is mostly Zaitsev orientated.

Bulky Bases in E2 Reactions

Bulky bases form the Hofmann product, which is less substituted than the Zaitsev product.
These reactions are less prone to substitution.

E2 Stereochemistry

The halide and the proton must be anti-coplanar for the elimination to take place.
This is necessary to ensure the hydrogen and halide orbitals can form a pi bond in the transition state.
The anti-coplanar arrangement minimizes steric hindrance between the base and the leaving group.

Substitution or Elimination

Strong nucleophiles or bases promote bimolecular reactions.
Primary halides usually undergo SN2 reactions.
Tertiary halides are more complex and can undergo a mixture of SN1, E1, or E2 reactions.
Secondary alkyl halides are most challenging and the reaction depends on the strength of the nucleophile.

Dehydration of Alcohols

Concentrated H2SO4 or H3PO4 are used for dehydration.
The reaction proceeds via an E1 mechanism.
Dehydration often results in rearrangements.
The reaction follows Zaitsev's rule

Introduction to Alkenes

Alkenes contain a carbon-carbon double bond.
They are also known as olefins, meaning "oil-forming gas."
The carbon-carbon double bond is the functional group responsible for the reactivity of alkenes.

Structure of Alkenes

Sigma bonds around the double-bonded carbons are sp² hybridized.
The molecular geometry is trigonal planar with bond angles of approximately 120°.
Unhybridized p orbitals overlap to form the pi bond of the double bond.

Bonding in Alkenes

sp² hybrid orbitals have more s character than sp³ hybrid orbitals, leading to shorter C-C bond lengths in alkenes (1.33 Å) compared to alkanes (1.54 Å).
The pi bond is formed by the overlap of unhybridized p orbitals from each carbon, requiring the molecule to be coplanar.

Cis-Trans Isomers

Cis and trans isomers are not interconvertible due to the rigidity of the pi bond.
Rotation around the double bond is not possible without breaking the pi bond.

Elements of Unsaturation

Unsaturation refers to a structural feature that reduces the number of hydrogen atoms in a molecule by two.
Rings and double bonds are considered elements of unsaturation.
Also called the index of hydrogen deficiency.

Calculating Unsaturation

For a saturated compound, the maximum number of hydrogens is calculated as (2xC) + 2.
To find the unsaturation, subtract the actual number of hydrogens from the calculated maximum and divide by 2.

Unsaturation with Heteroatoms

Halogens count as hydrogen atoms when calculating unsaturation.
Oxygen atoms do not affect the calculation.
Nitrogen atoms count as half a carbon atom.

IUPAC Nomenclature of Alkenes

Identify the longest continuous carbon chain containing the double bond.
The suffix "-ane" is replaced with "-ene".
Number the chain to give the double bond the lowest possible number.
In rings, the double bond is assumed to be between carbons 1 and 2.

Cycloalkenes

Cycloalkenes behave like straight-chain alkenes unless they experience ring strain.
Small rings, like cyclopropene, have highly strained double bonds due to compressed angles (60°).

Stability of Cycloalkenes

Cis isomers are more stable than trans isomers in small cycloalkenes.
Rings larger than eight carbons can accommodate stable trans double bonds.
Bredt's rule states that bridged bicyclic compounds cannot have a double bond at a bridgehead position unless one ring has at least eight carbons.

Alkene Synthesis

Elimination reactions are the primary methods for alkene synthesis.
These reactions involve the loss of two groups, often a hydrogen and a halide (dehydrohalogenation).

E1 Reaction

Unimolecular elimination reaction that proceeds in two steps.
Requires a weak base.
The rate-determining step is the formation of a carbocation.

E2 Reaction

Bimolecular elimination reaction that occurs in one concerted step.
Requires a strong base.
The halide and proton must be anti-coplanar (180°) for the elimination to occur.

Zaitsev's Rule

In elimination reactions, the most substituted alkene (most stable) is the major product.

Hofmann Orientation

Bulky bases can favor the formation of the less substituted alkene (Hofmann product).

E2 Stereochemistry

The anti-coplanar geometry of the halide and proton minimizes steric hindrance in the transition state.

E2 Reactions on Cyclohexanes

A trans-diaxial conformation of the hydrogen and halide is required for E2 reactions.

Substitution vs. Elimination

Strong nucleophiles/bases favor bimolecular reactions (SN2/E2).
Weak nucleophiles favor unimolecular reactions (SN1/E1).
Primary halides usually undergo SN2.
Tertiary halides undergo a mixture of SN1, E1, and E2 reactions.
High temperatures and bulky bases favor elimination reactions.

Dehydration of Alcohols

Involves the loss of water from an alcohol using concentrated acid catalysts (H₂SO₄ or H₃PO₄).
Proceeds via an E1 mechanism.
The reaction obeys Zaitsev's rule, leading to the formation of more substituted alkenes.
Rearrangements are common in dehydration reactions.

Introduction to Alkenes

Alkenes are hydrocarbons with carbon-carbon double bonds, also called olefins.
The functional group of alkenes is the carbon-carbon double bond, which creates reactivity.

Sigma Bonds of Ethylene

The sigma bonds around the double-bonded carbon atoms are sp2 hybridized.
Angles are approximately 120°, with trigonal planar molecular geometry.
Unhybridized p orbitals overlap, forming a pi bond.

Bond Lengths and Angles

sp2 hybrid orbitals have a higher s character than sp3 hybrid orbitals.
Pi overlap brings carbon atoms closer, shortening the C—C bond from 1.54 Å in alkanes to 1.33 Å in alkenes.

Pi Bonding in Ethylene

The pi bond in ethylene forms from the overlap of unhybridized p orbitals of the sp2 hybrid carbon atoms.
Each carbon atom has one unpaired electron in the p orbital.
The overlap requires the two ends of the molecule to be coplanar.

Cis-Trans Interconversion

Cis and trans isomers cannot interconvert without breaking the pi bond (264 kJ/mole).
Rotation around the carbon-carbon bond is not possible.

Elements of Unsaturation

Unsaturation decreases the number of hydrogens in a molecule by two.
Also called the index of hydrogen deficiency.
Double bonds and rings are elements of unsaturation.

Calculating Unsaturations

To calculate the number of unsaturations, first find the number of hydrogens the carbons would have if the compound were saturated.
Subtract the actual number of hydrogens then divide by 2.

Multiple Double Bonds

Use di-, tri-, or tetra- before the suffix "-ene" to specify the number of double bonds.
Double bonds are assigned the lowest possible numbers.

Cis-Trans Isomers

Also called geometric isomerism.
If similar groups are on the same side of the double bond, the alkene is cis.
If similar groups are on opposite sides of the double bond, the alkene is trans.
Not all alkenes exhibit cis-trans isomerism.

Cyclic Compounds

Trans cycloalkenes are only stable when the ring has at least eight carbons.
All cycloalkenes are assumed cis unless named trans.

E-Z Nomenclature

The Cahn–Ingold–Prelog rules are used to assign priorities to groups attached to each carbon in the double bond.
If the high-priority groups are on the same side, the configuration is Z (zusammen).
If the high-priority groups are on opposite sides, the configuration is E (entgegen).

Cyclic Stereoisomers

Double bonds outside the ring can exhibit stereoisomerism.

Stereochemistry in Dienes

If there is more than one double bond, specify the stereochemistry at all double bonds.
Commercial Uses of Ethylene
Common uses include: polyethylene production, food ripening, and ethylene oxide production.

Commercial Uses of Propylene

Common uses include: polypropylene production, acrylonitrile production, and synthesis of other chemicals.

Addition Polymers

Polymers formed by adding monomers to a growing chain through double bond cleavage.

Physical Properties of Alkenes

Low boiling points, increasing with mass.
Branched alkenes have lower boiling points.
Less dense than water.
Slightly polar due to the polarizable pi bond and instantaneous dipole–dipole interactions.

Polarity and Dipole Moments of Alkenes

Cis-alkenes have a greater dipole moment than trans alkenes, making them slightly polar.
Cis-alkenes have higher boiling points than trans alkenes.

Heat of Hydrogenation

Combustion and hydrogenation of an alkene can be used to determine the stability of the double bond.
More substituted double bonds have lower heats of hydrogenation.

Relative Heats of Hydrogenation

More substituted double bonds are usually more stable.

HINT

Heats of hydrogenation are exothermic.
A larger amount of heat given off implies a less stable alkene.

Substituent Effects

The isomer with the more substituted double bond has a larger angular separation between the bulky alkyl groups, increasing its stability.

Disubstituted Isomers

Stability: cis < geminal < trans isomer
The less stable isomer has a higher exothermic heat of hydrogenation.

Cycloalkenes

Small rings have additional ring strain.
Five-membered or larger rings can easily accommodate double bonds and react much like straight-chain alkenes.

Cyclopropene

Cyclopropene has bond angles of about 60°, compressing the bond angles of the sp2 carbon atoms.
The double bond in cyclopropene has high strain.

Stability of Cycloalkene

Cis isomers are more stable than trans isomers in small cycloalkenes.
Rings must have at least eight carbons to form a stable trans double bond.
In cyclodecene and larger, the trans double bond is almost as stable as the cis.

Bredt’s Rule

A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.

Solved Problem 1

Analyze the bridged bicyclic systems to determine if they violate Bredt's rule and if the trans double bond is stable.

Alkene Synthesis Overview

E2 dehydrohalogenation (–HX)
E1 dehydrohalogenation (–HX)
Dehalogenation of vicinal dibromides (–X2)
Dehydration of alcohols (–H2O)

Elimination Reactions

Elimination reactions produce double bonds, also known as dehydrohalogenation.

The E1 Reaction

Unimolecular elimination.
Two groups are lost: a hydrogen and a halide.
The nucleophile acts as a base.
The E1 and SN1 reactions occur under the same conditions, leading to a mixture of products.

E1 Mechanism

Step 1: Ionization to form a carbocation.
Step 2: The solvent abstracts a proton to form an alkene.

E1 Energy Diagram

The E1 and SN1 reactions share the same first step: carbocation formation.
Carbocation formation is the rate-determining step.

Zaitsev’s Rule

When more than one elimination product is possible, the most substituted alkene (most stable) is the major product.

Alkene Stability

More substituted alkenes are usually more stable.

The E2 Reaction

Bimolecular elimination.
Requires a strong base.
Concerted reaction: The proton is abstracted, the double bond forms, and the leaving group leaves in one step.

The E2 Mechanism

Reactivity order for alkyl halides: 3° > 2° > 1°
A mixture may form, but the Zaitsev product predominates.

Bulky Bases in E2 Eliminations; Hofmann Orientation

Use a bulky base to minimize substitution products when the substrate is prone to substitution.
Bulky bases can achieve dehydrohalogenations that do not follow Zaitsev's rule, yielding the Hofmann product.

Zaitsev and Hofmann Products

Zaitsev's rule predicts the most substituted alkene as the major product.
Hofmann orientation predicts the least substituted alkene as the major product, often achieved with bulky bases.

E2 Stereochemistry

The halide and the proton must be anti-coplanar (180°) to each other for elimination to occur.
The orbitals of the hydrogen atom and the halide must be aligned for pi bond formation in the transition state.
The anti-coplanar arrangement minimizes steric hindrance between the base and the leaving group.

Stereochemistry of E2 Elimination

Most E2 reactions go through an anti-coplanar transition state.
This geometry is clearest when the alkyl halide is viewed in a Newman projection.

E2 Reactions on Cyclohexanes

An anti-coplanar conformation (180°) is only achieved when both the hydrogen and the halogen occupy axial positions.

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Description

This quiz covers the fundamentals of alkenes, including their structural characteristics and reactivity due to carbon-carbon double bonds. It also explores the concepts of pi bonds and cis-trans isomerism, crucial for understanding the behavior of these compounds. Test your knowledge on how these factors influence the geometry and classification of alkenes.