Organic Chemistry- Exam 3: Reaction Mechanisms

Functional groups of alcohols and alkyl halides (def, structural, polarity, carbon charge)

• Definition: An atom or group of atoms where all members exhibit similar chemical and physical properties.

• Structural Similarity: Both alcohols (R-OH) and alkyl halides (R-X) feature a carbon atom bonded to an electronegative heteroatom (oxygen or halogen) via a single bond.

• Polarity: The C-heteroatom bond is polar, with a partial negative charge on the heteroatom and a partial positive charge on the carbon.

• Electrophilic Carbon: The partially positive carbon is electron-deficient and acts as an electrophile (Lewis acid).

Alkyl halides naming

• Substitutive Naming (Haloalkane): Treat the halogen as a substituent (e.g., chloro-, bromo-) on the parent alkane chain (e.g., chloroethane, 2-chlorobutane).

• Functional Class Naming: Name the alkyl group followed by the halide (e.g., ethyl chloride, sec-butyl chloride). Primarily used for simpler alkyl halides.

• when there’s multiple chlorides or florides, becomes dichloro or difluro

Alcohols naming

• Suffix Change: Replace the "-e" of the parent alkane with "-ol" (e.g., ethanol).

• Parent Chain Selection: The longest carbon chain containing the carbon bonded to the -OH group.

• Numbering: The carbon bearing the -OH group receives the lowest possible number.

• Location of -OH: Indicated by a number, either before the parent name (e.g., 3-hexanol) or directly before the "-ol" suffix (e.g., hexan-3-ol, preferred IUPAC).

• 2 alcohols= dihexanol

• 3 alcohols= trihexanol

Classification of Alcohols and Alkyl Halides

• Classified based on the number of carbon-containing groups (R) attached to the carbon atom bearing the -OH or halogen (Z).

  • Methyl: Carbon bonded to Z and three hydrogens (e.g., methyl halide, methyl alcohol).

  • Primary (1°): Carbon bonded to Z, one R group, and two hydrogens.

  • Secondary (2°): Carbon bonded to Z, two R groups, and one hydrogen.

  • Tertiary (3°): Carbon bonded to Z and three R groups.

  • Note: Quaternary alcohols/halides are not possible as the carbon can only form four bonds

Physical properties- IMFs

Stronger IMFs require more energy to overcome, leading to higher boiling points.

• Propane (Alkane): Nonpolar, only dispersion forces. Boiling point: -42 °C (gas at room temperature).

• Chloroethane (Alkyl Halide): Polar, dipole-dipole forces. Boiling point: 12 °C (gas at room temperature, used as numbing agent).

• Ethanol (Alcohol): Polar, hydrogen bonding (strongest IMF due to O-H bond). Boiling point: 78 °C (liquid at room temperature, used as antiseptic).

Substitution reactions

• Definition: A reaction where an atom or group of atoms is replaced by another.

• Example: Converting an alcohol into an alkyl halide by reacting it with a hydrogen halide (HX), resulting in the substitution of -OH by -X and the removal of water.

• This overall reaction requires understanding the underlying reaction mechanism.

Substitution reaction formula

R-OH + H-Cl → R-Cl + H2O

Alcohol + hydrogen halide → alkyl halide + water

What is a reaction mechanism

• A detailed, step-by-step account of how bonds are broken and formed to transform starting materials into products.

Elementary step

• A single step in a mechanism that occurs in one collision.

Curved arrows

• Used to depict the movement of electron pairs during bond making and breaking.

Experimental Evidence

• Any proposed mechanism must be consistent with experimental observations, particularly kinetic data (rate laws).

Stepwise reactions

• Reactions that occur in multiple elementary steps, often involving reactive intermediates.

What are reactive intermediates and examples

Transient species formed during a stepwise reaction

• carbocation

• carbanion

• radicals

What is a carbocation

In simples terms:

• C-Z bond is between a carbon and another atom

• When the bond breaks, both electrons go to the z atom to create a -1 charge while the carbon is now electron deficient and has a +1 charge

• The carbon is now sp2 hybridized with an empty p orbital

• the carbon is also electrophilic meaning that is wants electrons and is considered a acid

what is a carbanion

• Formation: Heterolytic bond cleavage of a C-Z bond, where both bonding electrons go to carbon, leaving carbon with a negative charge (a lone pair).

• Structure: Typically sp³ hybridized.

• Reactivity: Nucleophilic (Lewis base), donating electron pairs.

Simple terms:

• C-Z bond is between a carbon and another atom

• When the bond breaks, both electrons go to the carbon atom to create a -1 charge while the z atom is now electron deficient and has a +1 charge

• The carbon is now sp3 hybridized

• the carbon is also nucleophilic meaning that is wants to give electrons and is considered a Lewis base

Radical (free radical)

• Formation: Homolytic bond cleavage, where the C-Z bond splits, with one electron going to carbon and one to Z.

• Curved Arrows: Represented by single-headed (fishhook) arrows.

• Structure: Species with an unpaired electron. (Discussed in Chapter 10).

Homolytic bond cleavage

electrons split between atoms, meaning that when the bond breaks, a 1 electrons is sent to each of the departing groups

heterolytic bond cleavage

electrons from the covalent bond go to only 1 of the departing atoms or groups

Reaction Classification: "2 P's" vs. "4 P's"

• A departmental shorthand to indicate the level of understanding required for a specific reaction.

• 4 P's Reaction: Requires proposing starting materials/reagents, predicting products, identifying electrophile/nucleophile, and providing the arrow-pushing mechanism.

• 2 P's Reaction: Requires proposing starting materials/reagents and predicting products, but not the detailed stepwise mechanism.

SN1 reaction- with alcohol (RX)- what does SN1 mean

• This is a "4 P's" reaction, meaning the mechanism is required.

• SN1 (Substitution Nucleophilic Unimolecular):

  • S: Substitution.

  • N: Nucleophilic (a nucleophile attacks an electrophilic center).

  • 1: Unimolecular rate-determining step (reaction rate depends on only one species).

SN1 reaction- with alcohol (RX)- Step 1

Proton Transfer (Rapid step)

• Do this to make a better leaving group that is more stable on it’s own

• 2 electons from the Oxygen on the alcohol go to the H in the hydrogen halide

• the bond in the hydrogen halide goes to the halide

• Thus, the hydrogen joins the OH to make a H2O group attached to the larger alcohol molecule (+1 charge)

• The halide is on it’s own now (-1 charge)

SN1 reaction- with alcohol (RX)- Step 2

Carbocation formation (slow, rate determining step)

• The water molecule in step 1, will break away from the carbon chain to form a carbocation (+1) and a water molecule with a (-1)

• This step is endothermic and requires significant energy input.

• Get sp2 hybridized carbocation and H2O

SN1 reaction- with alcohol (RX)- Step 3

Nucleophilic Attack (rapid)

• The halide from step 1 comes back to join the carbocation, forming a Carbon- halide bond and making the carbon sp3 hybridized now

• Nucleophile attacks electrophile

• This bond formation step is energetically favorable, leading to the final substituted product.

Energy Diagram for SN1

How to distinguish transition states, intermediates, and rate-determining step?

• Visualization: Energy diagrams plot potential energy against the reaction coordinate, illustrating the energy changes throughout the reaction.

• Key Features:

  • Transition States: High-energy points representing species with partial bonds being formed or broken.

  • Intermediates: Energy troughs representing stable, isolable species (e.g., alkyl oxonium ion, carbocation).

  • Rate-Determining Step: The step with the highest activation energy barrier.

Carbocation stability trends

The stability of the carbocation intermediate is critical as its formation is the rate-determining step.

• Stability Trend: Tertiary > Secondary > Primary > Methyl (most stable to least stable).

How does carbocation stability effect reaction rate for SN1

More stable the carbocation, the lower the Ea (activation energy) → faster the reaction

• Activation Energy: A more stable carbocation intermediate leads to a lower activation energy for the rate-determining step.

• Reaction Rate: A lower activation energy results in a faster reaction rate.

• Conclusion: Tertiary alcohols react fastest in SN1 reactions due to the formation of the most stable tertiary carbocation, followed by secondary alcohols.

Stabilizing factors for carbocations- inductive effect

• Inductive Effects: Alkyl groups are electron-donating through sigma bonds, dispersing the positive charge on the carbocation. More alkyl groups lead to greater charge dispersal and stability. Hydrogens are not electron-donating.

• Think of the number of R groups or classification of the carbon

Stabilizing factors for carbocations- hyperconjugation

• Overlap between the empty p-orbital of the carbocation and adjacent sigma bonds (C-H or C-C). This delocalizes the positive charge, further stabilizing the carbocation. More alkyl groups provide more adjacent sigma bonds for hyperconjugation.

• Sharing through space; stabilization of the +1 charge by neighboring C-H sigma bond

Stabilizing factors for carbocations- resonance

• The more resonance structures that exist, the more stable the carbocation becomes due to the charge being able to spread out

Reaction rate of SN1

rate = k[RX]

where RX is the halide

bond breaking is before bond making

How to speed up or slow down reaction rate?

Since the rate depends on how easily the carbocation forms, SN1 will be faster when:

1. the carbocation is more stable

• tertiary vs secondary vs primary

2. Better leaving group

• a longer and weaker bond will break more easily and make the reaction occur faster

• HI > HBr> HCl

• *no HF because it’s a poor leaving group due to the very strong bond

3. Solvent is polar protic ( water, methanol, ethanol) details to come later

Stereochemistry in SN1 reactions

• Nucleophile can attack from either face (top or bottom) of the carbocation… same or opposite face as the leaving group is approaching

• Preference is for the nucleophile to attack the face opposite to where the leaving group is leaving due to less steric hinderance

Stereochemistry in SN1 reactions- Chiral center

• Attacking from opposite face yields inversion of configuration (R → S)

• Attacking from same side yields retention of configuration (R→ R)

• Leads to the formation of enantiomers and diastereomers

Stereochemistry in SN1 reactions- Chiral center- racemization

• Racemization: More stable carbocations (e.g., tertiary) allow more time for the leaving group to fully diffuse, leading to a closer to 50/50 mixture of enantiomers. Secondary carbocations often show a major (inversion) and minor (retention) product ratio.

Stereochemistry in SN1 reactions- No chiral center

• prefer attack from opposite face, but will not change if attacking from either side

Carbocation Rearrangements- why do they occur

• Carbocation rearrangements are possible in SN1 mechanisms and can lead to unexpected products where the carbon skeleton has changed.

• Driving Force: Rearrangements occur to form a more stable carbocation (e.g., a secondary carbocation rearranging to a tertiary carbocation).

Carbocation Rearrangements- What are they?

• 1,2-Shift: This is an elementary step where a group (hydride or alkyl) shifts from an adjacent carbon (position 2) to the carbocation center (position 1).

  • 1,2-Hydride Shift: Migration of a hydrogen atom with its pair of electrons ((\text{H}^-)). These are generally more rapid.

  • 1,2-Alkyl Shift: Migration of an alkyl group (e.g., methyl) with its pair of electrons.

Evidence: Rearrangements are indicated by unexpected product structures, spectroscopic data showing a changed carbon skeleton, or explicit hints in problem statement

Overall goal: make the carbon with the halogen more stable

SN1 Reaction with neutral nucleophiles

• additional 3rd step after slow rate determining step and fast nucleophile attack; only used for neutral nucleophiles to achieve a neutral product

• Neutral Nucleophiles (e.g., water):

  • The nucleophile attacks the carbocation, forming a protonated intermediate (e.g., protonated alcohol). (OH2)

  • A subsequent deprotonation step (proton transfer) by a base (e.g., water or the leaving group) yields the neutral product.

• Form pronated alcohol onto carbocation, then use water to remove extra hydrogen, produce H3O (hydronium)

Nucleophile definition

Donates electron pair to form a new bond

will have a negative charge

Lewis base

common ones include

OH

NH3

H2O

Cl

Br

CN

Leaving group trends

1) Weakest halogen bond makes the best leaving group

• longer and weaker bonds are easier to break

• I> Br > Cl > F

2) Weaker base

• The weaker base makes the better leaving group

Alternative Alkyl Halide Synthesis from Alcohols

• When HX reaction have strongly acidic conditions, the conditions are not suitable for all functional groups

• goal is to develop alternative methods to convert alcohols into alkyl halides to expand synthetic opportunities.

Alternative Alkyl Halide Synthesis from Alcohols- SoCl2 + Pyridine

• Start with carbon chain and alcohol

• Use SoCl2 and pyridine for substitution

• The Cl replaced the OH

  • Always use opposite attack to get major inversion

• Produce SO2 gas, Pyridine with additional H attached to N and Cl (-1)

• Note: This is a "2P reaction," meaning the detailed arrow-pushing mechanism is not typically required for exams.

Alternative Alkyl Halide Synthesis from Alcohols- PBr3 or PCl3

• Reagents: Phosphorus tribromide (PBr3) for alkyl bromides, or phosphorus trichloride (PCl3) for alkyl chlorides.

• Reaction: Alcohol reacts with PBr3 or PCl3 to form the corresponding alkyl halide.

• Pyridine: Not typically required as a base.

• Stereochemistry: Results in predominant inversion of configuration.

• Byproduct: The alcohol's OH group substitutes onto the phosphorus.

• Note: Also a "2P reaction."

Essentially one of the CL or BR switch places with the OH

Alternative Alkyl Halide Synthesis from Alcohols- Alkyl Sulfonates def.

• Purpose: To create a derivative that reacts like an alkyl halide but allows for better control over stereochemistry.

• Key Advantage: The carbon-oxygen bond of the alcohol is not broken during sulfonate formation. This preserves the original stereochemistry.

• Stereochemistry: The stereochemistry at the carbon bearing the oxygen is retained because the C-O bond remains intact. This is crucial for multi-step syntheses.

Alternative Alkyl Halide Synthesis from Alcohols- Alkyl Sulfonates MsCl and TsCl

• Reagents: Sulfonyl chloride (e.g., mesyl chloride or tosyl chloride) and pyridine.

• Sulfonyl Chlorides:

  • Methane Sulfonyl Chloride (Mesyl Chloride, MsCl): Contains a methyl group. Forms an alkyl mesylate.

  • p-Toluene Sulfonyl Chloride (Tosyl Chloride, TsCl): Contains a p-toluene group. Forms an alkyl tosylate.

  • Either get OTs or OMs on the alcohol group, only the OH bond breaks

Alkyl Sulfonates

• Tosylates as Leaving Groups:

  • Alcohols, which are poor leaving groups, can be converted into tosylates (good leaving groups) using tosyl chloride (TsCl) and pyridine.

  • Tosylates behave similarly to alkyl halides in SN1/SN2 reactions.

• Stereochemical Advantage:

  • The key advantage of using tosylates is that the carbon-oxygen bond of the alcohol does not break during tosylate formation, preserving the stereochemistry of that carbon.

  • This allows for subsequent SN2 reactions with a strong nucleophile (e.g., cyanide), ensuring inversion of configuration and precise stereochemical control, which would be difficult to achieve directly with the alcohol (e.g., reacting with HCN would likely lead to SN1 and loss of stereochemistry).

SN2 Mechanism: what is it?

Concerted (bond breaking and bond forming occur simultaneously) rate determining step

• No carbocation

• Nomenclature: "SN2" stands for Substitution, Nucleophilic, and Bimolecular (meaning the rate-determining step is second order).

SN2 Mechanism- Step 1

• Step 1 (Proton Transfer): Alcohol is protonated to form an alkyloxonium species (similar to SN1).

• Same as SN1 where the O in the OH gains the H from the hydrogen halide and the halide forms a -1 charge

SN2 Mechanism- Step 2

• Step 2 (Concerted Displacement): Bromide attacks the electrophilic carbon as the water molecule (leaving group) simultaneously departs. This is the slow, rate-determining step.

• Nucleophile attack is on the carbon to kick out the leaving group

Possible mechanisms for nucleophilic substitution

R-x +Nu → R-Nu + X

Requirement for Sn1 and Sn2 reaction with alkyl halides (starting material)

• Unreactive Halides (for SN1/SN2):

  • Vinyl Halides: Halogen attached to an sp2 hybridized carbon of an alkene.

  • Aryl Halides: Halogen attached to an sp2 hybridized carbon of an aromatic ring.

  • Reasons for Unreactivity: Unstable carbocations in SN1, and inappropriate geometry/pi bond repulsion in SN2.

• Reactive Halides (with sp3 spacer):

  • Allylic Halides: Halogen on an sp3 carbon adjacent to an alkene.

  • Benzylic Halides: Halogen on an sp3 carbon adjacent to an aromatic ring.

nucleophile

A Lewis base, electron pair donor, attacks the electrophilic carbon.

substrate

The alkyl halide, where the carbon bonded to the halogen is electrophilic due to bond polarity.

leaving group X

• The group that departs during the substitution.

Leaving group trends

• Order: I- > Br- > Cl- > F- (Iodide is the best leaving group, Fluoride is the worst).

• Rationale: Based on periodic properties, atomic size, and bond strength.

  • C-I bond is longer and weaker, making it easier to break.

  • C-F bond is shorter and stronger, requiring more energy to break.

• Basicity Correlation: A better leaving group is a weaker base. I- is the weakest halide base, F- is the strongest.

Why pronate alcohols?

• Hydroxide (OH-) is a strong base and a poor leaving group.

• Protonation converts OH- into water (H2O), which is a weaker base and thus a good leaving group.

Sn1 vs Sn2 basic comparison (bonding, rate law)

• SN2 Process:

  • Bonding: Nucleophile attacks and leaving group departs simultaneously (concerted).

  • Rate-Determining Step: The single elementary step is rate-determining.

  • Rate Law: Second order (rate depends on both halide and nucleophile concentrations).

• SN1 Process:

  • Bonding: Leaving group departs first to form a carbocation (slow step). Then, the nucleophile attacks the carbocation.

  • Rate-Determining Step: The formation of the carbocation is the slow, rate-determining step.Rate Law: First order (rate depends only on halide concentration, not nucleophile, as the nucleophile is not involved in the rate-determining step).

Sn2 Transition State- what does it mean?

• transition State:

  • Highest point of potential energy.

  • Bond between nucleophile and carbon starts to form, while the bond between carbon and leaving group starts to break.

  • Partial negative charges develop on the nucleophile and leaving group.

  • Represented by square brackets and a double dagger symbol ((\ddagger)).

Sn2 Transition State

• Nucleophile approaches 180 degrees opposite to the leaving group (backside attack).

• This is due to steric repulsion and electron-electron repulsion between the nucleophile and leaving group.

• Results in inversion of configuration at the chiral center.

• SN2 is stereospecific: a single stereoisomer starting material yields a single stereoisomer product (e.g., S to R, or R to S).

• If a racemic mixture is used, the ratio of R to S is maintained, but each enantiomer inverts its configuration.

• SN2 only uses backside attack

why does sn2 only have backside attack where as sn1 can have wither one?

because the leaving group and the nucleophile attack occur during the same step

Factors Affecting SN2 Rate- Alkyl Halide Structure

• Reactivity trend: Methyl > Primary > Secondary > Tertiary.

• Tertiary alkyl halides are least reactive due to steric hindrance.

• More R groups on the carbon bonded to the leaving group increase steric hindrance, impeding the nucleophile's approach.

• Increased steric hindrance raises the activation barrier, decreasing the reaction rate.

Factors Affecting SN2 Rate- Nucleophile Characteristics

• Definition: Lewis bases that attack an electrophilic (electron-deficient) atom, typically carbon.

• Nucleophilicity vs. Basicity:

  • Alignment: Stronger bases are generally stronger nucleophiles when comparing:

    • Species with the same nucleophilic atom (e.g., hydroxide vs. acetate).

    • A neutral molecule vs. its conjugate base (e.g., water vs. hydroxide).

    • Elements within the same period (e.g., CH(_3)⁻ vs. NH(_2)⁻ vs. OH⁻ vs. F⁻).

  • Weak bases make great nucleophiles and thus better leaving groups

  • Across a period, basicity decreases

Factors Affecting SN2 Rate- Nucleophile Characteristics- Exception

• Down a group

  • Nucleophilicity increases down a group, but basicity decreases

  • I⁻ is a better nucleophile than F⁻, but F⁻ is a stronger base

  • This is due to polarizability- the ability to distort electron cloud

  • Larger atoms have more diffuse electron clouds, making them more polarizable and better able to donate electrons to the electrophilic carbon. Also, larger ions interact less strongly with solvent molecules, making them less shielded and more accessible.

  • F vs I in H20

    • I is large and less En, not as sheilded by solvent, more able to donate e- pair, better leaving group

Factors Affecting SN2 Rate- Nucleophile Characteristics- steric hinderance

Bulky bases can be strong bases but poor nucleophiles (e.g., tert-butoxide vs. ethoxide). The bulky groups physically block the nucleophilic atom from attacking the electrophilic carbon, hindering its ability to act as a nucleophile

• essentially the ability to act as a nucleophile decreases due to size despite being a strong base

Other Nucleophilicity Rules

1. For 2 Nu with the same nucleophilic atom, stronger base= stronger Nu

2. Negatively charges Nu are stronger nucleophiles than their conjugate acids

3. Across a period, nucleophilicity increases as basicity increases (R → L)

Examples

OH vs COOH3 → OH is the stronger one because of resonance stabilized

CH3 vs F → CH3 is stronger because negative charge is less stable on the less EN atom

Solvents- protic

• contain NH or OH bond → allows for hydrogen bonding

• H20 and Alcohols

Solvents- aprotic

• dipole-dipole w/ no hydrogen bonding

• Acetone, DMSO

• Dipole- dipole only exists in a polar molecule when the dipoles do not cancel

  • OH, C=O, CCl, NH

Solvents Overview

• Affects rate of product formation but not identity

• R-OH and R-X are polar starting materials, just need a polar solvent

• increase in polarity = increase in dielectric constant

Protic solvent affect on nucleophile- sn1

polar protic solvents increase rate of SN1 due to stabilizing H- bonds as LG leaves

Protic solvent affect on nucleophile- sn2

• cations solvated by ion-dipole

• anion solvated by hydrogen bonding

• Solvent decreases the nucleophiles strength, becomes less sheilded by solvent

• weaker nucleophiles slows rate of SN2 reaction

Determining SN1 vs SN2- Alkyl Halide Identity

Methyl of 1 → SN2 (less steric hinderance + unstable carbocation

2 → either

3 → SN1 (lot of steric hinderance + stable carbocation)

Determining SN1 vs SN2- nucleophile strength

Strong Nu favors SN2 b/c rate law depends on nu

• Weak Nucleophile: Favors SN1 because the nucleophile attacks after the rate-determining step.

• Example: A secondary alkyl halide with a strong nucleophile (hydroxide) undergoes SN2, yielding only the inverted product. The same secondary alkyl halide with a weak nucleophile (water) undergoes SN1, yielding a mixture of stereoisomers.

Determining SN1 vs SN2- solvent

protic→ SN1

• Polar Protic Solvents:

  • Contain N-H or O-H bonds, capable of hydrogen bonding (e.g., water, alcohols).

  • Can "screen" or diminish nucleophile strength by solvating anions, decreasing the SN2 rate.

  • Increase the SN1 rate by stabilizing the transition state (leaving group departure) through hydrogen bonding.

  • Solvolysis occurs when the solvent acts as the nucleophile in an SN1

aprotic → SN2

• Have dipole-dipole interactions but no hydrogen bonding (e.g., acetone).

• Do not diminish nucleophile strength as much, thus favoring SN2 reactions.

• Note: While solvent influences rate, substrate and nucleophile are primary considerations for determining the mechanism.

Determining SN1 vs SN2- leaving group

• A good leaving group (a weak base) increases the reaction rate for both SN1 and SN2 mechanisms.

• Example: Alkyl iodides react faster than alkyl bromides because the carbon-iodine bond is longer and weaker, making it easier to break.

organic synthesis

• The process of making molecules, viewed as a creative endeavor.

retrosynthesis

• A planning tool for organic synthesis, involving working backward from the target product.

  • Represented by an open arrow (⇒).

  • Starts from the target product and identifies reactants.

• Use strong nucleophiles (e.g., CN⁻ instead of HCN) to enhance SN2 rates, especially for primary alkyl halides.

Elimination reactions overview

• Role of Bases: Bases attack protons, unlike nucleophiles which attack electron-deficient atoms.

• Mechanism: A base abstracts a proton, the C-H electron pair forms a pi bond, and the leaving group departs.

• Product: An alkene (carbon-carbon double bond).

Alkene structure + properties + types

• Structure: Carbon atoms in a double bond are trigonal planar and sp2 hybridized, with bond angles around 120 degrees.

• Formula: CnH2n for alkenes with no other units of unsaturation, indicating one degree of unsaturation.

• Bonding: Composed of one sigma bond (end-on-end overlap of sp2 orbitals) and one pi bond (side-to-side overlap of unhybridized p orbitals).

• Restricted Rotation: The pi bond prevents rotation around the carbon-carbon double bond axis.

• Pi Bond Weakness: Pi bonds are weaker than sigma bonds, making alkenes reactive in addition reactions.

• Types:

  • Terminal Alkene: Double bond at the end of the chain.

  • Internal Alkene: Double bond within the chain.

  • Cycloalkene: Double bond in a ring.

Naming Alkenes

• Suffix: "-ene" indicates a double bond.

• Numbering Priority: The double bond receives the lowest possible number in the carbon chain.

• Location: The position of the double bond is indicated by the lower of the two carbon numbers it connects (e.g., hex-3-ene).

• Multiple Double Bonds: Use "diene" for two, "triene" for three (e.g., 1,4-pentadiene).

• Functional Group Priority: Alcohols have higher priority than alkenes; the chain is numbered to give the alcohol the lowest number.

• Cycloalkenes: The double bond is implied to be between carbons 1 and 2.

• Alkenyl Substituents: Common names include vinyl (CH₂=CH-) and allyl (CH₂=CH-CH₂-).

Alkene spectroscopy

• ¹H NMR: Vinyl protons (directly attached to sp2 carbons) typically appear in the 4.5-6.5 ppm range due to deshielding.

• IR Spectroscopy:

  • Carbon-carbon double bond stretch (C=C) near 1600 cm⁻¹.

  • sp2 C-H stretch around 3100 cm⁻¹.

Alkene stereoisomerism (Cis and Trans)

• Restricted Rotation: Leads to stereoisomers that cannot interconvert by simple bond rotation.

• Cis/Trans Isomers: Used for simple alkenes where identical groups are either on the same side (cis) or opposite sides (trans) of the double bond (e.g., cis-2-butene, trans-2-butene). These are diastereomers.

Alkene stereoisomerism (E/Z)

• E/Z Isomers: Used for ambiguous cases where cis/trans labels are insufficient (e.g., when different groups are attached to the double bond carbons).

  • Assignment: Assign priorities (1=high, 2=low) to the two groups on each carbon of the double bond based on atomic number (Cahn-Ingold-Prelog rules).

  • E (entgegen): High-priority groups are on opposite sides of the double bond.

  • Z (zusammen): High-priority groups are on the same side of the double bond.

Alkene classification by substitution in order of stability

• Evaluation Method: Stability is evaluated by the number of carbon atoms (R groups) bonded to the carbons of the C=C double bond.

• Classification by Substitution:

  • Ethylene (Unsubstituted): Four hydrogens attached to the double bond carbons.

  • Monosubstituted: One R group, three hydrogens.

  • Disubstituted: Two R groups.

    • Cis: R groups on different carbons, on the same side of the double bond.

    • Trans: R groups on different carbons, on opposite sides of the double bond.

    • Geminal: Both R groups on the same carbon

  • Trisubstituted: Three R groups, one hydrogen.

  • Tetrasubstituted: Four R groups.

Reason for increased stability in alkenes (2)

• Inductive Effects: Sp3 hybridized alkyl groups are electron-donating, stabilizing the sp2 hybridized carbons of the double bond by donating electron density.

• Hyperconjugation: Electron density from neighboring C-H bonds (on sp3 carbons) is shared through space into the empty (\pi^*) orbital of the double bond.

Cis vs Trans alkenes

• Acyclic Systems: Trans alkenes are generally more stable than cis alkenes due to reduced steric hindrance between R groups.

• Cyclic Systems:

  • Fewer than 8 carbons: Only cis double bonds are observed due to ring strain preventing trans geometry.

  • 8 to 11 carbons: Cis is still favored, but trans becomes possible.

  • 12 or more carbons: Trans isomers become more stable and favored.

Elimination Reaction- start materials and general scheme

• Starting Materials: Alcohols and alkyl halides.

• General Scheme: Removal of elements X and Y from adjacent carbons, forming a (\pi) bond between those carbons.

  • One carbon is bonded to X, the other to Y.

  • X and Y are eliminated, and a (\pi) bond forms between the two carbons.

• Beta Elimination (1,2-Elimination):

  • Alpha Carbon: The carbon bonded to the leaving group (X).

  • Beta Carbon: A carbon bonded to the alpha carbon, which must have at least one hydrogen atom (beta hydrogen).

  • Process: The leaving group (X) is removed from the alpha carbon, and a beta hydrogen is removed from a beta carbon, forming a (\pi) bond between the alpha and beta carbons.

Dehydration of Alcohols

• Reaction Type: Elimination of a water molecule from an alcohol.

• Leaving Group: Hydroxyl group ((-\text{OH})) is a poor leaving group.

• Strategy: Protonate the hydroxyl group to convert it into a good leaving group ((-\text{OH}_2^+), which leaves as water).

• Reagents: Strong acids with non-nucleophilic conjugate bases (e.g., (\text{H}_2\text{SO}_4), TsOH) are used to promote elimination over substitution.

  • Non-nucleophilic conjugate bases (e.g., (\text{HSO}_4^-), tosylate) are typically large and sterically hindered.

• Maximizing Alkene Yield:

  • Remove water from the product side (Le Chatelier's principle).

  • Use concentrated acid (dilute acid would push equilibrium back to alcohol)

Regioselectivity in Dehydration (Zaitsev’s Rule)

• Definition: When an elimination reaction can form multiple constitutional isomers, one is favored as the major product.

• Zaitsev's Rule: The more substituted alkene is the major product because it is more stable.

  • To apply: Identify all unique beta carbons with hydrogens. Removal of a beta hydrogen from different beta carbons can lead to different alkene products. The most substituted alkene formed will be the major product.

stereoselectivity in Dehydration

• Definition: When an elimination reaction can form multiple stereoisomers (e.g., cis/trans), one is favored.

• Rule: The trans stereoisomer is generally more stable than the cis stereoisomer and will be the major product, if its formation is geometrically possible.

E1 Mechanism- Alcohol Dehydration: Step 1

• Characteristics: Unimolecular, involves a carbocation intermediate.

• Applicability: Typically for secondary and tertiary alcohols. Primary alcohols generally follow an E2 mechanism. Methanol cannot undergo this reaction as it lacks a beta carbon.

• Steps:

  1. Proton Transfer: The alcohol's oxygen is protonated by the strong acid (e.g., (\text{H}_2\text{SO}_4)) to form a protonated alcohol ((\text{ROH}_2^+)). This step is rapid and reversible.

E1 Mechanism- Alcohol Dehydration: Step 2

1. Loss of Leaving Group: The protonated alcohol loses a water molecule ((\text{H}_2\text{O})), forming a carbocation. This is the slow, rate-determining step.

E1 Mechanism- Alcohol Dehydration: Step 3

1. Deprotonation: A weak base (e.g., (\text{HSO}_4^-) or water) removes a beta hydrogen from an adjacent carbon, forming the (\pi) bond and regenerating the acid catalyst.

• No Competing SN1: Because the conjugate bases of the strong acids used (e.g., (\text{HSO}_4^-)) are non-nucleophilic, substitution reactions are disfavored.