Organic Chemistry I – Alkyl Halides, Substitution & Elimination

Vocabulary flashcards summarizing core terms and definitions from the lecture on alkyl halides, nucleophilic substitution (SN1/SN2), elimination (E1/E2), stereochemistry, synthetic methods, and related concepts.

Alkyl Halide (Haloalkane)

An organic compound in which a halogen atom (F, Cl, Br, I) is bonded to a sp³-hybridized carbon of an alkyl group (R–X).

Vinyl Halide

A compound where a halogen is bonded directly to an sp² carbon of a C=C double bond (R–CH=CH–X).

Aryl Halide

A compound with a halogen attached to an sp² carbon of an aromatic ring (Ar–X).

C-X Bond Polarization

In alkyl halides the halogen is more electronegative, giving the carbon a partial positive (δ+) and the halogen a partial negative (δ−) charge.

Electronegativity Effect

Greater halogen electronegativity increases C–X bond polarization, enhancing electrophilicity of the carbon.

Primary Alkyl Halide (1°)

Halogen-bearing carbon is attached to one other carbon atom (RCH₂X).

Secondary Alkyl Halide (2°)

Halogen-bearing carbon is attached to two other carbons (R₂CHX).

Tertiary Alkyl Halide (3°)

Halogen-bearing carbon is attached to three other carbons (R₃CX).

Methyl Halide

Halogen attached to a CH₃ group (CH₃X).

Geminal Dihalide

Two halogen atoms bonded to the same carbon (e.g., CH₃–CBr₂–CH₃).

Vicinal Dihalide

Two halogen atoms bonded to adjacent carbons (e.g., CH₂Cl–CH₂Cl).

HX Addition to Alkenes

Electrophilic addition of hydrogen halides (HX) across C=C to form alkyl halides.

Halogen Addition to Alkenes

Addition of X₂ across a double bond yielding vicinal dihalides.

HX Addition to Alkynes

Electrophilic addition giving vinyl or geminal dihalides depending on stoichiometry.

Halogen Addition to Alkynes

Reaction of X₂ with C≡C to form tetra- or di-halogenated products.

Allylic Halogenation

Radical substitution where a halogen replaces an allylic hydrogen adjacent to a double bond.

N-Bromosuccinimide (NBS)

A reagent that maintains low Br₂ concentration, promoting selective allylic bromination.

Nucleophilic Substitution

Reaction where a nucleophile replaces the leaving group on an electrophilic carbon.

Elimination Reaction

Reaction where a base removes H and a leaving group departs, forming a π bond.

Nucleophile

Electron-rich species that donates a pair of electrons to an electrophile.

Basicity

Thermodynamic tendency of a base to accept a proton; measured by pKₐ of its conjugate acid.

Nucleophilicity

Kinetic measure of how rapidly a species attacks an electrophilic carbon.

Charge Effect on Nucleophilicity

Negatively charged species are stronger nucleophiles than their neutral conjugates (e.g., HO⁻ > H₂O).

Periodic Trend (Row)

Nucleophilicity decreases left-to-right across a period due to increasing electronegativity.

Periodic Trend (Column)

Nucleophilicity increases down a group owing to larger size and polarizability (I⁻ > Br⁻ > Cl⁻ > F⁻).

Leaving Group

Atom or group that departs with an electron pair; good leaving groups are weak bases (e.g., I⁻, Br⁻, TsO⁻).

SN1 Mechanism

Unimolecular nucleophilic substitution involving carbocation formation then nucleophilic attack; rate depends only on substrate.

SN2 Mechanism

Bimolecular nucleophilic substitution that occurs in one concerted step with backside attack; rate depends on substrate and nucleophile.

Carbocation Rearrangement

Migration (hydride or alkyl shift) that forms a more stable carbocation during SN1 or E1.

SN1 Stereochemistry

Gives racemic mixture due to planar carbocation allowing attack from either face.

SN2 Stereochemistry

Backside attack inverts configuration at the reacting carbon (Walden inversion).

Steric Hindrance (SN2)

Bulky groups around the electrophilic carbon slow or prevent SN2 reactions.

Polarizability (Nucleophile)

Ability of electron cloud to distort; more polarizable nucleophiles (I⁻) stabilize transition state and react faster in SN2.

Energy Profile of SN2

Single transition state; activation energy lowered by strong nucleophile and good leaving group.

Energy Profile of SN1

Two transition states; first (ionization) is rate-limiting and highest in energy.

E1 Mechanism

Unimolecular elimination: carbocation formation followed by base removal of β-H to form alkene.

E2 Mechanism

Bimolecular elimination: base abstracts β-H while leaving group departs in one concerted step; requires anti-coplanar geometry.

Saytzeff Product

The more substituted, thermodynamically favored alkene formed in elimination.

Hofmann Product

The less substituted alkene produced when bulky base or bulky leaving group controls elimination.

Anti-Coplanar Requirement (E2)

β-H and leaving group must be anti-periplanar (180°) for optimal orbital overlap in E2.

Bulky Base

Sterically hindered base (e.g., tert-butoxide) that favors Hofmann elimination over substitution.

Competition SN1 vs E1

Both share carbocation intermediate; product ratio depends on nucleophile strength versus basicity.

Competition SN2 vs E2

Strong, unhindered bases/nucleophiles may cause both; steric bulk and heat favor E2.

Geminal Dihalide Reduction

Elimination of X₂ (often with I⁻) via E2 mechanism, viewed as a formal reduction (deshalogenation).

Allylic Radical Resonance

Stabilization of an allylic radical by two resonance forms, explaining selectivity in allylic halogenation.

Trihaloethane (1,1,1-Trichloroethane)

Chlorinated solvent historically used for metal cleaning (CCI₃CH₃).

Chloroform (CHCl₃)

Dense, volatile solvent and anesthetic once used medically.

Freon-22 (CHClF₂)

Chlorofluorocarbon refrigerant; phased out due to ozone depletion.

Halothane

CF₃CHClBr, a non-flammable inhalation anesthetic.

Vinyl Chloride

CH₂=CHCl; monomer used to make poly(vinyl chloride) (PVC).

Tetrafluoroethylene (TFE)

F₂C=CF₂; monomer for Teflon® (PTFE).

Para-Dichlorobenzene

An aryl halide (1,4-dichlorobenzene) used as deodorizer and moth repellent.

Thyroxine

Iodine-containing aryl halide hormone regulating metabolism.

Good Leaving Group Examples

I⁻, Br⁻, Cl⁻, tosylate (TsO⁻), water (from protonated alcohol).

Strong Nucleophile Examples

I⁻, HS⁻, HO⁻, RO⁻, CN⁻, N₃⁻, PR₃.

Weak Nucleophile Examples

H₂O, ROH, F⁻, NH₃, alcohols.

Solvent Effect (SN1)

Polar protic solvents stabilize ions, accelerating SN1 reactions.

Solvent Effect (SN2)

Polar aprotic solvents (DMSO, acetone) enhance nucleophilicity and accelerate SN2.

Transition State (SN2)

Pentacoordinate carbon with partial bonds to nucleophile and leaving group.

Transition State (E2)

Base, β-H, C=C forming, and leaving group aligned in a single anti-coplanar arrangement.