Chapter 7 - Alkenes: Structure and Reactivity

Vocabulary practice flashcards covering nomenclature, unsaturation rules, stereochemistry (cis/trans and E/Z), alkene stability, electrophilic additions, Markovnikov's rule, carbocation stability, the Hammond postulate, and carbocation rearrangements from Chapter 7 of McMurry's Organic Chemistry.

Alkene

A hydrocarbon containing a carbon–carbon double bond, also sometimes referred to as an olefin.

Olefin

An alternative historical and industrial term for an alkene.

Thermal Cracking (Steam Cracking)

An industrial preparation method carried out at $900\,^\circ\text{C}$ without a catalyst to convert light alkanes into light alkenes such as ethylene, propylene, and butane.

Degree of Unsaturation

The total number of multiple bonds or rings present in a molecule, where each ring or multiple bond replaces $2\text{ H}$ atoms compared to a saturated acyclic compound ($C_nH_{2n+2}$).

Degree of Unsaturation Rule for Organohalogens

Because a halogen ($\text{F, Cl, Br, I}$) replaces a hydrogen atom, halogens are counted as hydrogens when converting the formula to an equivalent hydrocarbon formula.

Degree of Unsaturation Rule for Organooxygen Compounds

Because oxygen forms two bonds, it does not alter the hydrogen-to-carbon ratio of equivalent hydrocarbons and is ignored when calculating degrees of unsaturation.

Degree of Unsaturation Rule for Organonitrogen Compounds

Because nitrogen forms three bonds and possesses one more hydrogen than a related hydrocarbon, the number of nitrogen atoms is subtracted from the number of hydrogens to determine the equivalent hydrocarbon formula.

Isobutylene

The common name for $2\text{-methylpropene}$.

Isoprene

The common name for $2\text{-methyl-1,3-butadiene}$.

Cis-Trans Isomerism in Alkenes

Stereoisomerism arising because rotation around the carbon–carbon double bond is restricted by the $\pi$ bond; it occurs whenever both double-bond carbons are attached to two different groups.

cis-Isomer

An alkene stereoisomer in which two identical or similar substituent groups are located on the same side of the carbon–carbon double bond.

trans-Isomer

An alkene stereoisomer in which two identical or similar substituent groups are located on opposite sides of the carbon–carbon double bond.

Barrier to Double-Bond Rotation

The energetic cost required to break $\pi$-orbital overlap in an alkene, approximately equal to the strength of the $\pi$ bond ($\approx 350\,kJ\,mol^{-1}$).

E,Z System

A comprehensive stereochemical nomenclature method based on the Cahn–Ingold–Prelog sequence rules used to describe configurations of tri- and tetra-substituted double bonds.

Cahn–Ingold–Prelog Sequence Rules

A set of priority rules that ranks substituents attached to double-bond carbons by atomic number: the higher the atomic number of the directly attached atom, the higher the priority.

Z Geometry

The stereochemical configuration of an alkene where the higher-ranked priority groups on both double-bond carbons reside on the same side of the double bond.

E Geometry

The stereochemical configuration of an alkene where the higher-ranked priority groups on each double-bond carbon reside on opposite sides of the double bond.

Alkene Stability Order

The thermodynamic stability ranking of alkenes based on alkyl substitution: tetrasubstituted > trisubstituted > disubstituted > monosubstituted.

Heat of Hydrogenation ($\Delta H^\circ_\text{hydrog}$)

The enthalpy released upon catalytic reduction of an alkene to an alkane; smaller negative values indicate greater thermodynamic stability of the starting alkene.

Hyperconjugation (in Alkenes)

A stabilizing electronic interaction between electrons in a neighboring filled $\sigma$ orbital and a vacant antibonding $\pi^*$ molecular orbital of the double bond.

Electrophilic Addition Reaction

A two-step reaction wherein an electrophile is attacked by the nucleophilic $\pi$ electrons of an alkene to form a carbocation intermediate, which is subsequently captured by a nucleophile.

Regiospecific Reaction

A reaction in which only one of two possible constitutional isomers or orientations of addition is exclusively formed.

Markovnikov’s Rule

In the addition of $HX$ to an unsymmetrical alkene, the hydrogen attaches to the double-bond carbon with fewer alkyl substituents, and $X$ attaches to the double-bond carbon with more alkyl substituents.

Markovnikov’s Rule (Mechanistic Restatement)

In the addition of $HX$ to an alkene, the reaction proceeds through the more stable (more highly substituted) carbocation intermediate rather than the less substituted one.

Carbocation

A planar, trivalent reaction intermediate containing a positively charged carbon that is $sp^2$-hybridized with six valence electrons and an unoccupied $p$ orbital.

Carbocation Stability Order

The stability ranking of carbocations based on alkyl substitution: tertiary ($3^\circ$) > secondary ($2^\circ$) > primary ($1^\circ$) > methyl ($\text{CH}_3^+$).

Inductive Effect (Carbocation)

The electronic stabilization of a carbocation resulting from polarizable alkyl groups shifting electron density through $\sigma$ bonds toward the positively charged carbon.

Hammond Postulate

A principle stating that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product.

Hydride Shift

A carbocation rearrangement involving the migration of a hydrogen atom along with its electron pair ($:H^-$) from an adjacent carbon to a positively charged carbon to yield a more stable carbocation.

Methyl Shift (Alkyl Shift)

A carbocation rearrangement involving the migration of a methyl or alkyl group with its electron pair ($:R^-$) to an adjacent carbocation center to form a more stable carbocation.