Conjugated Systems, UV Spectroscopy, Ethers, Epoxides, and Thioethers

Vocabulary practice flashcards covering conjugated dienes, orbital symmetry, UV spectroscopy, Diels-Alder reactions, ethers, epoxides, silyl protecting groups, and thioethers.

Conjugated System

A molecular structure occurring whenever $p$ orbitals overlap across three or more adjacent atoms, such as in $1,3\text{-butadiene}$.

$$s\text{-cis}\text{ Conformation}$$

A diene conformation with a $0^\circ$ dihedral angle around the central single bond, placing the double bonds on the same side and enabling parallel $p\text{-orbital}$ overlap required for the Diels-Alder reaction.

$$s\text{-trans}\text{ Conformation}$$

A diene conformation with a $180^\circ$ dihedral angle around the central single bond, where the double bonds are oriented on opposite sides.

Allylic Carbon

A carbon atom directly attached to an $sp^2$ hybridized carbon atom.

Kinetic Product

The reaction product formed faster through a pathway with lower activation energy, dominating at low temperatures (such as $-80\,^\circ\text{C}$ in electrophilic addition to conjugated dienes).

Thermodynamic Product

The reaction product that is lower in overall energy and more stable, dominating at higher temperatures (such as $40\,^\circ\text{C}$) under reaction equilibrium.

N-Bromosuccinimide (NBS)

A solid reagent providing a constant low concentration of bromine in solution, allowing efficient allylic radical halogenation while avoiding alkene addition.

Diels-Alder Reaction

A concerted $[4 + 2]$ cycloaddition reaction between a $1,3\text{-diene}$ and a dienophile that yields a new six-membered ring (cyclohexene derivative).

Dienophile

An alkene or alkyne component in a Diels-Alder reaction, whose reaction rate is increased by attached electron-withdrawing groups.

$$\text{endo Product}$$

The major product of a Diels-Alder reaction with a cyclic diene, formed when the electron-withdrawing group ($Z$) is oriented under the diene in the transition state, placing $Z$ closer to the two new $\sigma$ bonds.

$$\text{exo Product}$$

The minor product of a Diels-Alder reaction with a cyclic diene, formed when the substituent ($Z$) is oriented away from the diene in the transition state, placing $Z$ above the two new $\sigma$ bonds.

Frontier Molecular Orbital (FMO) Theory

A theory (Fukui Postulate) stating that the interaction between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is critical to stabilizing the transition state during a chemical reaction.

$$\lambda_{\text{max}}$$

The specific wavelength at which ultraviolet or visible light absorption by a chemical compound reaches its maximum.

Beer's Law

The equation governing absorbance magnitude, given by $A = \epsilon \times c \times l$, where $A$ is absorbance, $\epsilon$ is molar absorptivity, $c$ is concentration, and $l$ is path length.

Ether

An organic compound containing an oxygen atom flanked by two alkyl or aryl groups ($\text{R–O–R'}$), featuring $sp^3$ oxygen hybridization, a bent geometry with a bond angle of $110^\circ$, and acting as a hydrogen bond acceptor.

Crown Ether

A cyclic polyether capable of complexing metal cations in its central ring cavity, facilitating the dissolution of polar inorganic salts in nonpolar organic solvents.

Epoxide (Oxirane)

A cyclic ether contained within a three-membered ring, possessing approximately $25\,\text{kcal/mol}$ ($105\,\text{kJ/mol}$) of ring strain that drives ring-opening reactions.

Williamson Ether Synthesis

An $\text{S}_{\text{N}}2$ reaction between an alkoxide or phenoxide nucleophile and an unhindered alkyl halide or tosylate to yield symmetrical or unsymmetrical ethers.

Alkoxymercuration-Demercuration

A two-step reaction sequence that converts an alkene and an alcohol into an ether using mercury(II) acetate $\text{Hg(OAc)}_2$ followed by sodium borohydride $\text{NaBH}_4$ reduction with Markovnikov regioselectivity.

Silyl Ether

A functional group with general structure $\text{RO–SiR}_3$ (such as TBDMS ether) widely used as a protecting group for alcohols in organic synthesis.

Tetrabutylammonium Fluoride (TBAF)

A reagent ($(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2)_4\text{N}^+\text{F}^-$) supplying fluoride ions to cleave silyl ethers and regenerate parent alcohols during deprotection.

Halohydrin

An organic compound containing a hydroxy group and a halogen atom on adjacent carbon atoms, converted into an epoxide via base-induced intramolecular $\text{S}_{\text{N}}2$ reaction.

Sulfide (Thioether)

The sulfur analogue of an ether with general structure $\text{R–S–R'}$, synthesized via Williamson ether synthesis using a thiolate anion as a nucleophile.

Sulfonium Salt

An ionic compound containing a tri-substituted, positively charged sulfur atom ($\text{R}_3\text{S}^+$) produced when a sulfide acts as a nucleophile and reacts with an alkyl halide.