Organic Chemistry: Reaction Mechanisms, Functional Groups, and Aromaticity

Vocabulary flashcards covering key organic chemistry concepts including reaction mechanisms ($$S_N1$$, $$S_N2$$, $$E1$$, $$E2$$), functional groups, reactions of alkenes/alkynes, oxidation/reduction, radicals, dienes, and aromaticity.

Alkyl halide

An organic compound containing a halogen atom bonded to an $sp^3$ hybridized carbon atom ($R-X$).

Vinyl halide

An organic compound featuring a halogen atom bonded directly to an $sp^2$ hybridized carbon of an alkene ($C=C-X$).

Aryl halide

An organic compound featuring a halogen atom bonded directly to an $sp^2$ hybridized carbon of an aromatic ring ($Ar-X$).

Allylic halide

A compound containing a halogen atom bonded to an $sp^3$ hybridized carbon directly adjacent to a carbon-carbon double bond.

Benzylic halide

A compound containing a halogen atom bonded to an $sp^3$ hybridized carbon directly adjacent to a benzene ring.

Bromomethane ($CH_3Br$)

A naturally occurring atmospheric alkyl halide formerly used as a soil fumigant, phased out in developed countries due to ozone-depleting properties.

Dichloromethane ($CH_2Cl_2$)

An important organic solvent previously used in industrial processes to decaffeinate coffee.

Halothane ($CF_3CHClBr$)

A safe general anesthetic compound compared to historical anesthetics like chloroform ($CHCl_3$) and diethyl ether.

Leaving group

An atom or group of atoms that departs with an electron pair during heterolytic cleavage; weaker bases form more stable and better leaving groups.

Polar protic solvent

A solvent containing hydrogen-bond donors (such as $H_2O$ or $ROH$) that solvates anions via hydrogen bonding, decreasing nucleophilicity.

Polar aprotic solvent

A solvent lacking hydrogen-bond donors (such as acetone, DMSO, DMF, or THF) that fails to solvate anions strongly, leaving nucleophiles unshielded and highly reactive.

$S_N2$ mechanism

A concerted, single-step bimolecular nucleophilic substitution mechanism displaying second-order kinetics, backside nucleophilic attack, and stereochemical inversion.

$S_N1$ mechanism

A stepwise unimolecular nucleophilic substitution mechanism involving a rate-determining carbocation intermediate, first-order kinetics, and loss of stereospecificity (racemization).

Hyperconjugation

The delocalization of electron density from adjacent $\text{C-H}$ or $\text{C-C}$ \text{\sigma} bonds into an adjacent empty $p$ orbital, stabilizing carbocations and substituted alkenes.

Carbocation stability order

The relative stability of carbocations, which increases with alkyl substitution due to hyperconjugation and inductive effects: $\text{methyl} < 1^\circ < 2^\circ < 3^\circ$.

Dehydrohalogenation

An elimination reaction of alkyl halides using a base in which elements of $HX$ are lost to form an alkene containing a new $\pi$ bond.

Zaitsev rule

A rule stating that the major product in an elimination reaction is the most substituted, thermodynamically most stable alkene.

$E2$ mechanism

A concerted, one-step bimolecular elimination mechanism exhibiting second-order kinetics, requiring anti-periplanar alignment of the $\beta$-hydrogen and leaving group.

$E1$ mechanism

A stepwise unimolecular elimination mechanism exhibiting first-order kinetics and proceeding through a rate-determining carbocation intermediate.

Anti-periplanar geometry

A stereochemical arrangement where the $\beta$-hydrogen and leaving group lie in the same plane with a dihedral angle of $180^\circ$, allowing optimal orbital overlap during an $E2$ reaction.

Williamson ether synthesis

An $S_N2$ reaction between an alkoxide nucleophile ($RO^-$) and an unhindered primary alkyl halide to prepare symmetrical or unsymmetrical ethers.

Crown ether

A cyclic polyether with a central cavity that selectively binds specific alkali metal cations through ion-dipole interactions (e.g., 18-crown-6 complexes $K^+$).

1,2-Hydride shift

The migration of a hydrogen atom with its pair of bonding electrons from an adjacent carbon to a carbocation, forming a more stable carbocation.

1,2-Alkyl shift

The migration of an alkyl group (such as a methyl group) with its bonding electrons from an adjacent carbon to a carbocation, generating a more stable carbocation.

Tosylate ($OTs$)

An alkyl $p$-toluenesulfonate derivative formed from an alcohol and $p$-toluenesulfonyl chloride ($TsCl$) in pyridine, converting a poor $-OH$ leaving group into an excellent leaving group with retention of configuration.

Thiol

An organic compound containing a sulfhydryl group ($-SH$) bonded to an $sp^3$ hybridized carbon atom.

Sulfide

An organic compound containing a sulfur atom bonded to two alkyl or aryl groups ($R-S-R'$).

Degree of unsaturation

A calculated index representing the total number of rings and $\pi$ bonds in a structure, determined relative to a fully saturated alkane formula ($C_n H_{2n+2}$).

Markovnikov's rule

A rule governing electrophilic addition to unsymmetrical alkenes stating that the electrophile ($H^+$) adds to the carbon with more hydrogen atoms to yield the more stable carbocation intermediate.

Halohydrin

A functional compound containing a halogen atom ($X$) and a hydroxyl group ($-OH$) on adjacent carbon atoms.

Hydroboration-oxidation

A two-step addition sequence converting an alkene or alkyne into an alcohol, aldehyde, or ketone with syn stereospecificity and anti-Markovnikov regioselectivity.

9-borabicyclo[3.3.1]nonane (9-BBN)

A sterically hindered organoborane reagent ($R_2BH$) utilized in hydroboration reactions to achieve enhanced anti-Markovnikov regioselectivity.

Keto-enol tautomerization

An equilibrium process converting an enol (vinylic alcohol) into a constitutional isomer containing a carbonyl group (keto form), strongly favoring the carbonyl compound.

Acetylide ion

A strong nucleophile and base ($R-C\equiv C^-$) generated by deprotonating a terminal alkyne with a strong base like sodium amide ($NaNH_2$).

Lindlar catalyst

A deactivated catalyst comprised of palladium on $CaCO_3$ poisoned with $Pb(OCOCH_3)_2$ and quinoline, which selectively reduces alkynes to cis-alkenes.

Dissolving metal reduction

A reaction using sodium in liquid ammonia ($Na / NH_3$) that reduces internal alkynes specifically to trans-alkenes via radical anion intermediates.

Peroxyacid ($RCO_3H$)

An oxidizing reagent containing an unstable $-O-O-$ bond (such as peroxyacetic acid or $mCPBA$) used to convert alkenes into epoxides via syn addition.

Ozonolysis

An oxidative cleavage reaction that cleaves both $\sigma$ and $\pi$ bonds of alkenes or alkynes using ozone ($O_3$), producing carbonyl or carboxylic acid compounds.

N-bromosuccinimide (NBS)

A reagent that maintains a low concentration of $Br_2$ in solution, allowing selective radical substitution at the allylic position over ionic addition across double bonds.

Antioxidant

A compound (such as Vitamin E or BHT) that intercepts free radicals and forms stable, unreactive radical species, terminating radical chain reactions.

Conjugated diene

A diene in which two carbon-carbon double bonds are separated by a single $\sigma$ bond, permitting contiguous $p$ orbital overlap and electron delocalization.

Thermodynamic product

The most stable product of a reaction (such as 1,4-addition to a conjugated diene) formed preferentially at higher temperatures under reversible reaction conditions.

Kinetic product

The product that forms fastest due to a lower activation energy barrier (such as 1,2-addition to a conjugated diene), dominating at lower temperatures.

Diels-Alder reaction

A thermal $[4+2]$ cycloaddition reaction between a conjugated diene in an s-cis conformation and a dienophile to yield a cyclohexene adduct.

Endo rule

A stereochemical outcome in Diels-Alder reactions with cyclic dienes where electron-withdrawing substituents on the dienophile orient beneath the diene $\pi$ system in the transition state, giving the endo product as major.

Hückel's rule

A requirement for aromaticity stating that a planar, cyclic, completely conjugated system must possess $(4n + 2)$ $\pi$ electrons, where $n$ is a non-negative integer.

Antiaromatic compound

A cyclic, planar, completely conjugated molecule containing $4n$ $\pi$ electrons, rendering it unusually unstable compared to an open-chain counterpart.