Chapter 3: Unsaturated Hydrocarbons

Vocabulary flashcards covering key terms and definitions from Chapter 3: Unsaturated Hydrocarbons (alkenes, alkynes, aromatics, nomenclature, isomerism, and addition reactions).

Unsaturated hydrocarbon

A hydrocarbon with one or more carbon–carbon multiple bonds (double or triple); more reactive than saturated hydrocarbons due to the C=C or C≡C bonds.

Functional group

Part of an organic molecule responsible for most of its chemical reactions.

Alkene

An acyclic unsaturated hydrocarbon with one or more C=C double bonds; suffix -ene; formula CnH2n; examples: ethene (ethylene) and propene (propylene).

Cycloalkene

A cyclic unsaturated hydrocarbon with a carbon–carbon double bond in the ring; general formula CnH2n−2 for one double bond; simplest example: cyclopropene.

Alkyne

A hydrocarbon containing a C≡C triple bond; noncyclic alkynes with one triple bond have formula CnH2n−2; acetylene (ethyne) is the simplest; terminal vs internal alkynes; sp hybridization.

Aromatic hydrocarbon (arene)

An unsaturated cyclic hydrocarbon based on a benzene ring; very stable due to conjugation; typically undergo substitution, not addition.

Benzene

A cyclic aromatic hydrocarbon with the formula C6H6; planar ring with delocalized electrons; substitutions are favored over additions.

Arene substitution

Reactions in arenes where a hydrogen is replaced by a substituent (e.g., halogenation with Lewis acids, Friedel–Crafts alkylation).

Ortho/Meta/Para (o/m/p)

Prefixes describing relative positions of substituents on a benzene ring: ortho (1,2-), meta (1,3-), para (1,4-).

Phenyl group

A benzene ring acting as a substituent, written as C6H5−.

Xylene

Dimethylbenzene; three isomers (o-, m-, p-); used as solvents.

Toluene

Monosubstituted benzene derivative with a methyl group; common name for methylbenzene.

Friedel–Crafts alkylation

Alkylates an aromatic ring using an alkyl halide in the presence of a Lewis acid to form a carbocation electrophile.

Resonance

Delocalization of electrons in benzene; real structure is a hybrid of two Kekulé forms rather than a single structure.

Kekulé structure

A proposed alternating single/double-bond structure for benzene that cannot explain equal reactivity of all C–H bonds.

CIP system (priority rules)

Cahn–Ingold–Prelog rules for assigning E/Z configurations and R/S stereochemistry by ranking substituents by atomic number and proceeding outward.

E/Z notation

A stereochemical system for double bonds: E = higher-priority groups on opposite sides; Z = on the same side.

Cis–Trans isomerism

Geometric isomerism around a C=C double bond; cis = same side, trans = opposite sides; more complex with multiple double bonds.

Markovnikov’s rule

In electrophilic addition of HX or H2O to alkenes, the hydrogen adds to the carbon with more hydrogens; the substituent (X or OH) adds to the other carbon.

Hydrogenation

Addition of H2 to alkenes to form alkanes; often catalyst-assisted and typically syn addition.

Lindlar catalyst

A poisoned Pd catalyst on CaCO3 used to hydrogenate alkynes to cis-alkenes selectively (syn addition).

Halogenation (alkenes)

Addition of X2 (X = Cl, Br) to alkenes to form vicinal dihalides; anti addition via halonium ion intermediate.

Hydration (alkenes)

Acid-catalyzed addition of water to alkenes to form alcohols; Markovnikov’s rule applies; proceeds via carbocation intermediate.

Alkyl hydrogen sulfate

Product formed when alkenes react with concentrated sulfuric acid; can be hydrolyzed to alcohols.

Addition polymerization

Polymer formation by successive addition of monomer units with no small-molecule byproducts.

Polyethylene (HDPE/LDPE/LLDPE)

Addition polymers of ethene; HDPE is high-density, linear and rigid; LDPE is branched and flexible; LLDPE is linear with short branches.

Copolymer

Polymer formed from two different monomers (A and B); e.g., Saran Wrap from vinyl chloride and 1,1-dichloroethene.

Alkenyl group

Noncyclic hydrocarbon substituent containing a C=C; examples: ethenyl (vinyl), methylidene.

Terpenes

Natural compounds built from isoprene (C5H8) units; widespread in nature and responsible for many odors.

Isoprene

2-methyl-1,3-butadiene; five-carbon diene; fundamental building block of terpenes.

Aromatic fused-ring hydrocarbons

Arene systems where two or more benzene rings are fused (share carbon atoms); examples include naphthalene and anthracene.

Acetylene (ethyne)

Simplest alkyne, C2H2; terminal vs internal alkynes; sp hybridization; acidic C–H on terminal alkynes.

Acidity of alkynes

Alkynes are more acidic than alkanes/alkenes due to greater s-character in the C–H bond of the terminal triple bond.

Triple-bond (sp) hybridization

Each carbon in an alkyne is sp-hybridized, giving linear geometry about the C≡C bond.

Nomenclature for alkynes

Like alkenes: change -ane to -yne; choose longest chain containing both triple-bond carbons; number to give the triple bond the lowest number; diynes/triynes for multiple triple bonds.

Nomenclature for benzene derivatives

Substituents on benzene are named with prefixes; if a substituent gives a common base name (e.g., toluene), use that base; otherwise, list substituents alphabetically before benzene.

Phenyl vs. benzyl

Phenyl: benzene ring as a substituent (C6H5−); benzyl: benzene ring attached via a CH2 group (C6H5−CH2−).