Alkenes and Alkynes

Alkene

1+ C-C double bond

Alkyne

1+ C-C triple bond

Nomenclature

1. Name Parent Compound

• longest chain containing double or triple bond

• -ene or -yne ending

• if multiple double/triple bonds, indicate # w/ numerical prefix (diene, triene)

2. # C atoms in main chain so that multiple bonds have lowest #s

3. Write full name

• assign #s to branching attachments

• list substituents alphabetically

• indicate position of multiple bonds by giving # of first C — if multiple then use commas and appropriate prefix

Common names

HC=CH — acetylene

H2C=CH2 — ethylene

CH3CH=CH2 — propylene

Bond angles of alkenes and alkynes

alkene: 120

alkyne: 180

Cis-Trans isomerism

same formula & connections btwn atoms, but diff 3d structures since double bonds do not allow rotation around bonds, creates rigidity and possibility for different structures

• cis: groups are on the same side of the double bond

• trans: groups are on opposite sides of the double bond

Properties

• nonpolar

• insoluble in water

• less dense than water

• flammable

• nontoxic

• multiple bonds are chemically reactive

Types of reactions

• Addition

  • Reagant X-Y

  • Hydrogenation

  • Halogenation

  • Hydrohalogenation

  • Hydration

• Elimination

• Substitution

• Rearrangement

Reagant X-Y addition

reagant X-Y adds to multiple bond in unsaturated reactant to yield saturated product w/ only saturated bonds

Hydrogenation addition

Addition of H2 to Alkenes in presence of a solid state metal catalyst (Pd,Pt,Au), yields corresponding alkane product

Halogenation addition

Addition of Cl2 or Br2 to alkenes, use UV and heat

Hydrohalogenation addition

Addition of HBr or HCl to Alkenes to yield alkyl bromides/chlorides

Can have a major product—> use Markovnikov’s rule to determine

Hydration addition

Addition of water to alkenes

think of water as H-OH

Not the same as X-Y addition, alkene will not react with pure water alone —> needs a strong acid catalyst (H2SO4)

Yields an alcohol (R-OH)

Can have a major product —> use Markovnikov’s rule to determine

Elimination Reaction

single reactant splits into 2+ products

converts starting material to product w/ 2 fewer single bonds & a C-C multiple bond in their place +XY

If Hydrogens— needs a solid state catalyst and yields an alkene + H2

Substitution Reaction

2 reactants exchange parts to give 2 new products, uses UV and heat

Br2 (less reactive, more selective) or Cl2 (more reactive, less selective)

If reactant is alkane—one attaches and yields new product + HBr/Cl

If reactant is alkene—two attach and yields new product + H2

Rearrangement reaction

bonds and atoms in reactant are rearranged to yield single product that is an isomer of reactant

breaks, rotates, and reforms the =

uses H2SO4 catalyst

Can switch cis and trans

Markovnikov’s Rule

When adding HX to an alkene—major product has H attaching to = C with larger # H atoms directly attached, and X attaches to C with smallest # H atoms directly attached

Only happens in with molecules that are asymmetrical for hydrogen

• Symmetrical for H: each = C has the same # H directly attached—> products form in equal amounts

• Asymmetrical for H: one = C has more H directly attached than the other—> forms a major product