Chapter 3 Alkenes and Alkynes

Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds. ethylene is the simplest alkene. Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds. acetylene is the simplest alkyne.

Structure: The VSEPR model predicts bond angles of 120° about each carbon of a double bond. In ethylene, the actual angles are close to 120°. The VSEPR model predicts bond angles of 180° about each carbon of a triple bond.

In substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion between groups bonded to the double bond.

Cis-trans isomerism because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.

Nomenclature To name an alkene; The parent name is that of the longest chain that contains the C=C. Number the chain from the end that gives the lower numbers to the carbons of the C=C. Locate the C=C by the number of its first carbon. Use the ending -ene to show the presence of the C=C Branched-chain alkenes are named in a manner similar to alkanes; substituted groups are located and named.

Examples

Alkynes follow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond.

Common names are still used for some alkenes and alkynes, particularly those of low molecular weight.

To name a cycloalkene: number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first. number and list substituents in alphabetical order.

Dienes, Trienes, and Polyenes Alkenes that contain more than one double bond are named as alkadienes, alkatrienes, and so on. Those that contain several double bonds are referred to more generally as polyenes (Greek: poly, many).

Physical Properties Alkenes and alkynes are nonpolar compounds. The only attractive forces between their molecules are London dispersion forces. Their physical properties are similar to those of alkanes with the same carbon skeletons.

Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids. Alkenes and alkynes that are liquid or solid at room temperature have densities less than 1.0 g/mL; they float on water.

Reactions of Alkenes

Most alkene addition reactions are exothermic. The products are more stable (lower in energy) than the reactants.

Just because they are exothermic doesn’t mean that alkene addition reactions occur rapidly. reaction rate depends on the activation energy Many alkene addition reactions require a catalyst.

Addition of HX Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane. H adds to one carbon of the C=C and X to the other.

reaction is regioselective. Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon.

Chemists account for the addition of HX to an alkene by a two-step reaction mechanism. We use curved arrows to show the repositioning of electron pairs during a chemical reaction. The tail of an arrow shows the origin of the electron pair (either on an atom or in the double bond). The head of the arrow shows its new position. Curved arrows show us which bonds break and which new ones form.

Common Mechanism Steps Pattern 1: Add a proton Pattern 2: Take a proton away

Pattern 3: Reaction of an electrophile and a nucleophile to form a new covalent bond An electrophile is an electron-poor species that can accept a pair of electrons to form a new covalent bond. A nucleophile is an electron-rich species that can donate a pair of electrons to form a new covalent bond.

Variation on a Pattern: Add a proton to a carbon-carbon double bond Simplified version:

Addition of HCl to 2-Butene Step 1: Add a proton reaction of the carbon-carbon double bond with H+ gives a secondary (2°) carbocation intermediate.

Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond reaction of the carbocation intermediate with chloride ion completes the addition.

Addition of H2O Addition of water is called hydration hydration is acid catalyzed, most commonly by H2SO4.

hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.

Addition of H2O to Propene Step 1: Add a proton

Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond

Step 3: Take a proton away

Addition of Cl2 and Br2 Addition takes place readily at room temp. reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent

Addition of H2--Reduction Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni.

The addition of hydrogen to an alkene involving a transition metal catalyst.

Polymerization polymer: Greek: poly, many and meros, part monomer: Greek: mono, single and meros, part

Show the structure of a polymer by placing parentheses around the repeating monomer unit. Place a subscript, n, outside the parentheses to indicate that this unit repeats n times.

The structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions.

Summary of Topics: Chapter 3 Nomenclature Properties (mp/bp; solubility) Addition reactions (Markovnikov’s rule, carbocation intermediates, mechanisms) Hydrohalogenation Hydration Halogenation Hydrogenation (reduction) Polymerization reactions