Alkenes and Alkynes Alkenes Alkenes are unsaturated hydrocarbons

Alkenes and Alkynes

Alkenes • Alkenes are unsaturated hydrocarbons. They contain at least one double bond. • The double bond is made of two different bonds, one sigma , σ, and one pi, π, bond. The carbon atoms in the double bond are sp 2 hybridized, forming a trigonal planar shape with 120 o angles.

Naming Alkenes • 1. Find the longest chain containing the double bond and name it according to the number of carbon atoms, and add the suffix ene. The number that indicates the position of the double bond is placed in front of the parent name. In IB, the position of the double bond is shown before the functional group ending. For example: 1 -butene 2 -butene

Naming Alkenes 2. Number the carbon atoms in the chain beginning with the end closest to the double bond. 3. Assign numbers to the branching substituents and list them in alphabetical order. • For example: 5 -methyl-2 -hexene

Naming Alkynes • The rules are the same as for naming alkenes, except that the suffix should be changed to “yne” 3 -methyl-1 -butyne

Cis –Trans Isomers • Because the carbon atoms in double bonds are not able to rotate, the groups attached to the double bond remain fixed. This situation leads to cis-trans isomers, where the only difference in the structure is the position of the atoms about the double bond. • Cis – same group on same side • Trans – same group opposite sides.

Cis –Trans Isomers • Examples: Cis-3 -hexene Trans-3 -hexene

Cis –Trans Isomers • Draw the Cis and Trans structural formulas for: a. 1, 2 -dibromo-1 -butene b. 3 -hexene

Cis –Trans Isomers • No cis-trans isomers occur if either carbon atom in the double bond is attached to identical groups or atoms. • For example: 2 -bromo-1 -butene

Properties of Alkenes and Alkynes • Non-polar, they are influenced by London dispersion forces. Alkenes and Alkynes with 2 -4 carbons are gases. Their boiling points increase with the size of the molecule. • Insoluble in water, but soluble in non-polar organic solvents. They are less dense than water. • Display cis-trans isomerism • Alkenes and alkynes with low molecular weights are volatile and their vapors are flammable. Mixtures of alkane vapors and air can explode when ignited by a single spark. • Chemically reactive at the multiple bond

Reactions of Alkenes

Hydrogenation – addition of hydrogen • 1. An alkene reacts with H 2 in the presence of a nickel catalyst, at about 150 o C. The product is an alkane. | | —C=C— Example: | + H 2 | — C—C — | | H H

Hydrogenation • This process is used in the margarine industry to convert oils into more saturated compounds, which have higher melting points. This process causes margarine to be a solid at room temperature. • The partially hydrogenated oil is less likely to spoil, so foods made with it have a longer shelf life. • There are now widespread concerns about the health effects of the so called trans fats, which are produced by this process.

Halogenation – addition of halogens • An alkene reacts with a halogen (F 2, Cl 2, Br 2 or I 2) to produce a dihalogenated hydrocarbon. | | —C=C— Example: | + X 2 | — C—C — | | X X

Hydrohalogenation – addition of hydrogen halides • An alkene reacts with a hydrogen halide (HF, HCl, HBr, HI) to produce a halogenoalkane. | | —C=C— Example: | + H-X | — C—C — | | H X

Hydrohalogenation • If the alkene is not symmetrical, Markovnikov’s rule is used to determine how the H-X is added to the double bond. The rule basically states that the H is added to the carbon that has the most H’s. • For example: H H | | CH 3 — C = C — H + H-Cl CH 3 — C—C — H | | Cl H

Hydration- addition of water • An alkene reacts with water in the presence of an acid catalyst ( conc. sulfuric acid) to produce an alcohol. Alcohols contain C—OH. | | —C=C— | + H-OH | — C—C — | | H OH

Hydration

Polymerization of alkenes

Arenes • Arenes are a class of compounds that are derived from benzene. The belong to special branch of organic compounds known as aromatic compounds. • Benzene is the simplest aromatic compound. • Benzene is a flat, symmetrical molecule with the formula C 6 H 6. •

Benzene • It is often represented with a 6 carbon ring with alternating double bonds. • The problem with this representation is that it gives the wrong impression about benzene’s chemical reactivity and bonding, as one might expect that it reacts with hydrogen, halogens, hydrogen halides, and water to give the same products that alkenes do.

Benzene • Benzene and other aromatic compounds are much less reactive than alkenes and don’t undergo addition reactions. • Benzene’s relative lack of chemical reactivity is a consequence of its structure, which is very stable.

Benzene’s Stability

Benzene’s Stability • The sp 2 hybridization, leaves one unhybridized p orbital with one electron on each carbon. • These unhybridized p orbitals do not form alternating pi bonds. Instead, the p orbitals overlap in both directions, allowing these electrons to be shared by all six carbons. This forms a delocalized π electron cloud, in which the electrons are concentrated into two donut-shaped rings above and below the plane of the ring. This is a very stable arrangement.

Benzene • To illustrate the structure of benzene, use a hexagon with a ring inside.