Chapter 12: Alkenes and Alkynes

Oxonium ion

An ion in which oxygen is bonded to three other atoms and bears a positive charge

Describe the reaction of an electrophile and a nucleophile to form a new covalent bond.

Another characteristic pattern is the reaction between an electrophile (an electron-poor species that can accept a pair of electrons to form a new covalent bond) and a nucleophile (an electron- rich species that can donate a pair of electrons to form a new covalent bond). The driving force behind this reaction is the strong attraction between the positive and negative charges of the reacting species and the energy released when the covalent bond forms.

Peroxide

Any compound that contains an -O-O- bond as, for example, hydrogen peroxide, H-O-O-H

Describe cis-trans isomerism of an alkene

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. For example, 2-butene has two cis-trans isomers. In cis-2-butene, the two methyl groups are located on the same side of the double bond and the two hydrogens are on the other side. In trans-2-butene, the two methyl groups are located on opposite sides of the double bond. Cis-2-butene and trans-2-butene are different compounds and have different physical and chemical properties.

How do carbon-carbon double bonds effect the structure of the molecule? Angles and 3D? How is increasing level of unsaturation effecting the physical properties of the molecule

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. The VSEPR model predicts bond angles of 120° about each carbon of a double bond. In ethylene, the actual angles are close to 120°. 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. The melting points decrease as the number of double bonds increases.

Monomer

From the Greek mono, single, and meros, part; the simplest nonredundant unit from which a polymer is synthesized

Polymer

From the Greek poly, many, and meros, part; any long-chain molecule synthesized by bonding together many single parts called monomers.

Explain chain-growth polymers

From the perspective of the chemical industry, the single most important reaction of alkenes is the formation of chain-growth polymers (Greek: poly, many, and meros, part). In the presence of certain compounds called initiators, many alkenes form polymers made by the stepwise addition of monomers (Greek: mono, one, and meros, part) to a growing polymer chain, as illustrated by the formation of polyethylene from ethylene. In al- kene polymers of industrial and commercial importance, n is a large number, typically several thousand.

What are characteristic addition reactions of alkenes?

Hydrohalogenation (Hydrochlorination shown in example picture) Hydration Halogenation (Bromination shown in example picture) Hydrogenation (reduction)

What are common mechanism steps for an addition reaction in an alkene?

1. Add a proton 2. Take a proton away 3. Reaction of an electrophile and a nucleophile to form a new covalent bond.

How do we name ALKENES using IUPAC names?

1. Find the longest carbon chain that includes the double bond. Indicate the length of the parent chain by using a prefix that tells the number of carbon atoms in it and the suffix -ene to show that it is an alkene. 2. Number the chain from the end that gives the lower set of numbers to the carbon atoms of the double bond. Designate the position of the double bond by the number of its first carbon. 3. Branched alkenes are named in a manner similar to alkanes; substituent groups are located and named.

Regioselective reaction

A reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions.

Carbocation

A species containing a carbon atom with only three bonds to it and bearing a positive charge

Markovnikov's rule

In the addition of HX (where X = halogen) to an alkene, hydrogen adds to the doubly bonded carbon that already has the greater number of hydrogens bonded to it; halogen adds to the other carbon. Markovnikov's rule is often paraphrased as "the rich get richer."

Reaction mechanism

A step-by-step description of how a chemical reaction occurs. Step 1 is the addition of H+ to an alkene. To show this addition, we use a curved arrow that shows the repositioning of an electron pair from its origin (the tail of the arrow) to its new location (the head of the arrow). We use curved arrows to show bond breaking and bond formation in a reaction mechanism. Step 1 results in the formation of an organic cation. A species containing a positively charged carbon atom is called a carbocation (carbon + cation). Carbocations are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbon groups bonded to the carbon bearing the positive charge.

Hydration mechanism of an alkene

In the presence of an acid catalyst, most commonly concentrated sulfuric acid, water adds to the carbon-carbon double bond of an alkene to give an alcohol. In the case of simple alkenes, hydration follows Markovnikov's rule: H of H2O adds to the carbon of the double bond with the greater number of hydrogens, and OH of H2O adds to the carbon with the smaller number of hydrogens. The mechanism for the acid-catalyzed hydration of an alkene is similar to what we proposed for the addition of HCl, HBr, and HI to an alkene. This mechanism is consistent with the fact that acid is a catalyst. One H+ is consumed in Step 1, but another is generated in Step 3.

Thermal cracking

In thermal cracking, a saturated hydrocarbon is converted to an unsaturated hydrocarbon plus H2. Ethane is thermally cracked by heating it in a furnace to 800-900°C for a fraction of a second.

High-Density Polyethylene (HDPE)

Polyethylene from Ziegler-Natta systems, termed high-density polyethylene (HDPE), has little chain branching. Consequently, its chains pack together more closely than those of LDPE, with the result that the London dispersion forces between chains of HDPE are stronger than those in LDPE. HDPE has a higher melting point than LDPE and is three to ten times stronger. Approximately 45% of all HDPE products are made by the blow-molding process . HDPE is used for consumer items such as milk and water jugs, grocery bags, and squeezable bottles.

Describe the Hydration mechanism by showing the acid-catalyzed hydration of propene

Step 1: Add a proton. Addition of H+ to the carbon of the double bond with the greater number of hydrogens gives a 2° carbocation intermediate. Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond. The carbocation intermediate completes its valence shell by forming a new covalent bond with an unshared pair of electrons of the oxygen atom of H2O to give an oxonium ion. Step 3: Take a proton away. Loss of H1 from the oxonium ion gives the alcohol and generates a new H+catalyst

Describe the reaction mechanism process by using the addition of HCl to 2-Butene as an example.

Step 1: Add a proton. Reaction of the carbon-carbon double bond of the alkene with H1 forms a 2° carbocation intermediate. In forming this intermediate, one bond of the double bond breaks and its pair of electrons is used to form a new covalent bond with H+. One carbon of the double bond is then left with only six electrons in its valence shell and therefore has a positive charge. Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond. Reaction of the 2° carbocation intermediate with a chloride ion completes the valence shell of carbon and gives 2-chlorobutane.

How do we designate CIS and TRANS configurations of ALKENES?

The orientation of the carbon atoms of the parent chain determines whether an alkene is cis or trans. If the carbons of the parent chain are on the same side of the double bond, the alkene is cis; if they are on opposite sides, it is a trans alkene.

How do we show the structure of a polymer?

To show the structure of a polymer, we place parentheses around the repeating monomer unit. The structure of an entire polymer chain can be reproduced by repeating this enclosed structure in both directions. A subscript n is placed outside the parentheses to indicate that this unit is repeated n times, as illustrated for the conversion of propylene to polypropylene.

What are the physical properties of alkenes and alkynes?

Alkenes and alkynes are nonpolar compounds, and the only attractive forces between their molecules are very weak London dispersion forces. Alkenes and alkynes that are liquid at room temperature have densities less than 1.0 g/mL (they float on water). They are insoluble in water but soluble in one another and in other nonpolar organic liquids.

Are addition reactions of alkenes endothermic or exothermic?

Almost all addition reactions of alkenes are exothermic, which means that the products are more stable (have lower energy) than the reactants. Just because an alkene addition reaction is exothermic, however, doesn't mean that it occurs rapidly. The rate of a chemical reaction depends on its activation energy, not on how exothermic or endothermic it is. Many alkene addition reactions require a catalyst.

Describe an alkene reaction when a proton is added. Describe an alkene reaction when a proton is removed.

An acid is a proton donor, a base is a proton acceptor, and an acid-base reaction is a proton-transfer reaction. Curved arrows to show how a proton-transfer reaction takes place as, for example, in the acid-base reaction between acetic acid and ammonia to form acetate ion and ammonium ion. If we run the above reaction in reverse, then it corresponds to taking a proton away from the ammonium ion and transferring it to the acetate ion. We can also use curved arrows to show the flow of electron pairs in this type of reaction.

Describe the two-step mechanism for the addition of a HX to an alkene.

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.

Halogenation mechanism

Chlorine, Cl2, and bromine, Br2, react with alkenes at room temperature by addition of halogen atoms to the carbon atoms of the double bond. When Br2 or Cl2 are added to a cycloalkene, one halogen adds to each carbon of the double bond, regardless of if there is already a substituent on one of those bonds. This reaction is generally carried out by mixing the pure reagents together or by mixing them in an inert solvent such as dichloromethane, CH2Cl2. Addition of bromine is a useful qualitative test for the presence of an alkene. If we dissolve bromine in carbon tetrachloride, the solution is red. In contrast, alkenes and dibromoalkanes are colorless. If we mix a few drops of the red bromine solution with an unknown sample suspected of being an alkene, disappearance of the red color as bromine adds to the double bond tells us that an alkene is, indeed, present.

How many stereoisomers can polyenes have?

For an alkene with one carbon-carbon double bond that can show cis-trans isomerism, two stereoisomers are possible. For an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n stereoisomers are possible. To show cis-trans isomerism, each carbon of the double bond must have two different groups bonded to it.

Haloalkanes (alkyl halides)

Formed when a hydrogen halide HX (HCl, HBr, and HI) is added to an alkene. Addition of HCl to ethylene, for example, gives chloroethane (ethyl chloride).

How do we name a polymer?

The most common method of naming a polymer is to attach the prefix poly- to the name of the monomer from which the polymer is synthesized— for example, polyethylene and polystyrene. When the name of the monomer consists of two words (for example, the monomer vinyl chloride), its name is enclosed in parentheses.

Describe the reaction of a proton donor with a carbon-carbon double bond to form a new covalent bond.

The double bond provides the pair of electrons that forms a new covalent bond. This pattern is typical in all alkene reactions in which the reaction is catalyzed by an acid. In this step, the carbon-carbon double bond serves as the nucleophile that provides the electron pair to form the new covalent bond. Remember that in a carbon-carbon double bond, two pairs of electrons are shared between the two carbons. An acid-base reaction in which a double bond provides the pair of electrons for the hydrogen transfer creates a carbocation. And remember that a proton, H+, does not exist as such in aqueous solution. Instead, it immediately combines with a water molecule to form the hydronium ion, H3O+.

Low-Density Polyethylene (LDPE)

The first commercial process for ethylene polymerization used peroxide initiators at 500°C and 1000 atm and yielded a tough, transparent poly- mer known as low-density polyethylene (LDPE). At the molecular level, LDPE chains are highly branched, with the result that they do not pack well together and the London dispersion forces between them are weak. LDPE softens and melts at about 115°C, which means that it cannot be used in products that will be exposed to boiling water. Today, approximately 65% of all LDPE is used for the manufacture of films by the blow-molding technique. LDPE film is inexpensive, which makes it ideal for packaging such consumer items as baked goods and vegetables and for the manufacture of trash bags.

How do we name ALKYNES using IUPAC names?

The key to the IUPAC name of an alkyne is the ending -yne, which shows the presence of a carbon-carbon triple bond. In higher alkynes, number the longest carbon chain that contains the triple bond from the end that gives the lower set of numbers to the triply bonded carbons. Indicate the location of the triple bond by the number of its first carbon atom.

What are characteristic reactions of alkenes?

The most characteristic reaction of alkenes is an addition to their carbon-carbon double bond: The double bond is broken, and in its place, single bonds form between the carbons and two new atoms or groups of atoms.

Mechanism: Catalytic Reduction

The transition metal catalysts used in catalytic hydrogenation are able to absorb large quantities of hydrogen onto their surfaces, probably by forming metal-hydrogen bonds. Similarly, alkenes are absorbed on metal surfaces with the formation of carbon-metal bonds. Addition of hydrogen atoms to an alkene occurs in two steps: (a ) Hydrogen and the alkene are absorbed on the metal surface, and (b) one hydrogen atom is transferred to the alkene, forming one new C-H bond. The other carbon remains absorbed on the metal surface. (c ) A second C-H bond is formed, and the alkene is released.

How do we name CYCLOALKENES?

To name a cycloalkene: 1. 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. 2. Note that it is not necessary to explicitly number the position of the double bond in a cycloalkene as in linear alkenes. 3. Number and list substituents in alphabetical order.

Alkynes

Unsaturated hydrocarbons that contain one or more carbon- carbon triple bonds. The simplest alkyne is acetylene. Alkynes are not widespread in nature and have little importance in biochemistry.

Alkenes

Unsaturated hydrocarbons that contain one or more carbon-carbon double bonds. Compounds containing carbon-carbon double bonds are especially wide- spread in nature. Furthermore, several low-molecular-weight alkenes, including ethylene and propene, have enormous commercial importance in our modern industrialized society. The simplest alkene is ethylene. The organic chemical industry world-wide produces more pounds of ethylene than any other organic chemical

Addition of hydrogen: Reduction (hydrogenation)

Virtually all alkenes react quantitatively with molecular hydrogen, H2, in the presence of a transition metal catalyst to give alkanes. Commonly used transition metal catalysts include platinum, palladium, ruthenium, and nickel. Because the conversion of an alkene to an alkane involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or, alternatively, catalytic hydrogenation.

How do we name alkenes that contain more than one double bond?

We name alkenes that contain more than one double bond as alkadienes, alkatrienes, and so on. We often refer to those that contain several double bonds more generally as polyenes.

Using the VSEPR model, predict the bond angles for Alkenes

We predict bond angles of 120° about each carbon in a double bond. The observed H-C-C bond angle in ethylene, for example, is 121.7°, close to the predicted value. In other alkenes, deviations from the predicted angle of 120° may be somewhat larger because of interactions between alkyl groups bonded to the doubly bonded carbons. The C-C-C bond angle in propene, for example, is 124.7°.