This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds.

This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds.

Mechanism for the addition of a hydrogen halide

What happens if the alkene does not have the same substituents on both of the sp 2 carbon? Which sp 2 carbon gets the hydrogen?

The sp 2 carbon that does not become attached to the proton is the carbon that is positively charged in the carbocation. Why is the tert-butyl cation formed faster than the isobutyl cation? We need to take a look at the factors that affect the stability of carbocations.

Alkyl groups bonded to the positively charged carbon decrease the concentration of positive charge on the carbon.

The more stable the transition state, the smaller is the free energy of activation, and therefore, the faster is the reaction.

In both cases, the more stable tertiary carbocation is formed more rapidly than the less stable secondary carbocation, so the major product of each reaction is the one that results from forming the tertiary carbocation.

The two different products of each of these reactions are constitutional isomers. A reaction in which two or more constitutional isomer could be obtained as products, but one of them predominates, is called a regioselective reaction. Regioselectivity is the preferential formation of one constitutional isomer over another. The addition of HBr to 2-pentene is not regioselective. Because both carbocation intermediate have the same stability.

Markonikov s rule The electrophile add to the sp 2 carbon that is bonded to the greater number of hydrogens. H + adds preferentially to C1 because C-1 is bonded tot two hydrogens, where C-2 is bonded to only one hydrogen. H + adds to C1 because that results in the formation of a secondary carbocation, which is more stable than the primary carbocation that would have to be formed if H + added to C-2.

An alkene does not react with water because there is no electrophile present to start a reaction by adding to the nucleophile alkene. The O-H bonds of water are too strongwater is too weakly acidic-to allow the hydrogen to act as an electrophile for this reaction. If an acid is added to the solution, a reaction will occur because the acid provides an electrophile (H + ). The addition of water to molecule is called hydration. An alkene will be hydrated.

Because the catalyst in the hydration of an alkene is an acid, hydration is an acid-catalyzed reaction. Mechanism for the acid-catalyzed addition of water.

Alcohol react with alkenes in the same way that water does, so this reaction too requires an acid catalyst. The product of the reaction is an ether. The mechanism for the acid-catalyzed addition of an alcohol is the same as that of water.

The few drugs in clinical use that contain alkyne functional group are not naturally occuring compounds: they exist only because chemists have been able to synthesize them.

Replacing the ane ending of the alkane with yne. If the triple bond is at the end of the chain, the alkyne is classified as a terminal alkyne. Alkynes with triple bonds located elsewhere along the chain are called internal alkynes.

If counting from either direction leads to the same number for the functional group suffix, the correct systematic name is the one that contains the lowest substituent number.

Each of the two p orbitals on one carbon overlaps the parallel p orbital on the other carbon to form two bonds.

All hydrocarbons (alkane, alkene, and alkyne) have similar physical properties. All are insoluble in water and soluble in nonpolar solvents such as hexane. They are less dense than water. They have boiling points that increase with increasing molecular weight. Alkynes are more linear than alkenes, causing alkynes to have stronger van der Waals interactions. Therefore, alkyne has a higher boiling point than an alkene with the same molecular weight.

Alkynes, like alkenes, undergo electrophile addition reactions. The mechanism for electrophilic addition to an alkyne is the same as the mechanism for electrophilic addition to an alkene. Mechanism for the addition of a hydrogen halide

The product of the addition of an electrophilic reagent to an alkyne is an alkene, a second electrophilic addition reaction can occur if excess hydrogenhalide is present.

If the alkyne is a terminal alkyne, the H + will add to the carbon bonded to the hydrogen.

A second addition reaction will take place if excess hydrogen halide is present Addition of a hydrogen halide to an internal alkyne forms two products, because the initial addition of the proton can occur with equal ease to either of the sp carbons.

However, that if the same group is attached to each of the sp carbons of the internal alkyne, only one product will be obtained.

Alkyne also undergo the acid-catalyzed addition of water. The initial product of the reaction is an enol.

The enol immediately rearranges to a ketone A ketone and an enol differ only in the location of a double bond and a hydrogen.

The addition of water to an internal alkyne that has the same group attached to each of the sp carbons forms a single ketone as a product. If the two groups are not identical, two ketones are formed because the initial addition of the proton can occur to either of the sp carbons.

Terminal alkynes are less reactive than internal alkynes toward the addition of water. Terminal alkynes will add water if mercuric ion (Hg 2+ ) is added to the acidic mixture. The mercuric ion is a catalyst. It increase the rate of the addition reaction.

In the presence of a metal catalyst such as platinum or palladium, hydrogen (H 2 ) adds to the double bond of an alkene to form an alkane.

The addition of hydrogen is called hydrogenation. They are catalytic hydrogenations. A reaction that increases the number of C-H bonds in a compound is called a reduction reaction. Hydrogen is adsorbed on the surface of the metal and that all the bond-breaking and bond-forming events occur on the surface of the metal.

The initial product is an alkene, but it is difficult to stop the reaction at that stage because of hydrogen s strong tendency to add to alkenes in the presence of these efficient metal catalysts. The product of the hydrogenation reaction, therefore, is an alkane. The reaction can be stopped at the alkene stage if a partially deactivated metal catalyst is used. The most commonly used catalyst is Lindlar catalyst.

Because the alkyne sits on the surface of the metal catalyst, both hydrogens are delivered to the same side of the triple bond. Therefore, the addition of hydrogen to an internal alkyne in the presence of Lindlar catalyst forms a cis alkene.

An sp carbon is more electronegative than an sp 2 carbon, which is more electronegative than an sp 3 carbon. Because the most acidic compound is the one with the hydrogen attached to the most (for the same size atoms), ethyne is a stronger acid than ethane, and ethane is a stronger acid than ethane.

The stronger the acid, the weaker its conjugate base. To remove a proton from an acid in a reaction that favors products, the base that removes the proton must be stronger than the base that is formed. An amide ion cannot remove a hydrogen bonded to an sp 2 or an sp 3 carbon. Only a hydrogen bonded to an sp carbon is sufficiently acidic to be removed by an amide ion.

Reactions that form C-C bonds are important in the synthesis of organic compounds. One reaction that forms a C-C bond is the reaction of an acetylide ion with an alkyl halides or methyl halides should be used in this reaction.

We can convert terminal alkynes into internal alkynes of any desired chain length simply by choosing an alkyl halide.

Synthetic chemists consider, cost, and yield in designing systheses. In the interest of time, a well-designed synthesis will consist of as few steps as possible (and shoul be easy to carry out). Sometimes a synthesis involving several steps is preferred because the starting materials are inexpensive, the reactions are easy to carry out, and the yield of each step is high. Example 1. starting with 1-butyne, how could you make the ketone shown below?

Work backward Decide how to do the last step. The only reaction you know that forms a ketone is the addition of water to an alkyne. If the alkyne used in the reaction has identical substituents on both sp carbons, only one ketone will be obtained. Thus, 3-hexyne is the best alkyne to use.

3-hexyne can be obtained from the four-carbon starting material by removing the proton from its sp carbon, followed by reaction with a two-carbon alkyl halide. Thus, the synthetic scheme for the synthesis of the desired ketone is given by

The desired product can be prepared from 1-pentene, which can be prepared from 1-pentyne. 1-pentyne can be prepared from ethyne and alkyl halide with three carbons.

Example 3. How could you prepare 3,3-dibromohexane from reagents that contain no more than two carbons?

A polymer is a large molecule made by linking together repeating units of small molecules called monomers. The process of linking them together is called polymerization. Classification of polymers: 1. synthetic polymers (e.g. films, compact discs, food wrap, toys, plastic bottles) 2. biopolymers (e.g. DNA)

Classification of synthetic polymers 1. chain-growth polymers 2. Step-growth polymers Chain-growth polymers are made by chain reactions -the addition of monomers to the end of a growing chain. The monomer used most commonly in chain-growth polymerization are ethylene and substituted ethylenes.

The two most common mechanisms for chain-growth polymerization are cationic polymerization and radical polymerization. Each of these mechanisms has three distinct phases; initiation steps, propagation steps, and termination steps

The initiator is a species that breaks into radicals. Most radical initiators have an O-O bond because such a bond easily breaks.

If the propagating site removes a hydrogen atom from the polymer chain, a branch can grow off the chain at that point. Removing a hydrogen atom from a carbon near the end of a chain leads to short branches, whereas removing a hydrogen atom from a carbon near the middle of chain results in long branches.

Branching greatly affects the physical properties of the polymer. Unbranches chains can pack together more closely than branched chains can. Consequently, linear polyethylene is a relatively hard plastic, whereas branches polyethylene is much more flexible polymer.

Radicals are extreamly reactive species. Radical reactions in biological system also have been implicated in the aging process. Unwanted radicals in biological systems must be destroyed before radical reactions have an opportunity to damage cells. Compounds known as radical inhibitors destroy radicals by creating compounds with only paired electron.