Chapter 4 1.The most important reaction of alkenes is the addition to the C=C double-bond of various reagents X-Y to yield saturated products 2.A second characteristic reaction of alkenes is the formation of chain-growth polymers Reactions of Alkenes and Alkynes

Electrophilic addition reactions Addition of HX (Hydrohalogenation) Addition of H 2 O Addition of X 2 Addition of H 2 Hydroxylation with KMnO 4 Oxidative cleavage of alkenes with acidic KMnO 4 Polymerization of alkenes Reactions of alkenes

Addition of HX to Alkenes: Hydrohalogenation The of halogen acids, HX, to alkenes is a general reaction that allows chemists to prepare a variety of halo-substituted alkane products

A regiospecific reaction: The reactions are regiospecific (regioselective) when only one of two possible directions of addition occurs Cl H H Cl | | | | CH 3 — C — CH 2 | | CH 3 CH 3

In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents, and the X attaches to the carbon with more alkyl substituents Electrophile; H + Orientation of Alkene Addition Reactions: Markovnilov’s Rule

Carbocation Structure and Stability The electronic structure of a carbocation 1.Bond angles about the positively charged carbon are 120° 2.Carbon uses sp 2 hybrid orbitals to form sigma bonds to the three attached groups 3.The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons

More highly substituted carbocation are more stable Alkyl groups tend to donate electrons to the positively charged carbon atom The more alkyl groups there are, the more electron donation there is and the more stable the carbocation

Addition of H 2 O to Alkenes: Hydration Addition of water is called hydration Acid-catalyzed hydration of an alkene is regioselective - H adds to the less substituted carbon of the double bond Require high temperature and strongly acidic condition

Other methods

Addition of X 2 to Alkenes: Halogenation Carried out with either the pure reagents or in an inert solvent such as CCl 4 or CH 2 Cl 2

A test for a double bond Br 2 (red) → no color

Stereoselective reaction: a reaction in which a single starting material has the capacity to form two or more stereoisomeric products but forms one of them in greater ammounts Anti stereochemistry

Addition of H 2 to Alkenes: Hydrogenation Most alkenes react with H 2 in the presence of a transition metal catalyst to give alkanes –commonly used catalysts are Pt, Pd, Ru, and Ni The process is called catalytic reduction or catalytic hydrogenation Oxidation: the loss of electrons Reduction: the gain of electrons

Syn stereochemistry

Oxidation of Alkenes: Epoxidation, Hydroxylation and Cleavage The addition of oxygen Alkenes are oxidized to give epoxides on treatment with a peroxyacid, RCOOOH

Epoxides undergo an acid-catalyzed ring-opening reaction with water (a hydrolysis) to give the corresponding dialcohol, or diol, also called a glycol Hyrdoxylation, the addition of an -OH group

The hydroxylation of the alkene can also be carried out by reaction with potassium permanganate, KMnO 4, in basic solution The reaction occurs with syn stereochemistry and yields a 1,2-dialcohol, or cis-diol, product (also called glycol)

When oxidation of the alkene is carried out with KMnO 4 in acidic solution, cleavage of the double bond occurs and carbonyl-containing products are obtained The double bond carbons –contain two substutuents: the products are ketone –contain one substutuent: the products are carboxylic acid –contain two hydrogens: the products are CO 2

Addition of Radical to Alkenes: Polymers A polymer is a large molecule built up by repetitive bonding together of many smaller molecules (called monomer) –Cellulose (sugar) –Proteins (amino acid) –Nucleic acid (nucleotide) –Synthetic polymers

Many simple alkenes undergo rapid polymerization when treated with a small amount of a radical as catalyst High pressure ( atm) High temperature ( ℃ ) (several thousand monomers)

Radical polymerization of an alkene involves three kinds of steps: 1.Initiation 2.Propagation 3.Termination In the mechanism, a curved half-arrow, or “fishhook,” is used to show the movement of a single electron

Step 1 Initiation: Reaction begins when a few radicals are generated by the catalyst Benzoyloxy peroxide is used as initiator, the O-O bond is broken on heating to yield benzoyloxy radicals The benzoyloxy radicals then adds to the C=C bond of ethylene to generate a carbon radical

Step 2 Propagation: Polymerization occurs when the carbon radical formed in step 1 adds to another ethylene molecule Repetition of this step for hundreds or thousands of times builds the polymer chain

Step 3 Termination: Polymerization eventually stops when a reaction that consumes the radical occurs Combination of two growing chains is one possible chain-terminating reaction 2 R-CH 2 CH 2 · → R-CH 2 CH 2 CH 2 CH 2 -R

Conjugated Dienes A compound has altering single and double bonds – so-called conjugated compound -- –If the double bonds are well separated in a molecule, they react independently, but they are close together, they may interact with one another Buta-1,3-diene is a conjugated diene, whereas penta-1,4-diene is a non-conjugated diene with isolated double bonds

There is an electronic interaction between the two double bonds of a conjugated diene because of p orbital overlap across the central single bond This interaction of p orbitals across a single bond gives conjugated dienes some unusual properties

HX adds to a conjugated diene, mixtures of products are often obtained 3-Bromobut-1-ene is the typical Markovnikov product of 1,2-addition, but 1-bromobut-2-ene appears unusual (1,4-addition)

Allylic carbocation –Next to the double bond –More stable than nonallylic

Stability of Allylic Carbocations: Resonance All three carbon atoms are sp 2 -hybridized, and each has a p orbital The p orbital on the central carbon can overlap equally well with p orbitals on either of the two neighboring carbons The two electrons are free to move about over the entire three-orbital array

The two individual structures of an allylic carbocation are called resonance forms –The only difference between the resonance forms is the position of the bonding electrons The atoms remain in exactly the same place in both resonance forms – connections and 3-D shapes An allylic carbocation has a single, unchanging structure called a resonance hybrid that is blend of the two individual forms The greater the number of possible resonance forms, the greater the stability – resonance leads to stability

Drawing and Interpreting Resonance Forms The lengths of the two C-O bonds are identical The acetate ion is simply a resonance hybrid of the two resonance forms, with both oxygens sharing the  electrons and the negative charge equally

1.Individual resonance forms are imaginary. ─The real structure is a resonance hybrid of the different resonance forms 2.Resonance forms differ only in the placement of their  or non-bonding electrons

3.Different resonance forms of a substance don’t have to be equivalent

1.Resonance forms must be valid Lewis structures and obey normal rules of valency 2.Resonance leads to stability –The greater the number of resonance forms, the more stable of the substance

Localized electrons –restricted to a particular locality –belong to a single atom or stay in a bond between two atoms Delocalized electrons –not localized on a single atom, nor localized between two atoms −  or non-bonding electrons can be moved to near atoms (sp 2 atoms) 1.Toward a positive charge 2.Toward a  bond 3.Toward the more electronegative of the atoms (only  electrons) A compound with delocalized electrons is said to have resonance

Alkynes and Their Reactions Alkynes are hydrocarbons that contain a carbon- carbon triple bond C ≣ C bond results from the overlap of two sp- hybridized carbon atoms and consists of one sp- sp  bond and two p-p  bonds The general formula is C n H 2n-2 Alkynes are named by general rules similar to those used for alkanes and alkenes The suffix –yne Internal alkynes and terminal alkynes

Compounds containing both double and triple bonds are called enynes (not ynenes) Numbering of the hydrocarbon chain starts from the end nearer the first multiple bond,whether double or triple If there is a choice in numbering, double bond receive lower number than triple bond

Common names: prefix the substituents on the triple bond to the name “ acetylene ” CH 3 C≡CH CH 3 C≡CCH 3 CH 2 =CHC≡CH IUPAC Propyne But-2-yne But-1-en-3-yne Common Methylacetylene Dimethylacetylene Vinylacetylene

Addition of H 2 Lindlar ’ s catalyst can be prepared by precipitating palladium on calcium carbonate and treating it with lead acetate and quinoline Syn-addition Converted into trans alkenes using Na or Li in liquid ammonia

Addition of HX Stopped after addition of 1 equivalent of HX An excess of HX leads to formation of a dihalide product Addition of X 2 Anti-addition

Addition of H 2 O The enol product rearranges to a more stable isomer, a ketone

A mixture of both possible ketones results when an internal alkyne is hydrated Only a single product is formed from reaction of a terminal alkyne

Formation of acetylide anions When a terminal alkyne is treated with a strong base such as sodium amide (NaNH 2 ), the terminal hydrogen is removed and an acetylide anion is formed Acetylide anions are both acidic and nucleophilic

Acetylide anion react with alkyl halides to subsitute for the halogen and yield a new alkyne product It is a very useful method for preparing large alkyne from small alkyne precursors Chapter 7