Alkenes : Structure and Reactivity

Alkene - Hydrocarbon With Carbon-Carbon Double Bond Includes many naturally occurring materials (beta carotene) The general formula of an alkene with 1 double bond is CnH2n

6.2 Degree of Unsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for saturated a acyclic compound is CnH2n+2 Each ring or multiple bond replaces 2 H's

Summary - Degree of Unsaturation

Example: C6H10 Saturated is C6H14 This has two degrees of unsaturation Therefore 4 H's are not present This has two degrees of unsaturation Two double bonds? or triple bond? or two rings or ring and double bond

Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I) Halogen replaces hydrogen C4H6Br2 and C4H8 have one degree of unsaturation Oxygen atoms - if connected by single bonds These don't affect the total count of H's C4H6O3 should be C4H8O3

Degree of Unsaturation and Variation Compounds with the same degree of unsaturation can have many things in common and still be very different

If C-N Bonds Are Present Nitrogen has three bonds So if it connects where H was, it adds a connection point Subtract one H for equivalent degree of unsaturation in hydrocarbon

Geometric Isomerism in Alkenes Geometric Isomers or Stereoisomers are isomers that differ only in the spatial orientation of different atoms or groups of atoms. They do not differ in the connectivity. Alkenes exhibit geometric isomerism when the two atoms or groups of atoms attached to each of the 2 C atoms of the double bond are different. When a H atom attached to each of the 2 C atoms of ethylene is substituted by a different atom or groups of atoms, a disubstituted alkene is obtained. When 3 of the 4 H atoms of ethylene is substituted a different atom or groups of atoms, a trisubstituted alkene is obtained. When all of the 4 H atoms of ethylene is substituted a different atom or groups of atoms, a tetraisubstituted alkene is obtained. . In a disubstituted alkene, when the 2 substituents are on the same C of the double bond, it is called a geminal disubstituted alkene Two geometric isomers (cis isomer & trans isomer) exist for disubstituted alkenes with substituents on both C’s The cis isomer has H atoms on the same side of the double bond while the trans isomer has H atoms on opposite sides of the double bond.

Disubstituted alkenes areclassified as being geminal or cis or trans. But for trisubstituted and tetrasubstituted alkenes, the terms cis and trans are replaced by Z (instead of cis) and E (instead of trans) respectively. The E isomer has SUBSTITUTENTS OF THE SAME PRIORITY on OPPOSITE SIDES of the double bond The Z isomer has SUBSTITUTENTS OF THE SAME PRIORITY on the SAME SIDE of the double bond The Cahn Ingold Prelog rules are used for assigning priorities for the substituents attached to the double bond Carbons. One substituent is assigned the lower priority and the other higher priority. The E isomer has groups of lower priority (and hence those of higher priority as well) on opposite sides of the double bond. The Z isomer has groups of lower priority (and hence those of higher priority as well) on the same side of the double bond.

Cahn-Ingold-Prelog Rules for Naming Geometric Isomers – E and Z isomers We first examine the immediate atoms attached to the C atoms of the double bond. Higher the atomic number, higher the priority. If the atoms directly attached to the double bond C are identical, examine the 2nd or the 3rd or the 4th and so on atoms until a difference is found. The substituent carrying the higher atomic number is assigned the higher priority. Multiple bonded atoms are treated as the same number of single-bonded atoms. That is, C=C is treated as if each C atom is bonded to 2 C atoms and CC is treated as if each C atom is attached to 3 C atoms. Therefore a triple bond substituent is assigned higher priority than a double bond substituent and a double bond substituent is assigned higher priority than a single bond substituent

6.3 Naming of Alkenes Find longest continuous carbon chain containing the double bond – parent chain. The name of this chain is similar to the name of the corresponding alkane with the same number of C atoms but with ending being ‘ene’ instead of ane. The position of the double bond is specified by the first C of the double bond encountered while numbering from end closest to double bond. 2. When there is more than one double bond in the compound, the ending diene , triene, tetraene etc are used. Numbers are required for all the double bonds 3. Number carbons in chain so that double bond carbons have lowest possible numbers. 4. When the double bond is in a cyclic alkene, the ring is numbered so that the double bond is between Carbons 1 and 2. Numbering is then clockwise or counterclockwise to assign lowest numbers to substituents. 5. Name substituents as you did for the alkanes and specify the numbers of the C atoms on the parent chain to which they are attached. 6. If alkene can exhibit geometric isomerism, use prefixes E or Z to specify arrangement of substituents relative to double bonds. If more than 1 double bond is present, the stereochemistry about each double bond should be specified.

Endocyclic double bonds have both carbons in the ring and exocyclic double bonds have only one carbon as part of the ring. Methylene cyclopentane & cyclohexene are constitutional isomers with the molecular formula C6H10. Methylene cyclopentane has an exocyclic double bond while cyclohexene has an endocyclic double bond. Methylene cyclopentane: Additional substituents to remember: Methylene: =CH2 Vinyl: -CH=CH2 Allyl: -CH2CH=CH2

6.6 Sequence Rules: The E,Z Designation

E,Z Stereochemical Nomenclature Priority rules of Cahn, Ingold, and Prelog Compare where higher priority group is with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side

Extended Comparison If atomic numbers are the same, compare at next connection point at same distance Compare until something has higher atomic number Do not combine – always compare

Dealing With Multiple Bonds Substituent is drawn with connections shown and no double or triple bonds Added atoms are valued with 0 ligands themselves

Alkene Stability Zaitsev’s rule or Saytzeff’s rule: More substituted double bonds are more stable. Therefore, Tetrasubstituted > Trisubstituted > geminal disubstituted, trans disubstituted > cis disubstituted > monosubstituted > ethylene One reason for this in the number of sp3C-sp2 C bonds. The more sp3C-sp2 C bonds, the more stable the alkene. The more substituted alkenes have more sp3C-sp2 C bonds. A sp3C-sp2 C bond is stronger than a sp3C-sp3 C bond. The other reason is the greater degree of Hyperconjugation in more substituted alkenes. Hyperconjugation is the overlap between the C=C pi bond and adjacent C-H sigma bonds. The more stable the alkene, the less exothermic its heat of hydrogenation. The heat of hydrogenation is the heat released per mole of H2 added to form the corresponding alkane.

Comparing Stabilities of Alkenes Evaluate heat given off when C=C is converted to C-C More stable alkene gives off less heat Trans butene generates 5 kJ less heat than cis-butene

Hyperconjugation Electrons in neighboring filled  orbital stabilize vacant antibonding  orbital – net positive interaction Alkyl groups are better than H

6.8 Electrophilic Addition of HX to Alkenes General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on  bond of alkene Produces carbocation and bromide ion Carbocation is an electrophile, reacting with nucleophilic bromide ion

Electrophilic Addition Energy Path Two step process First transition state is high energy point

Example of Electrophilic Addition Addition of hydrogen bromide to 2-Methyl-propene H-Br transfers proton to C=C Forms carbocation intermediate More stable cation forms Bromide adds to carbocation

Electrophilic Addition for preparations The reaction is successful with HCl and with HI as well as HBr Note that HI is generated from KI and phosphoric acid

6.9 Orientation of Electrophilic Addition: Markovnikov’s Rule In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other If one orientation predominates, the reaction is regiospecific Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) This is Markovnikov’s rule

Example of Markovnikov’s Rule Addition of HCl to 2-methylpropene Regiospecific – one product forms where two are possible If both ends have similar substitution, then not regiospecific

Energy of Carbocations and Markovnikov’s Rule More stable carbocation forms faster Tertiary cations and associated transition states are more stable than primary cations

Mechanistic Source of Regiospecificity in Addition Reactions If addition involves a carbocation intermediate and there are two possible ways to add the route producing the more alkyl substituted cationic center is lower in energy alkyl groups stabilize carbocation

6.10 Carbocation Structure and Stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals The fourth orbital on carbon is a vacant p-orbital The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents Therefore stability of carbocations: 3º > 2º > 1º > +CH3

Regioselectivity of Electrophilic Addition Reactions and The Hammond Postulate The transition state is more similar in structure to the species to which it is more similar in energy. In an exergonic reaction, the transition state is more similar to the reactants and in an endergonic reaction, it is more simlar to the products. The formation of cations in the electrophilic addition to alkenes is an endergonic process. The more stable carbocation will have a lower free energy. The formation of the corresponding transition state will have lower activation energy Therefore the more stable carbocation will be formed faster than the less stable carbocation. In an electrophilic addition reaction, the electrophile ( in the case of HX, electrophile = H+) will add to the less substituted C of a double bond which will result in the formation of the more substituted and more stable carbocation intermediate: Markvnikov’s rule. A reaction in which the formation of one constitutional isomer forms preferentially is called a regioselective reaction.

Competing Reactions and the Hammond Postulate Normal Expectation: Faster reaction gives more stable intermediate Intermediate resembles transition state

6.12 Mechanism of Electrophilic Addition: Rearrangements of Carbocations Carbocations undergo structural rearrangements following set patterns 1,2-H and 1,2-alkyl shifts occur Goes to give more stable carbocation Can go through less stable ions as intermediates