Chapter 6 Alkenes and Alkynes

Alkenes and Alkynes Alkene (or “olefin”) Alkyne Hydrocarbon that contains a carbon-carbon double bond Present in most organic and biological molecules β-Carotene Alkyne Hydrocarbon that contains a carbon-carbon triple bond Rarely occur in biological molecules or pathways

6.1 Calculating a Degree of Unsaturation Because of its double bond an alkene has fewer hydrogens than an alkane with the same number of carbons Alkane Alkene CnH2n+2 CnH2n C2H6 C2H4 (Saturated) (Unsaturated)

Calculating a Degree of Unsaturation Number of rings and/or multiple bonds present in the molecule Unknown hydrocarbon of formula C6H10with molecular weight of 82 has two fewer pairs of hydrogens (H14 – H10 = H4 = 2H2) than a saturated hydrocarbon Degree of unsaturation is two Possible structures for unknown

Calculating a Degree of Unsaturation Calculations for compounds containing other elements in addition to carbon and hydrogen: Organohalogen compounds (C,H,X, where X = F, Cl, Br, or I) Add number of halogens and hydrogens to arrive at an equivalent hydrocarbon formula

Calculating a Degree of Unsaturation Organooxygen compounds (C,H,O) oxygen does not affect the formula of an equivalent hydrocarbon.

Calculating a Degree of Unsaturation Organonitrogen compounds (C,H,N) Has one more hydrogen than a related hydrocarbon Must subtract the number of nitrogens from the number of hydrogens for equivalent hydrocarbon formula

6.2 Naming Alkenes and Alkynes Similar to the naming rules for alkanes Step 1 - Name the parent hydrocarbon Find longest carbon chain containing the double bond Step 2 – Number the carbon atoms in the chain Double-bond carbons should receive lowest possible numbers

Naming Alkenes and Alkynes Step 3 - Write the full name using suffix -ene in place of -ane If more than one double bond indicate position and use suffixes -diene, -triene, and so on

Naming Alkenes and Alkynes Cycloalkenes Number cycloalkene so double bond is between C1 and C2 and first substituent has lowest number possible

Naming Alkenes and Alkynes Common names that are often used and are recognized by IUPAC:

Naming Alkenes and Alkynes Named just like alkenes using suffix -yne Number main chain so triple bond receives as low a number as possible Alkyl, alkenyl, and alkynyl groups:

6.3 Cis-Trans Isomerism in Alkenes Carbon-carbon double bond description Valence bond language Carbons are sp2 hybridized Three equivalent hybrid orbitals that lie in a plane at angles of 120º to one another Carbons form a σ bond by head-on overlap of sp2 orbitals and a p bond by sideways overlap of unhybridized p orbitals oriented perpendicular to sp2 plane

Cis-Trans Isomerism in Alkenes Molecular orbital language Interaction between p orbitals leads to one bonding and one antibonding p molecular orbital p bonding MO contains no node between nuclei p antibonding MO contains a node between nuclei resulting from combination of lobes with different algebraic signs

Cis-Trans Isomerism in Alkenes Free rotation is not possible around a double bond The barrier to double bond rotation must be at least as great as the strength of the p bond itself (~ 350 kJ/mol)

Cis-Trans Isomerism in Alkenes Disubstituted alkene Two substituents other than hydrogen are attached to the double-bond carbons But-2-ene Two isomers cannot interconvert spontaneously Methyl groups are cis- or trans- to each other Bond rotation cannot occur – the two but-2-enes are cis-trans stereoisomers

Cis-Trans Isomerism in Alkenes Cis-trans isomerism occurs when both double-bond carbons are bonded to two different groups Carbons bonded to two identical groups cannot exist as cis-trans isomers

6.4 Sequence Rules: The E,Z Designation E,Z system Sequence rules used to assign priorities to the substituent groups on the double-bond carbons (alkenes) Z – for German zusammen “together” High priority groups on each carbon are on same side of the double-bond E – for German entgegen “opposite” High priority groups on each carbon are on opposite sides of the double-bond

Sequence Rules: The E,Z Designation Cahn-Ingold-Prelog rules Rule 1 – look at the two atoms directly attached to each double-bond carbon and rank them according to atomic number

Sequence Rules: The E,Z Designation Rule 2 – If a decision cannot be reached by ranking the first atom in the substituent, look at the second, third, and fourth atoms away from the double-bond carbons until the first difference is found

Sequence Rules: The E,Z Designation Rule 3 – multiple bonded atoms are equivalent to the same number of single-bonded atoms

Worked Example 6.1: Assign E and Z Configurations to Substituted Alkenes Assign E or Z Configuration to the double bond in the following compound:

Worked Example 6.1: Assign E and Z Configurations to Substituted Alkenes Strategy Look at the two substituents connected to each double-bond carbon, and determine their priorities using the Cahn-Ingold-Prelog rules. Then see whether the two high-priority groups are on the same or opposite sides of the double bond.

Worked Example 6.1: Assign E and Z Configurations to Substituted Alkenes Solution

6.5 Stability of Alkenes Stability of cis- and trans isomers Interconversion does not occur spontaneously Interconversion can be brought about by treating the alkene with a strong acid Cis isomers are less stable than trans isomers because of the steric strain between the two larger substituents on the same side of the double-bond

Stability of Alkenes Determining relative stabilities of alkene isomers Establish a cis-trans equilibrium through treatment with strong acid Hydrogenate alkene through a hydrogenation reaction to give corresponding alkane using H2 gas in the presence of a catalyst such as palladium or platinum

Stability of Alkenes Energy Diagram for Hydrogenation of cis- and trans- But-2-ene Cis isomer is higher energy than the trans isomer More energy released in the hydrogenation of the cis isomer than the trans isomer

Stability of Alkenes Alkenes become more stable with increasing substitution Stability order due to two factors: Hyperconjugation Stabilizing interaction between the C=C p bond and adjacent C-H σ bonds on substituents The more substituents there are the greater the stabilization of the alkene

Stability of Alkenes Bond strength A bond between an sp2 carbon and an sp3 carbon is somewhat stronger than a bond between two sp3 carbons

6.6 Electrophilic Addition Reactions of Alkenes Carbon-carbon double bonds behave as nucleophiles (Lewis bases) in polar reactions by donating a pair of electrons to an electrophile (Lewis acid) Successful with not only with H2O but also with HBr, HCl, and HI (addition reactions of halogen acids is not common in living organisms)

Electrophilic Addition Reactions of Alkenes Mechanism of the acid-catalyzed electrophilic addition of H2O to 2-methylpropene to give the alcohol 2-methylpropan-2-ol

Electrophilic Addition Reactions of Alkenes Alkynes also undergo electrophilic addition reactions Reactivity is substantially less than that of alkenes

6.7 Orientation of Electrophilic Addition: Markovnikov’s Rule Regiospecific reactions - reactions where only one of two possible orientations occurs Markovnikov’s Rule (Vladimir Markovnivov,1869) 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

Orientation of Electrophilic Addition: Markovnikov’s Rule (Restated) In the addition of HX to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results.

Worked Example 6.2: Predicting the Product of an Electrophilic Addition Reaction What product would you expect from reaction of HCl with 1-ethylcyclopentene?

Worked Example 6.2: Predicting the Product of an Electrophilic Addition Reaction Strategy Begin by looking at functional group(s) in the reactants Decide what kind of reaction occurs The reactant is an alkene that will probably undergo an electrophilic addition reaction with HCl Use knowledge of electrophilic addition reactions to predict product Electrophilic addition reactions follow Markovnikov’s rule

Worked Example 6.2: Predicting the Product of an Electrophilic Addition Reaction Solution The expected product is 1-chloro-1-ethylcyclopentane

Worked Example 6.3: Synthesizing a Specific Compound What alkene would you start with to prepare the following alkyl halide? (There may be more than one possibility.)

Worked Example 6.3: Synthesizing a Specific Compound Strategy Always work backward Look at product, and identify the functional group(s) it contains The product is a tertiary alkyl chloride Find all ways of preparing functional group Can be prepared by reaction of an alkene with HCl Draw and evaluate all possibilities

Worked Example 6.3: Synthesizing a Specific Compound Solution There are three possibilities, any one of which could give the desired product

6.8 Carbocation Structure and Stability Why Markovnikov’s Rule works Structure of carbocations Carbocations are planar The trivalent carbon is sp2 hybridized and has a vacant p orbital perpendicular to the plane of the carbon Carbon has three attached groups

Carbocation Structure and Stability Thermodynamic measurements show that the stability of carbocations increases with increasing substitution.

Carbocation Structure and Stability More highly substituted carbocations are more stable than less highly substituted ones Inductive effect Results from the response to the electronegativity of nearby atoms The more alkyl groups that are attached to the positively charged carbon, the more electron density shifts toward the charge and the more inductive stabilization of the cation occurs

Carbocation Structure and Stability Hyperconjugation the stabilizing interaction between a p orbital and properly oriented C-H σ bonds on neighboring carbons The more alkyl groups that are on the carbocation the more stable the carbocation

6.9 The Hammond Postulate Electrophilic addition reactions Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation intermediate A more highly substituted carbocation is more stable than a less substituted one There is an intuitive relationship between the stability of a carbocation intermediate and the rate of its formation Transition states represent energy maxima whose structures are similar to the structure of the nearest stable species

The Hammond Postulate Stability of the carbocation intermediate affects the rate of reaction When comparing two similar reactions the more stable intermediate forms faster than the less stable one

The Hammond Postulate Hammond postulate(1955) The structure of a transition state resembles the structure of the nearest stable species. Transition states for endergonic steps structurally resemble products, and transition states for exergonic steps structurally resemble reactants.

The Hammond Postulate Hammond postulate applied to electrophilic addition reactions Formation of carbocation by protonation of an alkene is an endergonic step Transition state of alkene protonation should structurally resemble the carbocation intermediate Any factor that stabilizes the carbocation should also stabilize the nearby transition state More stable carbocations form faster because their greater stability is reflected in the lower-energy transition state leading to them

The Hammond Postulate The transition state is stabilized by hyperconjugation and inductive effects The more alkyl groups that are present the greater the extent of stabilization and the faster the transition state forms

6.10 Carbocation Rearrangements Evidence to support carbocation mechanism for electrophilic addition reactions of alkenes Structural rearrangements often occur during the reaction of HX with an alkene HCl with 3-methylbut-1-ene

Carbocation Rearrangements Hydride shift The shift of a hydrogen atom and its electron pair (a hydride ion, :H-) between neighboring carbons The secondary carbocation intermediate formed by protonation of 3-methylbut-1-ene rearranges to a more stable tertiary carbocation through a hydride shift

Carbocation Rearrangements Alkyl shift The shift of an alkyl group with its electron pair between neighboring carbons A secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group

Carbocation Rearrangements In both cases a group (:H- or :CH3-) moves to an adjacent positively charged carbon taking its bonding electron pair with it A less stable carbocation rearranges to a more stable ion Rearrangements are a common feature of carbocation chemistry and are particularly important in biological pathways by which steroids and related substances are synthesized Hydride shift in biosynthesis of cholesterol