Introduction To Alkenes And Alkynes In An Alkane, All Covalent Bonds .

Introduction to Alkenes and AlkynesIn an alkane, all covalent bonds between carbon were σ(σ bonds are defined as bonds where the electron densityis symmetric about the internuclear axis)In an alkene, however, only three σ bonds are formed from the alkene carbon-the carbon thus adopts an sp2 hybridizationEthene (common name ethylene) has a molecular formula of CH2CH2Each carbon is sp2 hybridized with a σ bond to two hydrogens and the other carbonHybridized orbital allows strongerbonds due to more overlapHHC CHH

Structure of EthyleneIn addition to the σ framework of ethylene, each carbonhas an atomic p orbital not used in hybridizationThe two p orbitals (each with one electron) overlap to form a π bond(p bonds are not symmetric about the internuclear axis)π bonds are not as strong as σ bonds(in ethylene, the σ bond is 90 Kcal/mol and the π bond is 66 Kcal/mol)Thus while σ bonds are stable and very few reactions occur with C-C bonds,π bonds are much more reactive and many reactions occur with C C π bonds

Nomenclature of AlkenesAugust Wilhelm Hofmann’s attempt for systematic hydrocarbon nomenclature(1866)Attempted to use a systematic name by naming all possible structures with 4 ealkenealkine ted to use Quart from the Latin for 4 – this method was not embraced and BUT has remainedUsed English order of vowels, however, to name the groups (a, e, i, o, u)This naming is the reason why we still call functional groups alkanes (saturated),Alkenes (having one double bond, thus lost 2 hydrogens) or alkynes (a triple bond)(the –ine suffix was replaced with –yne for ease in pronunciation)

Nomenclature of AlkenesAlkenes use the same IUPAC nomenclature rules seen for alkanesMain difference is want to find the longest carbon chain that contains the double bondThe alkene is numbered to indicate the position of the alkene(want lowest number possible for alkene position)Br2-pentyl-1,3-butadieneIf multiple alkenes,find chain that contains allpolyenes as root2-ethyl-1-hepteneSome alkenes have isomers(use E or Z designation)XXallylOH(E)-3-penten-2-olSome common names:vinyl(E)-4-bromo-2-penteneAlcohols have higherpriority than alkenes

Double Bond Prevents RotationWhen a π bond rotates, the energy of the bond is lost when orbitals are orthogonalHWith a σ bond,rotation changes the stericenergy but all bonds are stillintactHHHWith a π bond,rotation breaks the bondH CH3EnergyHHHH 66 Kcal/mol 3.4 Kcal/molHCH3HH-60 HHHH0 60 HThere is not enoughthermal energy to allowbond rotation to occurH120 torsional angle180 240

Lack of Rotation Results in IsomersSince at room temperature a double bond is “locked” into one conformation,isomers may result when different substituents are placed on the double bond-if two substituents are on the same side of the double bond called CISH3 CHCH3cis-2-buteneH-if two substituents are on opposite sides of the double bond called TRANSH3 CHHtrans-2-buteneCH3

Cahn-Ingold-Prelog NamingIf there are more than two substituents on a double bond, cis and trans have no meaningThree chemists adopted a new naming method that is universally used(called Cahn-Ingold-Prelog or C-I-P)In this method the substituents are “ranked” by priorityTo rank priority:1) Consider the atomic number of the atom directly attached(higher the atomic number, higher the priority)2) For isotopes, atomic mass breaks the tie in atomic number3) If still tied, consider the atoms bonded to the tied atoms.Continue only until the tie is broken.4) Multiple bonds attached to an atom are treated as multiple single bonds. An alkene carbontherefore would consider as two bonds to that carbon43H D21Consider the starred atom in this compoundAtoms attached: C, C, H, D

Using Cahn-Ingold-Prelog in Assigning Alkenes-substituents are prioritizedConsider each end of the alkene separately-if the highest priorities are on the same side called Z2H C31BrH2CH3 1Z – zusammen – “together”Z-2-bromo-2-butene-if the highest priorities are on the opposite side called E2H C31BrCH3 1H2E – entgegen – “opposite”E-2-bromo-2-butene

Stability of AlkenesIn order to compare stability of different alkene structures,the structure must be consideredLet us reconsider structures of reactive intermediatesA carbocation has three bonds to other atoms and only 6 electrons in outer shellHHH HH2 CHHAnother type oforbital mixingcarbocationC-H bondMethyl cations are very unstable and rarely observed,but when extra alkyl substituents are attached the carbocation becomes more stableThe reason for this extra stability is due partly to the adjacent C-H bondThis bond can donate electron density to the empty p orbital of carbocation(hyperconjugation)With carbocations this electron donation will lower the energy of the structureAlkyl groups are often considered “electron donating” due to this effect and thus3 cations 2 cations 1 cations methyl cationsRarely observed

Stability of AlkenesRadicals are also electron deficient species,and thus more alkyl substituents will also stabilize them(3 radical 2 radical 1 radical methyl radical)Anions, however, are electron rich species-more alkyl substituents would raise the energy of an anion(methyl anion 1 anion 2 anion 3 anion)Alkenes are merely similar to two carbon radicals on adjacent atomsHHHHHRHHHRHRHRRHHRRRRRRRstabilitySubstituted alkeneis more stable(R group means anyalkyl substituent)E alkenes are morestable than ZAs number of substituentsincreases the alkene ismore stable

Alkenes in RingsA small ring (less than 8 carbons) can only have cis alkenescyclohexeneIt is sterically too high in energy to place a trans alkene in a ring smaller than 8 atoms(try building one with molecular models!)A similar strain energy is present when trying to place an alkene in a bridgehead positionbridgehead carbonNot allowedCalled Bredt’s rule(need at least 8 carbons in ring to allow an alkene in a bridgehead position)

Structure of AlkynesConsider Acetylene(common name for ethyne, the smallest possible alkyne)HHThe σ bond is formed by combining an sp hybridized orbital on each carbonThe second sp hybridized orbital is used to form σ bond to hydrogenEach carbon has 2 atomic p orbitals remaining which align to form two π bondsAs the number of bonds increases between two carbons, the bond length decreasesC-C triple bondbond length 1.20 ÅC-C double bondbond length 1.33 ÅC-C single bondbond length 1.54 Å

AlkynesMany aspects of alkynes are similar to alkenes:-A more substituted alkyne is more stable(there are only 2 possible substitution points for an alkyne though)-Nomenclature for alkyne follows same rules(use –yne suffix instead of –ene)(E)-3-methyl-3-decen-7-yne- Similar to trans alkenes, alkynes are not stable in a ring unless it is at least 8 carbons- Small substituted fragments often have common namesXpropargylXallyl

Acidity of AlkynesOne effect of the increase in number of bonds (and bond length changes)is the acidity of an sp hybridized carbon-hydrogen bondAs the resultant lone pair of electrons(the pair formed after the proton is abstracted in an acid-base reaction)becomes closer to the positively charged nucleus the lone pair is MORE STABLEAs the stability of the deprotonated form increases the conjugate is more acidicTherefore while abstracting an sp3 or sp2 hybridized C-H is difficult,an sp hybridized C-H bond can easily be abstracted with common bases

Acidity of AlkynesLearn the acidity of terminal alkynes versus other alkyl groups and basespKaCH3CH3HHHBCH3CH2HBHHHROHHBNH3BB 50-60HNH2443525HRO16Therefore while alkoxides cannot deprotonate an alkyne, amides can easily deprotonate

Degrees of Unsaturation (Ω)As we learned with saturated alkanes the molecular formula is CnH2n 2With a monoalkene, however, the molecular formula is CnH2nand with alkynes it is CnH2n-2π bonds are considered as a degree of unsaturation(each π bond results in 2 less hydrogens being attached to the molecule, thus unsaturation)A cycloalkane is also a degree of unsaturation(to form a ring from a straight chain alkane,two hydrogens are removed and the two radical sites combine to form the ring)The degrees of unsaturation (Ω) for a compound is thus statingthe combined total of number of π bonds and the number of rings in a compoundIf a molecular formula is given for a hydrocarbon, multiple the number of carbons by 2n 2,subtract the number of hydrogens in the actual compound from this number and then dividethe difference by 2 (because each unsaturation removes 2 hydrogens) to determine ΩIf C8H14: 1) 8 x 2n 2 18, 2) 18-14 4, 3) 4/2 2- Thus 2 degrees of unsaturation (compound has a combination of 2 π bonds and/or rings)

Introduction to Reaction of Alkenes and AlkynesAlkenes generally react in an addition mechanism(addition – two new species add to a molecule and none leave)If hydrogen halides react, then a H and Cl add to the two ends of the double bondCH3H3C! !H ClClCH3H3CHSince H-Cl is polarized, the H will be partially positively charged and Cl partially negativeThe alkene is thus the nucleophile and the proton is the electrophileAs we first learned with acid/base reactions with Lewis definition, any reaction can beconsidered as a nucleophile reacting with an electrophileelectrophileLUMOreactionEnergy gainHOMOnucleophileproduct

Introduction to Reaction of Alkenes and AlkynesWhen discussing mixing orbitals to form new orbitals there are two main considerations:1) Amount of overlap between the orbitals that are mixing2) How close in energy the two original orbitals are before mixingOrbitals that overlap strongly and are close in energy will form two new molecular orbitals,one higher in energy and one lower in energyWhen considering how a compound will react, and how readily it will react,the energy level of the HOMO and LUMO orbitals need to be determinedIf the HOMO is high in energy, the compound will react as a nucleophile(or base in acid/base terminology)If the LUMO is low in energy, the compound will react as an electrophile(or acid in acid/base terminology)In every reaction we will study, it is important to recognizewhat is the nucleophile and what is the electrophile(or whether the compound has a high energy HOMO or a low energy LUMO)But the question always becomes, “Compared to What?”What is a high energy HOMO and what is a low energy LUMO?

Introduction to Reaction of Alkenes and AlkynesAs organic chemists we often compare bonds to something that is known to be unreactive,a carbon-carbon single bond*σ C-Cπ* C-CC (p)C (sp3)C (p)C (sp3)π C-Cσ C-CC-C single bonds are relatively unreactivedue to large overlap of sp3 hybridizedorbital and energy match, therefore verylow HOMO and high LUMO energyAtomic p orbital is higher in energy than sp3(less s character) and the overlap for p orbitalsis much less to form π bondTherefore orbitals do not mix as well for π bondand thus HOMO does not lower in energy norLUMO raise in energy as much as σ bond(already had seen this with weaker π bond)C-C π bond will thus be far more reactive,and it will react preferentially as the nucleophile due to higher HOMO level

Introduction to Reaction of Alkenes and AlkynesThe reaction is thus a two step reactionThe first step will generate a carbocation as a reactive intermediateH3CCH3! !H ClClCH3H3 CClCH3H3 CHHAnd the second step will have the carbocation react with the chloride to yield the product(the chloride is the nucleophile and the carbocation is the electrophile)A reaction mechanism is often representedby a “reaction coordinate” diagram whichgraphs the energy of the species formed asthe reaction progressesReaction Coordinate

The reaction coordinate indicates all structures formed in the reaction in a proper tePotentialenergyStartingmaterialProductsReaction Coordinate

Equilibrium ConstantsEquilibrium constants (Keq) indicatethermodynamically whether the reaction isfavored in the forward or reverse direction andthe magnitude of this preferenceΔGKeq ([C][D]) / ([A][B]) ([products]) /([starting material])Reaction Coordinate