- Alkene Hydrocarbon With CarbonCarbon Double Bond
6. Alkenes: Structure and Reactivity
Based on McMurrys Organic Chemistry, 6th edition
Also called an olefin but alkene is better Includes many naturally occurring materials Flavors, fragrances, vitamins Important industrial products These are feedstocks for industrial processes
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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
Example: C6H10
Saturated is C6H14
Therefore 4 H's are not present
HH HH C C H3C C C H H HH
CH3
This has two degrees of unsaturation
Two double bonds? or triple bond? or two rings or ring and double bond
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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
H C H C H H H H H C C H H H H C C H H H C C F F H H C C H HO H H C C OH OH
Degree of Unsaturation and Variation
Compounds with the same degree of unsaturation can have many things in common and still be very different
C4H8
C4H6F2
C4H8O3
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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
Summary - Degree of Unsaturation
Count pairs of H's below CnH2n+2 Add number of halogens to number of H's (X equivalent to H) Don't count oxygens (oxygen links H) Subtract N's - they have two connections
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6.3 Naming of Alkenes
Find longest continuous carbon chain for root Number carbons in chain so that double bond carbons have lowest possible numbers Rings have cyclo prefix
H H 3C 5 1 2 CH3 H3C C H H C CH3 H C CH2
Many Alkenes Are Known by Common Names
Ethylene = ethene Propylene = propene Isobutylene = 2methylpropene Isoprene = 2-methyl-1,3butadiene
1,5-dimethylcyclohex-1-ene or 1,5-dimethylcyclohexene
3-methylpent-1-ene or 3-methylpentene
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6.4 Electronic Structure of Alkenes
Carbon atoms in a double bond are sp2-hybridized
Three equivalent orbitals at 120 separation in plane Fourth orbital is atomic p orbital
Rotation of Bond Is Prohibitive
Combination of electrons in two sp2 orbitals of two atoms forms bond between them Additive interaction of p orbitals creates a bonding orbital
Subtractive interaction creates a anti-bonding orbital
Occupied orbital prevents rotation about -bond Rotation prevented by bond - high barrier, about 268 kJ/mole in ethylene
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This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). Creates possible alternative structures
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6.5 Cis-Trans Isomerism in Alkenes
The presence of a carboncarbon double can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer similar groups on opposite sides Each carbon must have two different groups for these isomers to occur
Cis, Trans Isomers Require That End Groups Must Differ in Pairs
180rotation superposes Bottom pair cannot be superposed without breaking C=C
X
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6.6 Sequence Rules: The E,Z Designation
Neither compound is clearly cis or trans Substituents on C1 are different than those on C2 We need to define similarity in a precise way to distinguish the two stereoisomers Cis, trans nomenclature only works for disubstituted double bonds
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Develop a System for Comparison of Priority of Substituents
Assume a valuation system If Br has a higher value than Cl If CH3 is higher than H Then, in A, the higher value groups are on opposite sides In B, they are on the same side Requires a universally accepted valuation
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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
Ranking Priorities: Cahn-IngoldPrelog Rules
Must rank atoms that are connected at comparison point Higher atomic number gets higher priority Br > Cl > O > N > C > H
In this case,The higher priority groups are opposite: (E )-1-bromo-1-chloro-propene
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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
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6.7 Alkene Stability
Cis alkenes are less stable than trans alkenes Compare heat given off on hydrogenation: Ho Less stable isomer is higher in energy And gives off more heat tetrasubstituted > trisubstituted > disubstituted > monosusbtituted hyperconjugation stabilizes alkyl
Comparing Stabilities Alkenes
Evaluate of 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
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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
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Writing Organic Reactions
No established convention shorthand Can be formal kinetic expression Not necessarily balanced Reactants can be before or on arrow Solvent, temperature, details, on arrow
Electrophilic Addition Energy Path
Two step process First transition state is high energy point
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Example of Electrophilic Addition
Addition of hydrogen bromide to 2-Methylpropene H-Br transfers proton to C=C Forms carbocation intermediate More stable cation forms Bromide adds to carbocation
Energy Diagram for Electrophilic Addition
Rate determining (slowest) step has highest energy transition state Independent of direction In this case it is the first step in forward direction rate is not the same as rate constant
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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: Markovnikovs 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 Hs and X attaches to the other end (to the one with the most alkyl substituents) This is Markovnikovs rule
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Example of Markovnikovs 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 Markovnikovs Rule
More stable carbocation forms faster Tertiary cations and associated transition states are more stable than primary cations
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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: 3 > 2 > 1 > +CH3
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6.11 The Hammond Postulate
If carbocation intermediate is more stable than another, why is the reaction through the more stable one faster? The relative stability of the intermediate is related to an equilibrium constant (G) The relative stability of the transition state (which describes the size of the rate constant) is the activation energy (G) The transition state is transient and cannot be examined
Transition State Structures
A transition state is the highest energy species in a reaction step By definition, its structure is not stable enough to exist for one vibration But the structure controls the rate of reaction So we need to be able to guess about its properties in an informed way We classify them in general ways and look for trends in reactivity the conclusions are in the Hammond Postulate
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Statement of the Hammond Postulate
A transition state should be similar to an intermediate that is close in energy Sequential states on a reaction path that are close in energy are likely to be close in structure - G. S. Hammond
Competing Reactions and the Hammond Postulate
Normal Expectation: Faster reaction gives more stable intermediate Intermediate resembles transition state
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carbocation Reaction
In a reaction involving a carbocation, the transition states look like the intermediate
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Non-Hammond Behavior
More stable intermediate from slower reaction Conclude: transition state and intermediate must not be similar in this case not common
Transition State for Alkene Protonation
Resembles carbocation intermediate Close in energy and adjacent on pathway Hammond Postulate says they should be similar in structure
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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
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