Alkenes - Los Angeles Mission College

Alkenes

Structure, Nomenclature, and an introduc1on to Reac1vity

• Thermodynamics

and Kine1cs

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• Also called an olefin but alkene is be>er • Hydrocarbon that contains a C-‐C double bond • Includes many naturally occurring materials

– Flavors, fragrances, vitamins

Alkene -‐ Hydrocarbon With Carbon-‐Carbon Double Bond

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Industrial Prepara1on and Use of Alkenes

Ethylene and propylene are important organic chemicals produced

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Thermal Cracking

• Thermal cracking may take place w/o catalyst at very high temperatures

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Calcula1ng Degree of Unsatura1on

• Relates molecular formula to possible structures

• Degree of unsatura0on: number of mul1ple bonds or rings • Formula for a saturated acyclic compound is CnH2n+2

• Alkene has fewer hydrogens than an alkane with the same number of carbons —CnH2n because of double bond

• Each ring or mul1ple bond replaces 2 H's

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Vinylic and Allylic Carbons

vinylic carbon: the sp2 carbon of an alkene

allylic carbon: a carbon adjacent to a vinylic carbon 6

Naming Alkenes • Name the parent hydrocarbon

• Number the carbons in chain so that double bond carbons have lowest possible numbers

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Naming Alkenes Write the full name-‐ Number subs1tuents according to: 1) Posi1on in chain, 2) Alphabe1cally

Rings have “cyclo” prefix

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Cis-‐Trans Isomerism in Alkenes

• Carbon atoms in a double bond are sp2-‐hybridized

– Three equivalent orbitals at 120º separa1on in plane – Fourth orbital is atomic p orbital

• Combina1on of electrons in two sp2 orbitals of two atoms forms σ bond between them

• Addi1ve interac1on of p orbitals creates a π bonding orbital – Subtrac1ve interac1on creates a π an1-‐bonding orbital

• Occupied π orbital prevents rota1on about σ-‐bond • Rota1on prevented by π bond -‐ high barrier, about 268 kJ/mole in ethylene

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• Rota1on of π bond is prohibi1ve • This prevents rota1on about a carbon-‐carbon double bond

(unlike a carbon-‐carbon single bond). • Creates possible alterna1ve structures


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• The presence of a carbon-‐carbon double bond 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


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Cis-‐Trans Isomeriza1on requires that end groups differ in pairs

Bo>om pair cannot be superposed without breaking C=C

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Alkene Stereochemistry and the E,Z Designa1on

• Cis-‐Trans naming system discussed thus far only works with disubs+tuted alkenes

• Tri, Tetra-‐ subs1tuted double bonds, and conjugated systems require more general method, referred to as the E,Z system.

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Alkene Stereochemistry and the E,Z Designa1on (Con1nued): E,Z Stereochemical Nomenclature

• Priority rules of Cahn, Ingold, and Prelog

• Compare where higher priority groups are with respect to bond and designate as prefix

• E -‐entgegen, opposite sides

• Z -‐ zusammen, together on the same side

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Alkene Stereochemistry and the E,Z Designa1on Cahn-‐Ingold-‐Prelog Rules

RULE 1

• Must rank atoms that are connected at comparison point

• Higher atomic number gets higher priority

– Br > Cl > S > P > O > N > C > H

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RULE 2 • If atomic numbers are the same, compare at next connec1on point

at same distance • Compare un1l something has higher atomic number • Do not combine – always compare


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RULE 3 • Mul1ple-‐bonded atoms are equivalent to the same number of

single-‐bonded atoms • Subs1tuent is drawn with connec1ons shown and no double or

triple bonds • Added atoms are valued with 0 ligands themselves


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Isomers for Compounds with Two Double Bonds

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Stability of Alkenes • Cis alkenes are less stable than trans alkenes • Compare heat given off on hydrogena1on: ΔHo

• Less stable isomer is higher in energy – And gives off more heat – tetrasubs1tuted > trisubs1tuted > disubs1tuted > monosusb1tuted

– hyperconjuga:on stabilizes

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Rela1ve Stabili1es 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

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Stability of Alkenes-‐ Hyperconjuga1on

• Electrons in neighboring filled σ orbital stabilize vacant an1bonding π orbital – net posi1ve interac1on

• Alkyl groups are be>er than H

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Each Family Can Be Put in One of Four Groups

The families in a group react in similar ways.

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Electrophiles

An electrophile has a posi1ve charge, a par1al posi1ve charge, or an incomplete octet.

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Nucleophiles

A nucleophile has a nega1ve charge or a lone pair.

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Curved Arrows

Curved arrows are used to show the mechanism of a reac1on.

The mechanism of a reac1on is the step-‐by-‐step descrip1on of the process by which reactants are converted into products.

first step of the reac1on

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second step of the reac1on

The curved arrow shows where the electrons start from and where they end up.

Curved Arrows

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An Electrophilic Addi1on Reac1on

Transi1on states have par1ally formed bonds.

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Reac1on Coordinate Diagram for Each Step of the Addi1on of HBr to 2-‐Butene

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Reac1on Coordinate Diagram for the Addi1on of HBr to 2-‐Butene

The rate-‐limi1ng step of the reac1on is the step that has its transi1on state at the highest point of the reac1on coordinate diagram. 29

Electrophilic Addi1on of Alkenes • The reac1on is successful with HCl and with HI as well as

HBr

• HI is generated from KI and phosphoric acid

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Orienta1on of Electrophilic Addi1ons: 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 orienta1on predominates, the reac1on is regioselec0ve • Markovnikov observed in the 19th century that in the addi1on of

HX to alkene, the H a>aches to the carbon with more H’s and X a>aches to the other end (to the one with more alkyl subs1tuents) – This is Markovnikov’s rule

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• Addi1on of HCl to 2-‐methylpropene • Regiospecific – one product forms where two are possible • If both ends have similar subs1tu1on, then not regiospecific

Example of Markovnikov’s Rule

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Markovnikov’s Rule • More highly subs1tuted carboca1on forms as intermediate

rather than less highly subs1tuted one

• Ter1ary ca1ons and associated transi1on states are more stable than primary ca1ons

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Markovnikov’s Rule

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Carboca1on Structure and Stability

• Carboca1ons 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 carboca1on (measured by energy needed to

form it from R-‐X) is increased by the presence of alkyl subs1tuents

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Carboca1on Stability

A induc1ve stabilized ca1on species

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Carboca1on Structure and Stability

A plot of ΔH dissocia1on shows that more highly subs1tued alkyl halides dissociate more easily than less highly subs1tuted ones

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The Hammond Postulate

• If a carboca1on intermediate is more stable than another, why is the reac1on through the more stable one faster? – the rela1ve stability of the intermediate is related to an equilibrium constant (ΔGº)

– the rela1ve stability of the transi1on state (which describes the size of the rate constant) is the ac1va1on energy (ΔG‡)

– the transi1on state is transient and cannot be examined – What does the Hammond Postulate state? “the structure of a transi1on state resembles the structure of the nearest stable species. Transi1on states for endergonic steps structurally resemble products, and transi1on states for exergonic steps structurally resemble reactants”

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The Hammond Postulate (Con1nued): Transi1on State Structures

• A transi0on state is the highest energy species in a reac1on step

• By defini1on, its structure is not stable enough to exist for one vibra1on

• But the structure controls the rate of reac1on • So we need to be able to guess about its proper1es in an

informed way • We classify them in general ways and look for trends in

reac1vity – the conclusions are in the Hammond Postulate

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Examina1on of the Hammond Postulate

• A transi1on state should be similar to an intermediate that is close in energy

• Sequen1al states on a reac1on path that are close in energy are likely to be close in structure -‐ G. S. Hammond

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Compe1ng Reac1ons and the Hammond Postulate

• Normal Expecta1on: Faster reac1on gives more stable intermediate

• Intermediate resembles transi1on state

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Evidence for the Mechanism of Electrophilic Addi1on: Carboca1on Rearrangments

• Carboca1ons undergo structural rearrangements following set pa>erns

• 1,2-‐H and 1,2-‐alkyl shits occur

• Goes to give moststable carboca1on

• Can go through less stable ions as intermediates

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Hydride shits in biological molecules

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Alkenes - Los Angeles Mission College

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