chapter 7- alkenes: structure and reactivity · chapter 7- alkenes: structure and reactivity ......

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Chapter 7- Alkenes: Structure and Reactivity

Ashley Piekarski, Ph.D.

Alkene

•  What is an alkene func<onal group? •  hydrocarbon with carbon-carbon double bond •  occurs in many natural materials (flavors,

fragrances, vitamins)

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Why am I learning this, Dr. P?

•  C-‐C double bonds are present in most organic and biological molecules

•  Alkene stereochemistry

•  Focus on a general alkene reac<on: Electrophilic addi4on

Use of alkenes

•  Ethylene and propylene are the most important organic chemicals produced

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Degree of Unsaturation

•  Are alkenes saturated or unsaturated? •  Formula for saturated acyclic compound is

CnH2n+2

•  Degree of unsatura<on: number of mul<ple bonds or rings •  Each ring or multiple bond replaces 2H’s

Learning check

•  How many degrees of unsaturated are in C6H10? What are some possible structure for this molecular formula?

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With other elements

•  Organohalogens (X: F, Cl, Br, I) •  Halogen replaces a hydrogen

•  For example, C4H6Br2 and C4H8 have one degree of unsatura<on

•  Organoxygen compounds (C, H, O) •  These don’t affect the total count of H’s

Organonitrogen compounds

•  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

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Summary

•  Count pairs of H’s below CnH2n+2

•  Add number of halogens to number of H’s •  Ignore oxygens •  Subtract N’s-‐ they have two connec<ons

Learning check

•  Calculate the degree of unsatura<on for C10H12N2O3

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Nomenclature

•  Name the parent hydrocarbon •  Number carbons in chain so the double-‐bond carbons have the lowest possible numbers

•  Rings have a “cyclo” prefix

Common names

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Learning check

(E)-4-methylhept-2-ene(E)-hex-2-ene

4-ethylcyclopent-1-ene

Cl

(1S,3S)-1-chloro-3-ethylcyclopentane

(1R,3S)-1,3-diethylcyclopentane

Cl

(Z)-4-chloro-3-methyloct-3-ene (Z)-4-ethyl-2,3-dimethylhept-3-ene

+ HBr

Cl

•  Name the following alkenes:



Cl



Cl


+ HBr

Cl

Stereochemistry

•  Make a model of 1,2-‐dichloroethene. •  Can you rotate the double-‐bond, like an alkane?

•  This creates two possible isomers for alkenes!

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Cis- and Trans- Isomers

•  The presence of a carbon-‐carbon double bond can create two possible structures •  cis isomer- 2 similar groups on the 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

•  Now, draw the cis-‐ and trans-‐ isomers for 1,2-‐dichloroethene

Cis- and Trans- Isomers

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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 disubs<tuted double bonds

E, Z Stereochemistry

•  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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Ranking Priority Rules!

•  Rule 1 •  Rank atoms that are connected at comparison

point •  Higher atomic number gets higher priority


•  Rule 2 •  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

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•  Rule 3 •  Substituent is drawn with connections shown

and no double or triple bonds •  Added atoms are valued with 0 ligands

themselves

Learning check

•  Name the following alkenes using E, Z configura<on:



Cl



Cl


+ HBr

Cl

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Stability of Alkenes

•  Cis alkenes are less stable than trans alkenes •  Compare heat given off on hydrogena<on •  Less stable isomer is higher in energy

Comparing Stabilities of Alkenes

•  Evaluate heat given off when C=C is converted to C-‐C bond

•  More stable alkene gives off less heat

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Hyperconjugation

•  Hyperconjuga3on is the stabilizing interac<on between filled pi orbital and a neighboring filled C-‐H sigma bond on a subs<tuent. The more subs<tuents there are, the greater the stabiliza<on of the alkene

Electrophilic Addition of Alkenes- Mechanism

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Electrophilic Addition Energy Diagram

•  Two step process •  First transi<on state is high energy point

Electrophilic Addition Examples

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Markovnikov’s Rule

•  Addi<on is regiospecific •  Halide will attack the carbon that is more

substituted

Markovnikov’s Rule

•  Why? •  What is the intermediate of an electrophilic

addition reaction?

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Learning check

•  Predict the product for the following reac<on:



Cl



Cl


+ HBr

Cl

Learning check

•  Retrosynthesis: predict the reactants to make the following product



Cl



Cl


+ HBr

Cl

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Carbocation Structure and Stability

•  Carboca<ons 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

Carbocation Structure and Stability

•  The stability of the carboca<on (measured by energy needed to form it from R-‐X) is increased by the presence of alkyl subs<tuents

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Inductive stabilization

The Hammond Postulate

•  If carboca<on intermediate is more stable than another, why is the reac<on through the more stable one faster? •  The relative stability of the intermediate is related to

an equilibrium constant (ΔGº) •  The relative stability of the transition state (which

describes the size of the rate constant) is the activation energy (ΔG‡)

•  The transition state is transient and cannot be examined

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Transition State Structures

•  A transi3on state is the highest energy species in a reac<on step

•  By defini<on, its structure is not stable enough to exist for one vibra<on

•  But the structure controls the rate of reac<on

•  So we need to be able to guess about its proper<es in an informed way

•  We classify them in general ways and look for trends in reac<vity – the conclusions are in the Hammond Postulate

Hammond Postulate

•  A transi<on state should be similar to an intermediate that is close in energy

•  Sequen4al states on a reac4on path that are close in energy are likely to be close in structure -‐ G. S. Hammond

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Competing Reactions

•  Normal Expecta<on: Faster reac<on gives more stable intermediate

•  Intermediate resembles transi<on state

Rearrangements of Carbocations

•  Carboca<ons undergo structural rearrangements following set pajerns

•  1,2-‐H and 1,2-‐alkyl shiks occur •  Goes to give more stable carboca<on •  Can go through less stable ions as intermediates

chapter 7- alkenes: structure and reactivity · chapter 7- alkenes: structure and reactivity ......

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