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Organic REACTIONS:

ALkanes. Chapter 10.2:. Structure, bonding and chemical reactions involving functional group inter-conversions are key strands in organic chemistry. Chapter 20.1:. Key organic reaction types include nucleophilic substitution, electrophilic addition, electrophilic substitution, and redox reactions. Reaction mechanisms vary and help in understanding the different types of reactions taking place..

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Organic REACTIONS:






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Slide1

Organic REACTIONS: ALkanes

Chapter 10.2:

Structure, bonding and chemical reactions involving functional group inter-conversions are key strands in organic chemistry

Chapter 20.1:

Key organic reaction types include nucleophilic substitution, electrophilic addition, electrophilic substitution, and redox reactions. Reaction mechanisms vary and help in understanding the different types of reactions taking place.Slide2

Alkanes

Saturated

hydrocarbons where carbons in the chain are singly bonded to one another

Ex:

Methane

Ethane

Propane

Butane

Pentane

Reactivity

: relatively low

Carbon-hydrogen bond relatively strong (relatively high bond energy)

Only

slightly polar (electronegativity difference of

0.4)

Two

RXN types are

important:

1:Combustion

: rapid, exothermic oxidation of combustible

materials

2: Substitution: two main types

FRCR: free radical chain reaction

Nucleophilic (S

N

1, S

N

2)Slide3

Combustion: rapid, exothermic oxidation of combustible materials.

Most common alkane RXN

Requires:

oxidizer (oxygen)

fuel source (alkane)source of ignition (required to reach activation energy)

Alkanes: CombustionSlide4

Complete combustion of hydrocarbons produces CO

2

and

H

2OAll carbon converts to CO2 and all Hydrogen converts to H2O. When balancing:

 # of C in the alkane = # CO2

molecules produced  # of H in the alkane = 2 X H2

O molecules produced

In most situations, combustion of hydrocarbons is incomplete because of insufficient oxygen. Products of incomplete combustion are responsible for a large amount of urban pollution:  carbon monoxide (CO)  carbon (soot)

Alkanes: CombustionSlide5

Burning other hydrocarbons (unsaturated) is very similar

Alkanes

Alkene

Alkynes

ArenesThe more unsaturation (higher C:H ratio)  the higher the smokiness due to unburned carbon

CO

2 and H2O are greenhouse gases = absorb radiation and increase heat average world temp

CO toxin as binds irreversibly to hemoglobin in blood

C (soot) causes respiratory distress and contributes to smog and global dimmingAlkanes: CombustionSlide6

Free radical chain reaction:

alkane RXT with halogen = halogenoalkanes

One Hydrogen (H) in

the alkane is replaced by a

halogen (X)  reaction of ethane with chlorine: 

CH3

-CH3(g) + Cl

2(g

)  CH3-CH2-Cl(g) +

H-Cl

(g)

 

Ethane chlorine chloroethane hydrogen chloride

Alkanes-Substitution RXNs: FRCRSlide7

Reaction usually

brought about by exposure to

UV light

or

high temps (provides energy of activation)Chloroethane can RXT with more Cl2

 1,2-dichloroethane and

1,1-dichloroethaneHigh

amounts of

Cl2  eventually convert to hexachloroethane (substitute all H with Cl)

Alkanes-Substitution RXNs: FRCRSlide8

Free radical:

any molecule or atom with a single unpaired electron

 

highly reactive Reaction proceeds in 3 distinct phases. RXN of CH4 with Cl2

example:Initiation: free radicals are produced

Propagation: products are formed and radicals are reformedTermination: radicals are used up

Alkanes-Substitution RXNs: FRCRSlide9

1. Initiation phase:

Source

of

E (often

UV light) can break covalent bond between the 2 Cl atomsReleasing unpaired Cl atoms (free radicals)

Photochemical

homolytic fission: each atom results in one e- (“equal

splitting”).

Therefore, heterolytic fission is unequal splitting  both electrons result with one atomLarge reduction in stability for

Cl

when this

happens

Alkanes-Substitution RXNs: FRCRSlide10

2. Propagation:

Unstable Cl• readily forms

new covalent bond with whatever is

present

Here, H atom from CH4 Cl radical pulls H atom

(including its e- which is currently

shared with carbon atom) off of CH

4

This forms HCl and free radical, •CH3CH3• will then pull a

Cl atom

off a

Cl

2 molecule, reforming a chlorine radical. Continues in a chain reaction.

Alkanes-Substitution RXNs: FRCRSlide11

3. Termination

occurs when all of the radicals are consumed.

Cl

radicals can combine with each other to form a molecule of Cl2OR they can combine with

a CH3•

to form CH3Cl

OR 2 methyl

radicals can combine to form ethaneSince ethane found to be produced during the halogenations of methaneMechanism for

this reaction

is indeed the one illustrated in the

diagram.. So we know it’s good!

Alkanes-Substitution RXNs: FRCRSlide12

If

bromine were used instead of

chlorine

Dark

brown color provides simple visual method to monitor the progress of the reactionAs

the brown colored bromine is consumed, the color would gradually fade

Note: reaction is not

observed

in the darkThere is no source of energy to create the necessary radicalsAlkanes-Substitution RXNs: FRCRSlide13

Nucleophilic substitution of

halogenoalkanes

:

Nucleophile is e- rich and attack areas of e- deficiency

Nucleophile can be anything with a lone pair of electrons, but common examples are:Hydroxide ion:OH-Ammonia: NH

3Cyanide ion:

CN-Electrophile is e- deficient and accepts e- pairs from a nucleophileElectrophile

can be anything

e- deficientCommon examples are:Hydride ion: H+Bromide ion: Br+Nitrate ion: NO2

+

20.1: Alkanes-Substitution RXNs: NucleophilicSlide14

Nucleophilic substitution of halogenoalkanes:

Polar C-X bond means C atom is e- deficient=electrophile

It can be attacked by a nucleophile such as OH

-

General reaction:

20.1: Alkanes-Substitution RXNs: NucleophilicSlide15

Nucleophilic

substitution can occur by two distinct “

mechanisms”

Mechanism:

a step-wise model of how a reaction occursRate-determining step: In a chemical reaction with more

than one step (and many of them do)The

slowest step determines the overall rate of reaction

Balanced

equation implies that a reaction occurs in only one step – this is often not the case!Alkanes-Substitution RXNs: NucleophilicSlide16

Molecularity

:

#

of molecules involved in rate-determining stepUnimolecular: one molecule is involved Bimolecular: two are involved

Termolecular: Three involved, and so on

Termolecular steps and above are quite rare because the probability of three particles colliding simultaneously is very

low

Nucleophilic substitution can occur by two distinct “mechanisms”SN2SN1 Alkanes-Substitution RXNs: NucleophilicSlide17

S

N

2 type

mechanisms

Substitution, Nucleophilic, 2 (bimolecular). Nucleophilic substitution reaction that has two molecules in the rate-determining step.

Primary

Halogenoalkane

Subst

: SN2Slide18

Nucleophile attacking electrophile C on the opposite side of leaving group results in an inversion of the atoms around the carbon (stereospecific)

Primary

Halogenoalkane

Subst

: SN2Slide19

S

N

1

type mechanisms:

Stands for Substitution, Nucleophilic, 1 (unimolecular)Nucleophilic substitution reaction that has one molecule in the rate-determining step

Tertiary Halogenoalkane

Subst: SN1

Ex: a

haloalkane undergoes slow, heterolytic fission to produce a carbocation intermediate and a halide ion“X” is any halogenCarbocation means a positively charged carbon ion Slide20

Step 1:

Relatively slow due to the energy input required to break the carbon-halogen bond.

Curved arrow that starts on C and moves to the halogen (X) indicates that electrons move from carbon to the halogen.

Tertiary

Halogenoalkane Subst: SN1Slide21

Step 2:

Lone

pair

electrons on OH

- is attracted to this + carbocation

, and form a coordinate bond (dative)

2nd step is much quicker

so the 1

st step is rate-determining.1 molecule is involved in the rate-determining step = unimolecular, Therefore  SN1 mechanism

Tertiary

Halogenoalkane

Subst: SN1Slide22

S

N

1 or an S

N

2 mechanism depends on the nature of the haloalkane. 1° Primary haloalkanes tend to undergo SN2 substitutionEasy for the nucleophile (OH

- in ex) to access the carbon to attack it  No large carbon atoms in its

way 3° Tertiary

haloalkanes

tend to undergo SN1 substitutionDifficult for the nucleophile to access the carbon while the surrounding carbons “shield it” 2° Secondary halogenoalkanes, both mechanisms can occur

Comparing S

N

1

and SN2Slide23

Effect of the mechanism:

S

N

1

occur faster than SN2 In general: tertiary > secondary > primary. Influence of the leaving group:Polarity of

C–X bond C—F is most polar C—I is less polar

Would expect that C—F would be faster to leaveStrength of C–X bondStronger bonds take longer to break

C– I > C– Br > C– Cl >

C–FStrength predominate for rate of RXN (over polarity)

Speed of a nucleophilic substitution

reactionSlide24

S

N

2

Prefers solvents that are polar and

aprotic (no H-bonds in the solvent=no proton or H+)They tend to solvate the Na+ (kind of like dissolve) and leaves the nucleophile bare and more reactiveGood solvents are: propanone and

ethanenitrile

SN1Carbocation intermediate is planar, so nucleophile can attack from any position (not stereospecific and can result in racemic mixture)

Prefers polar and

protic solvents to help stabilize the carbocation intermediateGood solvents are: water, alcohols, carboxylic acidsSpeed of a nucleophilic substitution

reactionSlide25

Summary of Alkane Nucleophilic Substitution RXNS