Organic REACTIONS: Organic REACTIONS: - Start

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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.




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








: relatively low

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


slightly polar (electronegativity difference of



RXN types are



: rapid, exothermic oxidation of combustible


2: Substitution: two main types

FRCR: free radical chain reaction

Nucleophilic (S


1, S




Combustion: rapid, exothermic oxidation of combustible materials.Most common alkane RXNRequires:oxidizer (oxygen)fuel source (alkane)source of ignition (required to reach activation energy)

Alkanes: Combustion


Complete combustion of hydrocarbons produces CO2 and H2OAll 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 H2O molecules producedIn 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: Combustion


Burning other hydrocarbons (unsaturated) is very similarAlkanesAlkeneAlkynesArenesThe more unsaturation (higher C:H ratio)  the higher the smokiness due to unburned carbonCO2 and H2O are greenhouse gases = absorb radiation and increase heat average world tempCO toxin as binds irreversibly to hemoglobin in bloodC (soot) causes respiratory distress and contributes to smog and global dimming

Alkanes: Combustion


Free radical chain reaction:alkane RXT with halogen = halogenoalkanesOne Hydrogen (H) in the alkane is replaced by a halogen (X)  reaction of ethane with chlorine:  CH3-CH3(g) + Cl2(g)  CH3-CH2-Cl(g) + H-Cl(g)  Ethane chlorine chloroethane hydrogen chloride

Alkanes-Substitution RXNs: FRCR


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: FRCR


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: FRCR


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: FRCR


2. Propagation:Unstable Cl• readily forms new covalent bond with whatever is presentHere, H atom from CH4 Cl radical pulls H atom (including its e- which is currently shared with carbon atom) off of CH4This forms HCl and free radical, •CH3CH3• will then pull a Cl atom off a Cl2 molecule, reforming a chlorine radical. Continues in a chain reaction.

Alkanes-Substitution RXNs: FRCR


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: FRCR


If bromine were used instead of chlorineDark brown color provides simple visual method to monitor the progress of the reactionAs the brown colored bromine is consumed, the color would gradually fadeNote: reaction is not observed in the darkThere is no source of energy to create the necessary radicals

Alkanes-Substitution RXNs: FRCR


Nucleophilic substitution of halogenoalkanes:Nucleophile is e- rich and attack areas of e- deficiencyNucleophile can be anything with a lone pair of electrons, but common examples are:Hydroxide ion:OH-Ammonia: NH3Cyanide 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: Nucleophilic


Nucleophilic substitution of halogenoalkanes:

Polar C-X bond means C atom is e- deficient=electrophileIt can be attacked by a nucleophile such as OH-General reaction:

20.1: Alkanes-Substitution RXNs: Nucleophilic


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 reactionBalanced equation implies that a reaction occurs in only one step – this is often not the case!

Alkanes-Substitution RXNs: Nucleophilic


Molecularity: # of molecules involved in rate-determining stepUnimolecular: one molecule is involved Bimolecular: two are involvedTermolecular: Three involved, and so onTermolecular steps and above are quite rare because the probability of three particles colliding simultaneously is very lowNucleophilic substitution can occur by two distinct “mechanisms”SN2SN1

Alkanes-Substitution RXNs: Nucleophilic


SN2 type mechanismsSubstitution, Nucleophilic, 2 (bimolecular). Nucleophilic substitution reaction that has two molecules in the rate-determining step.


Halogenoalkane Subst: SN2


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


Halogenoalkane Subst: SN2


SN1 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


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: SN1


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

Tertiary Halogenoalkane Subst: SN1


SN1 or an SN2 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



and S




Effect of the mechanism:SN1 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 polarWould expect that C—F would be faster to leaveStrength of C–X bondStronger bonds take longer to breakC– I > C– Br > C– Cl > C–FStrength predominate for rate of RXN (over polarity)

Speed of a nucleophilic substitution



SN2 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 ethanenitrileSN1Carbocation 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 acids

Speed of a nucleophilic substitution



Summary of Alkane Nucleophilic Substitution RXNS



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