ALkanes Chapter 102 Structure bonding and chemical reactions involving functional group interconversions are key strands in organic chemistry Chapter 201 Key organic reaction types include nucleophilic substitution electrophilic addition electrophilic substitution and redox reactions ID: 405443
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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