Reactions of Hydrocarbons - Belle Vernon Area School District

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Transcript Reactions of Hydrocarbons - Belle Vernon Area School District

Reactions of Hydrocarbons
Unit 3
Review
Alkanes CnH2n+2
Alkenes CnH2n
Alkynes CnH2n-2
Combustion
HC’s burn in air and produce carbon
dioxide and water vapor
HC + O2 (g)  CO2 (g) + H2O (g)
Reactants
Products
Reactions must be balanced!
Example
Propane:
C 3H 8 +
Practice
Write and balance the following…
Pentane, 1-hexyne, octane, 2 decene,
2,5-heptadiene
Substitution Reactions
Alkanes – they are saturated HC’s so
they undergo substitution rxns where
an atom or group replaces a H atom
Halogenation – a substitution of a
halogen for a H in an alkane
Substitution Reaction
General Equation: R=Alkyl Group
RH + X2  RX + HX
Alkane
halogen
alkyl halide
hydrogen halide
Ex: Form 2-iodopropane (structural/condensed)
I
C-C-C + I2  C-C-C + HI
CH3CH2CH3 + I2  CH3CHICH3 + HI
Practice
Try 3-fluoropentane
(start with pentane then add F2)
Rules for Halogenation of Alkanes
Only 1 X can be substituted for each
X2 molecule used b/c the other has to
bond with the H that was replaced
Form 2,3-diiodopentane (start with
pentane)
Try 1,2,4 - trichlorohexane
Rules for Halogenation of Alkanes
If you aren’t given the location for the
X to substitute, then the X had to
bond to the carbon that is bonded to
the most other carbons.
1° = Primary C = Bonded to 1 other C
2° = Secondary C = Bnd to 2 other C
3° = Tertiary C = Bnd to 3 other C
Reactavity Order - 3°>2°>1°
C
Label each C as
C-C-C-C
1°, 2°, 3°
React 2-methyl butane with bromine
C
C
C-C-C-C + Br2  C-C-C-C + HBr
Br
CH3CH(CH3)CH2CH3 + Br2  CH3CBr(CH3)CH2CH3 + HBr
Practice
Try to react 3-ethylheptane with
fluorine
Rxns
Orgo Rxns are slow so they are easily
studied – there are steps that happen
b/t rcts and products.
Reaction Mechanism – the step by
step sequence of how a chemcial rxn
occurs
Examples
Halogenation of alkanes occurs by
free radical substitution
Free Radical – a highly reactive
species due to an unpaired e- as a
result of a homolytic bond clevage
Ex: Cl-Cl  2Cl•
The • represents
the free radical
Free Radical Substitutions
All alkyl groups are free radicals
Free radical substitution is a “chain
reaction”
Chain Reaction – a reaction that
occurs through a series of steps
Free Radical Substitution
RH + X2  RX + HX
1.) X2 –(Light) 2X•
Chain Initiating Step – a molecule
forms a free radical to start the chain rxn
2.) X• + RH  R• + HX
3.) R• + X2  RX + X•
Free Radical Substitution
2& 3 are Chain Propagating Steps –
one free radical forms a different free
radical to initiate the next step and to
sustain the rxn.
4.) 2X•  X2
5.) X• + R•  RX
6.) 2R•  RR (rarely happens)
Free Radical Substitution
4, 5, and 6 are…
Chain Terminating Steps – two free
radicals form a molecule to stop the
reaction
Practice
Write the reaction mechanism for the
formation of 2-fluoropropane
1 – draw the molecule
2 – write the reaction (structural or
condensed)
3 – start the free radical substitution
with the F
Alkenes
Sigma (σ) bonds – have orbiral
overlap directly between the nuclei of
2 atoms
More [ ] e- density = stronger bonds
All single covalent bonds are σ bonds
Alkenes
Pi bonds (π) have orbital overlap
above and below the nuclei of the two
atoms
Less [ ] e- density and weaker bonds
The 2nd and 3rd pairs of multiple bonds
are pi bonds.
Alkenes
Pi Bonds
Alkenes
Functional Group – an atom or group
of atoms that defines a family or
organic compounds and determines
their properties
Alkanes FG – C-C single covalent bond
Alkenes FG – C=C - undergo addition
rxns where 2 atoms or groups are added
to the double bond (π weaker than σ)
Reactions of Alkenes
Hydrogenation – the addition of
hydrogen to the C=C bond
General Equation –
C=C + H2 –Pt & Pd  C-C
Example – Hydrogenate 2-Pentene
C-C=C-C-C + H2 –Pt&Pd  C-C-C-C-C
~YOU FORMED PENTANE 
Reactions of Alkenes
Halogenation – the addition of a
halogen to the C=C bond
General equation - C=C +X2  X-C-C-X
Ex: Chlorinate 2-heptene
Ex: Form 2,3-dibromopentane by
addition (also name the reactant)
Reactions of Alkanes
Now you know 2 ways of forming an
alkyl halide (with 2 consecutive X’s
attached)
1.Substitution of an alkane
•
(RH + 2X2  RX2 + 2HX)
2.Addition to an alkane
•
(C=C + X2 X-C-C-X)
Examples
Form 2,3 diiodopentane by additon
Form 2,3 diiodopentane by
substitution
Form 3,4 difluoroheptane by addition
and substitution (name reactants)
Hydrohalogenation
Hydrohalogenation – the addition of a
hydrogen halide to the C=C
General Equation - C=C + HX  H-C-C-X
How do you know where the H or X will
bond?
Hydrohalogenation
Markovnikov’s Rule – in addition
reactions of unsymmetrical reagents,
the H bonds to the C bonded to the
most other H atoms (only look at the
two atoms making the C=C)
Example
Hydrofluorinate 1-propene (str&cond)
C=C-C + HF  H – C-C-C
F
CH2CHCH3 + HF  CH3CHFCH3
Form 2-chlorobutane by addition
Hydration of an Alkene
Hydration – the addition of water to
the C=C
H2O = HOH (hydrogen & hydroxide)
Also follows Markovnikov’s rule
Hydrate 1 – Propene (str&cond)
C=C –C + H2O  H-C-C-C
OH
CH2CHCH3 + H2O  CH3CHOHCH3
Practice
Form 2 – Hydroxopentane (Str&Cond)
Mechanism
Hydrohalogenation & Hydration occur
by “Ionic Addition” (this is our 2nd
mechanism)
C=C + HX  + C-C-H + X-  X-C-C-H
(Think of water as H+ and OH-)
Hydrohalogenation
The X takes the e- from H so H
becomes H+1 which makes a
coordinate bond to C [for this to
happen, the 2nd pair of e-s in the C=C
bond move to one of the C’s]. This
leaves the other C as a C+1. Then the
X-1 bonds to the C+1.
Hydrohalogenation
The + C-C-H is a carbocation (a
group of atoms that has a C with only
6e-s, giving the group a + charge)
Example
Write the reaction mechanism for the
reaction of 3-ethyl – 3-hexene and
water
Alkynes
Alkynes undergo addition reactions
where 4 atoms or groups are added to
the C= C
Hydrogenation  addition of H2 to the
C=C
H H
C=C + 2H2 –Pt/Pd C-C
H H
Hydrogenate 4-decyne (str, cond, name)
Halogenation of Alkynes
Halogenation – addition of a halogen
to the C=C bond
X X
C=C =2X2  C-C
X X
Chlorinate 2-hexyne (str,cond,name prod)
Form 1,1,2,2-tetrafluoropentane by addition
(str,cond,name prod)
Hydrohalogenation of Alkynes
H X
C=C + 2X  C-C
H X
Follows Markovnikov’s rule
Examples
Hydrobrominate 1-butyne (str, cond,
name product)
Form 2,2 – diiodopentane by addition
Hydration of an alkyne
Hydration – the addition of water to
the C=C
Follows the Markovnikov’s rule
H OH
C=C + 2H2O  C-C
H OH
Examples
Hydrate 1-propyne (str, cond, name
product)
Form 2,2 - dihydrobutane
Alkadienes
Undergo addition reactions where 4
atoms or groups are added to the two
C=C
There are 3 different general
equations for each of the 4 types
because of the conjugated, isolated,
and allenes orientations.
1. Hydrogenation
C=C-C=C + 2H2 -Pt/Pd C-C-C-C
H
H
H
H
C=C-C-C=C + 2H2 -Pt/Pd C-C-C-C-C
H
H
C=C=C + 2H2 -Pt/Pd C-C-C
H
H
H
H
H
H
2. Halogenation
C=C-C=C + 2X2  C-C-C-C
X
X
X
X
C=C-C-C=C + 2X2  C-C-C-C-C
X
X
X
C=C=C + 2X2  C-C-C
X
X
X
X
X
3. Hydrohalogenation
C=C-C=C + 2HX  C-C-C-C
H
X
X
H
C=C-C-C=C + 2HX  C-C-C-C-C
H
X
C=C=C + 2HX  C-C-C
H
X
H
X
X
H
4. Hydration
C=C-C=C + 2H2O  C-C-C-C
H OH OH
H
C=C-C-C=C + 2H2O C-C-C-C-C
H
OH
OH
C=C=C + 2H2O  C-C-C
H
OH
H
OH
H
Benzene
Undergoes substitution reactions
where an atom or a group of atoms
replaces a H
Benzene
1. Halogenation – the substitution of a
halogen for a H on benxene
Gen Eqn: ArH + X2 –FeX3 ArX + HX
Benzene
Aryl Halide
Ar = C6H5 = Aryl Group
EX:Form 1,3,5tribromobenzene(str,cond)
Benzene
2. Nitration – the substitution of a nitro
group for a H on Benzene
Gen Eqn = ArH + HNO3 –H SO ArNO2 +HX
Ex: Form 1,2,4,5 Tetranitrobenzene (str,cond)
2
4
Benzene
3. Friedel-Crafts Alkylation – the
substitution of an alkyl group for an H on
Benzene
GenEqn: ArH + RCl –AlCl3 ArH + HCl
Ex: Form orthodipropylbenzene (str&cond)
Friedel-Crafts Alkylation
(Reaction Mechanism)
RCl + AlCl3  R+ + AlCl4-1
R+ + ArH  ArR + H+
H+ + AlCl4-1  AlCl3 + HCl
Write the reaction mechanism for the
formation of 2-ethylbenzene
(str&cond)