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Reactions of Hydrocarbons. Unit 3. Review. Alkanes C n H 2n+2 Alkenes C n H 2n Alkynes C n H 2n-2. Combustion. HC’s burn in air and produce carbon dioxide and water vapor HC + O 2 (g)  CO 2 (g) + H 2 O (g) Reactants Products Reactions must be balanced!. Example. Propane:

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review
Review
  • Alkanes CnH2n+2
  • Alkenes CnH2n
  • Alkynes CnH2n-2
combustion
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
Example
  • Propane:
  • C3H8 +
practice
Practice
  • Write and balance the following…
  • Pentane, 1-hexyne, octane, 2 decene, 2,5-heptadiene
substitution reactions
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
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

practice1
Practice
  • Try 3-fluoropentane
  • (start with pentane then add F2)
rules for halogenation of alkanes
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 alkanes1
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°
slide11
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

practice2
Practice
  • Try to react 3-ethylheptane with fluorine
slide13
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
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
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
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 substitution1
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 substitution2
Free Radical Substitution

4, 5, and 6 are…

Chain Terminating Steps – two free radicals form a molecule to stop the reaction

practice3
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
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
alkenes1
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.
alkenes2
Alkenes
  • Pi Bonds
alkenes3
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
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 alkenes1
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
Reactions of Alkanes
  • Now you know 2 ways of forming an alkyl halide (with 2 consecutive X’s attached)
  • Substitution of an alkane
    • (RH + 2X2 RX2 + 2HX)
  • Addition to an alkane
    • (C=C + X2X-C-C-X)
examples1
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
  • 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?
hydrohalogenation1
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)
example1
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 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

practice4
Practice
  • Form 2 – Hydroxopentane (Str&Cond)
mechanism
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-)

hydrohalogenation2
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.
hydrohalogenation3
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)
example2
Example
  • Write the reaction mechanism for the reaction of 3-ethyl – 3-hexene and water
alkynes
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 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
Hydrohalogenation of Alkynes

H X

  • C=C + 2X  C-C

H X

  • Follows Markovnikov’s rule
examples2
Examples
  • Hydrobrominate 1-butyne (str, cond, name product)
  • Form 2,2 – diiodopentane by addition
hydration of an alkyne
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

examples3
Examples
  • Hydrate 1-propyne (str, cond, name product)
  • Form 2,2 - dihydrobutane
alkadienes
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
1. Hydrogenation
  • C=C-C=C + 2H2 -Pt/Pd C-C-C-C

H HHH

  • C=C-C-C=C + 2H2 -Pt/Pd C-C-C-C-C

H HHH

H

  • C=C=C + 2H2 -Pt/PdC-C-C

H HH

2 halogenation
2. Halogenation
  • C=C-C=C + 2X2 C-C-C-C

XXXX

  • C=C-C-C=C + 2X2 C-C-C-C-C

XXXX

X

  • C=C=C + 2X2C-C-C

X XX

3 hydrohalogenation
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 XX H

X

  • C=C=C + 2HX C-C-C

H X H

4 hydration
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 H

OH

  • C=C=C + 2H2O C-C-C

H OH H

benzene
Benzene
  • Undergoes substitution reactions where an atom or a group of atoms replaces a H
benzene1
Benzene
  • 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)

benzene2
Benzene

2. Nitration – the substitution of a nitro group for a H on Benzene

Gen Eqn = ArH + HNO3 –H2SO4ArNO2 +HX

Ex: Form 1,2,4,5 Tetranitrobenzene (str,cond)

benzene3
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
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)