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: C3H8 +

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°

React 2-methyl butane with bromine C C C-C-C-C + Br2  C-C-C-C + HBr 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) Substitution of an alkane (RH + 2X2  RX2 + 2HX) 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 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 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

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 H H H C=C=C + 2H2 -Pt/Pd C-C-C H H H

2. Halogenation C=C-C=C + 2X2  C-C-C-C C=C=C + 2X2  C-C-C X X X X C=C-C-C=C + 2X2  C-C-C-C-C X X X X X C=C=C + 2X2  C-C-C 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 X H X C=C=C + 2HX  C-C-C H X H

4. Hydration C=C-C=C + 2H2O  C-C-C-C C=C-C-C=C + 2H2O C-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 Undergoes substitution reactions where an atom or a group of atoms replaces a H

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)

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)

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)