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 ProductsReactions 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 + HXAlkane 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

BrCH3CH(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

AlkenesSigma (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

AlkenesFunctional 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 AlkeneHydration – 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 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 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-CX 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 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

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 –H2SO4ArNO2 +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)