Organic Reactions

Addition, Oxidation, Reduction, Substitution and Elimination

1.) Addition

• Only occurs with double or triple carbon bonds (on alkenes + alkynes)– Double (and triple) C bonds are more reactive than

single bonds

• A bimolecular molecule (A—X) breaks, and each half attaches itself to a C on either side of a = bond– This breaks the = bond to make a single C-C bond

C C + A X C C

A X

Addition• So called because it makes a bigger molecule (adds

more stuff onto the alkene/alkyne)• Things that will undergo addition with an alkene/alkyne:

– H2 – hydrogenation (usually requires a catalyst & high temperature)

– X2 – I.e. Br2, Cl2, F2 – halogenation I.e. bromination (occurs rapidly at room temperature)

– HX I.e. HCl, HBr, HI (hydrohalogenation – occurs at moderate rates at room T)

– H2O – hydration (requires an acid catalyst, i.e. H2SO4)

• Addition is always exothermic – from energy released by breaking C=C bond

Markovnikov’s Rule• “The Rich Get

Richer”– Where Hydrogen =

“riches”

• I.e. – when HX is attached to an alkene or alkyne, the H is more likely to go on the C that already has the most Hs

Addition Examples

• CH2=CHCH3 + H2O + • CH3CH=CHCH3 + Cl2

• H-C≡C-CH2CH3 + HBr +

• H-C≡C-CH2CH3 + H2 H2C=CHCH2CH3

Addition Examples

• Benzene + Br2 no reaction!

• Aromatics don’t undergo addition!– Not just with X2 (I.e. Br2), but with anything.– Because the product would be less stable

than the resonance double bonds of the benzene!

Uses: Hydrogenated Oils• Unsaturated oils can be

hydrogenated to form solid, saturated fats by the reaction with hydrogen gas in the presence of nickel or platinum as a catalyst.

• Ie: Margarine, shortening (Crisco)

• Vegetable oils were first hydrogenated for easier transport and storage in 1911.

http://www.dvo.com/newsletter/monthly/2006/august/images/Crisco.jpg

Uses: Hydrogenated Oils

Uses: The Iodine Index• The degree of unsaturation in oils can be

measured by measuring the amount of iodine that can react with the unsaturated fat or oil by addition. Each mole of C=C requires one mole of I2 to react.

• The haloalkane chain is nearly colorless Therefore unsaturated hydrocarbon chains will destroy the purple brown color of iodine solutions as long as there are C=C bonds present.

Uses: Bromine Test for Unsaturation

• Like Iodine Index, we test for unsaturation with Br2

– Usually done for non-fats, however (i.e. cyclohexene, below, is a non-polar solvent)

H

H

+ Br2

H

BrH

Br

2.) (Organic) Oxidation

• When an organic molecule is oxidized, it forms more bonds to an Oxygen and fewer bonds to a Hydrogen– LEO says GER still applies, but it’s easier to think in

terms of # of bonds to an oxygen or hydrogen• I.e.

• For oxidation to occur, the C must be bonded to at least one H at the start

(Organic) Oxidation

• Organic oxidation reactions are written:

• The [O] implies that the number of bonds to an oxygen is increasing, but not necessarily the number of oxygens in the molecule

• I.e.

(Organic) Oxidation

(Organic) Oxidation

• Needs an oxidizing agent (which is reduced) – I.e. potassium dichromate (VI) solution (K2Cr2O7, in

the presence of H2SO4, KMnO4):

• Oxidizing agent pulls 2 Hs off primary alcohol– Therefore pulls off electrons with the Hs: LEO

(Organic) Oxidation

• Oxidizing agent then donates an O to the remaining H

• I.e.

Oxidation Examples

3.) (Organic) Reduction

• The exact opposite of organic oxidation• Going backwards along this chain:

• Forms more bonds to an Hydrogen and fewer bonds to a Oxygen

• Done by a reducing agent and a catalyst (I.e. H2 on Platinum catalyst)

(Organic) Reduction

• Organic reduction reactions are written:

• The [H] implies that the number of bonds to a hydrogen is increasing– This has to increase # of Hs in the molecule

• To be reduced, a molecule can’t be saturated with Hs!

Reduction Examples

A Note about Organic Redox

• It’s not always obvious where the electrons go (like regular redox)– Sometimes there is no direct transfer of

electrons

• But there is always a change in Oxidation state. I.e. Reduction:

Uses• Pretty much everything, ever • Bleach oxidizes pigment molecules until they are no longer

the same molecule – lose their colour

• Cellular Respiration/Photosynthesis– Cell Resp: Glucose oxidized to CO2

C6H12O6+ 6 O2 6 H2O + 6 CO2

– Each C molecule bonded to 2 Os in CO2, vs. 1 in glucose

– Photosynthesis: CO2 reduced back to glucose

6 H2O + 6 CO2 C6H12O6+ 6 O2

Uses• Ethanol conversion in the human body:

• People get sick from drinking because the enzyme converting ethanol to acetaldehyde (alcohol dehydrogenase) works faster than the one converting acetaldehyde to acetic acid (acetaldehyde dehydrogenase)

– So acetaldehyde builds up – bad stuff

Uses

• Old breathalyzer tests:• Ethanol in the bloodstream will come into equilibrium

with ethanol in the lungs– I.e. when there’s ethanol in the body, it gets breathed out too

• The breathalyzer uses H2O to oxidize ethanol: it loses electrons

• CH3CH2OH(g) + H2O(l) CH3CO2H(l) + 4H+(aq) + 4e-

• The 4e- power an electrochemical cell; how much voltage is produced is proportional to how much ethanol is in the lungs and body!

Uses

• Problem with old breathalyzer tests: will detect any kind of alcohol (I.e. ethanol from mouthwash, acetone breathed out by some diabetics, compounds in some asthma inhalers!)– New breathalyzers filter out acetone– Breathalyzer tests are confirmed with blood

tests – screens for false positives