IB Organic Reactions I

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10.2.1.Low reactivity of alkanes

Alkanes are very unreactive due to single C C bonds. All combustion reactions are highly exothermic.(low polarity &van der waals)

I. Complete Combustion: 2C8 H18 + 25O2 => 16CO2 + 18 H2 O

II. Incomplete CombustionC8 H18 + 9O2 => 2CO2 + 9 H2 O + C + 5CO

C and CO are emitted as particulates. Read page 193

Solve Hodder page 300, question 10.

I-Combustion of Alkanes

Question 9

Organic Packet :

Homolytic Fission:

When the cleavage of covalent bond between two atoms takes place in a manner, which enables each atom to retain one electron of the shared pair.

This fission is symmetrical and leads to the formation of atoms or groups of atoms having unpaired electrons, called free radicals.

Cl-Cl => Cl● + Cl● chlorine free radical ( Cl atom )

Homolytic X Heterolytic Fission

Heterolytic fission is unsymmetrical , one of the fragments takes both the electrons of the shared pair, leaving none on the other. This results into two charged particles as:

C6 H5CH2 Cl => C6 H5CH2+ + Cl-

Heterolytic Fission

ALKANES UNDERGO SUBSTITUTION

ALKENES UNDERGO ADDITION

Alkanes undergo free radical substitution. This reaction takes place in a series of steps. With excess of the halogen, it continues until all the hydrogens were substituted)

CH4 (g) + Cl2 (g) => CH3 Cl(g) + HCl(g)

(1) Initiation Step:

(2) Propagation Step:

(3) Termination Step:

With excess of the halogen, it continues until all the hydrogens were substituted)

Solve Hodder page 300, question #8 and 11

Free Radical Substitution

1. The active species in the chlorination of an alkane is  A. Cl2 molecules

 B. Cl atoms C. Cl– ions D. Cl+ ions

2. This question concerns several different compounds with two carbon atoms.Ethane, C2H6 , reacts with chlorine to produce chloroethane as the first product.

(i) Write a balanced equation for this reaction.

 (ii) This reaction will not occur simply by mixing the chemicals. What else is required?

 (iii) What mechanism is involved in this reaction?

IB Questions

ALKANES UNDERGO FREE RADICAL SUBSTITUTION.

ALKENES AND ALKYNES UNDERGO ADDITION !!

The main source of alkenes is from the cracking of the higher fractions.

C10H22 (g) => C8H18 (g) + C2H4(g)

About 10% of the crude oil is converted into alkenes.

They are used as a chemical feedstock: plastics, solvents,detergents.

You may have heard of biofuels being used to fuel automobiles. Biofuel, or gasohol as it is called in many countries, is a mixture of gasoline and ethanol.

Margarines

PVC

There are three key areas for the use of alkenes:

Hydrogenation of unsaturated lipids to make margarine, the production of ethanol and the production of polymers.

Alkenes undergo addition due to the presence of the double bond.

Addition of Alkenes

Summary of Addition Reactions of Ethene

Catalyst: Ni , 180◦C

1. Hydrogenation,add H2

Unsaturated fatty acids may be converted to saturated fatty acids by the relatively simple hydrogenation reaction.

Addition of hydrogen to an alkene (unsaturated) results in an alkane (saturated).

H2C=CH2 + H2 ---> CH3CH3

Hydrogenation of Unsaturated Fats

Vegetable oils are commonly referred to as "polyunsaturated". This simply means that there are several double bonds present. Vegetable oils may be converted from liquids to solids by the hydrogenation reaction. Margarines and shortenings are "hardened" in this way to make them solid or semi-solids.

Hydrogenation of a oleic fatty acid

It has long been recognized that saturated fats tend to increase the blood level of the "bad" LDL cholesterol. Monounsaturated (one double bond) and polyunsaturated fats (two or more double bonds) found primarily in vegetable oils tend to lower "bad" LDL cholesterol. An elevated LDL-C increases the risk of developing coronary heart disease.

http://www.elmhurst.edu/~chm/vchembook/558hydrogenation.html

Test for unsaturation: Bromine water is a yellow/orange solution. The color will disappear because the product is

colorless: alkenes with Br2 (aq)

2. Halogenation, addition of alkene to Br, Cl and I page 200

The hydrogen will tend to migrate to the side with the greater number of hydrogen atoms.

This preference is known as Markovnikoff´s Rule.

DO THE REACTION WITH But 2 ene and HBr !!!!

3. With Hydrogen Halides,HCl and HBr

Water does not react directly with alkenes under normal conditions, concentrated sulfuric acid is required:(page 201)

Reaction:

4. Hydration, with Water

Monomer: ethene Polymer: Polyethene

5. Addition Polymerization

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Oxidation of Alcohols

Primary (1º) Secondary (2º ) Tertiary (3º)

H CH3 CH3

│ │ │

CH3─C─OH CH3─C─OH CH3─C─OH

│ │ │

H H CH3

1 C 2 C 3 C

attached attached attached to C-OH to C-OH to C-OH

Different types of Alcohols

In the oxidation [O] of a primary alcohol 1, one H isremoved from the –OH group and another H from the Cbonded to the –OH.

primary alcohol aldehyde

OH O │ ║ CH3─C─H CH3─C─H +

H2O │ H ethanol ethanal (ethyl alcohol) (acetaldehyde)

Oxidation of 1° Alcohols

K2Cr2O

7H2SO

4

[O]

The oxidation of 2 alcohols is similar to 1°, except that a ketone is formed.

secondary alcohol ketone

OH O │ ║

CH3─C─CH3 CH3─C─CH3 + H2O │

H

2-propanol 2-propanone

Oxidation of 2° Alcohols

[O]

K2Cr2O

7H2SO

4

Tertiary 3 alcohols do not oxidize.

Tertiary alcohol no reaction

OH │ CH3─C─CH3 no product

│

CH3

no H on the C-OH to oxidize

2-methyl-2-propanol

Oxidation of 3° Alcohols

K2Cr2O

7H2SO

4

[O]

If we compare the physical properties of ethanol, ethanal and ethanoic acid we can devise a way of obtaining a good yield of ethanal before further oxidation to ethanoic acid.

Ethaol and ethanoic acid: hydrogen bonds Ethanal: dipole dipole , lower BP

Distillation apparatus

ethanol H+ /Cr2 O7

-2

Distillation& Reflux Apparatus

The scheme below can be used to devise reaction pathways. They should involve no more than two steps.

Converting 2 butene to butanone:

Reaction Pathways(SG 67)

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1. Which alcohol undergoes oxidation most easily? A. CH3CH2CH2CH2CH2OH B. CH3CH(OH)CH2CH2CH3

C. CH3CH2CH(OH)CH2CH3

D. (CH3)2C(OH)CH2CH3

A small amount of alkanol, RCH2OH, is added to a warm solution of potassium dichromate(VI) and sulphuric acid and left for some time. When the reaction is complete, which functional group will be present in the product?

  A. RCH2––O––CH2R   O   B. RC––H   O   C. RC––O––H   O O   D. RC––O––CH2R

Oxidation of Alcohols(Alkanols)