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CHEMISTRY FORM 5-ORGANIC CHEMISTRY-AROMATIZATION – ALKANES

CHEMISTRY FORM 5-ORGANIC CHEMISTRY-AROMATIZATION - ALKANES

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CHEMISTRY FORM 5-ORGANIC CHEMISTRY-AROMATIZATION – ALKANES

 

Alkanes containing six or more carbon atoms when heated under pressure in the presence of suitable catalyst get cyclised to give aromatic compounds.

E.g.

n – hexane gives benzene




USES OF ALKANES

Alkanes are the simplest organic compound containing carbon and hydrogen. Some major uses of these compounds are;

(i)     Lower alkanes occurring as natural gas and lighter petroleum fractions are used as fuels.

(ii)    Low – boiling liquid alkanes e.g. hexane are uses as solvents.

(iii) Heavy petroleum fractions are used as lubricants (grease) and for obtaining waxes and vaseline

(iv) The products of cracking process are generally used for producing linearalkyl benzene (LAB) used as a raw material for manufacturing detergents.

 

ALKENES

These are hydrocarbons which are unsaturated. They contain double bond between two carbon atoms.

Functional group is C = C and share the same formula with cycloalkanes (cyclalkanes) are known as fractional isomer.

General formula: Cn H2n

Cycloalkanes:  Cn H2n

The type of hybridization is sp2 hybridization since it is trigonal pyramidal shape.

Nomenclature:

The first member is Ethane since we can’t have double bond in a single carbon atom.

E.g. C2 H4 – Ethene

C3H6 -Propene



Isomerism:   
3 – methylbut -1- ene

Naming of different compounds:

3 – propylhex – 2 – ene

Functional isomers;- They have the same general formula but different functional group.  Alkenes and cyloalkanes both have general formula Cn H2n

 

3 – menthlyhex – 3 – ene

2, 3- dimethyl hex-2-ene

 

If more than 1 double bond, add a prefix (a) to hept

6 – methylhepta – 1, 3, 6 – triene

 

Types of Isomerism in alkenes:

Alkenes show 4 types of isomerism;

(i)     Chain isomerism

(ii)    Positional isomerism

(iii)   Geometrical isomerism

(iv)   Functional isomerism

Chain isomerism: (Skeletal isomerism)

This is due to the difference in the structure of carbon chain.

Example

Positional isomerism:

This arises from the difference in the position of the double bond.

 

Geometrical isomerism:

Definition:

–    Is brought in the difference in the spatial arrangement of atoms or group of atoms about the double bond.

 

Functional isomers:

These compounds have the same general formula but difference functional group.

E.g. Alkene and cycloalkanes i.e Cn H2n

Cyclobutane

Cyclobutene

 

Laboratory preparation of Alkenes:

(i)     Dehydrohalogenation of haloalkanes (alkyl halides)

–   There is elimination reaction.

–    The reaction is done in alcoholic basic medium.

Note:

General formula for alky halides is R – X, X = Cl, Br, F

–    When alky halide is heated with alcoholic solution of sodium hydroxide or potassium hydroxide, hydrogen and halogen will be eliminated and alkene is formed.

This is known as Basic induced elimination reaction

 

SAYTZEFF’S RULE

It states, “during elimination reaction, the electrophyl (H+) is removed from carbon atom with fewer number of hydrogen atom.

Example of reactions which apply Saytzeff’s rule;-

(ii) (a)    Dehydration of alcohol

This is done using concentrated sulphuric acid by warming about 175 – 180 . You react alcohol with conc. sulphuric acid.

Note:

Temperature is very important  this reaction is sensitive to temperature

NOTE:

H3PO4 (conc.) can be used as a dehydrating agent (373 – 383k)

(b)   Dehydration by passing the vapour of alcohol over aluminium oxide (alumina) at 350

 

iii)  Dehalogenation of vicinal dihalides

Vicinal means the halogens are on the adjacent carbon of the same carbon.

iv) (a) Controlled hydrogenation of alkynes

It is done under palladium catalyst which is poisoned by calcium carbonate and quinoline and this reagent is known as Lindlers catalyst.

Note:

Poisoned means that the palladium is not pure

(b) Sodium, Lithium and Ammonia

This is not complete hydrogenation.





Chemical properties of Alkenes:

NOTE: Alkenes are more reactive than alkenes due to the presence of pi bond which is relatively weak. Hence, can react easily

 

Addition of hydrogen halides (HX)

Alkenes undergo electrophilic addition reaction (electron loving/electron deficient).

Electrophilic addition reaction is the reaction in which electrophyl is added first followed by nucleophyl.

Addition reaction of  follows a rule known as Mark KovniKov’s rule

 

Mark KovniKov’s rule:

It states, ‘During the addition reaction the electrophyl (hydrogen) is added to carbon atom with more number of hydrogen atoms’.

Eg:

Why Mark Kovni Kov’s rule?

It is because of the formation of stable carbocation. Hence the rule is used so as to form a stable carbocation.

NOTE:

Stability of carbocation is due to the supply of electron from the alkyl group as shown above. The halogen is added to the stable carbon.

In long chains, a stable carbocation will be formed when the carbon is bounded by many alkyl groups (since the alkyl group will be supply electrons

Note:

Hydrogen is added to the more stable carbon.

 

Hydration of alkenes:

Hydration means addition of water.

–     This is addition of water in the presence of mineral acids. The most preferred acid is conc H2SO4. The mixture should be heated in order to form alcohol.

 

Carbocation will be formed in CH

Home Work:

Anti – markovnikov’s rule (organic peroxide HBr). In 1933 the American chemist M. S. Kharasch discovered that the addition of HBr to unsymmetrical alkenes in the presence of organic peroxide (R – O – O – R) takes a course opposite to that suggested by Markovnikovs rule.

 

NOTE:

It is strictly works using HBr with organic peroxide.

Mechanism:

  1. Peroxide dissociates to give alkoxy free radicals.

Weekly Test:

  1. (b) N2(g)   + 3H2(g)     2NH3(g)

Given ∆Hr = 92KJ

Since Hf you form 1mole

Hf  =

=

Halogenation

Addition of halogens to alkenes

–          This reaction is best carried out by simply mixing halogens in the inert solvents such as carbon tetrachloride (CCl4)

 

E.g.

The reaction of Bromine with alkene can be used to test for the presence of double bonds.

Reason:

Addition of bromine (brown) to alkene since it makes the solution colourless. This is how we test for presence of double bonds.

Bromine tests unsaturation of hydrocarbons (alkenes and alkynes).

 

HYDROGENATION

Addition of hydrogen:

–   Alkenes react with hydrogen in the presence of platinum or nickel catalyst to form alkanes.

 

v  This cannot take place without catalyst.

Hydration of Alkenes:

–          This follows Markrnkov’s rule.

Homework on hydration of alkenes

–          Find the reagent which should be added to alkene to follow Anti markornikovs rule.

Oxidation reaction of alkenes




Addition of Bromine water (Br2/H2O)

Alkenes decolourize Bromine water (Brown) apart from decolorizing Bromine solution.

Reaction with conc H2 SO4

Structure of sulphuric acid

 

Oxidation reactions of Alkenes

 Alkenes react with oxidizing agent to form diols. Oxidizing agent can be either KMnO4, K2CrO4.

– With cold dilute KMnO4 or cold alkalineKMnO4 you form diols.

 

NOTE: Diol means two OH

(b) When hot concentrated acidified KMnO4 or K2Cr2O4 is used. Alkenes are oxidized to carboxylic acid or oxidized to ketones or both.

Note:

If double bond is branched, you can’t form carboxylic acid.

Terminal alkene

 

OZONOLYSIS (O3)Ozono     ozone

Lysis      Breaking

–          This is a cleavage or breaking of carbon, carbon double bond by using ozone.

–          In ozonolysis C = C is completely broken to produce aldehydes or ketones or both depending on the primary structure of alkene.

 


NOTE:

Ozonolysis is the best method of locating the position of double bond in unknown alkene.

The oxygenated carbon in carbonyl compound obtained by ozonolysis is the one that were joined by double bonds in the original alkenes.

Ozonolysis has 2 major steps:

  1. i) 1st step:

NOTE:

Zn dust is added to prevent oxidation of aldehyde to carboxylic acid.

E.g.

 

Questions

  1. A certain compound A was unsaturated hydrocarbon with molecular formula C6 H12. During ozonolysis of A, two compounds C and D were formed with molecular formula C3H6O. Compound C and compound D was ketone. Identify A, C and D.

 

Solution:

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