CHEMISTRY FORM 5 ORGANIC CHEMISTRY-ALIPHATIC HYDROCARBONS

CHEMISTRY FORM 5 ORGANIC CHEMISTRY-ALIPHATIC HYDROCARBONS

UNAWEZA JIPATIA NOTES ZETU KWA KUCHANGIA KIASI KIDOGO KABISA:PIGA SIMU:0787237719

ALSO READ;

1. O’ Level Study Notes All Subjects
2. A’ Level Study Notes All Subjects
3. Pats Papers

CHEMISTRY FORM 5 ORGANIC CHEMISTRY-ALIPHATIC HYDROCARBONS

ALIPHATIC HYDROCARBONS

Nomenclature:

METHODS OF PREPARATION OF ALKANES

(a)    Hydrogenation of Alkenes and Alkynes

Alkenes and alkynes react with hydrogen in presence of catalyst eg Ni/Pt around 200 c or 300  c

From Alkenes

(b)   Method 2 of preparation

Alkyl halide (haloalkanes)

 

a)      Reduction of alkyl halide by metal and acid

The hydrogen displaces Bromine. Only done in the presence of Zn metal

 

b)       Reduction of Alkyl halide by using zinc and copper coupled with alcohol

Note:

Both zinc and copper must be present together with alcohol.

    

 c) Reduction of alkyl halide by using hydroiodic acid in the presence of red phosphorus

 

 

NOTE:

Function of red phosphorus is removing iodide so that hydrogen can react with the alkyl halide.

 

(c)     Wurtz Synthesis 

Alkyl halide in dry either solution react with sodium to produce alkane always the product has twice the number of carbon as that of alkyl halide.

ii)                  

NOTE:

Dry ether is very important condition.

(d)    Decarboxilation of sodium carboxilate salt

This is the reaction between carboxylic salt and sodium hydroxide in the presence of calcium oxide. The product will have 1carbon less than the reactant.

Exercise (H/W)

1.      Preparation of alkanes from petroleum, coal, natural gas.

2.      Read on method of preparation known as cracking. Preparation of alkanes from petroleum and natural gas.

–       Petroleum is formed from the remains of tiny marine organisms that died and sank to the bottom of the sea millions of years ago.

–      Petroleum is a mixture of many organic compounds, since the organic compounds are lighter than both the rock and the water they move upwards through the adjacent rock. Sometimes the organic Compound are trapped in porous rocks that are called reservoirs beneath impermeable rocks.

–     Example of reservoir is limestone.

–     Reservoirs from which petroleum can be extracted by drilling are referred to as oil fields.

–    The petroleum obtained is referred to as a crude oil.

Fractional distillation of crude oil:

–    Major components of crude oil are:-

(i)      Residuals (coke, asphalt, tar).

(ii)     Lubricating oils.

(iii)  Fuel oils.

(iv)  Diesels.

(v)    Kerosene.

(vi)  Naptha.

(vii) Petrol.

(viii) Petroleum gas.

The components of the crude oils are known as fractions and different fractions are separated by heating them in a process known as Fractional distillation and it is done in a distillation tower called a still.

The oil is first evaporated by heating . The vapour rises up and the tower acts as a giant heat exchanger removing heat from the gases as they rise up. Temperature falls to 20  by the time vapour reaches the top. The vapour condenses as they rise up.

The heavier ones i.e. those with higher boiling points condense first. Gaseous fractions pass out at the top.

Cracking

Some of the fractions obtained from the fractional distillation of the crude oil are converted into new products.

Cracking is the conversion of large molecules of organic compounds into compounds with smaller molecules.

There are two methods;-

(i)   Thermal cracking.

(ii)  Catalytic cracking.

In thermal cracking the large molecule organic compound is heated to a high temperature until its molecule break apart.

In catalytic cracking, a catalyst speeds up the cracking process.

(e)     Preparation of Alkanes from alcohols:

By reduction of alcohols:

When alcohols are hated with concentrated hydroiodic acid and red phosphorus at 423k under high pressure, alcohols can be reduced to alkanes.

Physical properties of alkanes:

(a)     Boiling point and melting point:

Alkanes have low melting point and boiling point.

Reason: Since alkanes are non-polar molecules with weak Van – der – Waal forces between them then low temperature is required to break the bond hence low melting and boiling point.

  Increase in molecular mass leading to the increase in melting & Boiling points

NB:

If you compare straight chain and branched chain of same molecular mass; straight chain has higher M.P &B.P. Branched chained isomers have lower boiling points and melting point than straight chain isomer.

Reason: Branched chains are more compact hence have less surface area. This is why they have low M.P and B.P. Straight chains have higher surface area.

(b)   Solubility:

They are soluble in non polar organic solvents but insoluble in polar compounds e.g. Water.

Chemical properties of alkanes:

In general, alkanes are non reactive (inert) compared to other classes of organic compounds.

Reason:

(i)    They don’t have a functional group.

(ii)   There bonds are quite strong i.e. c – c (strong bond). Large energy is needed to break the bond hence less reactive.

(iii)      These two bounds are almost non polar and therefore neither electrophilic nor nucleophilic substitution reaction can take place. Can’t react with electron – loving  species or a proton loving species (Nucleophilic).
–          Electrophilic reacts with a negatively charged species.

–          Nucleophilic reacts with a positively charged species.

Alkanes can undergo the following reaction;-

(i)     Substitution reaction.

(ii)    Oxidation reaction.

(iii)   Thermal decomposition (cracking).

All these reactions take place at high temperature or under the presence of light energy.

       I.            SUBSTITUTION REACTIONS

a)      Halogenations  

–     This is addition of halogens.

–     The reaction between alkane and halogen is known as free radical substitution reaction.

–     Free radical substitution reaction is the reaction in which a free radical substitutes atom/atoms in a molecule.

Q. What is a free radical?

A free radical is an atom or group of atoms which consist of unpaired electrons.

Example: 

  Function of UV – light is to give out a free radical.

Mechanism of reaction

Homolytic sharing of electron i.e. equal sharing of electrons go to each chlorine atom

NOTE:

Free radicals are very reactive. It wants to become stable.

 

NOTE:

With fluorine, the reaction is violet and yield hydrogen fluoride and carbon.

 

However, controlled fluorination in the presence of cobalt moderator the fluoral derivatives are formed.

NITRATION (With Nitric acid)

This involves the substitution of hydrogen atom in alkane with NO₂ group. This is done when a mixture of alkane and nitric acid vapour is heated at 400 c – 500 c

SULPHONATION (With sulphuric acid)

Alkanes when subjected to prolonged reaction with fuming sulphuric acid one hydrogen atom of alkane is replaced with – SO₃ H group known as sulphonic group.

 

Oxidation Reactions:

When alkanes are ignited in the presence of excess oxygen they burn to form carbondioxide and water only.

Reaction is highly exothermic:

Thermal decomposition (PYROLYSIS)

This is breaking down of higher alkane into lower alkane by heating alkanes in absence of air.

Alkane and alkene are the only possibilities.

NOTE:

No two alkenes will be formed.

 

Catalytic cracking (Isomerization)

When straight chains of alkanes are heated in aluminium chloride in the presence of dry hydrogen chloride at 300  gives a branched chain isomers. In this process there is no breaking of the compound  but it is changed to branched chain.

 

This process is used in petrol chemical industry. The branched chain alkane has higher octane number. Hence branched chain burns easily than straight chains.

 

Question:

–          How to form aromatic compounds i.e. Aromatizations

–          Uses of alkanes

Exercise:

Qn. How can the following conversions be achieved?

(a)    Propyne to propane

(b)   Hexane to 3methylpentane

(c)    Ethane to butane

(d)   Pentane to nitropentane

Solution:

AROMATIZATION

–          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: C_n H_2n

Cycloalkanes:  C_n H_2n

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 C_n H_2n

3 – menthlyhex – 3 – ene

2, 3- dimethyl hex-2-ene

 

4.      

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 C_n H_2n

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.

CHEMISTRY FORM 5 ORGANIC CHEMISTRY-ALIPHATIC HYDROCARBONS

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

1.      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.

2.     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) N_2(g)   + 3H_2(g)          2NH_3(g)

Given ∆H_r = ^– 92KJ

Since H_f you form 1mole

H_f  =

=

3. 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).

4. HYDROGENATION

Addition of hydrogen:

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

  This cannot take place without catalyst.

5. 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

6. 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

7. Oxidation reactions of Alkenes

_–                     Alkenes react with oxidizing agent to form diols. Oxidizing agent can be either KMnO_4, 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:

i)                    1^st 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: