ALKANE, ALKENE AND ALKYNES
Contents:
AKANES �
ALKANES
Introduction:
Alkanes are the simplest organic compounds made of carbon and hydrogen only, containing no double bond. They are called saturated hydrocarbons and are least reactive class or hydrocarbons that give substitution reactions only under special conditions.
General formula is CnH2n+2 where n=1,2,3 etc.
The first three members of this class can be represented as :
SATURATED HYDROCARBONS:
The carbon atoms in their molecules are bonded to each other by single covalent bonds .Each carbon is again bonded to enough hydrogen atoms to give maximum covalence of 4.Since the carbon skeleton of alkanes is fully 'saturated’ with hydrogens, they also called saturated hydrocarbons.
Paraffins:
Alkanes contain strong C-C and C-H covalent bonds. therefore, this class of hydrocarbons are relatively chemically inert. Hence they are sometimes referred to as Paraffins (Latin, para = little and affinis = reactive)
NOMENCLATURE:
The IUPAC system is much the same for all families of organic compounds.
The steps for alkanes are as follows:
1) Select the longest chain of carbon atoms that will be called as the parent chain. As treating with alkanes the suffix “ane” will be added at the end of name.
2)Number the chain of carbon such that the substituent is given the lowest possible number.
3)If more than one substituent is present, they are written in alphabetical order.
4)The cyclic hydrocarbons are given the prefix cyclo, which appears directly in front of the base name.
METHODS OF PREPARATION:
2-REDUCTION OF ALKYL HALIDES:
When alkyl halide is treated with Zn dust, the halogen atom is substituted by hydrogen atom and alkane is formed as a product.
3-DECRABOXYLATION OF CARBOXYLIC ACIDS:
When the sodium salt of a carboxylic acid is heated strongly with sodalime (NaOH+CaO),a molecule of CO2 is split off as carbonate and an alkane is formed.
4-HYDROLYSIS OF GRIGNARD REAGENT:
Alkyl magnesium halides (Grignard reagents) are obtained by treating alkyl halides with magnesium in anhydrous ether. These on treatment with water give alkanes.
5-WURTZ SYNTHESIS:
Higher alkanes are produced by heating an alkyl halide (RX) with sodium metal in dry ether solution. The general reaction and examples are given below:
6-COREY-HOUSE ALKANE SYNTHESIS:
An alkyl halide is first converted to lithium dialkyl cooperate ,LiR2Cu.This is then treated with an alkyl halide to give an alkane. The general reaction is given below:
7-KOBLE’S SYNTHESIS:
CHEMICAL REACTIONS OF ALKANES:
1) Halogenation:
This involves the substitution of hydrogen atoms of alkanes with halogen atoms.
Alkanes react with chlorine in the presence of ultraviolet light or diffused sunlight or at temperature of 300-400◦C yielding a mixture of products.
CHLORINATION
2)NITRATION :
This involves the substitution of a hydrogen atom of alkane_NO2 group.When a mixture of alkane and nitric acid vapours is heated at 400-500◦C,one H atom on the alkane is replaced by a nitro group.This process is also known as VAPOUR PHASE NITRATION.
3) OXIDATION (COMBUSTION):
When ignited in the presence of excess oxygen,alkanes burn to form carbon oxide and water.Large quantity of heat (heat of combustion) is also evolved. The general reaction is given below:
4) ISOMERISM:
Normal alkanes are converted to their branched –chain isomers in the presence of aluminium chloride and HCl at 25◦C . The reaction is given below:
PYROLYSIS(CRACKING�����5)PYROLYSIS(CRACKING):
It involves the breaking of c-c bond and C-H bond to form lower alkanes and alkenes. The reaction is given below:
6) AROMATIZATION:
PHYSICAL PROPERTIES OF ALKANES
IDENTIFICATION TESTS
PHARMACEUTICAL APPLICATIONS:
Following are the pharmaceutical applications of alkanes:
.
ALKENES�
Eman shahid
Humra Shehzad
ALKENES
Alkenes are the hydrocarbons that contain at least on carbon-carbon double bond, which is referred to as unsaturation. The double bonds in alkene are at high energy than single bonds that’s why alkenes are more reactive. The simplest alkene is Ethene:
IUPAC ALKENE NOMENCLATURE:
PREPARATION OF ALKENES:
Alkenes are generally prepared by ß-elimination reactions in which atoms from the adjacent carbons are removed resulting in the formation of double bond. Following are the methods for the preparation of alkenes:
1)Dehydration of Alcohols:
Heating alcohols with a strong acid cause them to lose a water molecule (to dehydrate) and result in formation of a double bond.
2)Dehydrohalogenation of Monohalides:
Removal of HX from monohalide by Alcoholic KOH or NaNH2 results in the formation of alkene.
5)PARTIAL HYDROGENATION OF ALKYNES:
Hydrogenation f alkynes in the presence of Lindler’s catalyst give alkene.
6)By Grignard’s Reagent:
Venyl chloride with alkylation with copper alkyl give alkene.
3)DEHALOGENATION OF VICINAL DIHALIDES:
Dihalides undergo dehalogenation when treated with Zn dust and form alkene.
4)Kolbe’s Electrolysis:
The electrolysis of sodium succinate gives alkene.
7)CATALYTIC CRACKING:
Cracking is given the name to breaking up large hydrocarbons to smaller hydrocarbons and more useful bits. This is achieved in presence of high temperature and high pressure. The larger hydrocarbons are broken down in fairly random ways into smaller hydrocarbons.
8)Witting Reaction:
REACTIONS OF ALKENES
Due to the presence of double bond, alkenes usually give Addition Reactions. Addition reactions are mostly Exothermic. The general electrophilic addition reaction for alkenes is as follows:
Following are the electrophilic addition reactions given by alkenes:
1)Addition of Hydrogen Halide:
When alkene is reacted with hydrogen halide (HBr) the double bond breakes and hydrogen and bromine atom attach to the carbon atoms forming the double bond according to Markovnikove’s rule:
MARKOVNIKOVE’S RULE:
It is stated as:
“The addition of proton acid to the double bond of an alkene results in the formation of product in which the acid hydrogen bound to that carbon atom that already has the greater number of hydrogen attach”
The general Reaction of addition of HX to alkene is:
The mechanism of the reaction is:
2)Addition of Hydrogen:
When hydrogen is added to alkene, the double bond of alkene is broken and alkane is produced. The catalysts used for this reaction are Ni, Pd or Pt.
The industrial application of this reaction is that we prepare vegetable ghee (saturated compound) by the hydrogenation of oil (unsaturated compound)
Oil Vegetable ghee
3)Addition of Halogens:
When alkenes are reacted with halogens, the double bond breaks and halogen atoms get attach to adjacent carbon atoms forming the double bond. The general reaction is as follows:
The mechanism of this reaction is given below:
4) Addition of Water:
The addition of water to alkenes in the presence of a strong acid as a catalyst results in the formation of alcohol as a product. The general reaction is as under:
The mechanism of the reaction is given below:
5) Sulphonation:
When alkene is treated with concentrated sulphuric acid the reaction is called as Sulphonation (addition of sulphuric acid):
Alkyl hydrogen sulphate can be converted into alcohol by boiling in water.
6) Epixidation of Alkenes:
An epoxide is a three membered ring containing oxygen. When alkenes are treated with peroxyacid, the double bond of alkene is broken and it results in the formation of epoxide.
The mechanism of the reaction is:
7) Ozonolysis:
Ozone also gives double and this mild oxidizing reagent gives adlehydes or ketones as products.
The mechanism of this reaction is given below:
8) Oxidative Cleavage:
If either acid or heat is used with permanganate, cleavage and further oxidation can occur producing ketones and aldehydes. The aldehydes further oxidized to carboxylic acid. The reaction is given below:
e.g.
9) Dimerization and polymerization of Alkenes:
Alkenes in the presence of strong acids can be made to produce polumers.
An electrophile (H+) can be added electrophylically to the double bond, the carbocation is attacked by another alkene double bond.
Depending on the reaction conditions dimers and polymers are formed.
Table for polymerization of alkenes:
IDENTIFICATION TESTS OF ALKENES�
Alkenes are unsaturatedhydrocarbons which contain one or more carbon-carbon double bonds. You might also meet the alkynes which contain a carbon-carbon triple bond. Unsaturated compounds are much more reactive than the corresponding saturated alkanes. This is the principle behind the first two tests used to identify unsaturated compounds. The most common test involves shaking the compound with some bromine water. With an alkene we see rapid decolourisation of the original orange solution. |
In both of the last tests it is important to avoid using too much solution. This is particularly important where the alkene being tested is a gas and there may not be much in the test tube. 1 centimetre depth of solution is easily enough to see any colour change. The colour changes are the results of chemical reactions. If you half fill your test tube with bromine water, there may be too much bromine for the alkene to react with. The reaction will be incomplete and the solution will not decolourise.
Also bear in mind that acidified potassium manganate(VII) is an oxidizing agent and would be decolourised on reaction with easily oxidized materials like aldehydes and some alcohols. However, this reaction is usually much slower than the reaction with alkenes and will usually require heating.
REACTION WITH ACIDIFIED POTASSIUM MANGANATE(VII) SOLUTION
A similar test involves shaking the compound with potassium manganate(VII) solution acidified with dilute sulfuric acid. The initially purple solution is decolourised by alkenes. It is important to ensure that the potassium manganate(VII) solution is well acidified or a brown colouration may result. A 50:50 mixture of 0.02M potassium manganate(VII) and 1M sulfuric acid will work well.
Another indication of unsaturation can be found when the substance is burnt. Unsaturated compounds tend to burn with sooty, luminous flames. This is perhaps not surprising as unsaturation has the effect of decreasing the number of hydrogen atoms in the compound. This increases the proportion of carbon, and it is the carbon which creates the sooty, luminous flame. This effect becomes more significant as the degree of unsaturation increases |
COMBUSTION
Applications:
ALKYNES
ALKYNES
IUPAC NOMENCLATURE
METHODS FOR PREPARATION:
2. DEHALOGENATION OF TETRAHALIDES:
3.REACTION OF CALCIUM CARBIDE WITH WATER:
5.REACTION OF SODIUM ACETYLIDE WITH 1 ALKYL HALIDE:
CHCCGGR - C ΞC- Na + R-X → H-CΞC-H +Na-X
REACTIONS OF ALKYNES
1.HYDROGENATION:
Hydrogenation is the addition of hydrogen to alkyne.by this reaction alkene and alkane both are prepared.
PPP
CHCHΞCH + H2 Ni/Pt CH2=CH2
CH2=CH2 + H2 CH3-CH3
HH2 / NI
2.HALOGENATION:
3.ADDITION OF HYDROGEN HALIDE:
Hydrogen chloride, hydrogen bromide and hydrogen iodide react with alkyne and form gem-dihalide.
Br
Ι
H-CΞC-H CH3-C-H (ethylidene bromide)
Ι
Br Gem- Bromide
HBR
4.HYDRATION OF ALKYNE:
Acetylene is used as raw material for the manufacturing of chemicals ,particularly acetaldehyde in case of terminal alkyne. Mono and disubsituted alkyne yield ketone . The reaction occur in the presence of dil. H2SO4 and Hg at 190 C. The reaction is given below:
5.ADDITION OF ACETIC ACID:
Acetic acid adds to acetylene to form vinyl acetate, a monomer which can be polymerized to polyvinyl acetate (PVA).
6.ADDITION OF HCN:
Several molecules like HCN are added to alkyne in the presence of a catalyst ( CuCl+HCl ). By this reaction, an important polymer acronytrile is obtained. The reaction is given below:
CHΞCH + CH3-COOH CH2=CH-O-C=O
Ι
CH3
H2SO4
150C
7.OZONOLYSIS:
Ozonolysis of alkyne produces two molecules of carboxylic acids with the cleavage of the carbon carbon triple bond. The reaction is:
H-CΞC-H + HCN CH2=CH-CΞN
CuCl in HCL
80 C
(Acrylonitrile)
During the reaction, an unstable five membered ring called ozonoid is form which is on further reaction produce carboxylic acid.
8.ALKYNIDE FORMATION WITH HEAVY METAL:
Acidic alkynes react with heavy metal ions , chiefly Ag+ and Cu+ to form in soluble alkynides . as given in the following reaction:
9.OXIDATION WITH KMNO4:
Alkynes undergo oxidation reaction with KMNO4 and yield acids and ketones as given in the reaction:
IDENTIFICATION TESTS FOR ALKYNES
Tests for the Presence of a Triple BondThe presence of a triple bond in any hydrocarbon makes it unsaturated and the compound gives Baeyer’s test. However, alkynes are characterised by certain specific tests described below.With ammoniacal silver nitrate: Alkynes give a white precipitate of silver acetylide with ammoniacal silver nitrate.With ammoniacal cuprous chloride: Alkynes give a red precipitate of cuprous acetylide with ammoniacal cuprous chloride…..
What happens when alkyne reacts with bromine water?These compounds undergo addition reaction with bromine water. Addition of bromine to an alkene or alkynes results in the formation of vicinal dibromide which decolourises bromine water.
PHARMACEUTICAL APPLICATIONS: