(I) Reduction of alkyl halides:
(RX where X = F, Cl, Br or I)
(Substitution of halogen by hydrogen)
This may be done in three different ways
(i)Reduction by for example, zinc and acetic or hydrochloric acid, zinc and sodium hydroxide, zinc-copper couple and thanol.
(ii) Reduction by reducing agents like LiAlH4, NaBH4 etc
(iii)Using organometallic compounds like Grignard Reagent.
(II) Hydrogenation of alkenes in the presence of Pd or Pt:
This addition is an example of heterogeneous catalysis involving syn- addition.
The stereospecificity of the reaction is that the addition of hydrogen to the double bond occurs in syn fashion without disturbing the configuration at the chiral carbon.
(a) Wurtz Reaction:
An ethereal solution of an alkyl halide (preferably the bromide or iodide) is treated with sodium, when alkane is obtained. For example,
R1 – X + R2 – X + 2Na → R1 – R2 +2NaX
(b) Corey-House Synthesis:
A superior method for coupling is the Corey-House Synthesis which could be employed for obtaining alkanes containing odd number of carbon atoms (unsymmetrical alkanes).
R -X +2Li → R– Li + + LiX
2R– Li + + CuX → (R)2 CuLi + LiX
R2 CuLi + 2R’ – X → 2R – R’ + LiX + CuX
(c) Kolbe’s electrolytic method:
A concentrated solution of the sodium or potassium salt of a carboxylic acid or a mixture of carboxylic acids is electrolysed. For example,
R1CO2K + R2CO2K + 2H2O → R 1 – R 2 + 2CO2 + H2 + 2KOH
Decarboxylation of carboxylate salts:
By heating a mixture of the sodium salt of a carboxylic acid and soda-lime, alkanes can be obtained.
R CO2Na + NaOH (CaO) →Δ RH + Na2CO3
(I)The first four alkanes (methane to butane) are colourless gases, the next thirteen (pentane to heptadecane) are colourless liquids and those containing 18 carbon atoms or more are solids at ordinary temperatures.
(II) Their boiling point show gradual rise as the carbon content increases.
(I) Nitration: Under certain conditions alkanes react with nitric acid, when a hydrogen atom will be replaced by a nitro-group, NO2. This process is known as nitration.
(II) Sulphonation: It is the process of replacing hydrogen atom by a sulphonic acid group, SO3H. Sulphonation of a normal alkane from hexane onwards may be carried out by treating the alkane with oleum (fuming sulphuric acid).
(III) Oxidation: All alkanes readily burn in excess of air or oxygen to form carbon dioxide and water.
On the other hand, controlled oxidation under various conditions, leads to different products.
(IV)Halogenation: Chlorination may be brought about by photo irradiation, heat or catalysts, and the extent of chlorination depends largely on the amount of chlorine used. A mixture of all possible isomeric monochlorides is obtained, but the isomers are formed in unequal amounts, due to difference in reactivity of primary, secondary and tertiary hydrogen atoms.
The order of ease of substitution is
Tertiary Hydrogen > Secondary Hydrogen > Primary Hydrogen
Alkenes are hydrocarbons whose molecules contain the carbon- carbon double bond. They are also called as olefins.
METHODS OF PREPARATION OF ALKENES
(I)Cracking of Petroleum: Cracking of petroleum hydrocarbons is the source of commercial alkenes.
(II) Partial Hydrogenation of Alkynes: Alkynes can be partially reduced to alkenes by the use of reagents like Lindlar’s catalyst, P-2 catalyst with H2, Na in liquid NH3 etc.
(III) Dehydrohalogenation of Alkyl Halides (1, 2 – Elimination):
When isopropyl bromide is treated with hot concentrated alcoholic solution of a strong base like potassium hydroxide, propylene is obtained.
(IV) Acid Catalysed Dehydration of Alcohols:
Heating an alcohol with a strong acid like H2SO4 causes elimination of water and form an alkene. Elimination of water is called dehydration.
(V) Debromination of Vicinal Dibromides:
Vic-dibromides undergo debromination when they are treated with a solution of sodium iodide in acetone or a mixture of Zn dust in acetic acid (or ethanol).
(VI) Heating Quaternary (tetra alkyl) Ammonium Hydroxides:
Alkenes can also be formed by heating tetra alkyl ammonium hydroxides with the simultaneous elimination of trialkyl amine. This reaction involves Hoffmann’s elimination and product will be governed by Hoffman’s rule.
The first three members are gases, the next fourteen members are liquids and the higher ones are solids. They are colourless and odourless (except ethylene which has a faint sweet smell), practically insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum ether, etc.
GENERAL CHEMICAL PROPERTIES OF THE ALKENE
The addition of hydrogen to unsaturated compounds normally involves heterogeneous catalysis by finely divided metals such as Ni, Pt, Pd, Ru, Rh etc.
CH3 – CH = CH – CH3 →H2/Ni CH3 (CH2)2CH3
(II) Addition of Hydrogen Halides
Hydrogen halides (HCl, HBr and HI) add to the double bond of alkenes.
The addition of HX to alkenes follow Markownikov’s rule, according to which “the negative part of the unsymmetrical reagent goes to that carbon atom which bears lesser number of hydrogen atoms”.
(III) Addition of Hydrogen Bromide In Presence of Peroxides
The addition of HBr to propene, MeCH = CH2(1), under polar conditions ( in absence of peroxide) yields 2 bromopropane. However, in the presence of peroxides (or under other conditions that promote radical formation) however, the addition proceeds via a rapid chain reaction to yield 1-bromopropane. This addition of HBr in presence of peroxide is generally referred to as the peroxide effect leading to anti-Markownikov addition.
(IV) Addition of Water
(a) Acid Catalyzed Hydration
The acid catalyzed addition of water to the double bond of an alkene is a method of preparation of low molecular weight alcohols. The addition of water to the double bond follows Markovnikov’s rule in those case where rearrangement is not involved
Alkenes react with mercuric acetate in the presence of water to give hydroxymercurial compounds which on reduction yield alcohols.
With the reagent diborane, (BH3)2 or disubstituted borane (R2BH) alkenes undergo hydroboration to yield trialkylboranes, R3B, which on oxidation gives alcohols. For
(V) Addition of Bromine and Chlorine
Alkenes react rapidly with bromine at room temperature and in the absence of light.
(VI) Halohydrin Formation
If the halogenation of an alkene is carried out in aqueous solution (rather than in CCl4),
(a) Syn hydroxylation:
KMnO4 can also be used for syn hydroxylation.
(b) Anti hydroxylation:
(VIII) Oxidative Cleavage By Hot Alkaline KMnO4
A more widely used method for locating the position of double bond in an alkene involves the use of ozone (O3).
Ozonolysis can be of either of reductive type or of oxidative type.
The alkynes are unsaturated hydrocarbons that contain one triple bond. They have the general formula CnH2n-2 and the triple bond is known as the ‘acetylenic bond’..
METHODS OF PREPARATION OF ALKYNES
1. Dehydrohalogenation of vic-Dihalides or gem-Dihalides
A gem-dihalide A vic-dihalide A vinyl
2. Dehalogenation of vic-Tetrahalogen Compounds
This reaction has the drawback that the halogen compound is itself prepared by halogen addition to alkynes. The mechanism for dehalogenation has been taken up in the topic “alkenes”..
3. Alkyl Substitution in Acetylene or 1-alkynes
4. By the action of water on calcium carbide
CaC2 +2H2O → CH ≡ CH +Ca(OH)2
5. By Heating chloroform or iodoform with silver metal
CHX3 + 6Ag +CHX3 →Δ CH ≡ CH + 6AgX
6. Kolbe’s electrolytic methods
The physical properties of alkynes are similar to those of the corresponding alkenes. The lower members are gases with boiling points somewhat higher than those of the corresponding alkenes. Terminal alkynes have lower boiling points than isomeric internal alkynes and can be separated by careful fractional distillation.
GENERAL CHEMICAL PROPERTIES
Acetylene adds on hydrogen in the presence of a catalyst, the reaction proceeding in two stages.
C2H2 →H2/Ni C2H4 →H2/Ni C2H6
(II) Addition of HX
Acetylene combines with hydrogen bromide to form vinyl bromide first and then ethylidene dibromide
CH ≡ CH + HBr → CH2 = CHBr →HBr CH3CHBr2
Peroxides have the same effect on the addition of hydrogen bromide to acetylene as they have on alkenes.
(III) Addition of Halogen (Br2, Cl2)
(IV) Addition of H2O
When passed into dilute sulphuric acid at 60°C in the presence of mercuric sulphate as catalyst, acetylene adds on a molecule of water to form acetaldehyde.
(VII) Oxidation to Carboxylic Acids
(IX) Polymerization Reactions
When acetylene is passed into dilute hydrochloric acid at 65°C in the presence of mercuric ions as catalyst, vinyl chloride is formed.
CH ≡ CH + HCL →Hg2+ CH2 = CHCl
Hydrocarbons that consists of both aliphatic and aromatic groups are known as arenes. Toluene, ethyl benzene and isopropyl benzene are alkyl benzenes while vinyl benzene styrene) is an alkenyl benzene. The aliphatic portion of these compounds is commonly called the side chain.
METHODS OF PREPARATION OF ARENES
Preparation of Alkyl Benzenes
(a) Friedel-Craft’s Alkylation
Substitution of hydrogen by alkyl group in aromatic compound is known as Friedel-Crafts alkylation.
(b) Friedel-Craft’s Acylation
Substitution of hydrogen by acyl group in aromatic compound is known as Friedel-Crafts Acylation.
Preparation of Alkenyl Benzenes
(a) Styrene (vinyl benzene) can be prepared as
(b) Alkenyl benzenes that have their side-chain double bond conjugated with the benzene ring are more stable than those that do not have.
(c) Alkenyl benzenes can also be obtained by the dehydrohalogenation of haloalkyl groups attached to benzene ring. The elimination is governed by Saytzeff rule i.e. more substituted alkene is formed in greater amount.
GENERAL CHEMICAL PROPERTIES
Halogenation of the Ring
When halogenation of the alkyl or alkenyl benzenes is to be carried out in ring, a lewis acid catalyst is required. Since alkyl and alkenyl groups are ortho and para directing, so the incoming electrophile (Cl+ or Br+ in chlorination and
bromination respectively) will be oriented at either ortho or para position.
Halogenation of the Side Chain
Chlorine and bromine can also be made to replace hydrogens of the methyl group of toluene. Side chain halogenation takes place when the reaction is carried out in the absence of Lewis acids and under conditions that favour the formation of radicals
Additions to the Double Bond of Alkenyl Benzenes
In the presence of peroxides, hydrogen bromide adds to the double bond of 1-phenyl propene to give 2-bromo-1-phenyl propane as the major product
In the absence of peroxides, HBr adds in just the opposite way.
Oxidation of the Side Chain
Strong oxidizing agents oxidize toluene to benzoic acid. The oxidation can be carried out by the action of hot alkaline potassium permanganate. This method gives benzoic acid in almost quantitative yield.