Alkanes Ashis Kumar Podder

Alkanes Alkanes are carbon compounds that contain only single bonds. The simplest alkanes are hydrocarbons – compounds that contain only carbon and hydrogen. Hydrocarbons are used mainly as fuels, solvents and lubricants:

ALKANES belong to a family of hydrocarbons that are expressed by a general formula: C n H 2n+2 CH 4, C 2 H 6, C 3 H 8, C 4 H 10 etc. All the bonds present in alkanes are σ-bonds (sigma) due to sp 3 hybridization. Bond angles = o Bond length = 1.09A ⁰ (C-H) Bond length =1.54A ⁰ (C-C) Sometimes referred as paraffins

First few members of Alkanes

3-Dimensional Structure of Alkanes

Molecules that differ only by rotations around single bonds are conformational isomers. The different arrangements of the atoms in space that result from the rotation of groups about C-C axis are called conformations or conformation isomers. 1.Staggered 3-Dimensional Structure of Alkanes

The barrier to rotation about the carbon-carbon bond in ethane is 3 Kcal/mole. 3-Dimensional Structure of Alkanes

Different formulas of Alkane

General naming of alkanes

hexanes C 6 H 14 common names CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 2 CH 3 n-hexane isohexane CH 3 CH 3 CH 3 CH 2 CHCH 2 CH 3 CH 3 CCH 2 CH 3 ????? CH 3 neohexane CH 3 CH 3 CHCHCH 3 CH 3 ???? General naming of alkanes

Here comes the need of a systematic naming system known as IUPAC nomenclature (Geneva, 1920) IUPAC = International Union of Pure and Applied Chemistry

IUPAC System of Naming Organic Compounds

IUPAC nomenclature names of radicals (alkyl groups): CH 3 - “methyl”CH 3 Cl methyl chloride CH 3 OH methyl alcohol, etc. CH 3 CH 2 - “ethyl” CH 3 CH 2 CH 2 - “n-propyl”

IUPAC rules for naming alkanes: 1.parent chain = longest continuous carbon chain  “alkane”. 2.branches on the parent chain are named as “alkyl” groups. 3.number the parent chain starting from the end that gives you the lower number for the first branch (principle of lower number). 4.assign “locants” to the alkyl branches. 5.if an alkyl group appears more than once use prefixes: di, tri, tetra, penta…; each alkyl group must have a locant! 6.the name is written as one word with the parent name last. The names and locants for the alkyl branches are put in alphabetic order (ignore all prefixes except iso) separating numbers from numbers with commas and letters from numbers with hyphens.

hexanes C 6 H 14 IUPAC names CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 2 CH 3 (n-hexane) (isohexane) n-hexane2-methylpentane CH 3 CH 3 CH 3 CH 2 CHCH 2 CH 3 CH 3 CCH 2 CH 3 (no common name) CH 3 3-methylpentane (neohexane) 2,2-dimethylbutane CH 3 CH 3 CHCHCH 3 CH 3 (no common name) 2,3-dimethylbutane

“classes of carbons” 1.primary carbon (1 o ) – a carbon bonded to one carbon 2.secondary carbon (2 o ) – a carbon bonded to two carbons 3.tertiary carbon (3 o ) – a carbon bonded to three carbons 4.quaternary carbon (4 o ) – a carbon bonded to four carbons 1 o 4 o CH 3 CH 3 CH 3 CHCH 2 CH 2 CCH 3 1 o CH 3 3 o 2 o

Classification of hydrogens, halides – hydrogens or halides are classified by the carbon to which they are attached. 1 o CH 3 CH 3 CHCH 2 CH 2 CH 3 1 o 3 o 2 o 2 o 1 o CH 3 CH 2 CHCH 3 sec-butyl bromide 2 o bromide Br CH 3 CH 3 CCH 3 tert-butyl chloride3 o chloride Cl

Some Methyl and Ethyl Compounds

 non-polar or only weakly polar,  cannot form hydrogen bond  relatively weak intermolecular forces  lower mp/bp;  mp/bp increase with size (MW); decrease with branching  Specific gravities also increases with increasing molecular weight.  At room temperature: C 1 – C 4 are gases C 5 – C 17 are liquids > C 17 are solids (wax like)  Alkanes are water insoluble  Soluble in non-polar solvents Physical properties of Alkanes

Petroleum Composition of natural gas: 1.Methane 80% 2.Ethane 13% 3.Propane 3% 4.Butane 1% 5.C 5 to C 8 alkanes 0.5% 6.Nitrogen 1.3% Composition of Petroleum: 1.Alkanes (30% - 70%) 2.Cycloalkanes (16% - 64%) 3.Aromatic Hydrocarbons (8% - 15%) 4.S,N,O compounds The oily mixtures of hydrocarbons in its crude form is called petroleum or crude oil. It is separated into fractions by fractional distillation. Accompany natural gas-largely methane

Products from fractional distillation of petroleum: Fractionb.p. rangeCarbons natural gasbelow 20 o C 1 – C 4 petroleum “ether”20 – 60 o C 5 – C 6 ligroin60 – 100 o C 6 – C 7 raw gasoline40 – 205 o C 5 – C 10 kerosine175 – 325 o C 12 – C 18 gas oilabove 275 o C 12 & up lube oilnon-volaltile liquids asphaltnon-volatile solids cokesolid carbon

1.Hydrogenation of alkenes and alkynes: Substrate: alkene & alkyne Reactant: hydrogen Catalyst: nickel/platinum/palladium Temperature: ⁰ C Methods of Preparation of Alkanes

2. Reduction of alkyl halides: Substrate: Alkyl halide Reactant: nascent Hydroden Catalyst: Ni/Pt Methods of Preparation of Alkanes

3. Decarboxylation of Carboxylic acids: Substrate: sodium salt of carboxylic acids Reactant: sodalime (NaOH+CaO) 4. Hydrolysis of Grignard reagents: Substrate: Alkylmagnesium halides (obtained by reacting alkyl halide with magnesium in anhydrous ether) Reactant: water Methods of Preparation of Alkanes

5. Wurtz synthesis: Substrate: alkyl halide Reactant: Sodium In presence of ether produce higher alkane Symmetrical alkyl halide are used to prepare even carbon number containing alkanes 6. Kolbe’s synthesis: Substrate: sodium salt of carboxylic acids Condition: Electrolysis Methods of Preparation of Alkanes

7. Corey-House synthesis Substrate: Alky halide (two different) Reactant: Lithium dialkyl cupreate (R´-X should be 1 o or methyl) This synthesis is important because it affords a synthesis of a larger alkane from two smaller alkyl halides. Used to prepare unsymmetrical alkanes Methods of Preparation of Alkanes

Chemical Properties 1.Chlorination: alkanes with chlorine in presence of UV light / a temperature of ⁰ C. Methyl chloride and hydrochloric acid Methylene chloride and hydrochloric acid Chloroform and hydrochloric acid Carbon tetrachloride and hydrochloric acid Mechanism: Chain initiation Chain propagation Chain termination

2. Nitration: alkane and nitric acid vapor at ⁰ C produces nitroalkanes. 3. Sulphonation: prolonged reaction with fuming sulphuric acid gives alkanesulphonic acid. 4. Oxidation (Combustion): produces carbon dioxide, water and large amount of heat. Reaction should be in presence of excess oxygen Chemical Properties

5. Isomerisation: normal alkane converted into their branched chain isomer in the presence of aluminium chloride and HCl at 25 ⁰ C. 6. Pyrolysis (Cracking): decomposition by heat is called pyrolysis. When this process is applied to alkane then it is known as cracking. In absence of air alkanes are heated at high temperature, lower alkane, alkene and hydrogen are produced Temperature ranges from ⁰ C. Chemical Properties

7. Aromatisation: 6 to 10 carbon atoms containing alkanes are converted into benzene and its homoloues at high temperature and pressure and in presence of catalyst When n-hexane is passed over platinum supported over alumina 600 ⁰ C, benzene is produced. Chemical Properties

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