Dr Manal F. AbouTaleb Alkanes 1 Introduction 2 Nomenclature of Alkanes 3 Physical Properties of Alkanes 4 Preparation of Alkanes 5 Reactions of Alkanes Dr Manal F. AbouTaleb

Alkanes Alkanes are the hydrocarbons of aliphatic row. Alkanes are hydrocarbons in which all the bonds are single covalent bonds (-bonds). Alkanes are called saturated hydrocarbons. The simplest one, methane (CH4), is also the most abundant. All carbon atoms are sp3-hybridized All bond angles are 109.5°

Alkyl groups Alkyl groups are formed by loss of a hydrogen atom from the corresponding alkane . General formula of alkyl group : CnH2n+1 ( e.g. CH4 Methane – 1 H = -CH3 Methyl group ) Alkyl groups are named by dropping the -ane suffix of the alkanes and adding the suffix -yl. Methane becomes a methyl group, ethane an ethyl group, etc.

Unbranched alkyl groups

Nomenclature Of Branched Alkyl Groups Three-Carbon Groups - Methylethyl is the systematic name; isopropyl is a common name. - Numbering always begins at the point where the group is attached to the main chain.

Propane Butane

2. Four-Carbon Groups

2. Four-Carbon Groups

IUPAC Nomenclature IUPAC (International Union of Pure and Applied Chemistry) names: 1- The unbranched alkanes (homologous series) 2- Branched alkanes Suffix of Alkanes added ane

1. Straight-Chain Alkanes Nomenclature of Alkanes 1. Straight-Chain Alkanes Name Number of carbon atoms Structure Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane 1 2 3 4 5 6 7 8 9 10 CH4 CH3CH3 CH3CH2CH3 CH3(CH2)2CH3 CH3(CH2)3CH3 CH3(CH2)4CH3 CH3(CH2)5CH3 CH3(CH2)6CH3 CH3(CH2)7CH3 CH3(CH2)8CH3

Alkanes are either acyclic or cyclic. Nomenclature of Alkanes Alkanes Alkanes are molecules(hydrocarbons) that contain only C-C and C-H  bonds. Alkanes are either acyclic or cyclic. Acyclic alkanes (not cyclic) have the formula CnH2n+2 (where n = an integer)

2. Branched-Chain Alkanes Nomenclature of Alkanes 2. Branched-Chain Alkanes 1. Select the longest possible straight chain; this gives the parent name for the alkane 2. Number the parent chain beginning with the end of the chain nearer the branched chain 3. Use the number obtained by application of rule 2 to designate the position of the branched chain When two or more branched chains are present, give each branched chain a number corresponding to its position on the parent chain When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on

Nomenclature of Alkanes 1. Select the longest possible straight chain; this gives the parent name for the alkane 2- methyl hexane 3,4- dimethyl heptane 3- methyl heptane 2. Number the parent chain beginning with the end of the chain nearer the branched chain 4- ايثيل –3- ميثيل هبتان 7 6 5 4 3 2 1 4-ethyl-3- methyl heptane

Nomenclature of Alkanes 3- Use the numbers obtained by application of rule 2 to designate the position of the branched chain In writing the full name the root name is placed last; the substituent group, preceded by the number indicating its location on the chain, is placed first. 14 14

Nomenclature of Alkanes 4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain. - The substituent groups are listed alphabetically. 5. When two substituents are present on the same carbon, use the number twice.

Nomenclature of Alkanes 6. When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on. e.g.

3,3- Dimethyl hexane 2,7,8-Trimethyl decane 1 2 3 4 5 6 Nomenclature of Alkanes 1 2 3 4 5 6 3,3- Dimethyl hexane 10 9 8 7 6 5 4 3 2 1 2,7,8-Trimethyl decane

Nomenclature of Alkanes 7. When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents.

Nomenclature of Alkanes 8. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference.

Nomenclature Of Cycloalkanes Monocyclic Compounds 1. Cycloalkanes with only one ring:

Substituted cycloalkanes: 1) Number the ring beginning with the substituent first in the alphabet, and number in the direction that gives the next substituent the lower number possible. 2) When three or more substituents are present, begin at the substituent that leads to the lowest set of locants.

Physical Properties Of Alkanes and Cycloalkanes 1. A series of compounds, where each member differs from the next member by a constant unit, is called a homologous series. Members of a homologous series are called homologs. 2. At room temperature ( 25 °C) and 1 atm pressure, the C1-C4 unbranched alkanes are gases; the C5-C17 unbranched alkanes are liquids; the unbranched alkanes with 18 or more carbon atoms are solids.

Boiling Point Higher members have higher boiling points Reason: Physical Properties of Alkanes Boiling Point Higher members have higher boiling points Reason: Increase in molecular mass Increase in intermolecular force

Physical Properties of Alkanes Branched-chain alkanes have lower boiling points than straight-chain alkanes ∵ molecule is more compact  surface area   van der Waals’ force   boiling point 

Higher members have higher melting points Physical Properties of Alkanes Melting Point Higher members have higher melting points Reason: Increase in molecular mass Increase in intermolecular force

insoluble in water and highly polar solvents Physical Properties of Alkanes Solubility Alkanes: non-polar compounds insoluble in water and highly polar solvents soluble in non-polar solvents like benzene, 1,1,1-trichloroethane Density All alkanes and cycloalkanes have densities less than 1 g cm–3 at 20°C.  Petroleum floats on water surface Alkanes are less dense than water and swim on top of water

Preparation of alkanes Catalytic Hydrogenation From Alkyl Halides Gringard reagent Reduction Wartz Reaction From Alkenes & Alkynes Catalytic Hydrogenation

From Alkenes & Alkynes Hydrogenation of unsaturated hydrocarbon:

A) Reduction of alkyl halides 2) From alkyl Halides A) Reduction of alkyl halides

B) Hydrolysis of Grignard Reagent Grignard reagent is an alkyl magnesium halide compound, R-Mg-X - The Grignard reagent is formed when a solution of an Alkyl Halide (R-X) is allowed to stand over a metallic magnesium in the presence of dry ether - Then Grignard reagent react with water or alcohol to form alkane

C) Wurtz Reaction تفاعل فورتز This is the reaction of two alkyl halides (R-X) with metallic sodium to give symmetrical alkanes. The wurtz reaction is a poor method for the preparation of unsymmetrical alkanes 

Preparation of Alkanes Reforming Reforming is a process in which straight-chain alkanes are heated under pressure in the presence of a platinum catalyst. The chains break up and reform to give branched-chain molecules.

Combustion Complete combustion : 29.5 Reactions of Alkanes Combustion Complete combustion : Alkanes react with sufficient oxygen to give carbon dioxide and water through a complex series of reaction with the release of a large amount of energy. General formula:

incomplete combustion CH3-CH2-CH3 + 4 O2 ——>  CO2   + 2 CO   +  4 H2O   + heat

Reactions of Alkanes Chlorination Methane reacts with chlorine under diffuse sunlight or heating but not in dark A mixture of products (CH3Cl, CH2Cl2, CHCl3, CCl4) is formed with the replacement of hydrogen by one or more chlorine atom If methane is in excess, CH3Cl is major product If chlorine is in excess, CCl4 is major product

Reaction Mechanism: Free Radical Substitution Reaction Reactions of Alkanes Reaction Mechanism: Free Radical Substitution Reaction Mechanism of reaction : 1. Chain initiation homolytic fission of chlorine molecules by heat or light into two chlorine radicals Step 1:

Reactions of Alkanes 2. Chain propagation Step 2: Step 3: steps 2 and 3 repeat hundreds or thousands of time due to formation of the reactive intermediate in each step  chain reaction

Reactions of Alkanes Further substitution occurs:

Reactions of Alkanes

Reactions of Alkanes

two free radicals combine to form a neutral molecule Reactions of Alkanes 3. Chain termination two free radicals combine to form a neutral molecule the chain reaction is terminated

The reaction mechanism is shown as follows: 1. Chain initiation Reactions of Alkanes Example Write down the reaction mechanism involved in the chlorination of ethane in the presence of diffuse sunlight. Answer Solution: The reaction mechanism is shown as follows: 1. Chain initiation Chain propagation

Reactions of Alkanes Solution: Chain termination

Mechanism For the Radical Halogenation of Methane