Alkane

Introduction Alkanes are saturated hydrocarbon, that is they contain only carbon-hydrogen (C-H) and carbon-carbon (C-C) single bonds The general formula of alkane = CnH2n+2 Where n is the number of carbon

Nomenclature The IUPAC names of alkanes with unbranched chain of carbon atoms consists of two parts: A prefix: Indicates the number of carbon atoms in the chain The suffix –ane to show the compound is a saturated hydrocarbon; alkane

The prefix that correspond to the number of carbon atoms in a chain 1 Meth- 2 Eth- 3 Prop- 4 But- 5 Pent- 6 Hex- 7 Hept- 8 Oct- 9 Non- 10 Dec- 11 Undec- 12 Dodec-

Molecular formula and structural formula Name Molecular formula Condensed structural formula Methane CH4 Ethane C2H6 CH3CH3 Propane Butane Pentane Hexane Heptane

Alkyl group An alkyl group is a radical that has general formula CnH2n+1, formed by removing a hydrogen atom from alkane Alkyl group are named by dropping the suffix –ane from the parent alkane and adding the suffix –yl. For example alkyl group CH3-CH2- is named ethyl

Alkyl groups Alkane Name of alkyl group Condensed structure Methane Methyl CH3- Ethane Ethyl CH3-CH2- Propane Propyl CH3-CH2-CH2- Isopropane Isopropyl CH3-CH- | CH3 Butane Butyl CH3-CH2-CH2-CH2- Isobutane Isobutyl CH3-CH-CH2- Pentane Pentyl CH3-CH2-CH2-CH2-CH2-

Rules for naming branched Locate the longest continuous chain of carbon atoms which determines the parent name for the alkane For example substituent Parent chain

4. Thus the alkane named is 3-methylhexane and not 4-methylhexane Number the carbon atoms in the parent chain beginning from the end that is nearest to the substituent so that the substituent is attached at the lowest number carbon atom. 3. Use the number obtained in 2, to state the location of the substituent group. 4. Thus the alkane named is 3-methylhexane and not 4-methylhexane Methyl group 1 2 3 4 5 6 6 5 4 3 2 1

5. When two or more alkyl groups are present are of the same kind, use the prefix di-, tri-, tetra-, and so on. The numbers indicating the position of the alkyl groups are separated by commas. 1 2 3 4 5 6 2,4-dimethylhexane

6. When two or more alkyl groups are present, each group is given a number corresponding to its location on the longest chain. The groups should be listed alphabetically in the name of the alkane. i.e. ethyl before methyl. 1 2 3 4 5 6 4-ethyl-2-methylhexane

Write the IUPAC names for the following alkanes

Naming cycloalkanes Alkanes having one or more rings of carbon atoms are called cycloalkanes. Cycloalkanes are named by placing the prefix cyclo- to the names of the corresponding alkane. For example Cyclopropane (Skeletal formula)

1-ethyl-3-methylcyclohexane 1,3-dimethylcyclohaxane 1-ethyl-4-methylcyclohexane

Name the following cycloalkanes

Classification of carbon and hydrogen atoms A carbon is classified as primary, secondary, tertiary or quaternary depending on the number of carbon atoms bonded to it. Primary Primary Secondary Tertiary Quaternary

Exercise How many primary, secondary, tertiary and quaternary carbon atoms are there in 2,2,4-trimethylpentane? 5-primary carbon atoms 1-secondary carbon atoms 1-tertiary carbon atoms 1-quaternary carbon atom

General preparation of alkanes Alkanes can be prepared by decarboxylation of the sodium carboxylate salt with soda lime. Decarboxylation involves the removal of the carboxyl group, -COO- and replacing it with a hydrogen atom RCOONa + NaOH  RH + Na2CO3

2. Wurtz reaction: The action of sodium on alkyl halide in ether 2 R-X + 2 Na  R-R + 2NaX For example

Physical properties The lower alkanes (methane to butane), are colorless gases at room conditions The higher member (C5 to C18) are liquids and solids with pleasant smell

Physical properties Boiling point Alkane molecules have low polarity. Only weak van der walls forces are present between molecules. Hence the boiling point of alkane is lower than those of other organic compounds As the number of electrons in the molecule increases (with increasing number of carbon atoms), the strength of these attractive forces also increases. More energy is needed to break this forces between the molecules when boiling. Hence the boiling point rises with increasing number of carbon atoms in the molecule. The boiling points of unbranched alkanes show a regular increases with increasing number of carbon atoms in the molecules

Physical properties Melting point Like boiling points, the melting points of alkanes also increase with an increase in the number of carbon atoms Density Alkanes are the least dense of all groups of organic compounds and have densities less than water Solubility Alkanes are almost insoluble in water because of their low polarity and their inability to form hydrogen bonds. However liquid alkanes are micscible with each other

Exercise Arrange the compounds below in order of increasing boiling point As the number of carbon atoms increases, intermolecular forces between molecules become greater, resulting in increasing boiling point. Therefore propane<pentane<nonane

Chemical properties Combustion of Alkane Alkanes do not react with air at room temperature but if heated, they burn readily to give carbon dioxide and water in an exorthermic reaction. C3H8 + O2  3CO2 + 4H2O Try: C2H6 C4H10

Halogenation of alkanes Substitution is a reaction where an atom or a group of atoms in a molecule is substituted by a different atom or group of atoms. The reaction takes place in light (uv light) due to chemical inertness For example: CH4 + Cl2  CH3Cl + HCl CH3Cl + Cl2  CH2Cl2 + HCl CH2Cl2 + Cl2  CHCl3 + HCl CHCl3 + Cl2  CCl4 + HCl

Exercise Write halogenation substitution for the reaction of one mole of propane with one mole of chlorine

The mechanism of chlorination of methane:- Free radical substitution The mechanism consists of three steps:- Initiation, Propagation and termination Initiation: Involves the photochemical homolytic fission of the Cl-Cl bond in the chlorine molecule The two chlorine atoms in the bond. The bond will break homolytically producing two chlorine atoms, each with one unpaired electron. Known as a free radical. It is easier to break the Cl-Cl bond (+242 kJ/mol) than the C-H bond (+435 kJ/mol). Thus in the initiation step only chlorine radicals are produced although both the chlorine and methane molecules are exposed to sunlight.

The dot indicates the unpaired electron Step 1 The dot indicates the unpaired electron The half arrows shows the movement of the electrons

Chain propagation The chlorine radical formed is very reactive. It is reactive enough to break the C-H bond when it attacks the methane molecules, producing hydrogen chloride, HCl and a methyl radical

The methyl free radical can react with a chlorine molecule to produce chloromethane, CH3Cl and a new chlorine free radical.

This chain reaction continues as follows

Chain termination Free radicals can combine with each other and bring the chain to an end –termination steps

Summary Alkanes are saturated hydrocarbons and may exist as unbranched chains, branched chain or rings A mixture of alkanes is found in crude oil and natural gas

Summary Radical substitution reactions have three stages: Initiation – Where radicals are formed Propagation – Where products are formed and the radicals are regenerated Termination – Where radicals are removed