Alkanes - Lecture notes 1 PDF

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Summary

AlkanesStraight or branched chain hydrocarbons with only single covalent bonds between the carbon atomsSuccessive compounds in the alkane series differ by one carbon and two hydrogen atoms. Homologous seriesGeneral Formula: CnH2n+2 (open chain) where n corresponds to the number of carbon atoms in th...

Description

Alkanes Straight or branched chain hydrocarbons with only single covalent bonds between the carbon atoms Successive compounds in the alkane series differ by one carbon and two hydrogen atoms. Homologous series General Formula: CnH2n+2 (open chain) where n corresponds to the number of carbon atoms in the molecule Two main sources of alkanes 1.   

Natural Gas: Formed by anaerobic decay of plant and animal matter by microorganisms Composition varies but main constituent is Methane (85-90%) Economically significant amounts of methane are being generated from our waste

2. Petroleum:  Formed from the decomposition of plants and animals over millions of years  Composition varies widely, contains a mixture of hydrocarbons with smaller amounts of sulphur and nitrogen containing organic compounds  Mixtures are separated by fractional distillation Naming Alkanes     

Select longest continuous chain as parent, all other alkyl chains as branches Number carbon atoms in parent chain starting with end closest to the first carbon with branch Name each branch alkyl group and designate position on parent chain (e.g., 2-methyl means a methyl group attached to carbon 2) When several different alkyl groups attached list them alphabetically When same alkyl group occurs more than once, indicate with prefix (di, tri etc)

Constitutional (Structural) Isomer  

When two molecules have the same molecular formula but different 3D structure they are isomers When molecular formula are the same but arrangements of atoms differ they are called constitutional (structural) isomers

Branched Isomers     

Smaller surface area, don’t pack as well together Fewer intermolecular interactions so lower boiling and melting points cf straight chain Less Van de waal forces due to a lower surface area on branched molecules The longer the chain, the more interactions (van der Waal forces) hence more energy is required to disrupt At room temp – Small molecules = gases – Larger molecules = liquids or solids

Intramolecular Forces •

Organic molecules are drawn static, but they are constantly moving!

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Chemists speak of “free rotation” around single bonds. This is known as conformation (type of isomer)

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With alkanes of higher complexity than methane the number of conformations is infinite.

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Although there are infinite conformations, two are readily identifiable: -Eclipsed -Staggered

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Some arrangements of a molecule in space are energetically more favourable than others

Conformational Isomers   

Conformational isomerisation A form of stereoisomerisation Different isomers formed by rotations about single bonds

Cycloalkanes   

Hydrocarbons can form ring structures as well as straight and branched chains General formula CnH2n with sp3 hybridised carbons and are therefore not planar (cf benzene) Restricted rotation of C-C bonds in cyclic structures leading to two configurations.

Cyclic Compounds 

Chair (All six conformations are gauche (skew). Energetically more favourable)



Boat (4 gauche and 2 eclipsed conformations

Cis-Trans Isomers  

An added complication with cyclic compounds is seen with two or more substituents. Give rise to compounds with different physical properties – Geometric isomers

Reactions of Alkanes 

Combustion

CH4(g) + 2 O2(g)







→

CO2 + 2 H2O

ΔH= -213 kcal/mol

-Burning, not exploding -Always results in just CO2 and H2O Cracking C16H34 C8H18 + C8H16 -The breakdown of a large alkane into smaller alkanes and alkenes -Small alkanes and alkenes are more useful than long ones Dehydrogenation CnH2n+2 CnH2n + H2 - Removal of H2 to form unsaturated hydrocarbons (more on these in later lecture) Halogenation CH4 + 4 Cl2 CCl4 + 4 HCl -Replacing a H with a halogen -Radical Reaction (Homolytic Fission) -Alkyl halides are useful as intermediates for the manufacture of compounds

Classification of Carbon in Alkanes

  

A primary carbon is bonded to one other carbon A secondary carbon has two carbons attached to it A tertiary carbon is bonded to three other carbons

Primary

Secondary

Hyperconjugation    

Radicals are stabilised by Hyperconjugation As the radical forms the carbon centre transforms from sp3 hybridised to sp2 hybridised Delocalisation of the bonding electrons from carbon next door by overlap with partly filled p orbital Net stabilising effect (less energy required to generate the radicals)...