4 Allkanes Structure

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Lecture 4

Alkanes

Structure and Nomenclature

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Alkanes and Alkane Isomers

Alkanes: Compounds with C-C single bonds and C-H bonds only

(no functional groups) Connecting carbons can lead to small or large molecules

General formula CnH2n+2where the number of Cs is n

Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds

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Name These Shapes

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Name These Molecules

Decane

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HYBRIDISATION OF ORBITALS

The electronic configuration of a

carbon atom is 1s22s22p2

1 1s

22s

2p

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HYBRIDISATION OF ORBITALSThe electronic configuration of acarbon atom is 1s22s22p2

1 1s2 2s2p

If you provide a bit of energy you canpromote (lift) one of the s electrons

into a p orbital. The configuration isnow 1s22s12p3 1 1s2 2s2p

The process is favourable because the of arrangement of electrons;

four unpaired and with less repulsion is more stable

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Alkane IsomersCH4 = methane, C2H6 = ethane, C3H8= propane

The molecular formula of an alkane with more than threecarbons can give rise to more than one structure

C4 (butane) = butane and isobutane ( 2- methyl

propane)C5 (pentane) = pentane, 2-methylbutane, and 2,2-

dimethylpropane

Alkanes with Cs connected to no more than 2 other Cs arestraight-chain or normal alkanesAlkanes with one or more Cs connected to 3 or 4 Cs are

branched-chain alkanes

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Structural IsomersConstitutional isomers: Isomers that differ in the connectivity atoms

Branched vs straight chains

Cpds other than alkanes can be constitutional isomers of one anotherThey must have the same molecular formula to be isomers

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Structures of Alkanes

Condensed formulas an alkane can be represented in a brief/ condensed form or inmany types of extended form

A condensed structure does not show bonds but lists atoms,such as

CH3CH2CH3 (propane)

CH3C(CH2)2CH3 (2,2-dimethylpropane)

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Naming Alkanes

Compounds are given systematic names by a process that uses

Prefix-Parent-Suffix

Follows specific rules

Named as longest possible chain

Carbons in that chain are numbered in sequence

substituents are numbered at their point of attachment

Compound name is one word Complex substituents are named as compounds would be

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Nomenclature of alkanes

No. of Carbons Formula Name (CnH2n+2)1 Methane CH4

2 Ethane C2H6

3 Propane C3H84 Butane C4H10

5 Pentane C5H12

6 Hexane C6H14

7 Heptane C7H168 Octane C8H18

9 Nonane C9H20

10 Decane C10H22

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Alkyl Groups

Provides the substituent groups on the root name

Alkyl group obtained by removal of one H from an alkane (apart of a structure)

General abbreviation R (for Radical, an incomplete speciesor the rest of the molecule)

Name of the alkyl group is obtained by: replacing -aneending

of alkane with -ylendingCH3is methyl (from methane)

CH2CH3is ethyl from ethane

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Types of Alkyl groups Classified by the connection site (carbon type)-

Primary, secondary, tertiary and quartenary a carbon at the end of a chain (primary alkyl group)

a carbon in the middle of a chain (secondary alkyl group)

a carbon with three carbons attached to it (tertiary alkyl group)

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Properties of Alkanes

Called paraffins (low affinity compounds)because they do not react as most chemicals

They will burn in a flame, producing carbon

dioxide, water, and heat They react with Cl2 in the presence of light toreplace Hs with Cls (not controlled)

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Physical Properties

Boiling points and melting points increase as size ofalkane increases

Forces between molecules (temporary dipoles,dispersion) are weak

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Cycloalkanes

Cycloalkanes : alkanes that have carbon atoms thatform a ring (called alicyclic compounds)

Simple cycloalkanes rings ofCH2units, (CH2)n,or CnH2n

Structure is shown as a regular polygon with thenumber of vertices equal to the number of Cs (aprojection of the actual structure)

cyclopropanecyclohexanecyclopentane

cyclobutane

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Complex Cycloalkanes

Naturally occurring materials contain cycloalkane structures

Examples: chrysanthemic acid (cyclopropane),prostaglandins (cyclopentane), steroids (cyclohexanes andcyclopentane)

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Properties of Cycloalkanes

Melting points are affected by the shapes and theway that crystals pack so they do not changeuniformly

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Naming Cycloalkanes

Count the number of carbon atoms in the ring and the number inthe largest substituent chain.

If the number of carbon atoms in the ring is equal to or greaterthan the number in the substituent, the compound is named asan alkyl-substituted cycloalkane

For an alkyl- or halo-substituted cycloalkane, start at a point ofattachment as C1 and number the substituents on the ring sothat the secondsubstituent has as low a number as possible.

Number the substituents and write the name

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Cis-Trans Isomerism in

Cycloalkanes Rotation about C-C bonds in cycloalkanes is limited

by the ring structure

Rings have two faces and substituents are labeledas to their relative facial positions

There are two different 1,2-dimethyl-cyclopropaneisomers, one with the two methyls on the same side(cis) of the ring and one with the methyls on oppositesides (trans)

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Stereoisomers Compounds with atoms connected in the same order but which

differ in three-dimensional orientation, are stereoisomers The terms cis and trans should be used to specify

stereoisomeric ring structures

Recall that constitutional isomers have atoms connected indifferent order

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THE END